purpose of using two opamp together

Thread Starter

ep.hobbyiest

Joined Aug 26, 2014
171
Hi
i am referring one opamp based design (designed by friend of mine)where two op amps are used. one in inverting configuration and other one is in non-inverting configuration.

i am bit confused what would be the purpose of using two op amps together?
here is attached schematic. (Here R4 is load like motor).

two-opamps.JPG
 

ci139

Joined Jul 11, 2016
1,696
the differential input impedance for LM324 seems to be around 600kΩ
-- e.g. -- you can replace the upper opAmp with series 10µF+600kΩ RC chain -- but it does not define a stable operating point
 

crutschow

Joined Mar 14, 2008
25,460
understood the circuit this is voltage control current source circuit.
Yes, the transistor constant-current through R4 equals the pot pin 2 wiper voltage divided by the value of R2 (assuming 12V is sufficient to provide that current through R4).

U1a gives a voltage to the controller that's proportional to this current.
U1a's output voltage is the constant-current times the value of R2 plus the value of the resistor above it (can't decipher its reference number).

Don't know the purpose of the resistor in series with U1a's output.
 

AnalogKid

Joined Aug 1, 2013
8,608
U1A has no feedback, and is operating as a comparator.

If R4 is the motor, then 100% of the motor current is going through R1 and R2 to GND. That makes R2 the motor current sense resistor. I can see how this could be a constant-current circuit, but it certainly is not the traditional form.

If you add a current sense (shunt) resistor from the Q1 emitter to GND, then the circuit will be more efficient and easier to design.

AND - U1A becomes an overcurrent detector to flag the controller when there is a problem.

Conclusion: I think the latest schematic still is not complete.

ak
 

AnalogKid

Joined Aug 1, 2013
8,608
Yeah, I see that. So U1A is an overcurrent flag even without an explicit current sense shunt.

So it looks like the only value of R1 to the circuit is that it raises the U1A trip point voltage above the voltage at the pot wiper. So the R1-R2 combination sets a fixed *percentage* of overcurrent trip detection value, that tracks the operating current set point.

ak
 

Thread Starter

ep.hobbyiest

Joined Aug 26, 2014
171
one doubts is U1A will measure the voltage difference between pot and emitter.
And U1B will measure the voltage difference between pot and emitter+resistor.
Is that right?
 
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