Let's just use an example. Say you have a ceramic heat emitter or a heat panel in a cage. Place the sensor of the proportional controller where you want to measure the temp, say in the center of the cage in the air. Say your desired temp is 80 deg, and the current temp is 65 deg. When you first turn everything on, the controller will deliver constant voltage and current to the heater. As the temp approaches the range (called the proportional band) say for example 75 deg, the controller starts pulsing current to the heater. As the temp approaches 80 deg, the pulses get smaller and smaller in time length. When the temp hits 80, the controller delivers no power. As the temp just starts to drop again, the controller sends small pulses as necessary to reach 80 deg.



So you can see, with a proportional controller the heater stays very close to the setpoint (your dialed-in temp on the controller). This extends the life of the heater because the heating element is not warming up then cooling off then warming up then cooling off etc - it is just staying warm but controlled.



An ordinary ON/OFF controller (i.e. the $30 ones you see alot) applies full power until 80 deg, then shuts off and usually allows a drop of 4 deg then turns back on. So the heating element heats up, then cools alot (much faster than the cage temp cools) then heats up.. it does nothing to help heating element life but does control your temps.



ON/OFF control is the method of choice for heaters (or coolers) that heat very slowly or have moving parts like fans or else in insulated cages/containers (i.e. incubators) where the temp change is slow to cool. Proportional controllers are for heaters that heat fairly quickly and have no moving parts (and emit no light, because how annoying would a flashing light be) or in cages that cool quickly if the heater goes off for more than a few seconds.

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