Achieve precision temperature control with TEC Seebeck-voltage sampling
By periodically setting the TEC's drive to zero, a storage capacitor samples and holds the Seebeck voltage.
W Stephen Woodward, Chapel Hill, NC; Edited by Martin Rowe and Fran Granville -- EDN, November 27, 2008
TEC (thermoelectric-cooler) temperature-control systems often have limited stability. The causes of these limitations are the thermal properties of the system, not the performance of the control electronics. Real-world thermal-control systems incur nonzero thermal impedances in the heat-transfer paths between the TEC; the thermal load, which is the object of thermostasis; the temperature sensor—for example, a thermistor; and the ambient temperature.
If the ratios of these impedances don’t balance, then even perfect thermostasis of the sensor’s temperature doesn’t equate to adequate stability of the load’s temperature. The circuit in Figure 1 provides a thermoelectronic design that directly measures the heat flux through the TEC and then uses the measurement to better estimate and cancel the effects of thermal impedances. Its operation is based on the fact that the total voltage that every TEC develops is the sum of two components: an ohmic component proportional to drive current and the Seebeck voltage, VS, which is proportional to the temperature difference across the TEC and, therefore, to heat flux.
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In this circuit, however, the drive current switches to zero approximately every 100 µsec because of the asymmetrical sample-pulse waveform that multivibrator S2/S3 generates. Each sample pulse turns off 5V transistor Q1, which isolates the Seebeck voltage and allows its sampling through S1 and storage capacitor C1 to hold it. The duty factor of the sampling pulse, which the R1-to-R2 ratio sets, is less than 10% to avoid significantly reducing the TEC-drive capability of the circuit.
You apply the acquired Seebeck signal to the R3/R4/R5 adjustable-bridge circuit, which empirically determines the feedback ratio for both polarity and amplitude to provide best stability. With proper bridge adjustment, you can make gradient cancellation nearly perfect over a wide range of ambient temperatures. The TEC-control circuit in Figure 1 derives from a previous Design Idea because it eases the incorporation of Seebeck sampling (Reference 1). You can, however, adapt Seebeck sampling to virtually any TEC-drive topology. You can further enhance the circuit in Figure 1 by using nonvolatile, in-circuit-programmable resistors for the R3/R4/R5 bridge, automatically optimizing gradient cancellation. One attractive choice is the Rejustor family of monolithic resistors from Microbridge Technologies.
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