Again >> Comparator Circuit

[Hussain]

Hey guys . . .

I posted a thread month ago about a comparator circuit and Op. Amp.

Now ,

I need your help again with the same circuit . . .

I have a project which has a comparator circuit as main part of it . A signal comes from current transformer as voltage that is rectified to DC voltage. This DC voltage is fed to a comparator circuit that compares the incoming signal to the set value and act according to. The output signal is to energize a coil of a relay.

The comparator circuit is working fine but I have a small problem. This problem shows when the relay is oscillating - Not holding -.

I suspect that the signal has to be smoothed when then I installed a capacitor. It works again but the voltage hold the relay energized even when the input signal is away.

What to do ??!!

Ineed your help guys ...

Please see the drawing in the attachement

 

[crutschow]

Adding a large resistor between the op amp output and the +input will add hysteresis to eliminate the oscillation. The ratio of that resistance value to the +input resistor value determines the amount of hysteresis (difference in input voltage between the "on" point and the "off" point).

P.S. I don't see the rectifier in your circuit.

 

[ccurtis]

Perhaps the capacitor is holding a charge if it has only the comparator input to leak through. You show a resistor there, but the value of the capacitor, nor resistor is shown in your schematic, and yes, no rectifier either.

Another possibility is the comparator is latching up, which (with some comparators) can occur if the input voltage goes outside the voltage powering it.

If all you have is a rectified AC signal feeding the comparator (with no capacitor), even a large amount of hysterisis is not likely to prevent 50/60 Hz oscillation. I'd be suprised in your circuit doesn't oscillate at all times after the threshold is exceeded.

 

[Hussain]

Thanks a lot guys ...

Yes .. I 4got to draw the rectifier circuit ... IT IS AFTER THE RESISTOR IMMEDIATELY .

So,

You mean guys .. All what I have to do is to put another resistor @ pin # 6 ???

 

[ccurtis]

Again, you left out the value of capacitor. In addition, you did not address the efficacy of comments provided to you. It would be helpful if you redrew the circuit with all the parts shown, values included.