Ken adds:

You definitely don't want to be using bang-bang on-off switching=A0(or
worse=A0 - switching between opposite polarities) in order to control
temperature.=A0 For maximum longevity you need to be using a proper
continuously-variable drive under PID control=A0 - and for good measure
I'd incorporate some form of rate limiting so it can't degenerate to
bang-bang mode under unforeseen circumstances.

Just checked and it's Tellurex that says unfiltered rectified 60Hz is
OK=A0 - they even suggest using a battery charger.

Thay also produce a special range of Peltiers for applications
requiring wide temperature swings over repeated cycles (as many
biological application do).=A0 They are presumably designed to better
withstand the effects of thermally-induced stresses=A0 - and I note that
they say that the maximum temperature reached in the cycle is the
critical issue.=A0 It's well worth reading their design guidelines=A0 - as
it is for other manufacturers as well.

--=20
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