There are numerous devices on the market that read and monitor temperature. From simple thermometers to black body radiation optical measurements, many clever designers have invented techniques ranging from simple and intuitive, to complex and obscure. We're going to cover a common and simple technique of measuring temperature using standard low-cost diodes.
As one would imagine, the cost to implement a temperature sensor or limit detection circuit can be proportional to the accuracy needed. For example, a precision machine mixing chemicals may need to heat a solution to an exact and precise point before mixing in a reagent. Medical equipment, too, may need to be up to temperature before analyzing a blood sample.
Measurement of Temperature Using a Semiconductor Device
But, it is not crucial to have exacting temperature measurements in every design. In many cases a simple ballpark value can be used. Take, for example, an older-style thermostat. The manual and mechanical adjustments provided a needle gauge of ‘approximate’ temperature that a room is set to. It is way overkill and too costly to use a five-digit accurate calibrated and compensated temperature sensing system.
Fortunately, a common and simple technique using standard low-cost diodes can be used to make a relatively accurate temperature sensor. This hinges on the characteristic of a semiconductor junction under reverse-biased conditions. The reverse current flow is directly proportional to the temperature of the silicon. As a result, once biased properly, any inexpensive diode can generate a reverse voltage proportional to its temperature.
How to Build a Temperature Sensor Circuit
The exact characteristics of the circuit will depend on your choice of diode, but overall, the relationship of −x uV/˚C is a typical response of a reverse-biased silicon junction over temperature.
How to use this information will depend on your needs. For example, a thermal limit switch can use a diode and resistor bias circuit to feed a comparator (Figure 1). This will change state at a predetermined threshold allowing heater control, for example, to maintain a temperature range (note hysteresis applies here to prevent chattering oscillations around the trip point).
Figure 1: A reverse-biased diode junction acts as a current source (I) that’s current is proportional to the temperature and voltage. The resulting current will generate a voltage across a sense resistor that can be used to make this simple limit or threshold switch.
A more sophisticated op-amp circuit can provide a lower- and upper-level calibration point when combined with a variable resistor or digital trimmers (Figure 2). While the reverse current response is somewhat linear, there will be variations, so a look-up table can be referenced if you are using an embedded microcontroller and you need to find an exacting threshold.
Figure 2: Analog trim-pots or digital trimmers can be used to make a rudimentary calibration circuit that sets lower range and upper ranges of measurement.
Integrated semiconductor solutions and evaluation kits also exist, allowing designers to choose a diode for remote location. This can be handy since many metallic diode packages can thermally mount to metal machines that need to be monitored.
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An example kit from Texas Instruments is the LM95235EVAL evaluation and development system, which can use diodes or transistors as temperature sensors. Transistors can use the collector emitter junction diode as the sense element (Figure 3), and many use small metallic can packages that can also make a good thermal mount to a unit under test.
Figure 3: Based on diode and transistor thermal characteristics, IC’s like the LM95235 from Texas Instruments take advantage of the reverse-biased behavior of a P-N junction to provide 11-bit accuracies for remote temperature measurements. (Source: Texas Instruments)
While not as robust and accurate as an integrated calibrated and compensated monolithic solution, the TI development kit shows how to achieve an 11-bit accuracy using diodes and transistors as temperature sensors. This can provide the accuracy you need while keeping costs low.