Re: Heater rheostat


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Posted by Sherman in Idaho [72.47.153.158] on Wednesday, July 15, 2015 at 13:36:39 :

In Reply to: Heater rheostat posted by Bob in N. GA [71.28.171.205] on Wednesday, July 15, 2015 at 12:44:27 :

The only practical way to figure out what resistance values to use is to try various values and see what gives you the speeds you want. Resistors have to be able to handle quite a bit of power (10-30 watts, depending). All the military heaters I've seen originally came with a 2-position toggle-type blower switch that included a resistor for the low-speed position. You could try to find one. Your M37 likely has an M151 heater, since they were very cheap at one time and can be made to fit, albeit without any defrost.

Rather than get excessively technical, my suggestion is to first try any old ballast resistor you have laying around. They're about 1 ohm, and in my experience provide a reasonable low speed setting for a heater blower, at least on a 12 volt system. It may not drop enough on a 24 volt system. A better alternative would be the dual ballast resistor that's used with the slant 6. I believe it's 5 ohms on one side and 1 ohm on the other. That would let you have 3 speed settings, although you'd need a rotary switch rather than a double-pole toggle switch then. Ballast resistors have just enough power handling capacity to work okay in this application, the resistance is in the right range, and they're readily available. As for the switch itself, sure a cheap switch would be fine -- current rating would need to be maybe 5 amps or so -- but obviously as with anything mechanical a better one would likely last longer.

If you want to get fancy, you can even combine ballast resistors in series or parallel to get the speeds you want. The best way to do it is just try them and see what works for you. I wouldn't recommend getting a rheostat. You'd need something like a 50 watt 5 ohm rheostat. Those are hard to find and expensive. Even worse, rheostats are rated for power assuming they're at the maximum resistance position so the heat is dissipated evenly across the whole unit. But in use, maximum heat is generated when it's set at a small fraction of its full range, and that can burn out the portion of the coil that's carrying current even through it's within the overall wattage range of the unit.



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