I've been given a lab project for my Electronics I class. I am to design and build several circuits.

First circuit is basically just linearizing a thermisitor to output 4mA to 20mA. I think I've get this done.

Second circuit will take the outputs of two of the circuits designed above and when the temperature(current) has a 10° difference will turn on the motor of a pump. When the temperature is only 3° different the motor will be shut off.
I think I need to use a current comparator but can't find any good info on such a circuit. My partner says we can do it with op amps but I'm not sure because we are dealing with currents and not voltages.

Any advice would be much appreciated.

Thx, Lisa

P.S. I have a 24VDC Supply to work with.

 

Second circuit will take the outputs of two of the circuits designed above and when the temperature(current) has a 10° difference will turn on the motor of a pump. When the temperature is only 3° different the motor will be shut off.

What you want to do can be done using any op-amp connected as a comparator with some positive feedback.

 

Ok, but doesn't having only positive feedback on an op amp make it unstable?
What about using a schmitt trigger?