Could I make a simple tiny heater with 1/2 watt resistors?

[Jay112]

I have a CCTV dome camera that gets too cold in the winter time and produces blurry images until the temperature warms up. My idea is to put a tiny heater inside the dome. The dome is about the size of a human fist. I might only need to raise it a few degrees Fahrenheit for it to have a much longer working period during the day.

The camera is powered with 12 volts, and I think there are an extra 500mA on the power supply that's feeding it (I could replace it with a different supply if necessary).

I have a box of 1/2 watt resistors, ranging from 10 Ohms up to 1M Ohms. Is there any way I could use a few of these to try to raise the temperature of the dome by a few degrees?

I've seen many google results for "use resistor as heater", but the top results seemed to be about very specific situations, so that's why I was interested in hearing your advice here. Thanks!

[Gyro]

Certainly you can. The cabinet heaters available for larger cabinets are basically just power resistors in a convenient enclosure.

The dissipation you need may well be smaller than you think - a little experimentation is needed to achieve the desired result without overheating it in the summer.

You may find that a pysically larger resistor (such as 1 or 2W) under-run at 0.5W or less is more friendly due to lower surface temperature.

[Jay112]

Thanks! I'll actually be removing the heater in the summer.

I recently saved the PCB from a broken coffee maker. It has very large resistors on it, seeming physically 4 times larger than my 1/2 watt resistors. I haven't tested the Ohms yet though. Do you think some of those might make good stable heaters?

[Gyro]

Potentially, depending on their values and the space you have available. Remember that a resistor of the same resistance value will always dissipate the same power (heat) for a given voltage and current, it's just that larger the resistor package will do so at a lower surface temperature due to its larger surface area.

[Kleinstein]

You could also use something like 2 or 3 of the smaller resistors. The first part is to estimate how much power is really needed. Allready just a few degrees may be enough to keep humidity low enough and prevent condensation. So I would test it wtih something like 1/8 - 1/4 W or a 560 Ohms to 1 K resistor.

[Seekonk]

I've seen microprocessors with resistors glued on the top so they would still work at lower temperatures.

[Jay112]

Thanks for the info, everyone. So far I'm planning on trying Kleinstein's idea to use 2-3 of the smaller resistors first. I'm going to go on the roof now where the camera is, to see how much room I have to work in there. I'm pretty sure there's a nice flat metal part on the bottom, so I'm guessing I can probably have the resistors rest there. Maybe I'll try to take a few pics to show you guys the situation.

[Tom45]

If your 500ma estimate is correct, you have 6 watts available: 12 volts * 0.5 amps.

Power dissipated by a resistor is E² / R ,  or 144/R

To give a bit of safety margin, lets plan on 4 watts with the power supply you have now.

So that works out to R = 144 / 4 = 36 ohms.

Since you are using 1/2 watt resistors, you will need 8 of them for 4 watts. Using standard values in parallel, you need 8 270 ohm resistors in parallel. Or 8 4.7 ohm resistors in series. Or some other combination of series and parallel that combines to give around 36 ohms.

[Jay112]

Thanks Tom! That's very helpful!

I tried to get some more specifics on the current being drawn, but my multimeter kept telling me that the camera was only drawing 5.5mA. That seemed too low to me, so I looked at the camera's datasheet and it said the power consumption is 218mA at 12v.

The strange thing I just learned is that the camera still works if I disconnect it from ground at the power source. I've never seen that before. I wonder if it means there's a mistaken connection to ground somewhere, or if somehow that's normal behavior for CCTV cameras (like maybe they're connected to ground via the video connection?). I did find continuity between the power-supply ground and the center pin of the video cable.

When I go on the roof to open the camera (hopefully soon), I'll probably try to test the current draw from up there, if it's not too difficult.

Also I learned that the power source can supply 500mA (again I could swap that out if necessary). So if we assume that the datasheet is correct that the camera draws on average 218mA, could you guys please help me with the math again for choosing resistors? I was planning on researching how to do the math today, but I'd probably end up making mistakes anyway, so it would be great if I could get some input.

[Len]

The strange thing I just learned is that the camera still works if I disconnect it from ground at the power source. I've never seen that before.

You need to be more diligent with your EEVBlog watching! 

https://youtu.be/2yFh7Vv0Paw

[Jay112]

Thanks Len! I usually don't watch videos, because I prefer reading. That was the first EEVBlog video I've ever seen. The man has some amazing teaching skills!

[Jay112]

It ended up to be too cold and windy outside (20 degrees F), so I decided to put this off until tomorrow, when it's supposed to be in the 40s.

I think my fingers would've been frozen if I tried to do anything out there today. And also I was worried about my multimeter and camera suffering from the temperature changes.

[Lycaon]

Maybe one of these would work?
https://www.sparkfun.com/products/11289?gclid=CjwKEAiAndSzBRDp5P232v-qtHkSJABw-VdtT2V0BQG4bQ4BGjY4O8pi1kf5l3wlnYvkOFvt9hskwRoCqrjw_wcB

[fivefish]

or a tiny incandescent bulb?

[Ian.M]

A bulb is less reliable and introduces light pollution in the housing.  Simply add as many 560R or 470R resistors in parallel across the supply as it takes to prevent condensation. Each dissipates a little over 1/4W.

[Jay112]

@Lycaon: that looks like a good idea, but I would prefer to try using something I already have on hand first.

Would a regular 120v incandescent candelabra bulb (say a 5 watt version) still produce heat when powered by 12v DC? And if it does, I'm assuming the lifespan would be much longer than usual? I would prefer to not have to introduce 120v AC into the enclosure.

BTW I recently learned that the camera can optionally take 24v AC instead of 12v DC. I already have a 24v AC supply and the switch would be easy, if you think this might allow more heating options.

[LukeW]

If you go with a 24VAC power supply (or whatever you have on hand, closer to the upper end of the camera's allowed voltage input) you may find that the problem solves itself with no other components added, due to increased power dissipation in the camera's own power supply regulators.

Test it... maybe you don't need to do anything!

[Gyro]

I vote for the bulb solution, painted completely in black, or covered by black tin foil.

It gives much larger surface area to evenly radiate heat to the entire housing, rather than just some hot spots as if implemented with resistors.

How's a bulb going to be better than a few resistors running well within their dissipation ratings. You've got all the hassle of masking the bulb from All visible light, realizing you've picked the wrong wattage and doing it again, ANY light leak that develops (paint flake) is going to be a major problem. Arranging a holder (or directly soldering to the bulb). Just doesn't seem as practical a solution.

The OP started off by talking about a "small dome" and 1/2W resistors, these solutions seem to be getting bigger!

Edit: +1 for trying the higher supply voltage, the camera is probaby (hopefully) using a linear reg.

[Lee Leduc]

Here's a link for a simple resistor heater to keep a telescope from fogging up. I made one years ago and it worked great. I added a simple PWM circuit to adjust the temperature.
http://www.skyandtelescope.com/astronomy-equipment/diy-dew-heater/

[Jay112]

I finally got some pictures. It's dangerously windy today (gusts up to 35MPH), so I probably won't be able to do any precision work up there today.




I think there looks like room for a resistor heater on the "floor" on the side that doesn't have any wiring (the top of the 2nd picture). There's definitely no room for a lightbulb anywhere.

@LukeW and Gyro: I would love to do your idea first of trying the 24VAC power supply and seeing if that fixes the problem, but the problem is that if I see that it doesn't work, then already it will have become too cold for me to try out other ideas. So I think I'll try a small low-power resistor first, and then if that doesn't work then I can try with the 24VAC idea. It's supposed to be warm all week, so for an entire week I probably won't be able to see if I fixed the problem (it only shows up when it's below 20F).

@Lee Leduc: That's a great link! Thanks!

I'll probably make the resistor-heater easily removable, but I have a question: Is there a way to make the heater be able to handle both 12VDC and 24VAC?

[Jay112]

That DC fan on the left side of the lower picture was custom-installed by me. I was having problems with it overheating in the summer time, but when I installed that fan it fixed the problem.

Last winter I kept the fan unplugged. I can choose to keep it plugged or unplugged this winter. An interesting thing would be to have the resistor heater behind the fan, so that it cools down the resistors and blows the heat towards the PCB. But I think it would be way easier to just have the heater sitting on the floor of the unit.

[Jay112]

I just tested the current draw at the camera end, and it was only 3.9 mA. Does that make sense to anybody? Doesn't it seem very low?

I just made a heater according to the directions at Lee Leduc's link (http://www.skyandtelescope.com/astronomy-equipment/diy-dew-heater/), only I used 6 22 Ohm resistors. At 12 volts it should produce about 1.1 watts. I think I might try to go up there soon and see if it fits well in the dome. I'll take a picture if everything ends up fitting.

[Gyro]

I think you must have got your resistor values wrong. 4mA would imply a total of 3k, so 500R per resistor... assuming you've strapped it to 12V. Time to double check your values.

With regard to placement of the resistors. As you've got a fan in there the logical place would be to string them in front of it, that way air will blow over them and the warm air will circulate. No danger of hotspots and probably more effective too.  It looks as if you have unused mounting holes on the fan that you can use too.

It's possible that just running the fan would be sufficient to keep things clear but I'm sure the electronics and mechanics would appreciate a bit of warmth in winter too.

[Jay112]

I think you must have got your resistor values wrong. 4mA would imply a total of 3k, so 500R per resistor... assuming you've strapped it to 12V. Time to double check your values.

With regard to placement of the resistors. As you've got a fan in there the logical place would be to string them in front of it, that way air will blow over them and the warm air will circulate. No danger of hotspots and probably more effective too.  It looks as if you have unused mounting holes on the fan that you can use too.

It's possible that just running the fan would be sufficient to keep things clear but I'm sure the electronics and mechanics would appreciate a bit of warmth in winter too.

I'm sorry I didn't write clearly enough. I meant that when I didn't even have the heater connected, the camera alone was drawing only 4mA. That seemed low to me. I'll go test it now that I have the heater connected.

This is where I placed the heater. Looks like a pretty good fit, huh? Although in the picture it looks like the top row might be touching the bottom, they're not actually touching, and there's about 1/4" of space between them.


My biggest question is: What would happen if I ran 24VAC through those resistors?

I unplugged the fan for now, because if I end up switching to 24VAC, then I don't have to worry about blowing out the fan.

[Jay112]

Also: I tested the voltage at the camera end, and it's at 12.02 volts.