This information is provided for those with the proper electrical knowledge to build 12 Volt telescope heaters using Nichrome wire. If you do not have such knowledge then you should build heaters with 330 Ohm resistors. This page only covers aspects of Nichrome wire, refer to building heaters with 330 Ohm resistors for information on how to assemble the heaters.

Basically building Nichrome wire heaters involves figuring out what the resistance would have been using 330 ohm resistors and then making your heater the same resistance by using parallel lengths of Nichrome wire. It involves calculations specifically for the heater you are trying to build. You can obtain Nichrome wire out of an old blow dryer, toaster, heating pad, electric blanket. It can also be purchased from companies such as Scientific Instrument Services.

Do not cut any pieces of Nichrome wire until you get to step 7. These instructions deal with very low resistance values so it is important to subtract the resistance in the ohmmeter leads from the reading on the meter. Touch the meter leads together to measure the lead resistance. Subtract this lead resistance from every measurement taken in the following instructions. For example if touching the leads together reads 0.5 ohms, and when you measure a piece of Nichrome wire the meter reads 15.0 ohms, then the true resistance is 14.5 ohms.

1. Measure the circumference of what you want your heater to fit. For example a C11 measures 38".

2. Divide 190 by the circumference to find out the desired heater resistance. So    190 / 38"  =  5.0 ohms

The shorter a piece of Nichrome wire is, the hotter it will get when 12 Volts is applied to it. This is because shorter  lengths of Nichrome have less resistance so more current flows through the wire. We must use a long enough piece of Nichrome so that it will not get too hot and melt something. We want it to be warm enough so that it is uncomfortable to touch, but not so hot it will burn us or melt plastic. The next step will determine the minimum Nichrome wire length. The wire can be longer, but under no circumstances should you cut any piece of Nichrome wire shorter than this minimum length.

3. Without cutting the Nichrome wire, stretch it out and use alligator clips or some other means of temporarily applying 12 Volts to the full length of the wire (make sure it's over 30" long). Assuming the Nichrome barely gets warm, move one alligator clip  a few inches closer to shorten the portion of wire the 12 volts is being applied to. Keep moving it closer until you find a point that it gets warm enough to generate heat but not so hot as to melting anything in contact with the Nichrome wire (be careful not to burn yourself, the wire may get hot!). In the above example I found that a 30" length of wire was the shortest piece I could use without it getting too hot.

4. Remove power from the Nichrome wire and touch the meter leads to the places on the Nichrome wire where the alligator clips were placed in step 3 to measure the resistance of the minimum length of Nichrome wire.  In this example the 30" length of Nichrome wire read 15.0 ohms.

5. Divide the minimum Nichrome wire ohms (step 4) by the desired heater resistance (step 2).  Increase the result to the next higher whole number (do not round downward). This is the number of  pieces of Nichrome wire you will have to cut. In this example, 15.0 ohms divided by 5.0 ohms equals 3.0 which means we will need 3 pieces of Nichrome wire.

6. Multiply the desired heater resistance (step 2) by the number of pieces of Nichrome wire we will be cutting (step 5). In our example, 5.0 ohms times 3 pieces equals 15.0 ohms.

7. Connect an ohmmeter lead to one end of the Nichrome wire, then move the other meter lead along the nichrome wire  to find the length of Nichrome wire with the desired resistance (step 6). Cut the wire a little longer and you can trim it later. In our example,  a 30" length of Nichrome gave the desired 15.0 ohms so we cut 3 pieces of Nichrome wire 32 inches long.

8. Slide each piece of Nichrome wire into a nonconductive material that can take the heat without danger of melting. If you obtained the Nichrome from an electric blanket then it may already be enclosed in heat resistant sleeving. Nichrome doesn't solder well so use crimp terminals to connect to it. Common electrical "ring terminals" work well and you can attach your wire leads to it by soldering copper wire to the ring terminal.

10. Construct your heaters so that the Nichrome wires are in parallel as shown in the figures below. If the Nichrome wires are shorter than the length of the heater then you can stagger the Nichrome wires and connect them with copper wires. On the other hand, if the Nichrome wires are longer than the heater length, then you can loop the Nichrome wire back and forth. The main idea is to build your heater so that the heat is applied evenly along its length. Just make sure that no wires can make contact with each other or melt the insulating material and short out.