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Precision temp-control with TEC Seebeck-voltage sampling

04 Feb 2016  | Stephen Woodward

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Thermoelectric-cooler (TEC) temperature-control systems typically 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 the figure 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.


Figure: This circuit periodically sets the thermoelectric cooler's drive current to zero, sampling the Seebeck voltage and holding it in a storage capacitor to achieve stable temperature control with real-world heat sinks and thermocouples.


In this circuit, however, the drive current switches to zero approximately every 100µsec because of the asymmetrical sample-pulse waveform that multi-vibrator 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 the figure 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 by using nonvolatile, in-circuit-programmable resistors for the R3/R4/R5 bridge, automatically optimising gradient cancellation. One attractive choice is the Rejustor family of monolithic resistors from Microbridge Technologies.
Reference
Woodward, W Stephen, "Thermoelectric-cooler unipolar drive achieves stable temperatures," EDN, Dec 3, 2007, pg 98.


This article is a Design Idea selected for re-publication by the editors. It was first published on November 27, 2008 in EDN.com.




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