Comparator circuit not comparing

Thread Starter

tom7891

Joined May 16, 2011
26
Hi, i'm currently building a comparator circuit using a lm324n op amp.
The problem is, i get an output even when the voltage at the negative terminal is greater than at the positive terminal. I have 6.2v at the positive and 6.5v at the negative. I'm just wondering how accurate these op amps are, because i've simulated the circuit on multisim and it works fine.
 

ericgibbs

Joined Jan 29, 2010
18,766
Thanks for the reply. I'll have another check.

hi tom,
Recalc the voltage at the junction of the 10K and 2Meg, when supplied with 9 volts.

E

EDIT:
Remember your voltmeter could be loading the -Vin as it will be connected across a 2meg, so it could read lower than expected.
 

crutschow

Joined Mar 14, 2008
34,285
The data sheet states that the maximum input voltage should be no greater the 1.5V below the supply voltage or 7.5V.
According to your schematic the (-) input is well above that.
 

Thread Starter

tom7891

Joined May 16, 2011
26
Thanks for all the help. It is all wired correctly, i think you're right about the input voltage being too high for the op amp chip. I must have been looking at another parameter when i selected it. I'll have a look for a replacement.
 

crutschow

Joined Mar 14, 2008
34,285
Thanks for all the help. It is all wired correctly, i think you're right about the input voltage being too high for the op amp chip. I must have been looking at another parameter when i selected it. I'll have a look for a replacement.
Why do you need the reference voltage to be so close the the supply voltage?
If so, then you need a rail-rail input/output type op amp such as an LT1637.
 

AnalogKid

Joined Aug 1, 2013
10,987
Good question. The reference input is 99.5% of Vcc. Is it really the goal of this circuit to detect when the other input is between 8.955V and 9.000V? What is happening in those 45 millivolts?

ak
 
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