This is currently an on going experimental project but it works so well and efficiently that you will just have to make one if you are into Astrophotography. What is nice about this total system is that it controls four dew strips for the 10", 5" guidescope, finder and eyepiece. The 10" and 5" have separate settings of 2,4,8 & 16 watts. The finder and eyepiece have a common Off, 1 & 2 watts settings. One secret of good viewing and photogrphy is to use as little heat as possible and here you can make a total controlable anti-dew system for less than what a single fixed heat strip would cost.
To the Left here, we lay down a strip of fiberglass drywall tape on top of wax paper. In the photo, I was using aluminum foil with a release coat and that was not the thing to do.
While the two smaller strips use resistor ladders, the two longer dew strips use nichrome wire. This allows a more durable and flexible strip. I built a large resistor ladder but was unhappy with it. It was too much work and failed during testing. The idea does work very well on the small sizes as they only use ten, 680-1k Ohm resistors each.
Now we are starting the 36" long nichrome loops.
We use 0.012 nichrome wire that tested approximately 3-Ohms/ft so the total length in four loops has 72-Ohms of resistance. Using the E^2/R power formula we get 2 watts. The loop ends have wattages of 8/3, 4, and 8 watts. Using taps at the last two loop ends, we then have 2, 4 & 8 watts but with a little switching trick, we can also get 16 watts all from one strand of wire. It would be even possible to get more with extra switch positions but I found if 16 watts is not enough the air is too wet anyway.
Here we are using the fiberglass drywall tape to isolate the loops and to reinforce the ends. Limit the rest of the strip to one layer top and bottom for flexiblity.
The economy of this is that we use only one strand of wire in four loops, only four connecting wires and no electronics. The efficiency comes from the fact that the heat is controlled by switching taps and not by an external regulator or Power Resistor.
Use push pins to separate the wires and dabs of RTV rubber silicone compound to hold in place. When finished and after wire tying or crimping on the taps, cover boths sides with a smooth layer of silicone and press in the velcro strips. When dry, trim to finish.
I made the two long strips the same and just wrap one around the 5" twice. For the small strips, I took a piece of adhesive tape and taped the ends together to make a loop with the adhesive out and pressed it onto a ruler. I then pressed the 680-Ohm resistors onto the tape. I space the first ten for the eyepiece and later another ten for the finder.
After spacing the resistors the appropriate distance, strip a stranded wire and solder across the tops of every one. This will be a common ground. Then alternately solder two wires to the bottom ends.
I cut a six foot minature stereo phono plug cable in halves and soldered them into the centers of the resistor ladders. I then spread a thin layer of RTV about an inch wide and eight long on wax paper. After letting it set for about ten minutes I pressed the resistor ladder (cable up) into the silicone and let set for another half hour. I then came back and smoothed a finishing layer on and pressed velrco strips into each end.
Click on "Wiring the Box" pic for better view and wiring instructions.
I wired everything into the bottom of the box so the lid comes off tangle free. The only power dropping resistor is a 2.7K-Ohm for the LED power indicator that can be seen RTV'd into the case. With the exception of the nichrome wire, all the supplies can be bought at your local hardware and Radio Shack stores. You may be able to buy the wire from an Welding or Industrial Supplier as heat sealer or plastic thermo cutting wire.
Everything but the 12VDC power cord unplugs from the Control Box.
At times you can find Nichrome in 50 ft., 100 ft., and 250ft. spools. I bought about a mile as this size was stocked only in a huge spool.
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