Comparison of two ac signals of millivolt range and driving output

I cannot see this concept working reliably as discussed.

eg. There are many loads that use rectifiers at the power input - there could be a dead short after the rectifier, that will blow the breaker or fuses, but will still develop more than 1V with a very low current test.

As a possible alternative, how about a "test button" type system that temporarily allows eg. 100mA through the circuit, and looks for a significant voltage across the load, say at least 2V.

If properly designed that could also be a shared unit, with a test button for each output on a single phase distribution panel.
It would also need to be lockable so it cannot be operated while someone is working on the wiring etc..

You have to take all possible safety hazards in to account when designing anything for main voltages.


Regardless of all that, however you do it, you still have the problem that the user may switch an appliance off locally when they realise something is wrong, or a heater circuit may shut off due to a thermostat or timeswitch, so a test from the breaker end will not show a fault.

Sir,but if user switches off locally appliance and there is no status about fault.but it might be possible tgere is short circuit across terminals of device(after local switch).
currently if it is possible to compare millivolts signal to zero volts reference and drive somthng at output as indicator then it will be suitable fr my idea...but how to compare millivolts ac with zero volts

You can use a basic "differential amplifier" ro compare the voltages on live and neutral.
eg. https://www.electronicshub.org/wp-content/uploads/2015/01/1.-Differential-amplifier-circuit.jpg

R1 and R2 must be equal; R3 and R4 must be equal.
The ratio of the input resistors to the feedback & ground resistors sets the gain.
[Edit - added italicised part, missed from the original answer]

You would need to add diodes (eg. 1N4148) to limit the voltage on the opamp inputs, so no damage occurs if full mains supply voltage is present.

Values such as 100K for R1 & R2 with 1M for R3 and R4 would give 10x gain at the output. You need a high impedance input opamp, preferably a low offset type.

The "ground" is the zero reference for the rest of the circuit. eg. If you use a +/-15V supply, it is supply 0V (which would also be electrical ground).
If you use a single polarity, eg. +24V, use a voltage divider to give half supply, plus another opamp connected to that as a voltage follower. The opamp output is the zero reference for the analog circuitry. (And the PSU )v would be electrical ground).

See the left half of this image:


You can use another gain stage after the differential amp, if you want a higher voltage before rectification. That should be AC coupled as any DC offset is irrelevant and could cause problems. A high frequency cut filter may also help, either before or after the differential amp.


Then an active full wave rectifier; example:

The connections shown as ground connect to the reference zero, of whichever type that is.

What you do the voltage from that point is up to you...

ChrisP58 said:

He's got a neon lamp and a 230K resistor across the circuit breaker. This provides about 0.7mA of current to the load/short.

Click to expand...

that's commonly used as a tripped breaker indicator. very popular in military equipment.