Metering and Instrument CircuitsSensors - Tranducers Circuits

Remote Temperature Sensor for Digital Multimeters

National Semiconductor’s LM334Z is a temperature-dependent adjustable current source supplied in a plastic TO-92 package. In Fig.  a 226 Ω resistor, R1, is used to set a current gradient of 1 µA K-1.

The remote temperature sensor is formed by IC1, R1 and C1. Since its output is a temperature-dependent current, a simple two-wire connection may be used between the sensor and the DMM (digital multimeter) interface.

Constant current drive as applied here eliminates problems with voltage drop and expensive low-loss wiring associated with voltage drive. Also the voltage drop across a relatively long cable is temperature dependent, which calls for a fairly complex compensation circuit. By contrast, when the sensor is a constant current source, the length and the total resistance of the wire between it and the interface at the DMM side have virtually no effect on the output signal. This obviates a compensation circuit, and allows the sensor to be fitted at quite some distance (up to 25 m = 80 ft) from the DMM using inexpensive wiring.

Remote Temperature Sensor for Digital Multimeters Circuit Diagram:

Remote Temperature Sensor

Components P1 and R2 convert the current supplied by the sensor into a voltage with a gradient of 10 mV K-1. Capacitor C1 suppresses high-frequency interference which may be picked up by the cable. To prevent problems with ground levels, the current source must be powered by a separate 9-V battery as shown in the diagram. To allow temperature readings in degrees Celsius (°C), an adjustable high-stability voltage regulator Type REF-02 from Precision Monolithics Inc, is used to subtract a fixed amount of 2731.5 my from the converter output voltage. This is achieved by lifting’ the converter ground by 2731.5 mV (the REF-02 output voltage) when switch S2 is opened. When S2 is closed the converter produces a temperature reading in kelvin.

The remote temperature sensor circuit is switched ON and OFF by S1. A quick battery condition check is available by switching S-3a to position ‘a’ and S-3b to position ‘d’. Replace the battery if the DMM indicates less than 7.1 V.

Calibration of the converter is fairly simple. First, adjust multiturn preset P2 until a voltage of 2731.5 mV is obtained across R4 (open S2). Next, adjust the temperature gradient (preset P1) by comparing the DMM reading to that produced by a calibrated thermometer. Set the DMM to the 2 V range for degrees Celsius (°C) readings. An indication of, say, 0.217 V (on a 3.5 digit instrument) then corresponds to a measured temperature of 21.7 °C. Properly adjusted, the temperature sensor achieves a resolution of 0.1 kelvins. Finally, the current drain is about 2 mA.


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