Temperature Reference

It is often difficult to properly calibrate a temperature sensor since there is no suitable aid for doing so available. This article, which describes a temperature reference source, aims at putting this right. Since the source is made variable, the reference temperature may also be used for adjusting thermostats correctly. This may prove useful in the case of the recently published Titan 2000 audio power amplifier. The diagram shows how a Type BDV64 power transistor, T1, is used to provide a regulated-heat source and a calibrated sensor Type LM35 (IC2) monitors the resulting temperature. The two devices are mounted on a common heat sink.

temperature Reference circuit diagram
At the same time, good thermal coupling between IC2 and the sensor to be calibrated is of paramount importance. Circuit IC1 functions as an on/off switch and actuates the power transistor (heater) when the temperature drops below the set value. The desired temperature is set with potentiometer P1. The better the thermal coupling, the smaller the hysteresis of the system. The circuit operates as follows. The output of IC1 controls power transistor T1. The specified values of resistors R4 and R5 ensure that the current through the transistor is not greater than 0.5mA.

This results in a dissipation of not greater than 6W. Sensor IC2 is powered by a regulated 5 V supply. Its output is a direct voltage of 10 mV °C–1. With component values as specified, the temperature may be set with P1 between +20 °C and +74 °C. Given these data, it is fairly simple to construct a suitable scale for the potentiometer. Almost any power transistor in a TO3P case and an amplification factor of ≥ 1000 may be used for T1.

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