comparator switching prematurely

I have the circuit below. The comparator works as expected when ref (set by P2) is below ~3.6V. However, when ref is above 3.6V, the comparator still switches when Vin (set by P1) reaches 3.6V.

What am I doing wrong?

Thanks,
Rey

 

more info. DC and the comparator is a LM324 (which I'm now reading is a op-amp).

 

more info. DC and the comparator is a LM324 (which I'm now reading is a op-amp).

Why are you powering the OPAMP through a 1k ohm resistor. This just reduces the suppy voltage and allows power load surges to modulate the voltage.

Anyway, it is likely that you are hitting the maximum allowed common mode voltage range. The specification says that the input range is zero to Vcc-2V, although you may see Vcc-1.5V typically. This would put the voltage max at 4 V to 4.5 V with a 6V supply, but I'm estimating that your 1K resistor is dropping abut 0.7 V which brings you down to the 3.3V to 3.8 V common mode input range according to specs.