[time-nuts] Cycling of Peltier junction

[Robert Atkinson]

The PCR machine manufacturers just let them fail and then (when out of warranty) replace the whole themal transfer block, peltiers and heatsink at vast expense!
 
Robert G8RPI.

--- On Fri, 10/9/10, Hal Murray <hmurray at megapathdsl.net> wrote:


From: Hal Murray <hmurray at megapathdsl.net>
Subject: Re: [time-nuts] Cycling of Peltier junction
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Date: Friday, 10 September, 2010, 8:59



>> Does anybody know about using the same Peltier junction for both
>> heating and cooling? I'm concerned about thermal/mechanical shock
>> when changing the polarity back-n-forth between hot and cold.  Maybe
>> there needs to be a controlled ramp, if so then how do I figure out
>> the rate? 

> Not an issue.. there controllers that basically run AC through the thing,
> changing the relative amplitude of positive and negative pulses.

That seems like an interesting way to waste power, but not my problem...

I think the OP was interested in thermal cycling of the Peltier unit at time 
scales of an hour rather than PWM type rates.

Google found one web page that said, roughly:
  Yes, it's bad, but we won't tell you how bad because it depends...
The issue was thermal stress on the solder joints.

One thing to watch out for is overheating the unit.  Suppose you want to heat 
the chamber, so you turn on the juice.  If you are measuring temperature in 
the chamber, there is nothing to keep you from overheating the Peltier unit.


I'd probably do something like take at least 5 minutes for a big temperature 
swing, just to be nice to it.  Also, I'd use thermal grease rather than epoxy.

Another approach is to ignore the problem and just replace the units if/when 
they die.  That could be a disaster for long term customer relations, but 
might be appropriate for a home lab setup.




-- 
These are my opinions, not necessarily my employer's.  I hate spam.




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