I'm perplexed, and need a fresh pair of eyes and neurons. I've breadboarded the attached thermostat circuit - my design - and in general it's working very well, but the deadband temperature range seems to vary with even a small change in the temperature setting.
It was working great when I was controlling at about 20°C; the temp. stayed in a range of less than ±0.1°C and I was thrilled. But if I turn it down slightly to 15°C (by lowering the reference voltage at R9), the dead range grows to 1°C or more! This temperature setting change seems like minor change - the reference voltage at which the comparator switches drops from ~3v to ~2v. It doesn't make sense to me how this would destroy the tightness of the control.
My hunch is that there's something going on between the power supply voltages and how the comparator is feeding back. (It still makes no sense to me how the temperature change relates to this.) The 10M and 5.1K resistors are there to help insure solid switching of the FET gate voltage, to eliminate oscillation. I added them before I added the op-amp, and it helped. Maybe I don't need this feedback anymore. On the other hand, if I could get tight control at the comparator I could probably remove the op-amp altogether.
The power supply is an old computer PSU and has the odd behavior of increasing the voltage from 12v to 16v when the load is applied to the 5v part of the supply. I guess I can try using a battery for the 12v supply instead, to eliminate this effect. Thought I'd ask for ideas here first.
It was working great when I was controlling at about 20°C; the temp. stayed in a range of less than ±0.1°C and I was thrilled. But if I turn it down slightly to 15°C (by lowering the reference voltage at R9), the dead range grows to 1°C or more! This temperature setting change seems like minor change - the reference voltage at which the comparator switches drops from ~3v to ~2v. It doesn't make sense to me how this would destroy the tightness of the control.
My hunch is that there's something going on between the power supply voltages and how the comparator is feeding back. (It still makes no sense to me how the temperature change relates to this.) The 10M and 5.1K resistors are there to help insure solid switching of the FET gate voltage, to eliminate oscillation. I added them before I added the op-amp, and it helped. Maybe I don't need this feedback anymore. On the other hand, if I could get tight control at the comparator I could probably remove the op-amp altogether.
The power supply is an old computer PSU and has the odd behavior of increasing the voltage from 12v to 16v when the load is applied to the 5v part of the supply. I guess I can try using a battery for the 12v supply instead, to eliminate this effect. Thought I'd ask for ideas here first.
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