[time-nuts] Hobbyist grade or homebrew temperature testing chamber?
jimlux at earthlink.net
Tue Sep 6 07:54:01 EDT 2016
On 9/6/16 1:19 AM, Adrian Godwin wrote:
> On Tue, Sep 6, 2016 at 4:24 AM, Bob Darlington <rdarlington at gmail.com>
>> I'm going from memory here. My former business partner (now deceased) had
>> a "beer fridge" setup as an environmental chamber for our instruments. The
>> trick for stability was to run the AC compressor all the time, and push
>> against it with heat lamps.
> I had a commercial environmental chamber : it did exactly this. The
> fridge unit could be controlled from an auxiliary output of the PID
> controller but the basic principle was to heat from ambient, or heat
> against the fridge for lower temperatures. Maybe Peltier systems can have a
> cleaner rollover for cooling, but gas-based coolers have far too much
> settling time / transients / hysteresis to be actively in the control loop.
One thing we just found when using a Peltier type device is that the
stock controller typically PWM modulates the 12-24V supply voltage going
to the TE device. This produces enormous current/voltage transients
(many amps at 24V slamming on and off) at some audio frequency which is
sufficient to disrupt your carefully designed quiet device....
On our controllers (from TE Tech) you can use an external DC power
supply that has a remote programming input (e.g. a unipolar power
amplifier) and it just uses the H bridge to switch polarity to the
Peltier. This is MUCH quieter EMI/EMC wise.
The other issue is the condensation/frost when you go cold.
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