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Simple Transistor Thermometer Circuit

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This simple electronic transistor thermometer circuit can be used for measuring the temperature of the surrounding effectively using minimum number of components.

Introduction

All semiconductor materials are heat sensitive, some more 50 than others. Usually this is a disadvantage and many components in circuits especially those using germanium devices are included to overcome Hie effect on operation of heat.

There is however at least one semiconductor device which is designed specifically to take advantage of this property the thermistor, which is a resistor whose ohmic value varies with its temperature.

How Thermistor Works

Thermistors can have either a positive or negative temperature coefficient but most of those in common supply have a negative. coefficient that is their resistance falls with an increase in temperature.

The resistance/temperature curve for most thermistors is reasonably linear and it will be seen that quite a simple circuit can be made to take the place of a conventional mercury or spirit thermometer.

It is necessary to ensure that the applied voltage is absolutely constant, far more so than a plain battery would be and so a Zener diode is used in the supply circuit.

A Zener diode, if operated within its correct ratings will assure that whatever the condition of the power supply, there will always be exactly the same voltage across it (assuming that the supply voltage exceeds the operating working voltage).

Transistor Thermometer Circuit Description

With SW1 in the position shown, off, the meter terminals are shorted and no battery is connected. It is always a good idea to short out the terminals of a meter when not in use as this damps the movement in the case of accidental knocking and since there are spare switch positions, this facility is included.

In position B the battery is applied to the circuit through the resistor R4 on the other side of which is the Zener. The meter M1 is connected in a bridge circuit with R1 and the thermistor in one leg and with VRI across the supply in the other. At 25 degrees C the thermistor has a nominal resistance of about 500 ohms so there is about one third of the Zener voltage on that side of the meter.

If VR1 is set to a position so that the upper part of the track is twice that of the lower track there will be no voltage difference across the meter and so no current will flow.

If however the thermistor is heated above the nominal 25° C, the voltage on that side of the meter will fall and the meter will show a reading which will be directly proportional to the voltage which in turn is directly proportional to the temperature.

By using the setting of VR1 the meter can be made to read any range of temperature that is required. It is a fairly simple matter to calibrate the scale.

For instance, if it is required to measure weather temperatures we could say that the mean temperature is perhaps 15°C and we could make this the center scale. It Will be necessary to mix hot and cold water until this temperature is reached (this can be tested using a mercury thermometer).

The body of the thermistor is then put under water whilst making sure that the water does not come into contact with the wire leads.

Construction

The Construction of this transistor thermometer circuit is fairly easy task.

When the thermistor has reached a stable resistance, VR1 is set so that the meter reads center scale detection.

This control should not be touched again. To calibrate the scale it should then be a matter of adding hot water until the meter reaches full scale deflection and noting what this temperature is with the mercury thermometer.

The lower end of the scale can then be calibrated but this is not really necessary as the detection is constant. However many degrees full scale is over 15" C will indicate how many degrees zero deflection is below that temperature.

On the prototype with :1 15 degrees C center scale, the upper limit was 40 degrees C (105 °F) and the lower limit was -10 degrees C (15 degrees F) - quite a range.

For calibrating at 100 degrees C it is not necessary to put the thermistor into boiling water: it is quite sufficient to hold the thermistor in a jet of steam from a kettle. For low temperatures, 0 degrees C can be obtained by putting the thermistor into a bowl of melting ice.

The third position of the switch is for testing the battery voltage. The Zener ls bypassed and the battery is applied directly across meter with two accurate (at least 5%) resistors in series.

It is necessary to use two resistors to obtain the correct value. If the meter reads over half scale deflection, the battery is OK; if not it should be replaced. The battery will last a considerable time as current is only drawn while the unit is actually on.

Readings are shown instantly on switch on; there is no settling down time. The thermistor can be mounted anywhere in the circuit as the air temperature inside any cabinet will not be much different from that outside.

Alternatively the thermistor could be mounted outside a house with the meter inside, thus enabling the outside temperature to be seen at a glance. The resistance of even quite a long length of wire will make no difference to the readings.

Components List for the Electronic Thermometer

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