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Hello!

I have been looking through the forums sections of this site and found that nichrome wire is a very popular material for dew heaters. I was using my DSLR a several weeks back, and the dew was horrible and it encouraged me to build a nichrome heater. 

My first heater prototype consisted of 22" of wire with a resistance of 4.08 ohm per foot connected directly into a 12V power supply (consisting of 8 AA batteries in parallel). It was wrapped in duck tape. It worked for a while, but then the batteries began to overheat and I had to pull the heater out to let them cool. 

When I used the same prototype heater on 6V (4 AA batteries) it didn't have this same overheating issue, but it produced less heat then when it was on 12V. 

I'm not using a pulse width modulator, but would that solve this issue?

I'm worried as the heater is only really around 8 ohm, it may be short circuiting with such little resistance. The wire can't be terribly long as this is just to heat a DSLR lens. Any input on a solution would be greatly appreciated.

Thank you.

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All that PWM gives you is adjustability - heat output is dependant on current drawn so a PWM generator will not make the system more current efficient. To allow you to use a longer length of nichrome wire even on the small circumference of your DSLR lens, simply lay the wire in a 'sine wave' pattern on your duct tape. I use a really low cost PWM generator to warm the dome of my all sky camera and it is well worth having one so that you can obtain the exact temperature you need. Here is where I got mine from.

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Using Ohm's law you are drawing 1.5A (12V / 8R) which is 18 Watts (12V * 1.5A). That is a lot of heat to dissipate. Either use more wire as Steve mentions above or get some new wire with higher resistance.

As a comparison, for my 125mm refractor I usually generate about 3 to 4 Watts. It should be barely warm to the touch...in fact mine is difficult to tell if it is on or not (except for the lack of formation of dew).

Edited by StuartJPP
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I'm using 70 ohms on my all sky camera dome, run from 12v or so and that's more than enough.  But I'm using 7 10Ω resistors.  I have used nichrome wire in the past but electrically insulating it well without too much thermal insulation can be a problem.

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Thank you all for the quick replies! I really appreciate the input! If I were to use more wire or purchase wire that has a higher resistance, would this prevent my batteries from overheating? 

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your batteries are overheating because you a draining them to fast, you need a bigger a/hr battery supply

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