Tips for Using a Diode as a Thermometer

Diodes are typically used in electronics to move current in one direction, but this common component can also be used as a thermometer. Standard diodes made of silicon have a linear relationship between voltage and temperature. Here are some essential facts about how diodes can be used to measure temperature in a circuit.

  • Voltage and Temperature When forward current remains constant, forward voltage declines while the temperature rises in a standard diode. But when forward current drops, the forward voltage becomes more affected by temperature changes at lower forward currents. If the circuit can amplify the diode's forward voltage, then the diode begins to serve as a thermometer. The diode will generate voltage fluctuations as ambient temperature changes. As a digital multimeter will show, the diode acts as a temperature sensor. A diode can take the place of a thermistor and can be the most affordable solution for conducting such measurements with reasonable accuracy. It's best to avoid voltage excitation, which will reduce accuracy due to resistance.

  • How Thermal Diodes are Used As the diode heats up, the voltage drops while heat flows in one direction. Heat flows from one terminal of the diode to another, as long as the first terminal is hotter. A hotter second terminal will greatly reduce the effect. Silicon diodes are mass produced at low cost, which is why they are used frequently instead of more expensive thermistors for measuring temperature. Some diodes are manufactured specifically for this purpose, particularly the 1N4148 diode, which is appropriate for measuring temperature in the range of zero degrees to 160 degrees Celsius.

The technology of thermal diodes is used in microprocessors and placed where the highest temperature usually occurs. Modern Intel CPUs, for example, include thermal diodes embedded on the chips. Thermal diodes should be used when precision is not important. In other words, thermal diodes provide estimates that are in the ballpark.

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