PSN-L Email List Message

Subject: Re: Thermal Control for Sensors
From: Pete Rowe ptrowe@.........
Date: Fri, 16 Nov 2007 11:53:15 -0800 (PST)

Hi Jerry
Since we don't know how well your insulation works, it
is hard to define how many watts you need to dissipate
to keep the inside of the box at a certain
temperature. I doubt that you need even 10 watts if
your insulation is good. I would start with some
simple experiments. You'll need a thermometer inside
the box that you can read from the outside without
opening the box. Most any inexpensive indoor/outdoor
thermometers will be fine. Be aware that most of those
thermometers are only accurate to +/- 2 degrees C. For
this measurement that is ok.
Then get a 1 amp 12 volt DC wall wort and a 150 ohm 5
or 10 watt resistor. Put the resistor in your box and
hook up the 12 volts to it. The resistor will
dissipate about 1 watt (P= Esquared/R). Let it run for
a day and see if the temperature in the box goes up
and levels off. If this temp is ok, then you're done.
If you need more watts, calculate different resistors
for 2  or 5 watts and repeat the experiment. You
should find that you don't need a thermostat if you
sneak up on a fixed power dissipation that keeps the
temperature stable. If your insulation is very poor or
the outside temperature has large swings, then you'll
need more watts and a thermostat to cycle the heater.
It is best to avoid thermostats because they always
make electrical noise.
I hope this helps.
Pete Rowe
--- Jerry Payton  wrote:

> I am searching for wiring details for using Aluminum
> Housed Wirewound Power 
> Resistors used for thermal control inside an
> insulated housing for the 
> seismic sensors.  It has been suggested wiring
> several such resistors in 
> series to accomplish a 10-30 watts from the 110v AC
> house supply source. 
> I'm a little leery about the house current for
> safety reasons and possible 
> AC hum.
> In studying the Mouser Electronics catalog, I can
> find several possible 
> configurations using one to several resistors in
> series.  The resistors 
> specify their wattage capability, but have different
> resistance values to 
> choose from.
> If anyone has done this and has specific values
> etc., I'd appreciate hearing 
> from you.
> Light bulbs attract bugs and other unwanted
> critters!
> Thank you,
> Jerry 

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