peltier devices (any experts)

Don't they usually use a PWM signal with a frequency above a certain minimum to avoid thermal shock. You could just feed it with a 50% duty cycle squarewave above that set minimum frequency.

Regards Mark Harriss

snipped-for-privacy@spam.com (bitx) wrote in news: snipped-for-privacy@news.bigpond.com:

I have never seen comprehensive reliability data, but peltiers will subject to thermal cycling failure IMO. They are mechanically fragile, (if you pull one apart as I have, the little metallic bars that are the thermocouples, can easily be destroyed by rubbing your fingers on them). A moderate speed PWM of more than a few kHz is what I've used on my designs in the past, with field faults only due to overheating instances occuring. I would not rate them as relaible as a diode and I can't say how many thermal cycles they would last, but PWM is a reasonable approach.

If its a 12v device would running is at say 6-8v harm it in any way?

snipped-for-privacy@spam.com (bitx) wrote in news: snipped-for-privacy@news.bigpond.com:

I have always made sure that the RMS voltage is less than the nominal maximum voltage. Overcurrent should be avoided. Even if the surface temperature is within limits, hotspots probably will occur at the thermocouple junctions if higher than maximum current is used. Use a series resistor of the appropriate value if you can't be bothered with a PWM system.

snipped-for-privacy@spam.com (bitx) wrote in news: snipped-for-privacy@news.bigpond.com:

This is safe for the device, just less heat pumping power.

I remember reading somewhere that they shouldn't be used with PWM unless the PWM frequency is high (kHz) presumably due to thermal expansion and contraction causing fatigue.

You know those peltier fridges are really amazingly inefficient for what they do, many many times worse coefficient of performance than a household fridge with its mechanical compressor. If you are paying for solar panels to run this, then your total cost is probably less with a mechanical compressor fridge, preferably with added insulation on the outside and not a frost free one since they have heaters inside. Another alternative would be a thermos flask which you put some ice cubes (from your freezer) into every day or two. In fact if you don't need a really low temperature, you might be able to just dig a deep hole in the ground and keep the fish food there - what kind of food is it?

Yeh i realise they are terrbly inefficient for cooling... I have

200watts of panels which should do the job ...The cooler is just to keep the food fresh ..(yeh sounds like a over kill but its a serious fish tank ..1400L full of coral and reef fish).