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problem with LM324N opamp

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sina-VIP

New Member
hi
im trying to perform a voltage comparator circuit using lm324n opamp
and got these test results at the point that opamp begins to reflect to inputs
#1: Vcc=5 V+=3.2 V-=4.2
#2: Vcc=5.7 V+=4.2 V-=5.7
#3 Vcc=5.6 V+=4 V-=5.7
why the opamp is so sensitive to Vcc
also this circuit was supposed to be a comparator so why would it respond to the input which is 1.5 volts lower than V-
 

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crutschow

Well-Known Member
Most Helpful Member
It's not the op amp that's sensitive to Vcc. It's the trip point as determined by the pot, since it's also connected to Vcc. If you want the trip point to be stable with a Vcc change you need a stable reference voltage for the pot.

Don't understand your V+ and V- voltages. What do they refer to? Are they the trip voltages?

What trip voltage is the pot set to?

The LM324 will only work properly when both input voltages are less the 1.5V below Vcc or 3.5V for Vcc = 5V. (Maximum input common-mode voltage from the data sheet).
 

audioguru

Well-Known Member
Most Helpful Member
All your circuits have the inputs too close to the positive supply voltage. Then the PNP darlington transistors at the inputs are cutoff and don't work.
 
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