A peltier element can be looked upon as a resistance through which if you send current in one direction it heats up and if it is sent in the opposite direction it cools. Such an element is extremely important for situations where you need very precise temperature control and as far as I know Thorlabs has been able to achieve a temperature stability of .001 K using Peltier element.
However, the main challenge in working with a peltier element is that it has an extremely low resistance ( the one I am using has a resistance of 2 ohm) and often requires a high current ( of the order of 1A or more depending on the situation). Thus the challenge is to build effectively a voltage controlled current source but one which can give an output current of 1-2 A.
Normally one would think that a high power transistor would suffice. However, for such transistors, the hfe is often very small and hence one would require a boosting stage after the control circuit. This increases the complication because making a two stage amplifier is a really messy business.
Circuit diagram
The original concept for the following circuit was taken from a diploma thesis of a couple of students (George Kolling and Marting Warning) from University of Limerick. However, their circuit required the use of an OP-AMP which could deliver a high current ( of the order of hundreds of mA, normally 741 delivers of the order of 10-20 mA). Since that was not easy to get, the circuit was modified to be able to operate using 741. The circuit diagram is given below:
The initial inversion stage is added to make the output and the input to be of the same phase. The first stage of push-pull amplifier by the BC series of transistors is added because the second push-pull stage made by TIP series requires a base current of the order of 100 mA. In the original circuit there was only one stage push-pull amplifier made of TIP series and the required current was supplied by the OP-AMP LM6167. However, due to non-availability of OP-AMPs capable of delivering 100 mA, the first stage of the push-pull amplifier was used.
This circuit is capable of delivering upto 3-4 A current. However, till now it has been tested for a sinusoidal current with a maximum of 1.2 A, which was of course sufficient for the purpose I was using. The current was limited by the current supplying capability of the power source I had.
The 100K pot giving the feedback to the second Op-Amp controls the gain of the circuit.
Conclusion
This circuit can be used not only to drive a Peltier element, but also many other applications where a high current is required. It can act effectively as a VCCS with a maximum current of about 3-4 A or maybe more.
If high current Op-Amp is used then the first stage of the p-p amplifier as well as the initial inverting stage can be removed.
As with every electronic circuit, even this one held a few surprises, namely, the push-pull stage of the TIP transistors worked without the initial current boosting BC stages. So the good news is that the number of required components is reduced and hence the circuit becomes less complex and noisy
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