Temperature switching w/Thermistor

[davter]

I am experimenting with a temperature switch using a thermistor as the base switch. I will list details below, but the main problem is the differential switching point or as some call it, the hysteresis point. I am simply wanting to set up a fan switch relay to operate when the ambient goes above about 80 C, and to remain on for say 3-5 minutes after the cooling fans have lowered the ambient below the set point. I am using a suppliers PCB with a dual OP Amp (358) with the set-point set via a pot from a ref voltage, which is fed into the non inverting i/p. The thermistor is a 10K, which is about 8.7K @ 25C. I have used a couple of different therms, but as this is not a critical cct, well the exact trip point is not a problem. But the differential is. This cct has a 1M pot fed from the output of the OP amp thru a 1N4148 diode back to the non inverting I/p. I find that with this pot set high, ~ 800K, then the amp switches instantaneously, with a low setting, ~ 250K then simply does not reset, the amp does not return to low state O/P. Would appreciate advice please.

[pebe]

I don't see why there should be a diode in the feedback loop, or how the thermistor is wired into the op-amp, and why there are two op-amps.
Could you post a circuit diagram, or a photo of the PCB, please?