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I have a temperature controller with two SPST relay contacts, one each for heating and cooling, and I want to use them to control a Peltier thermoelectric cooler (TEC), plus a fan and pump and potentially a few other widgets.

When either the heating or cooling activates, the auxiliary devices (fan, pump, etc) need to all turn on, with a fixed polarity. However, the TEC needs to have its polarity reversed between heating and cooling, so it can pump the heat in the correct direction.

I have a circuit with the intended behavior, using a series of SPDT relays to handle the 'logic' of reversing the polarity, however there are some issues with it:

schematic

simulate this circuit – Schematic created using CircuitLab

The temperature controller is an ITC-1000F, and the cooler is a TEC1-12706.

I want to be able to run multiple coolers in parallel or perhaps just a single more powerful cooler, but the controller is only rated for 10A, so even two 6A TECs would be too much for this design.

If possible I'd also like to be able to simplify the logic and generally reduce the number of relays I need to use.

I came up with another design that solves some of the issues:

schematic

simulate this circuit

It still draws the current for the auxiliary equipment through the temperature controller's contacts, although in my case that's not an issue since the current is low enough.

In any case though, it feels like there ought to be a simpler way of going about this. How can I simplify or otherwise improve this design?

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2nd circuit is probably better, though I'd consider ditching RLY3 and use two diodes instead, assuming you can tolerate a diode drop in the power line to the accessories.

schematic

simulate this circuit – Schematic created using CircuitLab

ADDITION

However, I am a little concerned you don't appear to have shown any thermal sensing on the device itself to ensure the peltier does not cook.

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    \$\begingroup\$ The original version of circuit #1 did have a diode in place of RLY1 in my first iteration, but it burned nearly 8W in simulation due to the high current to the TEC. With the lower current to the accessories though using diodes looks a lot more feasible (only .5W dissipation there) and the voltage drop looks manageable. \$\endgroup\$ – AJMansfield Aug 25 '17 at 19:35
  • \$\begingroup\$ WRT thermal sensing, the temperature controller has an over-temperature alarm feature so if the temperature on the cooling target goes over 100 C it shuts everything off, but that wouldn't kick in until the TEC itself has gone far beyond it's safe range. Is there an easy way to add some kind of additional thermal protection to this? \$\endgroup\$ – AJMansfield Aug 25 '17 at 19:43
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    \$\begingroup\$ @AJMansfield as a minimum you could hook up a couple of appropriate thermal switches on either mechanical side of the peltier and wire them in series with it. digikey.ca/products/en/sensors-transducers/… \$\endgroup\$ – Trevor_G Aug 25 '17 at 19:50

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