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I've created a circuit that is powered with a floating ground off of a transformer. I'd like to count pulses from a meter by having the meter complete the floating ground side of the circuit but I'm concerned about running insulated floating ground through the earth and what the implications might be.

schematic

simulate this circuit – Schematic created using CircuitLab

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500 Ohm is a pretty strong pull-up, which is fine. The current would be limited to 10mA, which sounds adequate.

However, you have to consider voltage transients and potential EMI that come from "the outside" and could interfere with or even damage your digital logic (you probably have a controller connected to "PULSE_INPUT").

I assume the pulse from your meter are at least 20ms in length, so my advice would be:

  • Use some kind of low pass filtering (in the ballpark of 1kOhm and 1uF) on the PULSE_INPUT pin
  • Protect your PULSE_INPUT with a properly rated TVS diode or a Zener diode
  • Consider a small series resistance (100 Ohm) right before your micro controller input pin
  • Implement some simple software debouncing to prevent short voltage spikes to be detected, it comes for free.
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  • \$\begingroup\$ Thanks for the answer. I meant to include an opto-isolated filter with capacitance but I couldn't find one in circuit lab. What would happen if the upstream side of the pulse cable were to come in contact with the earth? Smoke? \$\endgroup\$ – Justin Manuel Oct 28 '16 at 19:00
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    \$\begingroup\$ It wouldn't hurt much if your isolated GND gets connected to "earth", if thats what you mean. \$\endgroup\$ – Rev1.0 Oct 28 '16 at 19:05

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