Hi Guys,

I am curious. I am trying to design a thermal differential controller and ended up with the circuits as in the attached picture below.
If the temperature on the Collector thermister is greater (about 5⁰C degrees or more) than the temperature on the Storage thermister, then the pump runs, otherwise it is off.
Thermisters are rated as 5k @ 25⁰C degrees
Presets (VR1 and VR2-in both circuits) are 2k ohms
Pump (MT1) is rated as 12vDC 6W 0.5A

I wanted to ask your opinion, which would you consider is the better circuit - Circuit 1 or Circuit 2 and why?
And is there anything that I have not thought of?

Many thanks for your comments and thoughts.

PS. I may use an LM393 instead of an LM358

 

My only suggestion is to flip the therms around electrically so they have one leg on ground instead of one leg on the positive battery bus.

 

I don't see any particular advantage of one circuit over the other, but the two pots in the first one could cause confusion in their adjustment.
Suggest you use one or two fixed resistors and one pot.

 

I vote for using the genuine comparator in this application. Not a big deal, just good form.

 

Circuit 1 has a subtle flaw: adjusting VR1 setting will also change the hysteresis, which you probably don't want.

 

I vote for using the genuine comparator in this application. Not a big deal, just good form.

Yes, this is what I was also thinking, but, if I did use an LM358, then I can take out the pull up resistors - or do I need them?
If I use an LM393 then I would need the pull up resistors.
Agreed?

 

My only suggestion is to flip the therms around electrically so they have one leg on ground instead of one leg on the positive battery bus.

This i can do very easily, but, if I did this, would it make any significant difference?

 

Circuit 1 has a subtle flaw: adjusting VR1 setting will also change the hysteresis, which you probably don't want.

Good point, but it may not be an excessive amount.... or will it?
I will think about this. Thanks.

 

If the sensors are remote mounted and must have wiring run to them. Keeping them ground referenced would simply mean no wire runs with a straight connection back to your 12 volt source.
Only a ground and signal wire. No worries if it is pinched, twisted, shorted, folded or mutilated.
Otherwise it is identical in function.

 

If the sensors are remote mounted and must have wiring run to them. Keeping them ground referenced would simply mean no wire runs with a straight connection back to your 12 volt source.
Only a ground and signal wire. No worries if it is pinched, twisted, shorted, folded or mutilated.
Otherwise it is identical in function.

Yes, thank you, good point!