At 04:15 PM 6/2/2011, Peter Loron wrote: >I may be showing my ignorance here, but why not use something *designed* >as a heating element? Or at least a simple cement power resistor? Its a reasonable question. I don't have much height to play with in=20 this enclosure. About the only suitable heating element is a Watlow=20 silicone-rubber thin, flat heating element - and these get expensive. That was going to be my first approach. But: the heater has to be=20 driven from a 120Vac power line. I was going to use a triac for=20 switching. The triac will be dissipating about a watt or so and=20 needs heatsinking - I was going to fasten it to the same aluminum=20 plate as the heating element. But: I've been making little heaters for years now based on using a=20 TIP122 power transistor as a heater. This is a little circuit board=20 powered from a 12V supply - the board is tiny (about 0.5" x 1") and=20 fastens to a metal plate or metal strip that is formed to fit inside=20 the enclosure. There is a bunch of these inside ceiling-mounted=20 surveillance cameras all over Northern Canada, for example. Another=20 bunch installed in the credit-card readers inside parking garage=20 entrance controllers. They work very well and we've sold a bunch of them. Anyway, it occurred to me that I could simply scale the same circuit=20 up to run from rectified, un-filtered AC and simply use the pass=20 device as the heating element. Like I said, I've got lots of=20 experience with this. Back-of-the-envelope calculations say this should work well. Now all=20 I need to do is find a suitable pass device. N-channel MOSFET,=20 because of price and voltage rating. The device will not be running hot - the unit turns on somewhat near=20 0C and is guaranteed to be off at 10C or so. Its only purpose is to=20 raise the internal temperature of the air inside the enclosure=20 significantly above the -45C that the outside of the enclosure is sitting a= t. Hope this makes sense! dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .