The circuit in "Peltier element controls itself," by Dmitrii Loukianov (EDN, October 27, 1994, pg 86) is elegant, but it suffers from two potential problems that could arise in some applications. The first is possible switching-noise injection from capacitive and inductive coupling at the front end of a sensitive analog-signal-conditioning chain (for example, in a CCD-cooling application). The second potential problem is Peltier-stack performance degradation from intermittent, fast-switching drive current. At least one UK-based Peltier-cooler manufacturer recommends uninterrupted dc drive.

The circuit in Fig 1 controls the temperature of a cooled CCD by using a two- or three-stage Peltier cooler running at about 50W. Usually, you would run a Peltier cooler at close to full cooling power (P1 in Fig 2). In a linear, series-mode controller, this full-power mode would entail high current and similar dissipation in the pass element. The controller described here works by heating the cold side of the Peltier element just enough to maintain stability-and consumes only about one extra watt to do so. The Peltier cooler has low efficiency; it needs a 50W input to produce about 1W of cooling.

R_H is a resistor (or pair of resistors for heat distribution) mounted on or near the cold face of the Peltier cooler. This resistor acts as a heater, which causes the temperature to rise until equilibrium occurs, as sensed by S₁ and the error-amplifier IC_1B. S₁, mounted on the cold end of the Peltier stack, is a low-cost temperature sensor whose current is proportional to absolute temperature to the amount of 1 µA/K. Because it's a current source, the sensor effectively rejects any supply ripple.

The dc supply voltage can be unregulated but should be reasonably well smoothed. R₁, C₁, and D₁ provide additional filtering for the control circuit. You set the temperature target by adjusting the potentiometer or, alternatively, by using a D/A converter to drive the error amplifier. You should adjust the operating temperature in your application to suit the range of control the application needs. The Peltier cold temperature varies with ambient changes, so you must set the operating offset temperature, T₀

T₀>(T_A(max)-T_A(min)),

where T_A(max)-T_A(min) is the maximum ambient-temperature range anticipated.

This circuit is a simple proportional controller with no integral or differential terms added. Therefore, it has an offset between the target and stable temperatures and is slow to settle. However, the thermal time constant is typically large, in the order of many minutes. You might need to adjust the gain of IC_1B by altering the value of R₄ to obtain optimum stability. IC₁ can be your favorite single-supply dual op amp; the LM158 or LM358 are suitable, for example. Q₁ is an enhancement-mode, n-channel, medium-power MOSFET, such as a VN66AF, mounted on a heat sink.

For optimum stability, you should mount the heater resistor(s) and the temperature sensor close together in an aluminum block installed between the cold end of the Peltier stack and the IC to be cooled, using a heat-sink compound (Fig 3). In our application, R_H is a pair of 200-Ohm, 0.5W metal-film resistors in parallel, yielding a maximum heating power of approximately 1.2W. Temperature stability is better than ±0.1K over an ambient range of 5 to 25°C. (DI#1712) EDN