Ac milli-voltmeter issue

[arvinfx]

Hi,

I am making a protection circuit for a "relay-contacts" that works as a ac mili-voltmeter .

for example if there is 0.6Vac on the relay-contacts so I need a signal . But problem is the contacts have 220Vac and I don't know where to connect the ground for opamp side!

 

[Nigel Goodwin]

Post a schematic for what you're trying to do - it seems to make little sense.

But if it's live mains, then everything is going to be live, so you can't ground it.

 

[ronsimpson]

I think you want to measure the "closed " voltage across the contacts. But if a contact opens while you are measuring it the meter circuit will die.

 

[Nigel Goodwin]

I think you want to measure the "closed " voltage across the contacts. But if a contact opens while you are measuring it the meter circuit will die.

That's what I presumed, but it makes little sense?.

 

[arvinfx]

I think you want to measure the "closed " voltage across the contacts. But if a contact opens while you are measuring it the meter circuit will die.

It will worse when an inductive load connected on the relay! A huge spark is second issue !

And no I want to measure the resistor on 2 contacts, not closed contacts

 

[arvinfx]

The schematic of main circuit:
It is an high - low ac voltage protection

 

[Les Jones]

You do not seem to have any means to control the power supply voltage to the op amps.
The current to the op amp supply is controled by the reactance of C5. I would normally expect to see a zener diode across the supply lines to control the voltage.
You say "I want to measure the resistor on 2 contacts, not closed contacts". You do not have any resistor connected to the relay contacts.
I am guessing that you want the relay to be energised only when the mains voltage is between two specified voltages.
In other words if the voltage is too high or too low you do not want the relay to be energised.
Is this guess correct.

Les.

 

[rjenkinsgb]

It's also drawn as switching the NEUTRAL side, which is a possibly lethally bad design!

 

[schmitt trigger]

I attempted to understand the main circuit, but it is too convoluted and confusing.