Telescope Reviews: Re: DIY Dew Controller Questions...

If the heater strips were very warm even when wrapped around the finder scope or eyepiece, it might be because you wired the heater in series. That is a mistake because as I found out, the heaters can get very hot. Instead, wire the heater strips in parallel, whose resistance is determined by the value of the resistors divided by their number. I made my heater strips from 330 Ohm, 1/2 watt resistors wired this way, and even when powered by 12-volt power directly, they keep dew away but only make the finder scope feel just a little warmer than it would be without the heater. Even when they're not on my finder, secondary mirror and coma corrector, they only get moderately warm. When I wired resistors in series, they got so hot they were enough to cause burns. I didn't build my controller from any plans, but I simply put two "bus bars" in the case, one for incoming power, the other for a ground. I connected leads to each switch, then from there power went to a variable resistor. At the load side of the switch, I connected a red LED and a 680 Ohm resistor to serve as a power on indicator, with a ground wire back to the ground bar. Power then flows to the RCA jack in the central terminal, and return current flows through the outer terminal which then goes back to the ground bus bar. A hot and ground wire goes from each bus bar to another RCA jack, with a fuse between the hot bus bar and the jack. I made the connections with forked lugs into terminal strips that acted as bus bars. I couldn't find any better pictures of my controller at the moment, but I'll post one as soon as I can find one.

Taras

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10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector

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