One application of a pulse width modulator is to drive a resistive load with a saturated switch. The power dissipated by the resistor (up to 5 watts) is converted to heat which raises the temperature of the object that the resistor is mounted to. The temperature of the body is monitored with an LM335A temperature sensor which has an accuracy of 1 degree C and is used to close the control loop.

1. To use this circuit, convert the desired temperature to Centigrade: C=(F-32.2)/1.8
2. Then convert from Centigrade to Kelvin: K=C+273
3. Divide the desired temperature in Kelvin by 100 to get a voltage in the neighborhood of 3 volts.
4. Adjust R7 until the voltage on the pin 3 of U3 is equal to the value calculated in step 3.

Note: be sure to use ample quantities of thermal grease between R6, D1 and the body being heated. Note also that this circuit heats only - if the ambient temperature is warmer than the desired setpoint then the circuit will do nothing.

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