Design Ideas:October 27, 1994
Peltier elements, or semiconductor heat pumps, combined with heat sinks are a designer's choice when controlled low temperature is required for CCDs and high-density ICs. These devices are suitable for device temperature cycling in reliability tests and for regular cooling. Because mounting inexpensive temperature sensors directly onto the controlled temperature surface isn't easy, a simple circuit (Fig 1b) provides the control. The circuit is successful because the Peltier element is a reciprocal device.
When current flows through the element's cells, the element pumps the heat from one surface to another, resulting in a temperature difference. But the element also generates a voltage proportional to that difference when no current is applied. For typical 12 and 24V elements, the voltage is about 0.1 to 0.3V/K. You can determine the actual controlled surface temperature by measuring that of another surface, such as the heat sink, and subtracting the Peltier element's difference from it (Fig 1a).
Because bulky elements are slow, you can power them with unregulated and rectified 50/60 Hz and 12/24-ac voltages. The circuit measures heat-sink temperature using IC1, which is an LM335 10-mV/K linear thermometer. ICamplifies IC1's output to 0.1 V/K and subtracts the 2.73V background signal to center the output around 0øC. ICcompares this voltage to the voltage across the Peltier element during the zero crossing of the ac input, and IClatches the result. This result is valid through the next 10-msec cycle and determines whether the circuit applies the next half-wave to the Peltier element. ICprovides the clock for ICwhen the ac source is at 1.2V. If the absolute voltage generated by the Peltier element exceeds 5V, ICswitches the current off to protect the element from stress.
A simple Darlington switch, Q1, conducts only when the half-wave voltage is greater than 5V, as determined by D2, and provides about a 2-msec gap between the current-off and Peltier voltage sampling. You should choose Qso that its current and voltage ratings are greater than or equal to the Peltier element. In this case, IC should be greater than 6 to 8A, and Vshould be greater than 30 to 50V. For a 12V, 5A Peltier element, you can use BDX34 by SGS-Thomson, but the TIP135 and 145 by Texas Instruments are also suitable. The temperature offset input V_Temp provides for external control. Although this circuit with the unipolar switch provides cooling only, you can build a controller for both negative and positive temperature swings. You build this controller by powering the Peltier element with a triac rectifier, using the positive output for cooling and negative for heating. ICshould still control the on/off state of the rectifier. No other circuit modifications are necessary, except for protection around IC2D, which must take the absolute value of the Peltier voltage. (DI# 1607)