Bang Bang Heater
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.