Comparing two temps

[Jerry In Maine]

I'd like to compare two temps using an op amp and two thermistors.
When temp 'A' is higher than 'B' I'd like to turn on a fan, which will run until temps at 'A' & 'B' are equal.

I've learned enough to know that some feededback is needed to establish hysterisis in order to keep the circuit from "chattering", and I'd like to make this adjustable.

Curious if someone could help me out with this aspect of the circuit.

Thanks

 

[Nigel Goodwin]

You simply add a little positive feedback to make it 'snap' - add a variable resistor in series with the feedback resistor to make it variable.

 

[ericgibbs]

I'd like to compare two temps using an op amp and two thermistors.
When temp 'A' is higher than 'B' I'd like to turn on a fan, which will run until temps at 'A' & 'B' are equal.

I've learned enough to know that some feededback is needed to establish hysterisis in order to keep the circuit from "chattering", and I'd like to make this adjustable.

Curious if someone could help me out with this aspect of the circuit.

Thanks

hi,
As Nigel says, add some hysteresis.

 

[Jerry In Maine]

But "hysteresis" is such a complicated sounding word...how can the implementation be so simple?

Thanks...I thought there'd be more to it.