Help with heater for all-sky camera - calling all electonics whizzkids!

[michaelmorris]

Dear Electronics Whizz Kids

I've recently completed building an all-sky camera based around a Samsung SDC-435 camera. It all seems to work okay but, I suspected it might, it is suffering from problems with dewing up on both the inside and outside of the dome.

The obvious answer is to install a small heater inside the dome to raise the temperature of the plastic dome a tad. The dome is only 8.5 cm across, so the volume of air to be heater is very small.

Having done a bit of reading on the subject, a few resistors mounted in a ring around the inside of the dome would seem a good strategy. I already have a 12v power supply feeding the camera housing, so a 12v system would be preferable.

I am an absolute numpty when it comes to electronics, so what I am after is an idiots guide to designing and building a very simple heater system. HELP!

Thanks

[JamesF]

Does the camera generate much heat itself? I seem to recall that you have it in quite a large box. I wonder if making a much smaller enclosure around the camera, perhaps lightly insulated, to contain the heat in that area might avoid the necessity for a separate heater?

James

[michaelmorris]

Does the camera generate much heat itself? I seem to recall that you have it in quite a large box. I wonder if making a much smaller enclosure around the camera, perhaps lightly insulated, to contain the heat in that area might avoid the necessity for a separate heater?

That's a clever idea John.

Unfortunately, the camera doesn't generate much heat and it wouldn't be practiable to sub-divde the box. Also, ideally I want to be able to prevent dew, not clear it when I decide to use the camera.

[Gina]

I would think something like a watt would be sufficient power to raise the temperature a few degrees to stop dewing up. So for a 12v supply that would be a total of 144 ohms. I suggest you use 4 resistors of 33 ohms connected in series. That will give just over a watt total. You'll want residtors rated at a quarter of a watt or more. See how that goes.

[faulksy]

or you could use nichrome wire as heating element, this would rasie the temperature and you could control it using pwm

[squeaky]

I'm using 6 off 22Ω resistors in series for the dew heater on my secondary and it works a treat (10cm x 7cm) which works out at around 1 Watt.

If you're not comfortable making one, and you don't need or want to buy a whole pack of resistors... gimme a PM and I'll put one together for you from my spares and post it to you.

[michaelmorris]

Thanks for the advice. A group of resistors in series it is. Now some more numpty questions.

Whaty should I mount the resistors on - an insulator (wood or plastic) or a something to conduct the heat (metal)?

Would a simple potentiometer be any good for fine tuning the heat output?

[squeaky]

I'd have thought that a ring of resistors placed around the inside of your dome's base would be all you'd need. No mounting plates at all. As for "fine tuning" the heater... again not really necessary at such low power. It's likely to only lift your dome temp by two to five degrees above ambient. If you do find that it's not warm enough, or too warm, just add or remove a resistor from your series chain to adjust slightly.

[rfdesigner]

just picking up on this..

Why not treat it the way most newt owners treat their scopes... install a fan... a simple 12V one should do the trick, then you should get no dew on any surface.

Derek

[michaelmorris]

Why not treat it the way most newt owners treat their scopes... install a fan... a simple 12V one should do the trick, then you should get no dew on any surface.

The biggest problem is dew on the outside of the dome (happened again last night). Would placing a fan so that it blew air across the outside of the dome really help?

[michaelmorris]

or you could use nichrome wire as heating element, this would rasie the temperature and you could control it using pwm

pwm?

[squeaky]

A bit over the top for a simple heater... http://en.wikipedia.org/wiki/Pulse-width_modulation

[rfdesigner]

The biggest problem is dew on the outside of the dome (happened again last night). Would placing a fan so that it blew air across the outside of the dome really help?

it should do. try it if you have a desk fan and an extension lead. If you can keep the dome within say 0.1 degrees of ambient then you won't get dew. You are only getting dew because it it cooling down below the atmospheric temperature due to radiation towards the -60C sky.

Derek

Edit.. if the dome is thin enough then blowing the inside will help as it will prevent the outside surface from over-cooling, which is the problem you're fighting.

Edit2: you could try painting the outside of the dome in heat reflecting paint, that may not be a complete solution but it would reduce the problem.

[Gina]

Sorry to interrupt but I'm puzzled. Why is dew on the outside of the dome a problem? Does it drip inside somewhere?

[michaelmorris]

Sorry to interrupt but I'm puzzled. Why is dew on the outside of the dome a problem? Does it drip inside somewhere?

The dome is question is the clear plastic dome sitting over the camera lens

[michaelmorris]

Edit2: you could try painting the outside of the dome in heat reflecting paint, that may not be a complete solution but it would reduce the problem.

How is the camera going to take images through a painted dome?

[Gina]

The dome is question is the clear plastic dome sitting over the camera lens

ooops! My mistake - got confused there - thought it was an observatory dome I really must read the subject before going off at half [removed word]

[rfdesigner]

How is the camera going to take images through a painted dome?

Of course. it's a clear dome!.. I was thinking it had a small slot in it Doh!

[michaelmorris]

I would think something like a watt would be sufficient power to raise the temperature a few degrees to stop dewing up. So for a 12v supply that would be a total of 144 ohms. I suggest you use 4 resistors of 33 ohms connected in series. That will give just over a watt total. You'll want residtors rated at a quarter of a watt or more. See how that goes.

Built the heater on Saturday. Remarkably I had no solder burns and it works, although the amount of heat it puts out is tiny. It's now installed in the camera housing and wired up. We'll see if it does the trick the next time we have a clear night.

[squeaky]

Great

If it doesn't quite cover your needs - just remove one of the resistors from the chain and try it again.

[russellhq]

Maybe you could try some of this:

http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_categoryId_255225_productId_222433_langId_-1

It is supposed to make the surface repellent to water.

[michaelmorris]

Just a quick update.

Last night I had the all-sky cam running all night. The temperature went down well below freezing and everything had a substantial layer of frost when I woke up this morning. Everything that is, except the dome on the camera, which was completely clear - RESULT!

Thanks for all the advice, it worked a treat.

[squeaky]

Which solution did you actually use?

[keithatrochdale]

What settings are you using on the camera?

[michaelmorris]

Which solution did you actually use?

#

Sorry - the four resistors wired in series - works a treat.