Constant Temperature Circuit

Published:2013/3/10 23:05:00 Author:Ecco | Keyword: Constant Temperature | From:SeekIC

The 7805 voltage regulator provides a reference voltage that is fed into a resistive bridge formed on one side by the 20K trimmer and the other side by the 3.3K resistor and the 1K/thermistor combination. The termistor is an NTC (Negative Temperature Coefficient) type. The op-amp is run in a differential mode and tries to keep its inputs at the same potential by the thermal feedback loop formed by the heater and the thermistor. The three 1N4001 diodes are used to bias the emitter of the transistor up enough that it can shut off fully with the limited voltage swing from the 741 op-amp. The heating indicator LED (a standard red LED) also taps off of the same diode ladder to enable it to shut off entirely. The value of the (1uF) capacitor in the op-amp feedback loop may need to be adjusted if the circuit rings , or swings back and forth before stabilizing on a temperature. The capacitor value is specific to the thermal mass that is being temperature stabilized. The heater resistor is rated at approximately 40 ohms and 5 watts. The value of the resistor determines the heating rate and the power consumption. The resistor value should not be too low or the resulting high current will damage the 1N4001 diodes and/or the TIP122 transistor.



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