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Simple fan heater

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danrogers

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Hi all, I need to make a simple heater, to raise the temperature of my camera lens enough to stop it from fogging up. I have a pc fan and heat sink that I would like to use, but I'm not sure what to use as the heat source - I was thinking of using resistors but the bottom of the heat sink is smooth so it would be hard to get the heat to transfer.

Any suggestions?
 
You could use an LM317 or a transistor in TO-220 case; just tap a hole and screw it on.
 
It's up to you. e.g. you could make a controller to keep the temperature of the heatsink constant, or you could control the amount of power you're feeding to it.
 
I've done this on a rain sensor, it was a tip transistor I think a tip122 darlington, a 1k pot accross the supply with the wiper going to the base, or something like that, the pot controls the amount of current dumped accross the transistor, worked a treat.
 
I will have to try out some circuits I guess. I could try and use a micro controller such as an arduino to monitor the temp but to be honest this only needs to be a quick and dirty solution so it might not be necessary :)

@ dr pepper, that doesnt sound to taxing, I may even have some tip122's somewhere. So I guess you simply connect the collector to vcc and emitter to gnd then vary the base? Sounds like a great idea
 
Yep, dont take the base up above 1.2v much or you'll quickly short the supply.
The only problem with a simple circuit is the temp isnt particularly stable, the transistors properties change as it warms up and that in turn changes the heat dissispated, worse case you can have runaway.
A simple circuit is ok on a simple application, if you want something more controlled this is a cool cicrcuit, but a lot more complicated, this circuit monitors and controls the temp:

**broken link removed**

Maybe a power resistor would suit your application better.
 
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a 25w or 50w power resistor in the metal package has a smooth surface made for a heatsink (they won't dissipate 50w unless installed on a 1 foot by 1 foot aluminum plate). Depending on your voltage source, you can pick the correct resistance to yield the amount of heat you need, from warm to HOT HOT HOT. You only have to dissipate about 1 w to get it very warm indeed. (I just did a project where I dissipated 2.76W into a 3W resistor and it gets very toasty, but doesn't burn up).

this resistor is about $3.50 from digikey.com: A102430-ND

power resistor.jpg
 
Or, with an LM34 and a PNP transistor plus a resistor pair, you could simply control the temperature. You'd want a PNP because the LM34 has a positive temperature coefficient. Use the resistor pair to set a bias on the transistor and then as the LM34 output goes up, the current through the transistor would go down. The LM34 is a linear temperature sensor whose output is 10mV per °F and works from 5-30V inputs. The size of the bias resistors would depend on what voltage you were using and what your control temperature is.
 
If you want a quick plug and play solution try a Google of "electric car window defroster". They come in 12 and 24 volt flavors and are very inexpensive. You would not be running it long but would still need a battery. Actually had a small hand held gun shaped one around here at one time.

Anyway, just a thought.

Ron
 
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