7.0 Power Supply Requirements
7.1 Thermoelectric coolers operate directly from DC power suitable power sources can range from batteries to simple unregulated “brute force” DC power supplies to extremely sophisticated closed-loop temperature control systems. A thermoelectric cooling module is a low-impedance semiconductor device that presents a resistive load to its power source. Due to the nature of the Bismuth Telluride material, modules exhibit a positive resistance temperature coefficient of approximately 0.5 percent per degree C based on average module temperature. For many noncritical applications, a lightly filtered conventional battery charger may provide adequate power for a TE cooler provided that the AC ripple is not excessive. Simple temperature control may be obtained through the use of a standard thermostat or by means of a variable-output DC power supply used to adjust the input power level to the TE device. In applications where the thermal load is reasonably constant, a manually adjustable DC power supply often will provide temperature control on the order of +/- 1°C over a period of several hours or more. Where precise temperature control is required, a closed-loop (feedback) system generally is used whereby the input current level or duty cycle of the thermoelectric device is automatically controlled. With such a system, temperature control to +/- 0.1°C may be readily achieved and much tighter control is not unusual.
7.2 Power supply ripple filtering normally is of less importance for thermoelectric devices than for typical electronic applications. However we recommend limiting power supply ripple to a maximum of 10 percent with a preferred value being < 5%.
7.2.1 Multistage cooling and low-level signal detection are two applications which may require lower values of power supply ripple. In the case of multistage thermoelectric devices, achieving a large temperature differential is the typical goal, and a ripple component of less than two percent may be necessary to maximize module performance. In situations where very low level signals must be detected and/or measured, even though the TE module itself is electrically quiet, the presence of an AC ripple signal within the module and wire leads may be unsatisfactory. The acceptable level of power supply ripple for such applications will have to be determined on a case-by-case basis.
7.3 Figure (7.1) illustrates a simple power supply capable of driving a 71-couple, 6-ampere module. This circuit features a bridge rectifier configuration and capacitive-input filter. With suitable component changes, a full-wave-center-tap rectifier could be used and/or a filter choke added ahead of the capacitor. A switching power supply, having a size and weight advantage over a comparable linear unit, also is appropriate for powering thermoelectric devices.
7.4 A typical analog closed-loop temperature controller is illustrated in Figure (7.2). This system is capable of closely controlling and maintaining the temperature of an object and will automatically correct for temperature variations by means of the feedback loop. Many variations of this system are possible including adaptation to digital and/or computer control.