Quote:
Originally Posted by Steffen
The use of a PID controller is unnecessary for (slow) temperature control, because in this application the plant already has integrating properties. A simple P controller will (eventually) lead to zero temperature error, since the controlled variable (temperature) is the integral of the controlling input (heat energy) over time.
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That is roughly the observed behaviour, somewhat modified by the circuit configuration.
All soldered, temperature dithers at an amplitude of 1.5C slightly above set point, over approximately 3 seconds. The mosfet gate pull down/up resistor value, presently 1M, resolved the large gap in setpoint vs actual temperature. No need to massage the reference voltage. I will experiment with different resistor values and see what gives.
Given that the test rig is not cooling a mass as it would be, connected to a cold finger, its very sensitive. The temperature oscillation will have little effect on temperature variation in practice because of the mass of the cold finger.
A bit more testing and I will post a complete schematic.