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Thermistor Type Temperature Monitoring Circuit

IP.com Disclosure Number: IPCOM000082434D
Original Publication Date: 1974-Dec-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

DeSilva, DK: AUTHOR [+2]

Abstract

This circuit provides high sensitivity, high reliability monitoring of the temperature sensed by a thermistor. This is accomplished by connecting he thermistor as part of an RC circuit, to control the charging of a periodically discharged capacitor in such RC circuit. If the capacitor voltage exceeds a given threshold level, an output signal is produced to provide an indication that the temperature has exceeded a selected level.

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Thermistor Type Temperature Monitoring Circuit

This circuit provides high sensitivity, high reliability monitoring of the temperature sensed by a thermistor. This is accomplished by connecting he thermistor as part of an RC circuit, to control the charging of a periodically discharged capacitor in such RC circuit. If the capacitor voltage exceeds a given threshold level, an output signal is produced to provide an indication that the temperature has exceeded a selected level.

Fig. 1 shows the circuit and Fig. 2 shows various waveforms appearing at different points in the circuit.

Thermistor 10 constitutes the temperature sensing element. The monitor circuit has an upper branch which includes a transistor 20, an RC circuit formed by a potentiometer 21 and a capacitor 22, a diode 23, transistors 24 and 25, a single-shot circuit 26, and a lamp driver circuit 27 which drives an indicator lamp
28. Lamp 28 is lit when the temperature sensed by thermistor 10 exceeds a first temperature level.

The monitor circuit also has a lower branch which includes a transistor 30, an RC circuit formed by a potentiometer 31 and a capacitor 32, a diode 33, transistors 34 and 35, a single-shot circuit 36, and a lamp driver circuit 37 which drives an indicator lamp 38. Lamp 38 is lit when the temperature sensed by thermistor 10 exceeds a second and different temperature level.

The thermistor 10 is connected to and forms part of the RC circuit in each branch. Except for different time constants in the RC circuits, the two branches are substantially identical.

Both branches are periodically pulsed by half-wave sinusoidal pulses from a 60 Hertz signal source 40. These pulses (waveform A) periodically turn on the transistors 20 and 30, to periodically discharge the RC circuit capacitors 22 and
32.

Considering the upper branch 20-28, the capacitor 22 is discharged by the 60 Hertz pulse 50 (waveform A). It then commences to be recharged by direct current flowing by way of the thermistor 10, the potentiometer 21 and the capacitor 22 to ground. The voltage waveform across capacitor 22 is indicated by waveform B. As this capacitor voltage crosses a threshold 51 (waveform B), the diode 23 is rendered conductive. This turns off the transistor 24 which, in turn, turns off the transistor 25 to produce, at the collector of the latter, the output signal represented by waveform C. This pulse-like output signal is lengthened by the single-shot circuit 26, and the lengthened signal is supplied to lamp driver circuit 27...