This information is provided for those with the proper electrical
knowledge to build 12 Volt telescope dew heaters using Nichrome
wire. If you do not have such knowledge then you should
build heaters using 330 Ohm resistors. Do not attempt
to run these heaters on continuous 12V as they may overheat.
These heaters must be used with a 12V dew heater controller
such as the DewBuster™ Controller.
Mark Kaye's instructions for building heaters with Nichrome
wire cover the construction details but leave it up to the
builder to determine the electrical aspects such as minimum
wire length and desired Ohmic value for your particular
telescope or eyepiece. The instructions below will insure
the wattage is consistent with a commercial heater of the
same size and allows using a mix of Nichrome heaters, 330
Ohm resistor heaters, and commercial heaters. These instructions
assume you are familiar with my instructions for
building heaters with 330 Ohm resistors, if not you
should review them first. This page only covers aspects
of Nichrome wire, refer to my
building heaters with 330 Ohm resistors and
Mark Kaye's instructions for information on how to assemble
Building Nichrome wire heaters involves calculating
the desired resistance and then using parallel lengths of
Nichrome wire to obtain that resistance. You can obtain
Nichrome wire out of an old blow dryer, toaster, heating
pad, etc. It can also be purchased from companies such as
Jacobs. 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, then a piece of Nichrome wire that reads
15.0 ohms is really only 14.5 ohms.
Measure the circumference of what you want your
heater to fit in inches. For example a C11 measures
Divide 190 by the inches of circumference to determine
the desired heater resistance. In this example:
190 / 38 inches = 5.0 Ohms
Because this ratio produces 0.76 Watts per inch of circumference
when operated at 12 Volts, matching the wattage of similar
The next step determines the minimum length that
any piece of Nichrome wire can be cut. If the Nichrome
is shorter than this minimum it has too little resistance
so too much current flows and the wire overheats.
The wire can be longer because this increases resistance
so less current flows and the heat is reduced.
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 feels very warm but not hot enough to melt anything
that contacts the Nichrome wire (be careful not to burn
yourself, the wire may take a few minutes to reach its
maximum temperature). In the above example I found that
30 inches was the shortest piece I could use without
it getting too hot. We do not want the wire to melt
the heater strip or any part of the telescope. The shorter
we cut a given piece of Nichrome wire the hotter it
will get because the shorter the wire the less resistance
it has and the more current flows through the wire.
Now that we know the minimum length of wire, as long
as the wires are this length or longer the wire will
not get too hot.
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 inch length of Nichrome wire
read 14.0 ohms.
Divide the minimum Nichrome wire ohms (step 4) by
the desired heater resistance (step 2) which in this
example was 2.8. Increase the result to the next higher
whole number (do not round downward) which in this example
was 3. This is the number of pieces of Nichrome wire
you will have to cut, but we do not know the
length so do not cut any Nichrome yet.
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.
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 32 inch length of Nichrome gave the desired 15.0 ohms
so we cut 3 pieces of Nichrome wire 33 inches long and
the length will be shortened to 32 inches when the terminals
Slide each piece of Nichrome wire into heat shrink (such as
Mouser 5174-11161) to prevent the wires from
touching themselves or any other wires. Test the
heater by applying 12V to the heater (as
in step 3) for 10 minutes to insure it does not melt
the material. NOTE: If the
nichrome wire is difficult to get through the heat
shrink tubing, first run a small diameter copper wire through
then twist the nichrome and copper wires together
and use the
copper wire to
pull the nichrome through the heat shrink tubing.
Solder will not stick to Nichrome so you must use
crimp terminals to connect the nichrome to your
copper wires. Common electrical
ring terminals work well. Twist the nichrome and
copper wires together then insert into terminal and
crimp. After crimping, you may solder the
connections for even better contact.
Construct your heaters so that the Nichrome wires
distribute the heat evenly along the length of the
heater as shown below (if portions of the heater
contain more the nichrome than other parts you will have hot spots). Also make sure that no wires can make electrical
contact with each other and short out.
If you only have one long Nichrome wire it can be
looped back and forth as shown in the first illustration.
If there is an odd number of passes, use a copper wire
to connect the end of the Nichrome to the opposite end
of the heater strip where the wire to the RCA plug attaches.
The copper wire should be covered in heat resistant
material so it won't melt and touch the nichrome wire.
If the Nichrome wires are shorter than the length
of your heater then you can stagger the Nichrome wires
and connect them with copper wires as shown in the second
If several parallel Nichrome wires are used and
they are longer than the heater length, then you can
put the loops side by side as shown in the third example.
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Copyright 2015 by Ronald & Teresa Keating. All Rights Reserved.