How do you measure temperature with a diode?

Question

Please, explain it also with the corresponding equations.

Answer 1

Generally speaking, an increase in temperature leads to an increase in electrical conductivity.

Many multimeters today include a feature that can allow us to use them as digital thermometers, by plugging in the appropriate probe.

If you have such a digital multimeter (try Radio Shack for an inexpensive one) they may sell probes ready made, or you can try to make your own using a typical diode (1N4004 silicon, or 1N34 germanium, or similar) as a probe.

The forward voltage drop across a silicon diode is about 0.6 volts, across a gemainium diode is about 0.34 volts. Calibrate your probe by dunking it into an ice bath (zero centigrade) and then into boiling water (200 centigrade). Record the voltage drop across the diode at those temperatures. Make sure it is properly encapsulated (use silicone rubber, if you are making your own) and let it return to ambient temperature before making subesquent measurements.

With a little effort, you should be able to develop a chart with which to convert the voltage reading to a temperature measurement. (Use graph paper.) If you have the right kind of diode, it should be fairly linear. If you are in doubt, try some measurements at temperatures in between the extremes and compare to a known good thermometer.

20 NOV 06, 0320 hrs, GMT.

Answer 2

I don't know of a particular equation. But Vf of a diode decreases with temperature. This is also true of Vbe of a transistor. If you search the component data sheets, the manufacture will give a graph plotting this voltage as a function of temperature. The result is not linear but you could make a look-up chart. FYI: Zener diodes, which operate in the reverse bias area, have an inverse characteristic. Vz increases with temperature. Neat tip: If you ever need a cheap temperature compensated Vref. Connect an ordinary silicon diode with a 5.6V Zener. Their combined temperature coefficients nearly cancel and give you 6.3V.
You can measure temperature with a germanium diode. But, they don't like much heat. If it's too hot to touch, use a silicon.