Olin I can confirm what you are saying from experimentation.  TEC's perform
poorly with thermostatic control.  They do not respond well to simple
current change to control them.  Apart from the inefficiency issues, one
other point is that unlike most heating and cooling systems that use
thermostats to control them, TEC's change tempurature too quickly for
thermostats to keep up with them.

However, using a well designed PID loop, running PWM at 5kHz on an F877, I
have obtained constant tempuratures between -30 and -25 degrees celcius in a
test chamber, using a single 8W TEC available from Oatley Electronics here
in Australia.

You should not try PWM frequencies below about 2kHz because it causes
thermal stress to the TEC.

I have brought this topic up before and generating a lot of criticism and
alternatives for driving TEC's.  Some will say that PWM is also not the best
way to control them.

I am not trying to rekindle that argument, just pointing out what I have
observed through a lot of empirical experimentation.

A few points to watch to get good performance from Peltier TEC's:
1) Large thermal mass, and good cooling for hot junction is essential.
2) Watch out for condensation !
3) Stacking Peltiers may be OK if you want lower cold junction tempuratures,
but the "cooling power" diminishes, so if you have any large mass to cool,
doing this generally does not help (i.e. no such thing as a free lunch).
Remember too that the hot junction of the top Peltier in a stack is going to
need a lot of cooling.  Better to use multiple Peltiers on multiple
locations of the mass to cool.  In a cool box for example, have a TEC on
each wall of the chamber, rather than stacking them.

Rgs
Ian

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Olin Lathrop
> Sent: Saturday, 17 January 2004 1:38 am
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE:] About test boxes (hot and cold)
>
>
> Michael Rigby-Jones wrote:
> > Unfortunately the author decided to use thermostatic control of the
> > TEC, which is definitely not recommended.  The constant thermal
> > cycling of the TEC will cause premature failure.
>
> It's also less efficient.  The efficiency of a peltier cooler
> decreases with
> increasing drive current.  You therefore want to use a linear
> drive with
> these devices, probably contolled by a PID loop.
>
>
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