A diode's forward voltage drop, VD, decreases by approximately 2 mV for each 1º C rise in temperature, assuming a constant current in the diode. The circuit shown in figure 1 uses this property as the basis of a crude temperature sensor, differential temperature actually. It is best if the diodes are of the same type, ideally from the same manufacturer. Both diodes are forward-biased using equal resistor values to establish the same current, at least when the diodes are at the same temperature. Diode Dsense serves as the temperature sensor while diode Dref serves as the temperature reference maintained at a constant temperature, say at room temperature (25º C) which is convenient. The difference in diode voltages VTemp is consequently proportional to the difference in temperature.
ADALM2000 Active Learning Module
2 - Resistors (1KΩ)
2 - small signal diode (1N914 or similar)
Connect scope input 1+ to the positive terminal of VTemp and connect scope input 1- to the negative terminal of VTemp. Use the Scopy Voltmeter or Oscilloscope instruments to monitor the value of VTemp using the True RMS measurement display. Use auto-range for the Voltmeter or set the “Volts/Div” scale for the Oscilloscope to its most sensitive value (10 mV) and ensure that Channel 1 is enabled. Connect Vp to the 5V Power Supply.
Figure 1 Differential Temperature Breadboard Circuit
1. Allow both diodes to reach the same temperature, i.e., Tsense = Tref . Measure and record the voltage offset as VTemp set; subtract this offset voltage from your later measurements.
Figure 3 Tsense = Tref Differential Temperature Waveform
2. Heat the sensor diode by squeezing it between your fingers. Wait for the voltage to stabilize, subtract VTemp set, and then record this value as the “body temperature” voltage. You might also try blowing through a straw to direct your warm breath at the sensor diode.
Figure 4 Tsense > Tref Differential Temperature Waveform
3. If available, wrap the sense diode in a thin plastic bag and submerge it in ice water to chill the sensor diode. Again, wait for the voltage to stabilize, subtract VTemp set, and then record its value as the “freezing point of water” voltage.
4. Determine the sensitivity of the temperature sensor output VTemp in millivolts per ºC.
Can you derive the sensitivity in mV/ºC you measured from the diode equation?
What is the purpose of the reference diode in this configuration?
This circuit only measures the difference in temperature not the absolute temperature of either diode. How could you use the temperature dependence of a simple diode circuit like this to determine the actual temperature of the sense diode ( i.e. relative to absolute zero )?
Try substituting a pair of diode connected (base and collector shorted together) NPN or PNP transistors from your ADALP2000 Analog Parts Kit. Does the differential voltage VTemp follow the same sensitivity in mV/ºC you measured from the diodes?
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