Here is an update on a dew heater, that I have now completed, that is made
from nichrome wire from an old heating blanket. I have tried building one
several ways including using a potentiometer to allow variable resistance
and have found an approach that what seems to work best. Here it is:

For my dew heater I wanted several power settings. The levels that I
eventually selected were 6.5W, 9W, 13W, 18W, and 28W. The nichrome wire
from the dead heating blanket that I used had a resistance of 0.33 ohms/ft.
In the heating blanket, each heating "wire" contains 2 separate nichrome
wires imbedded in insulation. the insulation is flat and about 1/8 inch
wide and 1/16 inch thick. The two wires are insulated from each other,
similar to what the old flat TV antenna cable was like. To get the lowest
setting, I needed a 75 ft length of nichrome wire. Higher power settings
required shorter lengths of nichrome. Since, the wire is doubled, I used
about 37 feet of cable, enough to do 14 wraps of my 10inch OTA and then used
a crimp connector to join both wires at one end giving me a total length of
75 feet. For the other power settings, I spliced into one side of the
heating blanket cable, at the appropriate lengths, to get shorter lengths of
nichrome wire. At each splice, I used crimp connectors to slice in a
supply wire.

Prior to wrapping the OTA, I put a wrap of duct tape around the OTA,
adhesive side out. I wrapped 2 layers of the heater cable on this, 7 wraps
to a layer. I used the duct tape to cover the wrapped heater elements.
With a few more layers of duct tape on it, it is pretty strong and holds its
shape well.

To supply the power to heater cable, I used some extra CAT 5 Ethernet cable
that I had available. This cable had 8 24 gauge wires in it, which are
thick enough to handle the 2.3-2.5 amps that the heater would draw at
maximum power. I ran the Ethernet cable back to a control box where I
connected the wires to a 6 position rotary switch I purchased at an
electronics store. For a power cord, I used a cigarette lighter connector
that I purchased at Radio Shack.

I find it works very well, is easily removed from my scope, and the control
box doesn't heat up, which is the problem I found when I tried to use a
potentiometer to vary resistance. the duct tape protects the OTA very well
and provides a very good enclosure for the heating elements.

I plan to write this up in a bit more detail, with pictures, and put it on
my web site when I have a few spare minutes.

Wendel Burkhardt

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