Okay, the first thing is that unless you are skilled with a soldering iron, a sewing machine, and happen to have more time than money and maybe also have access to the supplies to make these, you probably want to just fork over the money to buy a commercial solution. Far be it from me to discourage you, but I have made both my own heater control circuit and my own heaters. It takes time. For me it took a lot of time. After using them for about three years, I bought Dew Not heaters and A Thousand Oaks controller . I'm not going to show you want I did, because it was a hack using a CMOS 4093 as the timer for the pulse-width-modulation and a relay. The relay give nice audible feedback in the dark, but they really aren't designed for that sort of repetitive open/close cycle. Here are some links to sites where the folks actually know what they are doing.
- Wire Gauge and Current Limits has some basic information on what size wire will carry what size current load. You want to know this in building the different sized heaters. For example, to make the 1-1/4 inch size heaters, you can use 28-guage wire which will carry over 200 mA.
- Wire Resistance has some more information like the above link, including information on (surprise) wire resistance and current capacity.
- Home-Made Heaters
- Mark Kaye's Heaters shows how he built some very nice heaters. I've made some heaters using a variation on his technique.
- Ron Keating's Heaters use 330-ohm resistors wired in parallel to get whatever power rating you are aiming at. I've made on heater suitable for 2-inch eyepieces using this technique, but don't much care for it because the heater is stiff. But it does work. I also used denim rather than duct tape and had to do a bit of sewing.
- Heater Controllers
- Dewbuster is a controller made by Ron Keating that is thermostatically controlled. This is nice because it reduces power consumption by keeping the heater a fixed amount above ambient rather than just heating at a constant power rating.
- Don Clement's Controller is another one which uses automatic temperature control to avoid overheating and reduce power consumption. This one is strictly DIY; Don's site has schematics and a PCB layout.