On Mon, 3 Jan 2005 18:42:29 -0500, "Glenn Ashmore"
wrote:

Putting the final touches on RUTUs refrigerator and freezer. I want to use
digital thermostats but the Carel units are $150 each.

In my overflowing junk box I have a pair of 12VDC CN132 Omron process
controllers with type T thermocouples. These things are a lot smarter than
a plain old thermostat but I wonder if they would work? What worries me is
that as the box gets close to the target temperature the PID logic might
start trying to turn the compressors on and off as if they were heating
elements.

See if one of the available output modes is simple on-off with
deadband. You for sure don't want time proportioning. However, if
you're stuck with time proportioning PID, then set the proportional as
low as possible (or gain as high as possible), integral and derivative
to 0. This will make it act like an on-off controller, but without
deadband.

As a matter of fact, this will even work with an analog output
(0V-10V) if you can get a gain of about 100,000 to 1,000000. You just
have to use the output to drive a relay.

Deadband is a lot nicer, because there's no chance of flipping back
and forth right around setpoint. I'd guess a deadband of 5-10 deg.
would be fine if available.

Pete Keillor