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You probably don't need to vary the output current at all. The Peltier device almost certainly has a high enough thermal mass to simply switch it on and off to control the heat/cooling it produces. This is especially so if it is mounted to a conductive surface as it would further increase the thermal mass.

Given that the heat won't change rapidly you can pulse the current at quite low speed, maybe several seconds on and off by feeding the current through the relays. I don't know the configuration of your relay board but typically it would be wired like this:

power ---> on/off relay ---> polarity reversing relay ---> peltier.

The amount of heating or cooling is decided by the on to off ratio of the 'on/off' relay and whether it is heated or cooled by controlling the 'polarity' relay.

Brian.