How-To: Building a Pulse Width Modulated Dew Heater Controller

[iceman]

Al Sheehan (sheeny) has written a how-to article on Building a Pulse Width Modulated Dew Heater Controller.

You can read the article on the IceInSpace Projects (http://www.iceinspace.com.au/index.php?projects) page, or directly by clicking on the link below:

Building a Pulse Width Modulated Dew Heater Controller (http://www.iceinspace.com.au/index.php?id=63,285,0,0,1,0)

Thanks to Al for writing the article!

If anyone else would like to contribute a how-to, article, review or other content for the site, please contact me.

[iceman]

Article uploaded.

[barees63]

Nice Article Al! the kits are a much cleaner option than hacking veroboard that's for sure :thumbsup:

[sheeny]

Yes, could be Bruce! 20 years ago I might have had a go at "hacking" my own out of vero board but these days "insert tab A in slot B" suits me fine!:D

Al.

[Lester]

Okay you knowledgeable electricians. Why do you need one of these power controllers? I realise that they can alter the amount of power used, and therefore be turned up if needed to prevent dew. But wouldn't 2 seperate rings of resistors with one putting out 25% of total and another at 75% do the job? You could have either of the 2 on seperately, or both together to prevent dew.

I am always looking for the easy way.

[AndrewJ]

Gday Lester

This is my first post as i have just joined the IIS forums,
but i have been playing with designing and building PWM dewcontrollers
( manual and thermostatic ) for a few years now.

You dont "need" a fancy controller, but it makes life easier ( in some ways )
as you can set the heating level to suit the conditions.
Using seperate sets of switched resistors will work, but "you" need to switch them, you have limited settings ( hence can drain a battery when not reqd ), but as a benefit, they will produce no RFI.

PWM controllers allow you unlimited power levels ( almost ), can be easily converted into thermostatically controlled units ( so they only heat when required, thus preserving yr batteries ), but all PWM controllers put out RFI, depending on the type of heater element and wiring used.
This may or may not affect other things nearby.

As such, its a tradeoff in what you want.
Just remember, if you put too much heat into the OTA, you risk creating thermals.

Andrew

[Lester]

Welcome Andrew,

And thanks for your reply. Hope you enjoy your time in IIS as much as I am, there is always someone out there that can help.

[Geoff45]

I've just completed my controller. It heats the resistors OK, although I haven't used it under real conditions yet. One minor variation is that you can get a fused cigarette lighter plugs, so replacing the fuse is easier than opening the box.
Geoff

[Dennis]

I heard many years ago, that a popular dew heater controller had the design modified between the Mk III and Mk IV models to either minimise or remove RFI. It seems that some users reported “noise” in their CCD images when using the older controllers.

Cheers

Dennis

[iceman]

An addendum to this article has been supplied by Brendan (wasyoungonce).

Please check the article again if you have built, or plan on building this unit.

[sheeny]

Thanks to Brendan for doing this!:thumbsup:

He contacted me about the changes but I'm not an electronics wizz, and I suggested he submit it himself since I didn't feel comfortable trying to explain changes that I honestly don't understand too well myself.:whistle:

Onya Brendan!

Al.

[Astro78]

Awesome stuff. Followed these instructions here a few months ago and it works to perfection, plus looks really cool!

[troypiggo]

Sorry to resurrect such an old thread, but I'm looking at some dew heater solutions at the moment. I am a bit of an electronics noobie, but recently I successfully soldered my first Jaycar kit as a test run - it's a row of LEDs that indicate the charge level of my 12V battery. But I digress...

Understood Alan's article for the most part and reckon I could do that, but the addenda by Brendan was a little over my head. Could someone put that addenda in lay terms for a dummy like me? I mean, is there a problem if I build it as per the original article, or should I be making the adjustments in the addenda?



Does this mean you could, knowing the dew point, set a temperature somehow on the controller at which the heater would kick in? Any pointers in the right direction for that?

[wasyoungonce]

Hi Troy.

This circuit can be powered by anything up to around 28 volts but most people use 12 volts.

If you use a voltage higher than 12 Volts you can damage the Mosfets, that's why they use a 12 Volt regulator. It limits the amount of voltage to the gates of the Mosfets. This is stated in the documentation supplied with the kit.

However, if you are using a 12 Volt source, then you do not need the regulator. You can omit it (as it may interfere with the circuit) & just put a link wire as shown in the diagram (http://www.iceinspace.com.au/image_zoom.php?show=MmUxY2E3N2U1MGJ kYmY1ZmU0N2RjMTBjYjg1ZDE0YzUuanBnP3 dpZHRoPSI4MDAiIGhlaWdodD0iNTAzIg==) .

Hope this helps.:shrug:

edit:

Oooops forgot to mention that the circuit operates all the time when on at a level set by you. To make it "switch on" at a pre-set temperature would require a comparator circuit...a bit more complicated. You need this circuit "always on" as heat flows from hot to cold...you are constantly losing heat energy to the surrounding air (you not really losing energy but... close enough) & the heat is also flowing out from the optical tube.

[troypiggo]

Ok, so if I'm only ever powering from 12V deep cycle battery, I can just stick to the "original" kit. Thanks.

[AndrewJ]

Gday Troy



Sort of.
"YOU" dont need to know the dew point.
( and it changes over time anyway )
There are 2 basic ways to run a thermostatic heater
1) Keep corrector just hotter than ambient ( easy )
2) Keep corrector just above dew point ( harder )

I have made 2 types, one passive ( ie no PIC used ) and one PIC based
For the first, you set a temp differential, and the heater comes on only when reqd, and keeps the corrector X degrees above ambient.
This is simple to do but may heat when not reqd.

The Dewpoint version is more complicated ( and really needs a PIC controller ), as it has to measure relative humidity, and then calc dewpoint, and ensures the corrector stays X degrees above dewpoint.

The latter involves more calculations, but saves power as the heater is only used when absolutely necessary
For general use, just staying 1 to 2 deg above ambient is sufficient.
There are many ways to do this

Andrew

[troypiggo]

Gathering a shopping list at the moment for Jaycar when I get a chance to go in soon. The kit number is different to the Dick Smith one quoted in the tutorial. I think the Jaycar equivalent one is KC-5225 "10A 12VDC Motor Speed Controller" (http://jaycar.com.au/productView.asp?ID=KC5225).

I'm looking into the replacement for the trimpot as recommended by Alan. Looking in the Jaycar catalogue for potentiometers there are 2 different types of 24mm PCB mounting ones. "Log single gang (A)" and "Linear single gang (B)". Which one is correct? ie RP-3608 (http://jaycar.com.au/productView.asp?ID=RP3608) or RP-3508 (http://jaycar.com.au/productView.asp?ID=RP3508&keywords=rp-3508&form=KEYWORD)?

[peter_4059]

Troy - easiest way to find out is to get the dudes at Jaycar to test the trimpot in the kit with a multimeter and tell you what you need to replace it with. There are 2 variables - log/linear (not too important in this case) and resistance (very important to get the right level). I can check it out if you've already got the kit.

Peter

[troypiggo]

I went into Jaycar last weekend, and while the guy who served me was (trying to be) most helpful, he threw so many different alternatives at me that it confused me more than helped. Went in asking for something specific and he went round in circles trying to get me to use everything except what I asked for! I'm now back to sticking with the tutorial and will just ask for what's on the list, no variations from what I want. Now I am just doing the research on what I want :)

I'm pretty sure it's the 5k trimpot. That much I can sort out on the spot by opening up the kit in store. It's the log/linear thing that I'm not sure about.

[peter_4059]

Troy,

I finally found the catalog reference to the PWM unit I use - it's from RS Components and comes already assembled - Como Drills Motor Voltage Regulator. RS stock number 238-9816 $35.50. This also has a surface mounted trimpot that I replaced with a panel mount version. This unit is a bit smaller footprint than the Jaycar version.

http://australia.rs-online.com/web/search/searchBrowseAction.html?method=sear chProducts&searchTerm=238-9816

Peter

[mldee]

Peter, you're a lifesaver.
This is just the answer! I can't be bothered with kits anymore, or rather kit components can't be bothered with my eyesight, and this looks like the answer. Price is right too.

I can see uses for not only dew heaters, but motor controllers for focussers, etc.....

[peter_4059]

No problem Mike. Here's a pic of mine all dressed up....

[erick]

Troy, you want a 5k linear pot. I need one myself - just built up the Jaycar kit last night. It's been on the shelf for a couple of years - now I need it to control an individual heater. After the build (I havn't made up a circuit in years), it checked out fine.

[troypiggo]

Thanks for the confirmation. That's what I suspected but always better if someone with more experience agrees with me... :)

[erick]

I recall log pots were mostly for volume control in audio systems, but I'm sure there are many other uses as well.

[mldee]

Thanks Peter, looks good. Now if you could just find the RS number for a USB-controlled version.....:D

[Glenhuon]

I used a 5k linear on mine. Gives a good wide adjustment range.

Bill

[MrB]

Yes the 5k linear pot is the one you want. It is marked 'B5k'
Cat# RP3508 for the 24mm or RP7508 for the 16mm one.
Helped a customer with this very thing the other day... though not for a dew heater.
No it wasn't me that served you Troy :D

[troypiggo]

Sweet. Thanks again guys.

[hikerbob]

Has anyone played with one of the Arduino variants as the basis for a Dew heater controller. http://www.oceancontrols.com.au/controllers/arduino2.htm#Arduino%20Pro%20Mini for some info on them.

I've not tried one yet but with multiple analog inputs and digital input/output they seem like they could be combined with multiple temperature sensors and a power stage and possibly a humidity sensor to create a fairly potent controller.

The Arduino Pro Mini http://arduino.cc/en/uploads/Main/ArduinoProMini.jpghas the following specs
Summary

Microcontroller - ATmega168
Operating Voltage - 3.3V or 5V (depending on model)
Input Voltage - 3.35 -12 V (3.3V model) or 5 - 12 V (5V model)
Digital I/O Pins - 14 (of which 6 provide PWM output)
Analog Input Pins - 6
DC Current per I/O Pin - 40 mA
Flash Memory - 16 KB (of which 2 KB used by bootloader)
SRAM - 1 KB
EEPROM - 512 bytes
Clock Speed - 8 MHz (3.3V model) or 16 MHz (5V model)

Also a digital temperature/humidity sensor for $42US http://www.sparkfun.com/commerce/images/products/SHT15Breakout-01-L_i_ma.jpg
http://www.sparkfun.com/commerce/product_info.php?products_id=8257

I'm interested to find out if anyone else has played with this technology in case I'm missing something fundamental here.

Bob

[peter_4059]

Looks interesting Bob but will it handle the load alone or is something else needed (not sure what a power stage is?). The power requirement for an ED80 is about 6 Watts. Also looks like it needs some programming?

Peter

[hikerbob]

The output power is very low so I assume that you would then need to include something to turn the low power pulse width modulation (40mA I think) into something suitable for driving heaters. I've not done much looking for a circuit for that yet but for a start something like this might be useful http://www.sparkfun.com/commerce/product_info.php?products_id=8907. There are probably better options around and I don't think that one would use the Arduino's onboard PWM.

Definately some coding involved but if it's suitable that's something some of us may be able to brainstorm and share.

I've found another local supplier with a different range of boards and seemingly lower prices http://www.littlebirdelectronics.com/collections/arduino/. One is a bare bones board for $22.46 but I'm more tempted by the $44.99 version with the USb port etc. They also have the temperature/humidity sensor I mentioned earlier for $65. I'm assuming that you would use one of them to get the ambient temp and humidity and a straight temperature sensor to get the temperature near each of the heater strips.

I've not yet had much of a look around for a straight temperature sensor but this one from SparkFun looks promising (looks like it's out of stock at the moment though) http://www.sparkfun.com/commerce/product_info.php?products_id=245.
There is sample code for reading the sensor from an Ardunio at http://www.arduino.cc/playground/Learning/OneWire

I also just found this little device http://www.sparkfun.com/commerce/product_info.php?products_id=8855 which on first impressions might do a similar job but with a small LCD screen built in. I've not seen it before. I've not quite worked out if a seperate programmer device is needed for this.

Bob

[Nico]

Try this one, its cheap to build, and all parts probally set you back no more than $15. The heating element can be purchased from Jaycar, part no. WW4040. Also required is a length of heat resistant tubing to feed the element into, part no. WS5504.

The heating element is a special type of wire known as Nichrome wire whereby its resistance measured from one end increases as length increases. Its the same kind of stuff used in toasters, although you would definitely not want your element to glow red hot.

I feed the wire through the tubing then using some insulating type of flexible foam/rubber mat (roughly 25mm wide x dia of OTA) taped the tubing (with the element contained within) to the rubber/foam with a long strip of gaffa tape, then just folded the edges round the back to make it neat.

You'll need to solder the ends of the nichrome wire in order to make a good connection with the output of the PWM circuit. I tried a few ways (nichrome wire is difficult to solder) and found that by lightly removing the surface of the wire I was able to solder it to a couple of female spade terminals (remove the plastic insulation before soldering, and apply stacks of heat + solder) once cooled down just slide the insulation back on, or just use tape!

Stuff the circuit (made mine on a piece of viro board, also at Jaycar or dicksmith etc..) into some kind of plastic jiffy box and mount the led through the lid (so you can see the PWM in action), connect the load (element) and you're away.


http://www.backyard-astro.com/equipment/accessories/dewheater/dewheater.html

[montewilson]

I have two nichrome elements around my FSQ. It has never failed me and last year I upgraded the set-up so it has temp control for the tube and element.

I set the temp for the night and it stays at that temp! I focus it at the start of the night and it never varies - cool eh?

It has two digital temp controllers from a refrigeration unit and sensors on the metal of the tube. The whole tube is insulated. It looks ugly but is very effective.

[Omaroo]

It doesn't look "ugly" Monte - it looks positively industrial. :thumbsup: Doing the same to my 102 I think.

[montewilson]

The wiring is a bit of a mess but that's because I jerry-rigged it to use the 20Da someone lent me. You can see the controllers inside the box top right.

[montewilson]

Call me if you want any advice based on my experience of the set-up.

[Omaroo]

Yeah, cheers Monte :) I already have my home-made 4-channel PWM controller - all I have to do is build a feedback system for it.

Where's the foil look gone on yours? Both tubes were wrapped is Sisalation or something similar at IISAC, making them look like they were off a satellite. Very cool... :thumbsup: I take it that these images are old?

[montewilson]

Too much Shiraz mate! It has looked like this for a while including at IIS.

The big difference is the lack of the STL :sadeyes:

This was taken last week.

[Omaroo]

More likely not enough! LOL

Ok - my mistake. I can see your tubes are wrapped - but in a white material, not aluminium. I remember thinking that I had some foam rubber-backed aluminium that I was going to use on mine. Not yours. I became horribly confused and in a moment of indecision I posted what i did. Arrghhh!!

Maybe you're right.... to much shiraz...

[Glenhuon]

The resistance wire on those old boiling jug immersion heaters is ideal for the elements. You can pick them up pretty cheap at supermarkets and hardware shops. Cut off a length to give you 20 ohms resistance, about 9-10 watts at 12volt. Mine were made by cutting a 25mm wide piece off a PVC pipe and cutting it lengthwise to allow it to clip over the scope tube. Drill a couple of small holes in each end and and anchor one end of the wire by looping it through the holes. Wrap the wire evenly spaced around the 25mm (takes a couple of tries to get it right) and feed the other end through the holes again. Feed the connecting wires through the holes at each end to anchor them and solder to the element. I wrapped mine with some tubular hemming material from the local sewing shop and put a strip of waterproof tape on the outside. Works a treat and the soft material prevents scratching of the OTA. Does'nt get hot enough to damage the material but will melt plastic tape at full output if put in close contact with the wire. There should be enough length on the jug element to make at least 2 heaters for a 4" OTA.

Cheers
Bill

[Nico]

And if you want to take it a step further you could use a PIC to measure ambient temp and adjust o/p.

[montewilson]

Or measure the tube temp like I do this allows me to regulate the focus by regulating the tube temp.

I like the level of detail in the website. There is plenty of good information there. I have one comment about something that seems to get lost in all the effort to explain how to make the heaters and controllers. The effectiveness and efficiency of wire heaters is directly linked to how well the heat they generate gets into the metal of the scope. As many people use batteries this is super important. I have seen too many people with set-ups that warm the night air far more than the scope. By midnight they have flat batteries and a dewed up scope or worse they create heat shimmers that pass in front of the scope.

Long story short, consider installing the heater permanently by wrapping the wire around the tube (over a layer of tape to not short out on the tube) and then covering it with an insulating layer to not loose to much of the heat to air. When mine is heating, so much of the heat goes directly into the tube that the heating element never feels warm to touch. This is a pretty good test. If your element is warm to touch when it is on the scope the heat is not efficiently entering the scope.

[hikerbob]

I've made a start on a microcontroller based controller for dew heaters. Based on an Ardunio Duemilanovo and an SHT15 Temp/Humidity sensor. Fiddling with some code off the web I can measure Temp and humidity and calculate the dew point.

I'm also planning on using some Dallas 18B20 1-wire temperature sensors to measure the tube temp at the heaters. I've not yet tried out the 1-wire stuff but there is plenty of material on the web to support that. http://datasheets.maxim-ic.com/en/ds/DS18B20.pdf

I've not decided on how to control the heaters yet, an idea I've considered is for each heater unit to have 3 * 1-wire devices and a power switch device associeted with it. If I could get that to work I'd put one each of 1-wire eeprom, switch and temperature sensor at each heater. The eeprom would hold the addresses of the switch and temp sensor. The Arduino could search and find all the eeproms connected to the 1-wire bus and get the info it needs from them to know which temp sensor and switch were associated with each other.

The weakness in that is that I've not worked out how to take advantage of PWM to control the rate of heating. The strength is that I could have a number of heater unit's to plug into a common bus as suited the occasion.

I've still got to work out some aspects of the heaters, I found a supplier on ebay selling bulk rolls of nichrome wire and have made a start on one. I got some thermal transfer tape from Jaycar and wrapped it around some aluminium tube then wound nichrome wire onto that. I've covered that in some insulation sheet I have as leftovers from building a furnace. Some mechanical issues still bug me about that so I may revisit the whole thing.

I've attached a photo of the Ardunio, the SHT15 and an 18B20. I've also attached a screenshot of the software to control the Ardunio, in real use the Ardunio would run stand alone. I've attached the code I'm using at the moment as well for those who want an idea what's involved.

Bob

[tjmck]

Greetings from Canada!

Wow! All this time, I thought I was working on a uC - dew point based Dew Controller without any one else doing the same. I have chosen and aquired the Relative Humidity sensor (a Humirel HS1101) and temperature sensors (LM135s) and am now at the point of chosing a uC. This is where I hit a wall.

Do I choose a uC that can do the math (dew point calculations using relative humidity and ambient temperature requires logarithmic functions, especially at less than 50% RH), do I choose one that can do the A2D for the temperature sensors or do I choose one that can do the PWM for the O/P? The alternative is to choose one that can do it all - which is not really a problem (except for the math), if you are making a single O/P controller. I plan on making a 4x O/P controller (Dob primary, secondary, eyepiece and finder). I have now discovered http://www.awce.com/index.htm. They supply coprocessors for use with AVRs, PICs or any othe uC and have scaled my uP back to a simple I/O pin choice leaving the math and A2D up to the PAK-IX (combines ALU and 5x 10-bit A/D channels) and the PWM to the PAK-V (8 simultaneous PWM outputs). They do seem a bit expensive, however I am willing to pay more to simplify the design. The PWMs will need power switchs on the O/P - likely FETs.

The "stand alone" operation will raise the optics to a couple of degrees above the dew point temperature (or frost point temperature - which can be below the ambient air temperature below freezing) in a effort to preserve batteries and make the heating efficient. I believe I can fit the circuit onto a board about 2.5" by 4" in size and plan on mounting it in my power center (tool box with 2x 12v12AH batteries, power supply / charger) which travels with my scopes.

Regards,
Tom

[hikerbob]

Tom if you have not done so already have a look at the Ardunio.
It would seem to do the stuff you want to do. The basics don't need an external programmer and there is a big online community. There are other versions, some simpler and cheaper and I may eventually use one of them for the actual build and use the Duemilanove for learning and experimenting. A lot of work also seems to be happening with PIC's but
I've not worked out just what is needed to get going with them.

The broad specs of the Duemilanove are as follows
http://arduino.cc/en/Main/ArduinoBoardDuemilanove
Summary
Microcontroller ATmega168 (mines a 328)
Operating Voltage 5V
Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V
Digital I/O Pins 14 (of which 6 provide PWM output)
Analog Input Pins 6
DC Current per I/O Pin 40 mA
DC Current for 3.3V Pin 50 mA
Flash Memory 16 KB (ATmega168) or 32 KB (ATmega328) of which 2 KB used by bootloader
SRAM 1 KB (ATmega168) or 2 KB (ATmega328)
EEPROM 512 bytes (ATmega168) or 1 KB (ATmega328)
Clock Speed 16 MHz

After posting last night I breadboarded 4 or the temp sensors with a resistor and some jumpers, made three connections to the Ardunio and loaded some software from the web and was reading the temp from all four sensors with very little effort. This morning on the train I worked on merging the code for the SHT15 and the 18B20's but I've not had a chance to test the combined code yet.

The next step will be to make a decision about controlling the heaters.

Bob

[peter_4059]

Tom,

Do you have a feel for the accuracy of the temperature controller in terms of hysteresis? I suspect the difference between the complex (log) and simplified dew point calculations is only a couple of degrees and this is possibly within the control error of the temperature controller. You might be able to get away with the simple non-log dew point correlation opening up more options for your uC.

Peter

[hikerbob]

I have both types of sensors being measured now and have had it running in a loop for some hours without grief.

I'm still exploring options for controlling the heaters. If I skip the PWM idea there is a version of the temp sensor with two general purpose IO lines which could be used to drive a mosfet power switch - DS28EA00 and a few other options which might keep the circuits at the heater simple but still support fine control.

I've attached the current version of the code and one loop of output.

2-wire Temp = 20C
Humidity = 78%
Dew Point = 16C
1-wire DS18B20 Temp Sensor #1 = 20C
1-wire DS18B20 Temp Sensor #2 = 20C
1-wire DS18B20 Temp Sensor #3 = 20C
1-wire DS18B20 Temp Sensor #4 = 20C

[tjmck]

HikerBob, Peter,

Good points!
I will look at the differential between the simplified Dew Point Calculation and the more complex logarithmic method in the next couple of days. My application is more geared towards frost elimination which typically occurs with relative humidity well below the 50% which is the recommended floor for use of the simplified formula. I will fire up excel and do the comparison.
An no, I have not looked at the Arduino. I had planned on using a PIC or utilize the original Atmel MCU00100 Starter Kit which I have mothballed as well as AT90S8515s. Then I ran into the calculation complexity. Looks like I have some homework to do.
Will keep you posted.

Tom

[hikerbob]

Tom somehow I missed the attachment last night.
I've not checked on how this is implemented under the hood but the formula I'm using makes calls to a log function.

I've not got my head around the impacts of atmospheric pressure on dew point nor have I looked at wet bulb/dry bulb calcs but my impression is that I don't need great accuracy.

http://en.wikipedia.org/wiki/Dew_point#Closer_approximation

There is some notes on using a 1-wire device as an A/D for barometric pressure sensing at http://www.sensorsmag.com/articles/0501/34/

A range of I2C barometric pressure sensor's at
http://www.futurlec.com/Pressure_Sensors.shtml could work well if barometric pressure is important. Cheap and I'm already running a variant of the I2C for the Temp/Humidity sensor.

//--------------------------------------------------------------------
float calc_dewpoint(float h,float t)
//--------------------------------------------------------------------
// calculates dew point
// input: humidity [%RH], temperature [°C]
// output: dew point [°C]
{ float logEx,dew_point;
logEx=0.66077+7.5*t/(237.3+t)+(log10(h)-2);
dew_point = (logEx - 0.66077)*237.3/(0.66077+7.5-logEx);
return dew_point;
}

[tjmck]

HikerBob,

Looked into the Duemilanove. Wow! Have not dealt with uC for a while (evident from my AT90S8515 reference). Subsequently have ordered a Starter Kit and am downloading and trying to absorb the wide range of information available. Looks like this will make implementation simple on the hardware side.

All of my experience has been programming devices in Assembly language. I am looking forward to being able to use a higher level language. Am still looking into the LOG function call - not documented in the Atmel data sheet at first glance - which does not mean it is not a higher level translation of lower level commands. Do I remember my math on this - no!

Now for Dew Point calculations - I too was using the Wikipedia formulas as they are standard. On the praries of Canada, and especially during the winter months, it is unusual for the RH to get over 50%. However, with the memory of the Duemilanove (ATmega328), I can implement using the Simple approximation for RH above 50% and Closer approximation below 50%. I have discovered that there are no published approximations for frost point. This will likely have to be a table.

My eventual hardware implementation - final version - will likely be a custom PCB / Controller with bare minimum components supporting:
1) 4 channel control (with manual on/off selection).
2) Automatic dew/frost point temperature determination.
3) Optic temperature measurement.
4) Option for manual control by varying the PWM for each channel.
5) Channel on/off and heating on LED indicators.
6) Undecided on a possible 7 segment LED information readout (LCDs freeze here - not a good thing).

It will use the ATMEL ATmega328 with Duemilanove bootloader at the core. I sould probably review my sensor selections as well.

Thanks to you and Peter for setting me on a much simpler journey to the end goal. I find it so much better to develop and understand the details in something rather than to purchase a item that may not do exactly what I want it to do. As battery conservation during freezing temperature observing is a priority for me - no commercial product actually meets my needs.

Tom

[PeterN]

Hi Everyone,
Wow - everything from calculating dew and freze points to $15 do it on the cheap heater controllers in this thread...

What is my point? After people discover that I design electronics in my day job, I have been approched to suggest / help / design dew heater controllers for some of my Astronomy friends. (I actually designed a heater system for a telesope in Antarctica once, but that is another story)

OK, happy to help, but I am not going to build 5 different models on Veroboard in my kitchen. If we can get a minimum of 10 people interested in getting a professionally designed and built dew controller together, we could start a project.
Problem is can we find 10 people wanting the same requirements?
Goal is to get a quality controller for significantly less $ than buying one from the shop - final price depends on quantity and features as usual.

Post a quick reply or send me an email if you are interested and include a note on the key elements you need to have in your controller.
If there is enough interest I might open a new thread for this.

Clear skies,
Peter

[wasyoungonce]

Funny enough I am at this moment getting some cct boards made for a FET driven 4 channel PWM dew heater I designed & prototyped up.

I found that many FET (or transistor) PWM dew heaters switched the heater thru Vcc rail. Thus when the output driver device was off there is Vcc (in most cases +12V DC) sitting on the outer shell of the heater connector. Some/many of these commercial dew heater use a metal RCA connector & +12V sitting on this outer shell is recipe for trouble.

So I designed up a FET cct that switches heater thru earth with the outer shell connected to earth, thus the outer shell is always earth.

I am tossing up whether to market these as I could pump them out at pretty much $100 Aust.

A low tech (cheap) solution indeed.

[IvanTheTerrible]

New member here but I have made a few heaters before. I haven't read the whole thread so excuse me if this has been mentioned before. I use a PWM kit from Jaycar Electronics and for the element I salvage the core out of an electric blanket. My 10" LX50 has been using this setup for about 6 years without fail.

[wasyoungonce]

Problem is with the the jaycar cct & RCA connectors, 12V DC can be present on the outer shield of the RCA connectors (if the shell is metal)...see below. I knew this was a problem but really only did anything when I purchase some kendrick dew heater straps which had metal RCA plugs.

The Jaycar cct was not really designed for RCA type connectors more designed for something that is hardwired.

There are many Dew PWM ccts out there that switch Vcc & allow +12V on the RCA shield. In reality RCA plug connectors for this use are a poor choice adopted by amaturer astro nuts.

Some RCA plugs have plastic shells but usually even these have a metal skirt that can present the same problem, especially if you use an extender RCA cable wrapped around your tube.

An easy fix is to cover the RCA conectors with insulating heatshrink.

You will probably never have an issue but it would be very easy to short one of these RCA plugs to your mount or scope...passing +12V thru it.:scared:

Which would do wonders for all your valuable electronic equipment.



edit:

Oh damn....I forgot...and "Welcome Ivan"

[IvanTheTerrible]

Everything I have made is in plastic boxs and I use plastic RCA connectors and the heater element I use is encased in plastic too.



Ian

[astronut]

I use the multi pin/socket connectors, from Jaycar
The male/ female ends are shielded by the plastic housing that they are mounted into.
I've used this for many years with 100% success rate.:thumbsup:

(http://www.jaycar.com.au/productResults.asp?FORM=KEYWORD)

[peter_4059]

Is there a way to test the PWM dew controller (using a multimeter) to see if this is a problem? Would you see a voltage between the earth on the input to the PWM and the outer shell on the outlet when the load is turned down to minimum?

[IvanTheTerrible]

Considering my dew heater and scope share the same 12V source the only thing that will happen is the fuse in the heater will blow.

But the most interesting fact I'd like to make is that electric blankets are the best source of heater elements there is. It comes in a lattex rubber tube that makes it easy to use. Being nichrome means you have to crimp the ends because it's next to imposible to solder.

[PeterN]

Sounds good - can you give us some more info on the actual specs for your controller? A photo maybe? Max output currents, indicators, temperature controlled, under voltage protection/cutoff?

High-side switching - meaning you interrupt the 12V rather than the ground return - is the way to go and can avoid some EMC issues as well. Are the outputs in your controller short circuit proof? P-fets can be a bit sensitive and I have seen them go before the fuse at times.

Cheers,
Peter

[wasyoungonce]

Just measure the RCA outer shell to earth with the PWM on or off. If you see a pulsing 12V or 12V then just take care with your RCA connectors heat shrink cover them.

[wasyoungonce]

Hi peter...a very simple one at that.

I designed it for 5 Amps max. The P channel FETs can take much more but I limited it to this with the design of the PCB tracks in consideration & my needs in consideration This was originally just done for myself, cheap & simple & safe but it has taken on a life of itself.

PCB tracks can be changed but atm around 48 watts max is a fair power draw.

Just uses LEDs & pots for varying PWM output. Output is very linear from in I draw aqcross it's range. I mucked around with lots of PWM frequencies but settled on a low freq, below 10Hz as it provides the best visual feeback on pulsing LEDs. Low freq all so helps reduce noise onto the power lines.

No current foldback only fuse protection. I did consider it but decided not. There are current limit protected FTEs available but the I am really after a cheap simple solution.

No undervolts as well....although this is something I am going look into.

All is up in the air until I receive my cct boards.

I am looking at making this a 6 channel PWM, easy enough & allows more complexity with a bigger ccct board.

[wasyoungonce]

This is one of the major issues...heater elements & their cost. I suspect many people cringe at the cost of the big name dew heater straps.

I do!

I wish I could find the thin film resistor strips they use.

[AndrewJ]

Gday Peter



I have made a fair range of dew heaters now, both analog and digital control, with and without thermostatic control ( incl humidity / dew point control )
In nearly all cases, if you use PWM you will get RFI.
Even new commercial boxes marketed as having low RFI are affected
The main problem is not the box, its the wire going to the strap
as that radiates away like blazes, and PNP vs NPN wont fix it.
Secondly, with PNP switching, you need more circuitry if you want to use a microprocessor to drive it. Again a tradeoff

For anyone wanting a very simple 0-100% manual controller
an LM324, a pot, a few resistors and a mosfet is all thats required.
I have attached a piccy of the simplest one i make
Once you have the LM324 configured as a sawtooth generator
its a simple matter to add multiple outputs by just adding more
LM324s set up as comparators to drive the Mosfets.
Very cheap.

Andrew

[IvanTheTerrible]

Whats a cheap electric blanket worth? On ebay they go for $30. There is about 6 metres of insulated coiled nichrome in them.

[peter_4059]

Just tested my controller and sure enough as you decrease the output of the PWM unit you get increasing voltage (up to 11V) on the outer shell of the RCA outlet. Looks like I'll be changing out plugs this week! Thanks for the heads up.

[wasyoungonce]

It's a simple mistake by some heater designs that has propagated itself somewhat.

Glad to assist.

[wasyoungonce]

That is very true. Although I'm more inclined to work with the thin film stuff for ease of solder & they offer a bigger surface area of heat dissipation.

[hikerbob]

Peter, I'm doing this in part for the interest in doing so. I have the hope that I can get something out of it which is more effective than I can buy but mostly I'm enjoying the process of learning.

My preferences
- my own involvement for the fun of learning
- minimise power usage and don't do unnecessary heating (or miss heating when I should)
- extendable - I'd prefer not to be limited to a set number of ports although I can't see why I'd need many
- upgradable, if I find a better way of doing things I'd like to be able to go with it.
- I'd like to be able to just add in heater modules and have the controller manage them.

I've not yet decided if I'll add in digital readouts on the controller, I could add in an LCD screen (or 7-segment led panels) and show a few temperatures (ambient, dew point and individual element temps) but I've not put much thought into that yet. I have pondered small PCB's for the heater electronics but I'm still looking at options for that. If I do go PWM then I will probably need to do something for \\
that.

Bob

[PeterN]

Hi Andrew,

You are right, switching power causes interference. You are also right when you blame the cables for radiating most of it. Another possible path is throught the changing load on the battery depending where you connect your supply.
As far as reducing the radiation from the cable you could use shielded cable and connect the shield to ground - hence my preference for switching the high side and leave the gound connected through.
You could also add some softer switching at a lower frequency and reduce the spectrum you radiate - I guess that alone will take away a good deal of your problems.

If all that doesn't work and RFI is still critical you may have to suffer the loss of a linear regulator design.
I will be aiming to preserve battery power AND keep the design quiet. I am sure it can be done.

Cheers,
Peter

[Bassnut]

A PWM cycle time of minuites would do amply, to prevent RFI.

[wasyoungonce]

Lowering PWM freq should help a lot with EMI.

Most antennas use a standing wave design of 1/4wavelength. Thus making the power cable longer will reduce any standing wave forming even cancelling any waves forming.

Using 10Hz PWM and a 1/4 wave antenna (to optimise RFI) you would have a wave length of: C = fλ/4, if f = 10hz thus λ= 4C/10 = 1.2e^8m.. ( Edit: to form a 1/4 standing wave).

Obviously harmonics of 10Hz would form their own standing waves but I suspect it's more a EMI than RFI problem (edit: meaning noise feed back to the power supply as spikes rather than radiated interference).

Maybe using twisted pair power cable?

Any thoughts?

[MrB]

Yep. Higher frequency.
High frequency driver, say 10 or 20 kHz with decent low-pass L-C filter network on the output, all contained in a shielded case with shielded power cable = efficient, noiseless, variable DC power supply.

[Bassnut]

And why control a dew heater at higher frequency??. The thermal inertia would render that uneccesary I would have thought. More RFI, less efficient, no gain.

[MrB]

Less RFI actually, almost zero.
Waaaay more efficient than a linear regulator.
Thermal inertia? where did that come from... the heater would see close-to-pure DC.

[PeterN]

You are right Simon - higher frequency and filters and shielding would work to give you a near DC output - one small step to using a buck converter - meaning a type of switch mode power supply. Not the first thing I would consider for a heater given the more complex design, but actually worth looking at since you can design them to be reasonably quiet. (not on Veroboard though!)

Cheers,
Peter
P.S. twisted pair actually works quite effectively in reducing radiated emissions. Not much good for conducted emissions though, that needs filter components tailored to requirements.

[MrB]

Yes, exactly that, a buck converter.
Shouldn't be any more complex than what is already being discussed here.
The uC is already being discussed to generate the PWM.
The switching MOSFET is already there too, all that is needed is a freewheel diode, a cap and inductor(or more) for the filter network and it's done.
The purely resistive load simplifies things.

[wasyoungonce]

Yep...I was meaning "if the problem is unwanted RFI try twisted pair". All good.

[AndrewJ]

Gday Peter



Just for info, i have also played with these as well
I made a unit to drive my ETX to give a constant output irrespective of input ( piccy attached for info )
The problem with these for dewheaters is designing them to be "efficient" over a wide voltage range. ( And you still get RFI if you aren't careful )
I have some std purchased plugpacks that are noisy as hell.

I still reckon a high freq PWM with correct output filtering is a better bet for a dewcontroller. ( And its how nearly all the commercial ones work )

And as to RFI, i don't have any "problems" with it so far.
I know its there, but it hasn't affected my scope ( yet :whistle:)

Andrew

[tjmck]

Peter_4059,

Finally got around to comparing the Simple Approximation and the Better Approximation for dew point calculation. Below 50% RH, as stated for the Simple Approximation, the numbers get significantly out of wack. Example (for our somewhat extreme observing conditions on the praries of Canada - during the winter):

Ambient Temperature: -20 C
Relative Humidity: 10 % (not uncommon in the winter)

Simple Approximation: -38 C (frost point - not dew point)
Better Approxiation: -44 C

And of course the number diverge even more as the temperature drops further and the humidity gets lower.

I like observing during the winter months better than the summer as I can have the scope setup and be viewing by 6:00 PM. During our summer months, like now, it does not get dark until 10:00 PM.

I will be continuing on my project to build a dew / frost point dew heater and find a way to do the math with a uC. I have ordered an Arduino. It should be here next week. As I already have the sensors, I may have something breadboarded in a couple of weeks. Then I will make the decisions on the final temperature sensors and board modifications. I will produce PCBs (one off) once the design is settled. I expect I will have to make a further revision farther down the line with features I have overlooked.

I am using CAT-5 cable with RJ-45s to connect the heaters / temperature sensors. Using 3 conductors each for the power and ground to the heater and using 1 conductor for the sensor power and 1 conductor for the sensor data. The sensor ground is connected to the heater ground. Each RJ-45 contact is rated at 1A and the CAT-5 is already twisted pairs. The heaters are not drawing anywhere near what three conductors within the cable can handle.

Tom

[Rod]

Hi,

I was researching DIY dew controllers when I came across this thread. I have a few questions I hope someone here can answer. I have two Kendrick heaters - one for the eyepiece and the other for my quickfinder. I may add a resister to heat the secondary mirror. I want to experiment with some baffling first to see if that is necessary. So I need a controller that can potentially handle the three dew prevention devices.

Would the circuit described by Al Sheehan on the projects page be OK to use with these heaters? I was also considering the RS components version Peter uses:http://australia.rs-online.com/web/s...hTerm=238-9816 (http://australia.rs-online.com/web/search/searchBrowseAction.html?method=sear chProducts&searchTerm=238-9816)

I was a little concerned about the posts relating to RF noise. Is this a problem with either of these controllers?

BTW I have a 12 inch computerised truss Dob (Mel Bartels stepper system).

Thanks,

Rod.

[sheeny]

G'Day Rod,

The original PWM kit that I used is good to up to about 3A. For an EP, quick finder and secondary mirror heater it should do it easy.:thumbsup:

Al.

[disturbed13]

besides the fact that it keeps the dew off of the optics
whats the point?
its just a heater right?
(yes im a noob, dont kill me)

[sheeny]

Correct.

The key is to be able to adjust the amount of heat, so you can use as little as possible to prevent dew forming. It takes a lot more energy to get rid of due once it forms, than it does to prevent it. Use too much heat and you create thermal air flows that distort your vision/image.

Al.

[peter_4059]

Rod,

The RS controller will easily do the load you are talking about. I run four heater straps (2x4" and 2x2" diameter) no problems. You need to choose the type of connectors carefully as the -ve terminal goes to 12V relative to ground when you reduce the output to minimum (I think this is discussed in this thread).

Peter

[Mick]

Is it OK to fit a LED into the out put circuit to aid in adjusting the PW? The LED dims and brightens when the pot is adjusted and I've also added a on-off switch. Thanks for this well written article AL...:thumbsup:

[Rod]

Hi Peter and Al,

Thanks for the advice. My heatstraps have RCA connectors. One has exposed metal so I will cover that with heatshrink. I'll probably go with the RS components controller since it costs little more and saves time.

Rod.

[hikerbob]

I've got moving again on an Ardunio controlled PWM heater. I had the whole thing on hold for a while dithering over heater straps, I did some work on making my own and decided that I was going to spend a fair bit anyway and was no confident that I'd get a good result. I ended up getting a number of Dew-Not heater straps which so far seem great.

The controller is more complex than many find necessary but component count is low (and the ugly bit's are in the power stage which I'd need anyway) and I'm using it as a learning exercise to get across some of the technologies.

I've got the important parts of the dew heater controller working
- 4 independent PWM channels each with a dedicated temperature sensor.
- Dew point calculation based on ambient temp and relative humidity (I expect that the calculations will be suitable for the conditions I observe in)

I'm planning some extra work soon. A red on black serial LCD screen, a current shunt/sensor and an extra Ardunio are on order. I'm planning to
- Add voltage monitoring (2 resistors, some code and a hookup to an analog pin on the ardunio) to try and avoid running batteries right down.
- Add current monitoring, combined with the voltage monitoring and controlled cycling of the power channels and the controller should be able to work out what's hooked up to it and adjust the behaviour of the different channels to suit. I'm guessing that a 1.25" eyepiece will benefit from different heating patterns vs a large corrector plate or the front lens of a finder scope.
- Add the LCD so that I can see on the controller what's happening, I can monitor via a laptop currently but that's not always ideal or practical
- Tidy up some wiring
- Swap the ardunio over to a cheaper and slightly more basic model (and use the current one for developing something else)
- Fix some indicator led's which are not working
- Review the code to see if there are tidy up's I can do

Bob

[hikerbob]

I've now fitted the LCD screen and got the main values displaying on it. 16*2 characters is not a lot of space but I can fit the main values on at the same time (although I won't have space for supply voltage).

Next up will be fitting the current and supply voltage measuring components and making up a couple more temperature sensor leads.

Bob

[Tandum]

Here's one I'm knocking up for AlexN. The controller board is from Oatley Electronics, $25 delivered. 2 controlled outputs on top and 3 12v raw outputs on the left. Cheap and functional :)

[mldee]

Looks Like a nice kit. On a small side track, has anybody ever seen a gadget (kit or built) that is a USB-controllable PWM 12V supply? My thought is for a generic PC-controllable variable supply that can be used for focusers, dew heaters, lights, whatever.

I am aware JMI etc sell such an item for focusers, just wondered if there was a kit or similar project floating around so that a general purpose USB 'control box' could be built for the obs.

The world of RC hobby electronics has many such potential items, as does computer overclockers accessories.

If this is more suited to it's own thread, please feel free to let me know.

[erick]

This controller? K252 - DC MOTOR SPEED / HOT WIRE CONTROLLER KIT AUD18 - that's good. The Mosfet is in the kit?

http://secure.oatleyelectronics.com//product_info.php?products_id=625

[Starkler]

Why is rfi even being discussed as an issue here?

In any case interference to radio services is due to the speed of the switching transients, which can be smoothed out in circuitry. Whack a capacitor on each output.

If I were designing one I would prefer a faster cycle time in the hundreds of hertz to avoid transients on the dc supply affecting other electronics such as the argonavis.

[mill]

A couple of windings on a ferrite core will get rid of most of the rfi anyway.

[kinetic]

Geoff, possibly because it may show in your USB data feed coming
from your imaging device?

The last thing you want in your dome is something emitting RFI.
Modified webcams can pick up RFI really well because some of the
circuitry added to the CCD pins and timing chip act like wonderful
antennas.
The problem is made worse when you extend the CCD legs to
install it into it's own cold chamber or Peltier heatsink.
Trust me...been there.:sadeyes:

FWIW.

Steve

[Tandum]

Yep, that's the one. The mosfet on it is rated to 55amps :lol:

[AdrianF]

Sent for one of these about a week ago just waiting on delivery. Have you used one? Are they good? I have made a Dew Heater from some resistance wire (from JayCar) and some velcro strip and was going to use teh controller to control the heater.

Adrian

[Tandum]

Yep, I put a pair of them into my controller box and got the straps from hong kong along with some other stuff. However, last night I dewed up as I forgot to turn them on :rolleyes: I have an LED across the output with a 1.2K resistor and it is a good indicator of how much juice your pushing out.

[Starkler]

Steve thanks for the explanation. As I dont use cameras I didnt think of that.

I have worked in the radio comms business for many years and can tell you that rfi from a switching dew heater should be very easy to tame.

Its not so much the switching cycle speed, but radiated harmonics from the switching transient. This can easily be remedied.

[tjmck]

Bob,

I too have been busy on other projects (those pesky home improvement things) but aquired an Arduino and have been playing it. As you had stated previously, the Log function does allow me to calculate the dew point.

Curious about which temperature sensors you are using - the Dallas 1 wires or discrete? Are you measuring the relative humidity and again which sensor?

I had decided to use an LCD only for prove-in and not integrate it into the final project. I will integrate a ISP/communication port so I can monitor the project with the laptop. The final board will have the ATmega328 (with or without bootloader) implemented directly. May even be the possibility of a Astronomy Dew Heater Shield in the future!

Tom

[hikerbob]

Tom I've used Dallas 1 wire sensors for the strap temp measurements I've had a sensor wrapped in a small heater and I've not seen anything to suggest that the RFI is intefering with it.
I used the Sparkfun breakout board for SHT15 for the ambient temp/humidity sensor http://www.sparkfun.com/commerce/product_info.php?products_id=8257
Voltage sensing is a resistor divider circuit feeding an AD port on the Ardunio and the current sensing is a Sparkfun breakout for ACS712 http://www.sparkfun.com/commerce/product_info.php?products_id=8882 (I've not got that working correctly yet).
I'm using A Sparkfun Serial LCD unit http://www.sparkfun.com/commerce/product_info.php?products_id=812

Some overkill in this but I'm using it as a learning exercise, I've got a couple of other temp control projects in mind.

I'll attach my code (as is), as mentioned the current measurement stuff is not working properly and since adding in the current and voltage sensing I seem to have messed up the LCD (I suspect a connection problem).

Bob

[tjmck]

Bob,

I have already built all of my dew heater straps with an integral LM335 - before I new of the Arduino and the 1 wire temperature sensors. If I stay with the LM335, to keep ports free on the Arduino, I am thinking of multiplexing the inputs. Will likely use 3 straps at a time - corrector plate, eyepiece, x1 finder (Telrad or Rigel). My 4th output will be used to power a heater in my eyepiece case (will have to build an insulated eyepiece case). I have aquired a Humirel RH sensor but was looking at the Sparkfun SHT15 as an alternative as the interfacing is already in place.

Have you given any thought to the PWM code? Such as what conditions to change the PWM timing versus just on and off? I was contemplating trying to have the Arduino set the PWM timing so that each individual heater was on all of the time but just enough to reach the dew point plus a degree or two. this would allow for a more predicatable overall battery drain. the other scenario I was looking at was to intersperse the PWM over the heaters such that only one heater was "on" at any one time. I was trying to figure out which would be most efficient but have not as of yet ran the numbers.

Tom

[hikerbob]

Tom, with the existing code I start to ramp up the duty cycle of the PWM from 5 degree's above dew point .

I'll refine this after some field testing but at the moment it's pretty basic

if ( ChTempVal[TCount] <= dew_point + 5 ) {
ChPWMVal[TCount] = (dew_point + 5 - ChTempVal[TCount]) * 50;
if ( ChPWMVal[TCount] > 255 ) {
ChPWMVal[TCount] = 255;
}
}
else ChPWMVal[TCount] = 0;
analogWrite(ChPWMPin[TCount], ChPWMVal[TCount]);

Once I get the current sensing working properly I hope to use the current flows for each output to estimate the size of strap attached (and hence the size of the optics) and alter the behaviour of the channel to suit. I'm expecting that I might want to start heating a bit sooner and ramp up more quickly.

During the initialisation stage each channel is turned on individually and briefly and the current measured and recorded.

I've not put much thought into managing the peak power load yet, so far it's been a back of the mind thing. Once I get the electronics right the rest should be a matter of tweaking the code based on experimentation.

Bob

[hikerbob]

I've done some more work on the code for the Ardunio based dew heater controller and now have the current and voltage readings right for my setup. I want to add a brightness control for the LCD backlight and still want to do some work on the behaviour of the heater control for different sized heaters. I may also do some more work on what displays when in the LCD.

I've attached the latest code version.

[hikerbob]

I gave the heater controller a good run last night. I hooked 4 heater straps up 12", 2of2" and .96"and ran the whole thing of a selaed lead acid battery (the sort that goes in burgler alarms etc).

For most of the evening the ambient temp was a couple of degree's above the dew point but with the 5 degree margin I use so the heaters were running at a light duty cycle. With a dew point of 17 and ambient of 19 the sensors just under the heater straps were reporting a temp of 22.

The battery did not seem to have any trouble keeping it all going and showed only marginal voltage drop under load. The optics all remained dew free while others were having dew problems.

The ugly part was the wires running all over the scope, I do need to find a way to tidy that up.

Bob

[tjmck]

Bob,

Have been giving the wire routing some thought over the last few monthes. I have decided to make my strips with a short leads and permanently install jacks and wiring on my scopes.

Have been looking at using RJ style connectors (the pins are surprisingly rated at 1A!) mounted in close proximity to the point of use. This will allow one bundled or multiconductor cable to plug into the Dew Controller which I am mounting in or on my power center.

For my Newt, the primary and secondary dew heaters are permanently mounted already. The eyepeice and finder heaters are already in close proximity to each other so one small 2x jack panel will do the trick. I will likely add a bracket to the base to accomodate mounting the power center (on the front of the base).

For my Mak/Cass, the power center will sit under the tripod and again the wiring will be instaled permanently onto the scope with velcro attaching it to a tripod leg. This time there will be a 3x jack panel to for the eyepiece, finder and corrector.

I had decided on RJ-45s as they inexpensive are are widely available (CAT-5/6 cables), and have 8 pins (2 for +, 2 for -, with 4 left for sensor data and power).

Tom

[Shark Bait]

I have just taken my first steps into the 'dark art' of electronics, with the goal being to construct the project that is described in the original post. I have put together a couple of the Jaycar Short Circuits Series kits. The soldering was pretty rough to begin with but it improved fairly quickly.

I now feel ready to have a go at Motor Speed Controller (Jaycar kit KC-5225). I still have to work out the details for the resistor dew straps but I have a few areas to learn before I will understand what is needed prior to tackling that part of the build.

I think Jaycar is now my favourite store. So much great gear. I have to say that some of the customers looked like they walked off the set of Big Bang Theory. :thumbsup:

My thanks to Al for making the information available.

[sheeny]

Thanks Stu.

No worries mate! You'll be able to fix your broken "Green Lantern Lantern" in no time.;):lol:

Al.

[rcheshire]

Thanks Al. I used your sheet as a starting point and applied your calcs to Nichrome. Worked out well. PWM is from an Arduino with a humidity sensor proving the input. Basically, PWM is scaled to humidity input.

[Shark Bait]

PWM Build - 1st Stage

I have taken the plunge and constructed the 10A 12VDC Motor Speed Controller (Jaycar Kit KC-5225).

I thought I was ready to give this project a go, being the first time that I have moved away from following a kits instructions to the letter. Some of the kits components have been updated and that threw me. Initially I felt a little out of my depth, but after reading the papers a few more times things fell into place.

The first photo shows the testing using a 12v red LED with pot turned to maximum. The second is with the pot turned to minimum. It appears to work. I had some trouble soldering the power leads to the terminal posts. That part will have to be tidied up before it all goes into a jiffy box.

Jaycar had sold out of Nichrome Wire, so the dew strap for the primary will have to wait. In the meantime, the smaller dew straps to be constructed from resistors can be started.

If I can do this, anybody can. It's a very satisfying way to spend the evening after the kids have gone to bed and the clouds have ruined the nights observing.

[Shark Bait]

PWM Build - 2nd Stage

The heater board, RCA sockets, 12v red LED and associated wires have made it into a box to reduce the chaos. The dew heater straps will have to wait for another day.

I am using a 12VDC power supply, so I have used a link in place of the 12 Volt regulator as supplied with the kit. This is in accordance with Brendan's addendum to Al's original design. Brendan (wasyoungonce) discussed that this design needs 'headroom' voltage.

I have measured the voltage at the RCA sockets as it appears in the photos. With the dial turned all the way clockwise, the LED is dim and it reads as 1.95 Volts. A maximum of 12.5 Volts is reached by the time the pot is only half way through its travel and that is what it reads until the pot is turned all the way anti-clockwise.

This is the point where my lack of knowledge in this area lets me down. Any advice would be gratefully accepted.


Is the 1.95 Volts the 'headroom' voltage that I should expect to see?


When I construct the 4 dew straps and there is a larger load, can I expect to see 0 Volts as a minimum and the maximum voltage achieved when the pot is turned all the way anti-clockwise?


Thanks.

[rcheshire]

A neat way to do posts is to wind 3 or 4 turns around the post first.

[wasyoungonce]

Stu..the Headroom voltage is head room for the +12V regulator only. What this means is that for the 12V regulator to actually regulate voltage then the input voltage to it must be around 2.5V (actually some times 2.2V) above the voltage it is rated to regulate out.

For example a +12V regulator needs around 14.2 ~14.5V DC in (minimum) for it to regulate the output efficiently to +12V DC.

If the input to the same +12V regulator was lets say 12.5VDC ...then the regulator cannot regulate the output voltage efficiently, it may work but it may not and it is outside the mfgr's operating specifications.

Thus these type of 3 terminal regulators need an input voltage headroom above what their rated output is. It the case of the PWM controller you built...it was designed to operate a number of motors (or whatever) from a varying range of input voltages. But since most of the Astro guy's (and Gal's) are using +12V DC supplies already ...then you don't need the regulator, just install the link shown in the addendum. I see you have already done that so you are all thumbs up!:thumbsup: (note: if you using a higher DC supply like 18-24V then you will need the regulator).

The voltages you are measuring at the RCA are the minimum and maximum range of PWM (avg output) the controller can attain (due to it's design)...nothing wrong here.

[Scorpius51]

The 'headroom' voltage referred to probably refers to the fact that when you are using regulators, you are required to supply it with a couple of volts over the regulated voltage. Even when using low dropout regulators, a volt or so extra is usually a good idea - the data sheet will tell you.

The measured voltage at a PWM output depends on the particular circuit. Some PWM outputs will range over a (say) 5% - 95% duty cycle - not necessarily 0% - 100%. If this is the case, then you will get a small voltage at the 'minimum' setting, and near maximum voltage at the maximum setting. In your case, with the LED at max when you have the setting near 50%, it may depend on the series resistor you are using with the LED. The average of the PWM voltage at your 50% setting may be providing sufficient current to fully illuminate the LED.

By the way - you would normally wire the PWM control so that the max output occurs when the pot is fully clockwise.

Good luck.

Cheers
John

[Scorpius51]

Oops - I should have finished this response earlier! It has already been addressed by wasyoungonce.

John

[Shark Bait]

Good idea. That should tidy things up a little. Still, it will be a while before I will try soldering anything really delicate.



Thanks for the detailed reply Brendan. I understand what is going on now. This little project has been an eye opener.



Cheers John. I still appreciate the time taken to educate a beginner. I do have a better understanding about what is going on now.

I can only blame Mr Squiggle for the PWM control being back to front / upside down. If you ever need any electonic soldering done, send it my way. I can safely guarantee an end product that will be unique ......... I can't leave it that way. I will have to fix it. It is going to annoy me every time I use it if I don't. :doh: