 Peltier Controller
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Steve Conner
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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
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Even if they were linear resistors, my explanation about the cooling power being proportional to the average current, but the unwanted internal heating (which opposes the cooling effect) being proportional to the RMS current, still stands. Driving a Peltier with anything other than pure DC (with an RMS-to-average ratio of 1) makes it fight against itself harder than usual, and hence fail sooner.
The RMS-to-average ratio for a PWM waveform depends on the duty cycle: for 50% duty it's sqrt(2) or something, and for lower duty cycles it only gets worse.
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Bjørn
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Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
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I found this at some manufacturers website (Melcor): "Any A.C. component on the D.C. is detrimental. Degradation due to ripple can be approximated by: dT / dTmax = 1 / (1+N^2), where N is % current ripple."
They also said less then 1/60 Hz or more than 5 kHz for PWM.
They seem to fail by gradual diffusion of metals into the semiconductors and mechanical failure. Maybe the ripple can speed up the diffusion through some process?
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