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. These heaters are Kendrick compatible, but any mistake in
construction could damage
non-DewBuster™ Controllers and void their warranty. The
DewBuster™ Controller has
built-in fault protection and the warranty covers any damage cuased by mistakes
building these heaters. These heaters are only for use on 12-Volt Battery or
13.8V Power Supply and require a controller to
regulate the heat or they will generate too much heat and burn up. 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.
Homemade Heater Strips
by Ron Keating
Search the Internet and you will find many different arrangements
for building dew heaters.
While they do work, most of them wire the resistors in series and if you need a
different size heater then all the values change. You must also be careful not
to exceed the resistor wattage or it could overheat and possibly even cause a fire. To avoid such problems, the instructions
below will use 330-Ohm 1/2 Watt resistors wired in parallel. No calculations are needed with this method,
simply space the
resistors 5/8 inch apart. These heaters are compatible with the
DewBuster™ Controller
and other controllers that accept Kendrick heaters. There is no risk of
exceeding the wattage rating and each resister will generate its full rated
1/2 Watt of heat
when the controller is set to full power which means you will use the fewest possible resistors
for the heater. Any size heater can be built by simply adding more 330-Ohm resistors.
To determine the wattage of the heater simply multiply the number of resistors
used by 0.5 Watts. To determine heater current just multiply the number of
resistors used by 0.036 Amps. For example a 28 resistor heater will generate 14
Watts and draw 1 Amp. If you plan to build a very large heater be sure the
current does not exceed the capabilities of your controller. 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.
Materials needed:
330-Ohm, 1/2-Watt Resistors
-- Make sure you have the correct resistors. They must be 1/2 Watt and they
must have a value of 330-Ohm. A 330K-Ohm or 330M-Ohm will not generate any
heat. There should be only 4 color bands
and they should be
orange-orange-brown-gold meaning that the resistor is 330-Ohms
(orange-orange-brown) plus
or minus 5% (gold). If there are more than 4 bands or if
the 4th is not gold, then consult the
manufacturer for proper identification. 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 can save money by ordering resistors in quantity from an
electronics outlet such as Mouser Electronics (Mouser# 293-330-RC )
and the leftovers can be used whenever you need to build another heater. If you do not need enough to offset Mouser's shipping
costs, you may
purchase
resistors through me.
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.
Your local Radio Shack should carry "solid hookup wire" and a roll is under $10.
It is also available at Mouser Electronics (20AWG
Mouser# 602-1563-100-01 and 18AWG
Mouser# 602-1565-100-01). Solid wire is
preferred because with stranded wire the solder will bond the strands together
making the heater stiff and inflexible.
Speaker cable -- 18 AWG lamp cord or speaker cable is
recommended, but you may use 24 AWG for 4" and smaller heaters. You may solder
your own RCA plugs or use pre-made speaker cables with RCA plugs.
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. Radio Shack
has pre-made 18 AWG speaker cables with RCA plugs (RS#
42-2478)
that are ideal, but many Radio Shack stores do not keep them in stock (ask your
store if they will order them and let you buy them "in store" to save shipping).
Another option is to get a good quality RCA plug and make your own. Mouser has a
very nice all-metal solder-type RCA plug
Mouser# 17PP058 (the web site doesn't show a
picture but click on Data Sheet to see what it looks like). Black 18 AWG "lamp
cord" is available at any Home Depot (they have it in rolls and will cut a
length to your needs) and speaker wire is available at most department stores.
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.
HEATER CONSTRUCTION:
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 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.
If your DewBuster is mounted on the
telescope base and you want to reduce the number of wires running up to the
scope, run a single wire from the DewBuster's AUX output to a
4 RCA Jack Panel. You can then plug 4 heaters into it. Make sure the wire is
heavy enough gauge to handle the combined current of the heaters. Usually #18AWG
is sufficient.
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 putting them in front of the glass. For this you need a slim heater
so wire it as shown in the following diagram. It looks different but is actually
the same parallel wiring as in the above instructions. The main difference is
that you must use insulated wires to prevent shorts.
For a Rigel Quickfinder, the
resistors are wired as shown and glued to the inside of the black plastic
housing about 1/8" from the Rigel Window (to allow for window movement when
aligning). 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 is much easier than having to refit a heater
every time you change an eyepiece. Use about 20 resistors for a 2" diagonal,
about 15 resistors for a 1.25" diagonal. The resistors can be arranged similar to the Newtonian
Secondary Mirror heater shown below.
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. 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 center connection is +12V and all
outer connections are ground. Only 4 resistors are shown for clarity, but the
actual number is determined by dividing the circumference of the mirror by .625
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