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I'm trying to take dual 4-20 mA signals and use them as a heat/cool signal for a Peltier module.

I have an integrator but I'm trying to find the best way to decipher the dual signals and turn them into a heat or a cool signal. I've seen H-bridges for driving it, but I think I need something that's additive or subtractive as the signals will likely be coming from a user's Yokogawa or other temperature controller.

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  • \$\begingroup\$ I'm not sure what you mean by "dual 4-20mA" inputs. A single 4-20mA current loop signal requires 2 (or 3 or 4) wires. So, do you mean 2 wires, or 2 different signals? If two signals, what does each signal represent? \$\endgroup\$ Jan 31 at 4:10
  • \$\begingroup\$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. \$\endgroup\$
    – Community Bot
    Jan 31 at 4:10

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You should never get both signals > 4mA at once from your heat/cool process controller, but you should probably allow for that and have one take precedence. The Peltier has only one input (current).

So say the two input currents are i_h and i_c, which you can think of as having a useful range of 0~1 by subtracting 4mA and dividing by 16mA, to give you \$y_h\$ and \$y_c\$

Then, say, heating takes precedence your circuit supplies a current of \$y_h\cdot k_h\$ to the Peltier whenever \$y_h\$ > 0. Otherwise the circuit supplies a current of -\$y_c \cdot k_c\$ to the Peltier if y_c > 0. You may place limits on the current (or at least limit \$y_h\$ and \$y_c\$ to 1.0 maximum). \$k_h\$ and \$k_c\$ are positive gain constants that you can choose to drive the Peltier properly. They might be equal.

In any case, the controller tuning for the cooling will be radically different than for heating because the Peltier is far more effective at heating than cooling.

Then to drive the Peltier itself, probably a switching current feedback supply would be ideal. There are integrated drivers such as MAX1968 or you can roll your own:

MAX1968 typical operating circuit from its datasheet

Source: Analog Devices - MAX1968 datasheet

If you have a module that has some kind of analog input and heat/cool digital input you can detect heating with comparator and switch between the two inputs with a multiplexer.

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