[sdiy] don't get fooled with peltier specs

[Czech Martin]

So , these peltier specs read very good:

example: 

Voltage: 8.5V (max)
Curent: 4.6A  (max)
Pump Pow: 27 W (max)
delta T: 60K  (max)

Now this sounds great! Can I get 60K temperature difference
@ 8.5V/4.6A , thereby pumping 27W?

Nooooo!!!!!

Unfortunately the maximum temperature difference of 60K holds
only if NO power is pumped. OTOH, if 27W are pumped the
temperature difference is 0!
These two points can be drawn into a temperature difference / 
pump power diagram, it seems to be pretty linear in between.
So, the attempt to pump out 15W will give delta T of only 25K!

To make things worse:

Teh pumped power will add to the Joule power, which can be 
a maximum of 8.5V * 4.6A = 39 W in this case.
So the hot side has to get rid of 15W +39 W= 54 W.
This will not happen without serious temperature rise, perhaps
10K or 20K. 

This in turn together with the delta T of only 25K will have the
effect that the cold side will only go 15 or 5 K down.

Now, this IS disapointing. Physics is sometimes unfair ;->

Guess why there are so many peltier elements of that caliber
in EBAY? 

Cause people thought that they could freeze their CPU or whatever
and had to find out that this is not realy going to happen.

Gosh, this little peltier costs EUR 20 already...


m.c.