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I am creating a closed loop gycol heating/cooling system for beer fermentation and lagering. The system uses a ITC-1000 temperature controller and two power supplies to feed a heating circuit and a cooling circuit with peltier devices attached to a water block and pump. See below:

I need to have a single pump(to pump gycol) and fan(to cool internal components) powered when either circuit is active. The circuits will not be active at the same time. How do I wire these to not create a feedback loop and not supply power to both circuits? Do I need to use diodes, and if so how/where do I wire them?

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A cheap and easy solution you can try using diodes as you mentioned would be as the following schematic.

Its not the most efficient option but it would work if your fan and pump are not very high current (say less than ~3A).

Note that you have to take the voltage drop into account (somewhere close to 0.5V~0.7V depending on the diode you choose and load current), so your pump and fan would have to reliably turn on at your power supplies actual voltage minus ~0.7V.

schematic

simulate this circuit – Schematic created using CircuitLab

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