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====== Activity: Differential Temperature Sensor ======

===== Objectives: =====

A diode's forward voltage drop, V<sub>D</sub>, 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 D<sub>sense</sub> serves as the temperature sensor while diode D<sub>ref</sub> serves as the temperature reference maintained at a constant temperature, say at room temperature (25º C) which is convenient. The difference in diode voltages V<sub>Temp</sub> is consequently proportional to the difference in temperature.

===== Materials: =====
ADALM2000 Active Learning Module\\
Solder-less Breadboard\\
2 - Resistors (1KΩ)\\
2 - small signal diode (1N914 or similar)\\

===== Directions: =====

Construct the circuit of figure 1 using two 1N914 diodes. 

{{ :university:courses:electronics:a25_f1.png?600 |}}

<WRAP centeralign> Figure 1 differential temperature circuit </WRAP>

===== Hardware Setup: =====

Connect scope input 1+ to the positive terminal of V<sub>Temp</sub> and connect scope input 1- to the negative terminal of V<sub>Temp</sub>. Use the Scopy Voltmeter or Oscilloscope instruments to monitor the value of V<sub>Temp</sub> 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.

{{ :university:courses:electronics:difftemp-bb.png |}}

<WRAP centeralign> Figure 2 Differential Temperature Breadboard Circuit</WRAP>

===== Procedure: =====

1. Allow both diodes to reach the same temperature, i.e., T<sub>sense</sub> = T<sub>ref</sub> . Measure and record the voltage offset as V<sub>Temp</sub> set; subtract this offset voltage from your later measurements.

{{ :university:courses:electronics:difftemp-wav1.png |}}
<WRAP centeralign> Figure 3  T<sub>sense</sub> = T<sub>ref</sub> Differential Temperature Waveform</WRAP>

2. Heat the sensor diode by squeezing it between your fingers. Wait for the voltage to stabilize, subtract V<sub>Temp</sub> 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.

{{ :university:courses:electronics:difftemp-wav2.png |}}
<WRAP centeralign> Figure 4   T<sub>sense</sub> > T<sub>ref</sub> Differential Temperature Waveform</WRAP>

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 V<sub>Temp</sub> set, and then record its value as the "freezing point of water" voltage.

4. Determine the sensitivity of the temperature sensor output V<sub>Temp</sub> in millivolts per ºC.

===== Questions: =====

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 )?\\

<WRAP round download>
**Resources:**
  * Fritzing files: [[downgit>education_tools/tree/master/m2k/fritzing/diff_temp_sensor_bb | diff_temp_sensor_bb]]
  * LTSpice files: [[downgit>education_tools/tree/master/m2k/ltspice/diff_temp_sensor_ltspice | diff_temp_sensor_ltspice]]
</WRAP>

===== Further exploration: =====

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 V<sub>Temp</sub> follow the same sensitivity in mV/ºC you measured from the diodes?

**Return to Lab Activity [[university:courses:electronics:labs|Table of Contents]]**
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