Exploring the use of Peltier plates for a microscope heating stage. A cautionary tale.

by David Walker, UK

 

 

In the August 2015, April 2018 and May 2018 issues of Micscape I shared my trials of using Peltier plates, both solid and with a central hole to make cooling stages for hobbyist microscopy. A particular goal was to be able to study water freezing under a coverslip with indoor comfort which was successful with later designs. Studying frost formation on solid plates was also fascinating.

Peltier plates can also be used for heating, either by having the hot side uppermost or using a cooling stage design and reversing the polarity of the power supply. Early trials of using both a solid and annular plate in this mode are summarised below.

Peltier plate TEC1 04903, 5.8V, 3.3A solid square plate

The existing setup shown below had been used to study frost formation. A solid plate was tried first as they are cheap, had some spares and did not want to risk the single and more expensive examples of annular plates possessed. The polarity was reversed so the top plate was now the hot side. The copper plate and heatsink now assumed the role of a large thermal mass to heat the hot side. The data sheets warn of not having a bare plate on the hot side so a piece of slide with a well ring filled with immersion oil was used. A type K thermocouple dipped into the oil and attached via a USB interface to PC software was used to monitor the results in real time. The volts were slowly increased in steps and equilibrium was quickly reached at each setting in a few minutes.

Although in data sheets the ΔT temp was stated between the hot and cold side, to date had not found any figures for the maximum permissible temperature of the hot side. A commercially produced Peltier based heating /cooling stages by Linkam had a temperature limit of 120C so this temperature was the maximum tried and for this plate at least it seemed stable. The temperature also dropped rapidly when the current was switched off as heat is rapidly conducted through the plate, a feature that had noticed for cooling studies as well. These were encouraging results for good control of temps to 100C so prompted me to try an annular plate which would be much more practical for transmitted studies. The maximum current of 1.8A used was also barely half the plate's max. rating of 3.3A.

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