Justin Richards wrote:
>> I intend to supply it with a purely DC source. I'm going to take the
>> power straight from my +5 or +12 rail out of the computer's PSU, or
>> a spare PSU I have lying around. I love those PSUs. Lots of power,
>> convenient voltage, can be used for anything.
>
> Not sure if PSU supplies are that pure.  I see pure =3D $$$ and would
> suspect ripple on all the DC rails as most components can handle some
> ripple.
>
> But I think any ripple here would equal heat.
>
> Just looking at a specs for a brand name PSU and the quote 1% ripple

You are going a bit off the deep end.  You need to go back to the basics an=
d
see what this is all about rather than relying on rules of thumb with long
forgotten derivations.

This issue is that the efficiency (final resulting cooling power per
electrical input power) of Peltier coolers goes down the harder they are
driven, although the total cooling power still goes up to some maximum poin=
t
specified in the datasheet.  Let's say you want 50% of the total cooling
power the device is capable of.  You can get that cooling power by a 50%
duty cycle of full power or a steady input resulting in 1/2 output power.
Both cause the same cooling, but the first uses more electrical power
because the device is more efficient at the lower power operating point.

Taken to the extreme, any deviation from running the cooler with a steady D=
C
current causes inefficiency.  You can contort this to say that "ripple" is
bad in that it causes inefficiency, but small deviations from the average
operating point aren't going to significantly matter.  Any regulated power
supply is going to be good enough.

If you are doing active temperature control, what you don't want to do is
the otherwise obvious PWM control of a Peltier cooler (like would be
appropriate for a resistive heater).  You essentially want to smoothly
control a reasonably filtered analog current.  However, that presents some
problems in itself.  If you are going to derive this current by linearly
regulating a fixed voltage power supply, then you're just wasting the extra
power elsewhere.  Maybe that helps because it's physically in a different
place where it may not matter as much getting hot or it's easier to get rid
of the heat.  However, if power efficiency is the goal that's not going to
help.

This is why Peltier coolers are usually driven by what amounts to a variabl=
e
switching power supply, but with enough filtering to keep the total ripple
to a small fraction of the average output.  Anything that would qualify as =
a
regulated power supply for other purposes would likely be good enough.


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