Electronics > Beginners

Opamp won't agree with me + everycircuit !

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Odd:
Please see schematic tested in EveryCircuit:

The positive-side current sensing works, both inputs are pretty high, the amplification of 66.6 times works, and the output is 2.54v, well within the 15.3v that the opamp is hooked up to.

Now the reality:
LM324 is connected to full battery voltage.
Built, as in the drawing, the opamp's inputs in respect to it's GND are:

when idle:
+input is 224mV (in respect to opamp's VPP)   
-input is  214mV (in respect to opamp's VPP)
so input is only 20mv from VPP, and the difference is only 20mV
the output is about 78mV (relative to ground)   
(All good, I do not need a very precise measurement.)

with 10 A load:
+input is 224mV (in respect to opamp's VPP)   
-input is  235mV (in respect to opamp's VPP)
so input is only 21mV "in the other direction", and the difference is only 20mV
the output is about 78mV (relative to ground)   

What do I do wrong ? - Too close to VPP ?  - I've used the same opamp on the low side, having the -input pulled almost to the ground, and +input 1v above, and the difference works fine then..

Vovk_Z:
I don't know about simulation model (it depends) but real LM324 (LM358 etc) work with signal near ground, but not near positive supply rail.

magic:
Indeed, if both inputs are within 1V of VCC the input stage of LM358 turns off and stops controlling the output. The output goes down regardless of what's happening to the inputs, until they leave that region.

Try TL072 instead, it's the other way around: works close to VCC, doesn't work at ground.

Beware that many TL072 from auction sites are fakes, often relabeled LM358 |O

Odd:
Thank you all.
Yes, the "some will work high, some low" makes kind of sense with other circuits I've seen.

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