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Implementing temperature proportional circuit

28 Jan 2013  | R.O. Ocaya

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It is recognised that the p-n diode can be the basis of an accurate thermometer for cryogenic temperatures up to about 200°C. A constant current is maintained through the diode and the voltage across it gives an indication of the temperature. The magnitude of the constant current is typically chosen small to minimise diode self heating. The diode voltage falls linearly with temperature making it necessary to have an operational amplifier conditioning circuit for direct readout. Output calibration is achieved using a voltage reference.

In this design idea the concept of the p-n diode driven at constant current is extrapolated, potentially eliminating voltage references and op amps. The result is a unique implementation of a simple but highly accurate temperature proportional circuit. Interesting applications are also suggested. The operation of the circuit is based on a switched scheme that allows two alternate constant currents to flow through the diode. The method is well grounded in the mathematics but the sombre details are omitted in this article. The starting point is Shockley's diode equation:

The equation relates the current (I) flowing through a p-n diode to the voltage (V) across it, the temperature (T) and the ideality factor (n) of the semiconducting material of the diode. Boltzmann's constant is k, and the electronic charge is q. The term In is a non-temperature dependent part of the reverse saturation current of the diode. In a reported experiment [1] a silicon 1N4148 diode is alternately driven first at current I1, and then at constant current I2, with the changeover occurring at a temperature T.

Figure 1 shows the effect of the two currents on the diode voltage as it is cooled in a water bath from temperature T0. In principle, the segments AB and CD extrapolated backwards meet at absolute zero (-273°C). However, impurity and other effects in the diode limit circuit usefulness to about -200°C.

Figure 1: Operating principle of the direct RMS readout thermometer.

At temperature T the use of two sequentially switched currents gives the data pair: (T, I1, V1), and (T, I2, V2). Putting the pair into Shockley's equation allows the difference voltage ΔV = V1(T)—V2(T) to be found easily in terms of the ideality factor as

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