GB2109938A - Temperature measuring circuit using semi-conductor diode - Google Patents

Abstract

A temperature measuring circuit includes a single semiconductor diode 10, having a P-N junction, and there is supplied to the diode a square waveform varying between two currents, possibly, the currents for a silicon diode having the ration 3:1 approximately. The corresponding square waveform output of the diode is supplied to rectifying means 20, possibly, via an A.C. amplifier A2, and the output of the rectifying means is representative of the instantaneous temperature of the diode. It is not required to calibrate the diode, for example, if the diode is in parallel with an operational amplifier, having a virtual earth point associated therewith. No D.C. drift is associated with the A.C. amplifier, if provided. <IMAGE>

Description

SPECIFICATION Temperature measurement This invention relates to the measurement of temperature by circuits employing semiconductor diodes.

For semiconductor diodes there is a negative temperature coefficient of approximately 2 millivolts per "C, constant over a wide temperature range.

Such a D.C. voltage change with temperature, however, is based upon a forward bias potential drop of approximately 0.5 volt. This D.C. offset may vary from diode to diode, and may drift during the lifetime of a diode.

Further, the negative temperature coefficient referred to above may vary from diode to diode.

It is known to avoid the disadvantage of having to calibrate a single diode arrangement, because of the features referred to above, by using instead two matched diodes. Different, but constant currents are arranged to flow through the two matched diodes, and the differences between the simultaneous potentials across the diodes are directly proportional to the differences in the simultaneous temperatures of the two diodes. However, the arrangement is disadvantageous because it requires the two matched diodes. Further, it is disadvantageous because, if the differences between the simultaneous potentials across the two matched diodes are to be amplified, a stable D.C. amplification stage is required, and it is difficult to provide a D.C. amplifier the performance of which does not drift during its lifetime.

It is an object of the present invention to provide a novel circuit for measuring temperature which employs only one semiconductor diode; and, if amplification of the voltage proprotional to the diode temperature is required, the need to provide a D.C. amplification stage in the circuit is obviated; and, possibly, the circuit having the advantages of employing two matched diodes.

According to the present invention a temperature measuring circuit comprises a single temperature sensitive diode, having a P-N junction, means to supply to supply to the diode a square waveform varying between two finite currents, and the corresponding square waveform output of the diode is supplied to rectifying means, in response, the output of the rectifying means being representative of the instantaneous temperature of the diode.

An A.C. amplifier may be provided between the diode and the rectifying means, the A.C. amplifier not having any D.C. drift associated therewith.

The temperature sensitive diode may be connected in parallel with an operational amplifier having a virtual earth point associated therewith. Otherwise, the idode is required to be connected to earth, and the two finite currents are required to be provided by constant current sources, if calibration of the diode is to be avoided.

The current square waveform supplied to the diode may be provided by means including a square wave oscillator. When the square waveform output of the oscillator varies between zero potential and a finite voltage, the square waveform output of the oscillator may be supplied to an arrangement including two resistors. Possibly the two resistors are connected in series with each other, with the temperature sensitive diode connected to the point intermediate between the resistors, the ends of the resistors remote from the diode are connected to the output terminal of the oscillator, and the arrangement is such that the square waveform received by one resistor instantaneously is inverted in relation to the square waveform received by the other resistor, the arrangement including at least one inverter.Thus, there are no indeterminate resistances associated with the switching between the two finite currents, Alternatively, the square waveform output of the oscillator is supplied alternately to one resistor, and to both resistors, possibly the resistors being in parallel with each other, when the magnitude of the oscillator output changes.

The ratio of the two finite currents of the square waveform supplied to a silicon temperature sensitive diode may be 3:1 approximately, there being 0.1 millivolt change in the amplitude of the square waveform output of the diode per degree Kelvin change in the temperature of the diode. This follows from the juntion equation:: AV mKT 12 q Ii where AV is the difference between the forward voltage drops of the diode for the currents 12 and 11, m is a constant, which is approximately 1 for a silicon diode, K is Boltzmann's constant (1.38.10-23) T is the temperature in degrees Kelvin and q is the electronic charge (1.6 x 10-19) The present invention will now be described by way of example with reference to the accompanying drawing, which is a diagram of one embodiment of a temperature measuring circuit in accordance with the present invention.

The illustrated temperature measuring circuit includes a silicon semiconductor diode 10, having a P-N junction. The diode 10 is arranged to be subjected to the temperature to be measured.

The diode 10 is connected in parallel with an operational amplifier A1, the anode of the dide being connected to one input of the amplifier, the anode being connected to a virtual earth point 11 associated with the amplifier. Hence, any change in the performance of the diode has no effect on the temperature measuring circuit. The other input of the amplifier Al is connected to a point maintained at a reference potential, for example, at zero potential.

A varying current, in the form of a square wave, is supplied to the diode by an oscillator 12, the square waveform voltage output of the ocillator varying between zero and a predetermined finite value. The varying voltage of the output of the oscillator 12 is supplied to a first resistor R, and to a second resistor 3R, of treble the resistance of the first resistor R. The two resistors, Rand 3R, are connected in series, and the point between the two resistors is connected to the virtual earth point 11 associated with the amplifier Al. The end of the resistor R remote the virtual earth point 11 is connected to the oscillator 12 via two inverting gates 14 and 15; an the end of the resistors 3R remote from the virtual earth point is connected to the oscillator solely via the inverting gate 14.

Thus, there is a current Ii flowing through the resistor 3R, when no current is flowing through the resistor R; and there is a current 12, three times as big as the current Ii, flowing through the resistor R, when no current is flowing through the resistor 3R. In this manner a required square wave current waveform is supplied to the temperature sensing diode 10. This arrangement for supplying the desired current waveform is advantageous because there are not indeterminate resistances associated with the switching between the two supplied current values.

It is convenient to arrange that the ratio of the currents 12 and Ii, is approximately 3:1. Hence, at 25"C there is a square waveform of 29.8 millivolts amplitude across the P-N junction of the diode 10, At 1 00 C the square waveform across the P-N junction has an amplitude of 37.3 millivolts. The amplitude varies by 0.1 millivolt per change in the temperature of the diode.

The scaling factor of the circuit is independent of the amplitude of the output of the oscillator 12, the instantaneous temperature of the diode, and the currents Ii and 12 supplied to the diode, being only dependent upon the ratio of currents 12:11.

It is not necessary to calibrate the diode.

The square waveform output of the diode 10 is connected to an A.C. amplifierA2, and this varying voltage is amplified by a factor of 100. There is no D.C. drift associated with the operation of the A.C. amplifier A2.

The magnified varying voltage is now supplied to rectifying means, indicated generally at 20.

The output of the rectifying means 20 is supplied to a meter, 21, of any convenient form. The meter is calibrated such that, when a voltage of 2.98 volts is supplied thereto a temperature of 25"C is indicated by the meter, and when 3.73 volts is supplied a temperature of 100"C is indicated.

The rectifying means 20 may have convenient form, and may comprise a full-wave rectifier of a conventional construction, or a known form of peak detector. With a full wave rectifier a desired scaling factor may be introduced, but such a scaling factor cannot be introduced with a peak detector.

The scaling factors provided by the A.C. amplifier A2, and the meter 21, can have any desired values.

It is not essential that the ratio of currents 12:1" has a value of approximately 3, the ratio instead having any conveniently provided value.

The inverting gate 14 may be omitted.

The required ratio of currents 12:11 may be provided in any convenient way. Any arrangement including two resistors may be provided with a square wave oscillator the output of which varies between zero potential and a finite voltage. Thus, for example, the square waveform output of the oscillator 12 may be supplied alternately to one resistor, and to both resistors, possibly the two resistors being connected in parallel with each other, when the magnitude of the oscillator output changes, and so that the required current square waveform can be supplied to the temperature sensitive diode.

Further, the required current square waveform may be supplied to the diode in any convenient manner, instead of employing a square wave oscillator.

In other arrangements the output of the square wave oscillator may not vary between zero potential and a finite voltage.

The A.C. amplifier A2 may be omitted.

The temperature sensitive diode may not be in parallel with an operational amplifier. Otherwise the diode is connected to earth, and the currents Ii and 12 are provided by constant current sources, if calibration of the diode is to be avoided.

Claims (12)

1. A temperature measuring circuit comprising a single temperature sensitive diode, having a P-N junction, means to supply to the diode a square waveform varying between two finite currents, and the corresponding square waveform output of the diode is supplied to rectifying means, in response, the output of the rectifying means being representative of the instantaneous temperature of the diode.

2. A circuit as claimed in claim 1 in which an A.C. amplifier is provided between the diode and the rectifying means.

3. A circuit as claimed in claim 1 or claim 2 claims in which the temperature sensitive diode is connected in parallel with an operational amplifier having a virtual earth point associated therewith.

4. A circuit as claimed in claim 1 or claim 2 in which the temperature sensitive diode is connected to earth, and the two finite currents are provided by constant current sources.

5. A circuit as claimed in any one of the preceding claims in which the current square waveform supplied to the diode is provided by means including a square wave oscillator.

6. A circuit as claimed in claim 5 in which the square waveform output of the oscillator is supplied to an arrangement of two resistors, the square waveform output of the oscillator varying between zero potential and a finite voltage.

7. A circuit as claimed in claim 6 in which the two resistors are connected in series with each other, with the temperature sensitive diode connected to the point intermediate between the resistors, the ends of the resistors remote from the diode are connected to the output terminal of the oscillator, and the arrangement is such that the square waveform received by one resistor instantaneously is inverted in relation to the square waveform received by the other resistor, the arrangement including at least one inverter.

8. A circuit as claimed in claim 6 in which the square waveform output of the oscillator is supplied alternately to one resistor, and to both resistors, when the magnitude of the oscillator output changes.

9. A circuit as claimed in any one of the preceding claims in which the ratio of the two finite currents of the square waveform supplied to the temperature sensitive diode is such that there is 0.1 millivolt change in the amplitude of the square waveform output of the diode per degree Kelvin change in the temperature of the diode.

10. A circuit as claimed in any one of the preceding claims in which the rectifying means comprises a full wave rectifier.

11. A circuit as claimed in any one of claims 1 to 9 in which the rectifying means comprises a peak detector.

12. A temperature measuring circuit substantially as described herein with reference to the accompanying drawing.