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

iceman (Mike) · #1 16-03-2006, 06:38 AM

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 page, or directly by clicking on the link below:

Building a Pulse Width Modulated Dew Heater Controller

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 (Mike) · #2 16-03-2006, 08:06 AM

Article uploaded.

barees63 · #3 17-03-2006, 06:29 AM

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

sheeny (Al) · #4 17-03-2006, 08:40 AM

Quote:

Originally Posted by barees63

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

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!

Al.

Lester · #5 01-04-2006, 03:36 PM

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 · #6 01-04-2006, 04:19 PM

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 · #7 01-04-2006, 04:48 PM

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 (Geoff) · #8 07-04-2007, 02:28 PM

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 · #9 07-04-2007, 07:40 PM

Quote:

Originally Posted by AndrewJ

Gday Lester
>snip
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.
>snip
Andrew

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 (Mike) · #10 17-12-2007, 07:06 AM

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 (Al) · #11 17-12-2007, 08:11 AM

Thanks to Brendan for doing this!

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.

Onya Brendan!

Al.

Astro78 · #12 17-12-2007, 11:25 PM

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

troypiggo (Troy) · #13 26-03-2009, 03:02 PM

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?

Quote:

Originally Posted by AndrewJ

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 )

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 (Brendan) · #14 26-03-2009, 03:56 PM

Quote:

Originally Posted by 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?

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.

Hope this helps.

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 (Troy) · #15 26-03-2009, 04:03 PM

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

AndrewJ · #16 26-03-2009, 04:40 PM

Gday Troy

Quote:

Originally Posted by AndrewJ http://www.iceinspace.com.au/forum/....s/viewpost.gif
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 )
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?

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 (Troy) · #17 04-04-2009, 02:55 PM

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".

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 or RP-3508?

peter_4059 (Peter) · #18 04-04-2009, 03:28 PM

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 (Troy) · #19 04-04-2009, 05:36 PM

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 (Peter) · #20 04-04-2009, 05:38 PM

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/s...hTerm=238-9816

Peter