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

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


wrote:

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.

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

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.

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).

On Thu, 30 Jun 2005 23:56:55 +0100, Chris Jones