Home made Dew Heater strips

NOTE -- The following instructions will assist you in building dew heaters for your telescope. While this project is not difficult, use of these instructions is done entirely at your own risk. When properly constructed these heaters are compatible with Kendrick, Dew-Not, and Astrozap heaters, however a mistake in construction could damage non-DewBuster™ Controllers. DewBuster™ Controllers will not be harmed because they have short-circuit protection and the warranty covers any damage caused by home-made heaters even if due to a mistake on your part. These heaters can produce almost twice the required watts of heat so they must be used with a controller to regulate the amount of heat. They are intended to be used with a 13.8VDC Power Supply or 12V Battery (13.8V when fully charged), higher voltage will overheat and damage them, lower voltage may not produce enough heat. I will offer no advice on modifying these heaters for other voltages or applications, try searching the internet for alternate heater plans.
Homemade Heater Strips
These instructions will build heaters that are compatible with the DewBuster ™Controller and any other controllers that accept Kendrick, Dew-Not, or Astrozap heaters. They use parallel resistors so all resistors are 330-Ohm 1/2 Watt spaced equally 5/8 inch apart. Since only one size resistor is used, you can buy in quantity to save cost. Each resister will generate its full rated 1/2 Watt of heat when the controller is set to full power so it uses the fewest possible resistors for the heater. It is also easy to determine heater current (number of resistors times 0.036 Amps) and wattage (number of resistors times 0.5 Watts). For example a 28 resistor heater will draw 1 Amp and produce 14 Watts of heat. If you plan to build a very large heater be sure the current does not exceed the capabilities of your controller (the DewBuster™ Controller can handle 6 Amps per channel so you could build a heater for a 36 inch diameter optic using 166 resistors). Also, this type of heater can not be used without a controller as it would get too hot. To estimate the number of resistors you will need, divide the circumference of your telescope tube by 0.625. For example an 8" SCT will use about 45 resistors, a 10" about 55, an 11" about 60. A 4" Refractor would use about 22 resistors depending on the tube diameter. If you have a broken hair dryer, heating pad, or electric blanket from which to get it, Nichrome wire can also be used for building heaters. See building heaters with Nichrome wire for more information.
  • Solder -- This project requires soldering, so you will need a soldering iron and electrical solder. DO NOT use acid core solder, use rosin core solder. If you are new to soldering, take a look at the "Soldering is Easy" guide which is tailored for beginners.
  • 330-Ohm, 1/2-Watt Resistors -- Use only 330-Ohm resistors rated for 1/2 Watt or greater. Common 4 colour band resistors should be orange-orange-brown-gold meaning 330-Ohms +/- 5%. If this is not the case then consult the manufacturer for proper identification. Also be sure that you have 330 and not 330K (330,000 Ohms) or 330M (330,000,000 Ohms) resistors. A 1/2-Watt resistor should be about 1/8" diameter, do not use the smaller 1/4 Watt resistors because they will overheat and burn up. Before spending time building a heater, apply 12 volts to one of your  resistors, it should get hot to the touch but should not smoke or discolour. You may purchase resistors from an electronics outlets or on this web site.
  • Solid hook up wire -- this is the wire that the resistors are soldered to when building the "ladder". 24 AWG is fine for up to 4" heaters, 20 AWG for up to 8", 18 AWG for up to 14" heaters. Stranded wire may also be used but try to solder only at the resistors so you will not bond the strands together and make the heater stiff.
  • Speaker Cables with RCA plugs -- 18 AWG is recommended for 8" and larger heaters. On smaller heaters you can use smaller wire down to 24 AWG for eyepiece heaters. If you can find 12 foot cables you can cut them in half and get two heater cords from one. Do not use "shielded" or "patch cables" because even the premium ones are designed for low current and the small wires will get hot, melt, and short out. Unless the cable specifically says it has 24 to 18 AWG wires do not use it. Your best option is to get a good quality RCA plug and make your own. Mouser has a nice all-metal solder-type RCA plug Mouser# 17PP058 (click on Data Sheet to see what it looks like). If you prefer a connector that you do not have to solder, you may use a Screw Terminal to RCA adapter.
  • Duct tape -- Home Depot carries black duct tape that looks nice. An even better material is black nylon "camouflage tape" sold in hunting outlets (thinner so you may have to use two layers but it looks like commercial heaters). You can also sew a fabric covering on but make sure the material does not restrict heat flow toward the telescope. For ideas see Mark Kaye's instructions for making heaters with fabric covers.
  • Velcro™ -- Available at Home Depot and many department stores. Black 1/2" width suggested.
  • Insulation -- used on the outside of the heater to reduce heat loss to the air so more heat travels into the telescope. 1/2" wide foam weather strip (for sealing around doors) works well and can be found at Home Depot.  Since the controller regulates the heat, the resistors do not get very hot so high-temperature materials are not needed. NOTE: The insulation shown in the photos below is foam rubber door gasket material originally intended for electrical enclosures.
1. Cut two lengths of solid conductor hook up wire at least as long as the heater will be (long enough to go around the object the heater is for) and strip off any insulation. These will form the top and bottom wires to which the 330-Ohm resistors will be soldered. To make it easier, drive two nails into a board 5/8 inches apart and wrap one end of each wire around a nail. Drive two more nails into the board at the opposite end and stretch the wires between them as shown. Note: When building several small heaters, it is easier to make one long ladder of resistors and then cut it into smaller lengths afterwards.
2. Attach the resistors by wrapping the leads around the solid wire and then soldering (polarity does not matter). Wrap snugly to allow a good solder joint but not so tightly that it damages the wire. Space the resistors 5/8 inches apart. As shown above and below, the color bands on the resistors should be orange-orange-brown-gold, if not then you have the wrong size resistors and it will probably not work.
3. Clip off excess resistor leads leaving only the ladder of resistors for the heater.
4. Cut the speaker cable in half yielding 2 lead wires for heaters (set one aside for next time). Solder the speaker cable wires to the top and bottom wires of the string of resistors (polarity does not matter). You should now test the assembly by applying 12 volts across the resistors (be sure the power source is fused). The resistors should get hot to the touch but should not smoke or discolor. If they do not heat up, measure the voltage from the top to bottom wire, it should be 12 volts. If the voltage is low, you either have a bad connection, too small a wire gauge on the RCA cable, or your power source is not able to supply enough current.
5. Lay out a piece of black duct tape and stick the resistor string to it.
6. Wrap the assembly around a can or other round object of similar diameter to what the heater will fit. Wrap the insulating material (weather-strip) around the outside and stick it to the duct tape. Fold the edges of the duct tape over and stick it to the insulation.
7. Apply another layer of duct tape to the outside of the heater to cover the insulation.
8. Cut a length of Velcro™ a few inches longer than the heater and stick it to the outside of the heater with one end even with the end of the heater (the end with the speaker cable attached). The excess Velcro should overhang several inches past the end of the heater. Note: The illustrations show the "loop" side of the Velcro being used here, but you can use whichever you prefer.
9. Cut a 1/2 inch wide strip of duct tape or electrical tape and stick it to the very end of the overhanging Velcro™ on the adhesive side. This forms a a pull tab for easy removal of the heater.
10. Cut a piece of the opposite type Velcro™ ("hook" side in the diagrams) and stick its adhesive side to the overhanging Velcro's adhesive side. When the heater is placed on the telescope the mating Velcro™ surfaces will stick to each other.
11. Before connecting your heater to your controller, test it with a multimeter to insure it has no shorts. The expected heater resistance can be calculated by dividing 330 by the number of resistors in the heater. For example if you used 33 resistors, then 330 / 33 = 10 so the heater should read about 10 Ohms resistance. Note: After measuring the heater resistance, touch the meter leads together and subtract this "lead resistance" from what you measured on the heater to find the actual resistance of your heater.
A Few Tips:
On a finder scope heater, connect its eyepiece and objective heaters by a short length of wire so that they both use the same RCA plug. This means one less wire to deal with.
When making a Telrad heater, try to place the resistors on the back side of the glass near the edges. This will do a much better job of heating than if the resistors were placed on the front side of the glass. For this you need a slim heater so wire it as shown in the above diagram. Although  it may look different, it is actually the same parallel wiring as in the above instructions. With this arrangement you must use insulated wires to prevent shorts.
For a Rigel Quick finder, the resistors are wired as shown above and glued to the inside of the black plastic housing. You must allow about 1/8" separation from the Rigel Glass Window (the window moves when the alignment is adjusted). The wiring to the RCA plug can be routed through the inside of the Rigel and a small hole drilled near base to allow wires to exit.
You can build a star diagonal heater as shown above. This will warm the diagonal which will in turn warm the eyepiece, so an eyepiece heater is not needed. This makes changing eyepieces much easier. Use about 20 resistors for a 2" diagonal, or about 15 resistors for a 1.25" diagonal. The resistors can be arranged in an array as shown below to provide more surface area.
If you are building a heater for a Newtonian Secondary Mirror it is better to stagger the resistors evenly across the back side of the mirror in an array similar to the above diagram. If the back of the mirror is aluminized do not allow any electrical connections to touch it as the aluminium coating will conduct electricity. Shown below is a matrix of resistors. Notice how jumpers connect every second connecting wire so that odd numbered wires are +12V and even numbered wires are ground. Thus each resistor has 12 volts applied to it. To determine the number of resistors to use, divide the circumference of the mirror by .625
Above is a variation that works well with small Secondary Mirrors. Notice that the resistors are laid out in a radial pattern with centre connection +12V and the outer wire ground. Only 4 resistors are shown but the actual number is calculated by dividing the circumference of the mirror by 0.625