Temperature Control

First created: Sat Mar 3 23:54:41 EST 2001
Last updated: Sat Mar 3 23:54:52 EST 2001
temperature control circuit
Cadsoft EAGLE schema file.
power dissipation vs. Vin

The total amount of heat generated by this circuit is Vcc*IRes where Vcc is the 12 Volts from the power supply and IRes is the current through the load resistor (R2). This current is controlled by the input signal Vin. Some of that heat is generated by the transistor WTrans, the rest by the load resistors WRes.

If Vin = 0 there is no power dissipated and no heat generated.
If Vin = Vcc/2 (6V), WTrans is about equal to WRes.
As Vin approaches Vcc, WRes rises and WTrans falls.

Clearly we want to operate in the region where Vin > Vcc/2, so that most of the heat generated is generated by the load resistors. If the base resistance R1 is 0, the circuit is quite independent of the transistor's gain. This graph was generated for Rbase = 0, Rload = 25. If Vin is around 10 V, then the transistor dissipates about 1 W, and the heating resistor about 4 W.
circuit model

The heat generated increases when Vin rises, so we need to couple this circuit to one that measures temperature and produces a higher signal Vout when the temperature drops. That's precisely the circuit used to monitor the temperature.

Coupling these 2 circuits produces the complete temperature control circuit. This circuit must remain completely independent from the monitoring circuit, and is housed in a separate enclosure.

temperature control circuit
Cadsoft EAGLE schema file.