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Circuit Notebook

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Using the LM335Z for temperature measurement

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This circuit was designed for use with a weather station that required a 0-0.7V signal for a temperature range of -7°C to +43°C. The temperature sensor chosen for the measurement was an LM335Z (TS1). This has an output of 0V at -273.15°C (absolute zero) and typically varies by 10mV/°C above absolute zero.

The change with temperature could be between 9.79mV/°C to 10.2mV/°C depending on the actual sensor. At 0°C, the sensor typically provides an output of 2.73V.

To use the LM335Z for the particular weather station, the signal needs to be level shifted and amplified so that the output voltage is 0V at -7°C.

The LM335Z is supplied with a nominal 1mA of current and a CMOS op amp (IC1b) monitors the voltage across it. VR3 sets the gain to about 1.4 to convert the nominal 10mV/°C from the sensor to 14mV/°C. This provides a 0.7V range in voltage for the 50°C temperature range from -7°C to +43°C

IC1a and the LM336-2.5 voltage ref­erence (REF1) provide the offset adjustment for IC1b’s output so that it is 0V at -7°C. REF1 has a nominal output of 2.490V and is temperature-compensated by two 1N4148 diodes connected in series with a 10kΩ trimpot (VR1). VR1 is adjusted to set the voltage across REF1 to 2.490V at 25°C.

Trimpot VR2 provides an adjust­able reference to IC1a, ranging from 0V to 2.490V. IC1a amplifies this voltage by three and thereby provides an offset for IC1b.

For the weather station, IC1b’s output needs to be 0V at -7°C. At -7°C, the LM335Z’s voltage will be about (273 - 7) x 10mV or 2.66V. To reduce IC1b’s output to 0V, we set IC1a’s output to 6.65V to get the desired result.

Calibration of the unit is as follows:

(1) A multimeter is used to measure the TP1 to TP GND voltage and VR1 is adjusted for 2.49V at a temperature of about 25°C.

(2) VR2 is adjusted so that TP2 is at 0V.

(3) The LM335Z is immersed in a container full of stirred ice in distilled water, to provide a 0°C reference. VR3 is then set to give 3.822V at the output of IC1b. This is a setting of 14mV/°C at the output. Since the voltage is 0V at -273°C, the 0°C voltage will be 273 x 14mV or 3.822V.

(4) Finally, VR2 is readjusted so that the output is at 98mV. This is the 0°C reading, so the reading at -7°C will be 0V.

The circuit can be configured for a different temperature and output voltage range. For example, if the output is required to cover a 0-5V range for 0-100°C, then the gain of IC1b needs to be set to about 5. Resistor R1 should be changed to be 10kΩ. VR3 would then be set to about 40kΩ. At 0°C, the output from the LM335Z is about 2.73V and IC1b multiplies this by 5. To offset this voltage, IC1a’s output would need to be (2.73V x 5)/4 or 3.41V.

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