I was just thinking about electric ranges and wondering how the
controls for the heating elements actually work. Does anyone know?
They can't be variable resistors because they would dissipate too much
power. They obviously aren't variable autotransformers. They can't be
triac-dimmers because they pre-date that technology. So what are they?
Some sort of duty-cycling switch? Thanks.
They contain a small heater, which is energized whenever the element is
energized, and a bimetallic strip thermostat mechanism. When you first
turn it on, the element and heater are energized. The heater warms up
the innards of the control, which bends the bimetal strip and shuts off
the element and heater. Then the control cools down, the switch closes
again, and the whole process repeats over and over. The higher the
contol setting, the larger the fraction of time the heater needs to be
on to get the control to cycle, and the more heat you get from the
The idea is rather similar to triac dimmers, but because the element has
so much thermal mass, it's OK for the on/off cycle to last 10 seconds or
so instead of being 1/120 second. It doesn't distort the AC supply
waveform by turning on in mid-cycle like a triac, but it does produce a
load that cycles between zero and a couple of kW.
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