I want to test how long it takes for four breakers in series to trip. I want to be able to identify which breaker first tripped.

I came up with this circuit. S1 to S4 are the breakers. Only S1 is pulsed (to simulate a trip). The differential circuit is the "detection" circuit. Hence it will be placed in parallel across all four breakers. I just have it across S1 for simulation.

enter image description here

When the breaker trips I accepted a binary output. However I get this on the output:

enter image description here

This is clearly not a binary output I can feed into a micro.

Is there a circuit that I can use that can give me two clear DC voltages for when the breaker is tripped and when it is not tripped?

  • \$\begingroup\$ What is the voltage across R7? Is it enough to operate an optoisolator LED? \$\endgroup\$ Commented Oct 9, 2023 at 18:32
  • \$\begingroup\$ @AndrewMorton R7 is used to set the current for the breakers. \$\endgroup\$
    – JoeyB
    Commented Oct 9, 2023 at 18:34
  • \$\begingroup\$ What do you mean which one "first" tripped? Only one of them will ever trip. \$\endgroup\$
    – Hearth
    Commented Oct 9, 2023 at 19:08
  • \$\begingroup\$ @Hearth but not all the breakers are exactly the same (even though they can be all 10A). S2 can trip first. Breakers have a min and max time to trip. Hence one can trip before the other. S2 may by the one that has the quickest trip time, and S4 may the longest trip time. \$\endgroup\$
    – JoeyB
    Commented Oct 9, 2023 at 19:12
  • 2
    \$\begingroup\$ @Hearth but even once a breaker "decides" to trip it still takes a finite amount of time to actually trip - time in which another breaker might also "decide" to trip. End result being 2 tripped breakers. \$\endgroup\$
    – brhans
    Commented Oct 9, 2023 at 20:16

2 Answers 2


I would never connect an MCU to a mains circuit. I would isolate with an opto-isolated transistor.

This simulator doesn't have an optocoupler. But you can see the output by looking at the current in D5. C1 isn't necessary if you are only looking for the rising edge.

You probably wouldn't want to waste this much power in R2 if this was a permanent circuit, but this is fine for a one-time test. R2 should be at least a 1 Watt resistor.


simulate this circuit – Schematic created using CircuitLab

Plot of the current through D5 and the power in R2.

enter image description here

  • \$\begingroup\$ Unfortunately, this is not a binary output, hence the down vote. I have added my answer to what I needed. \$\endgroup\$
    – JoeyB
    Commented Oct 10, 2023 at 16:12
  • 1
    \$\begingroup\$ @JoeyB - I did the hard part, I assumed that you had enough experience to add a comparator. \$\endgroup\$
    – Mattman944
    Commented Oct 10, 2023 at 16:42

Below is the final circuit, the signal it first feeds into a differential op-amp

enter image description here

The output of U1 looks like this: enter image description here

I then used a peak hold detector to convert this sine wave to a DC equivalent signal. The output of this looks like this: enter image description here

Finally, the signal is fed into a comparator circuit. The output looks like this: enter image description here

This was then fed into the opto and we got a nice clean binary output. enter image description here

The delay is due to the peak hold circuit, which can be fine-tuned for the application.


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