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 only for use on a 12-Volt Battery or 13.8VDC
Power Supply, higher voltage will overheat and damage them.
I will offer no advice on modifying these heaters for other
voltages or applications, try searching the internet for alternate
heater plans. NEVER POWER A DEW HEATER ON 120 VOLT DUE TO THE
RISK OF ELECTROCUTION.
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
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
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
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
"Soldering is Easy" guide which is tailored for
330-Ohm, 1/2-Watt Resistors -- Use only 330-Ohm
resistors rated for 1/2 Watt or greater. Common 4
color 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 discolor. You may purchase resistors from an
electronics outlets or
this web site.
Solid hookup 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
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
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
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
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
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
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
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
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 finderscope 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 fabove
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 Quickfinder, the resistors are wired as shown
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
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 aluminum
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 center 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
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Copyright 2015 by Ronald & Teresa Keating. All Rights Reserved.