Voltage Divider (straight off of the mains) trips GFCI... Why?

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Thread Starter


Joined Apr 13, 2010
I think I may understand what's happening here but it's just a theory. Here's the scenario...

I have a logic gate sensing when a 1/2hp 120V AC motor turns on. To get the 5Vdc logic signal (that the motor has come on), I have the following circuit:

My thinking is that since it's a motor starting, it's probably pretty noisy and with transients. Possibly a few milliamps of this signal is getting lost and confusing the GFCI. I tried the circuit tapped right off of the line (same 120VAC outlet, but before I had connected the voltage divider circuit to the motor) and no trouble. So it seems to be related to the motor and start up.

Any other theories besides my thought with the transients?


Joined Jul 18, 2013
You should be using an isolation circuit, also why is the N earth grounded?
That should never be connected in most jurisdictions.
The only time you connect N to Earth is when some deliberate isolation has occurred.


Joined Mar 30, 2015
Any other theories besides my thought with the transients?
The current that isn't going back to the power panel through the neutral wire is high enough to trip the GFCI.

Connecting the neutral wire to earth ground is only allowed at the circuit breaker panel in my area.


Joined Jun 26, 2012
This thread has been locked partially because it violates our Terms of Service / UA (direct to mains) but also because it is dangerous. If you would like to proceed with an opto-isolated or other safe way to interface / detect the AC line PM a moderator and we can go from there.
Thanks for your understanding.
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