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# Differential Heat Detector

IP.com Disclosure Number: IPCOM000091776D
Original Publication Date: 1968-May-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 31K

IBM

Reed, JW: AUTHOR

## Abstract

Linear, low-drift, temperature compensating resistors form the basis of this circuit for detecting temperature differences. Bridge circuit 10 comprises resistors 11...15. Temperature compensating resistors 11 and 12 are located in different environments for which the difference in temperature is significant. For example, one resistor can be set in an air flow duct and the other in a location reflecting room temperature. Circuit 10 can be calibrated or balanced by adjustment of the arm on tapped resistor 14.

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Differential Heat Detector

Linear, low-drift, temperature compensating resistors form the basis of this circuit for detecting temperature differences. Bridge circuit 10 comprises resistors
11...15. Temperature compensating resistors 11 and 12 are located in different environments for which the difference in temperature is significant. For example, one resistor can be set in an air flow duct and the other in a location reflecting room temperature. Circuit 10 can be calibrated or balanced by adjustment of the arm on tapped resistor 14.

Across the diagonal of the bridge between points 21 and 22 a voltage E(0) is derived equal to E(22) - E(21). Voltage E(0) varies from zero upwards as the temperature differs at resistors 11 and 12. This voltage difference is amplified in operational amplifier 20. A monolithic amplifier having a high, open-loop gain and high-input impedance is suitable.

Resistor 28 couples amplifier 20 to detector trigger circuit 30. Resistors 31, 32, and 33 determine the triggering level applied through resistor 34. The reset level is determined by resistors 31...34. Logic level conversion circuit 40 follows circuit 30. Normally, circuit 30 provides ground or zero potential at output terminal
41. It provides a logic level of +3 volts, for example, upon triggering of circuit 30 when the temperature difference rises beyond the limit set. A temperature controlling device of conventional configuration is connected to output terminal
41. By replacing resi...