Reverse engineering a GFI

Discussion in 'Test & Measurement Forum' started by E_Weisbard, Oct 18, 2017.

  1. E_Weisbard

    Thread Starter New Member

    Oct 17, 2017
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    I am using 2 CR8401-1000-A current transformers in opposite polarity to sense current on either side of a load. The Idea is that when the load is not in failure mode that the sensed current waveforms will be exact opposites and when "added together" will cancel each other out. In failure mode, or ground fault, there will be an alternate path for current. The wave currents will no longer cancel and yield a net current. This signal is amplified by an op amp circuit, and rectified by a precision full wave rectifier. The now DC signal is sent to an ADC. A micro controller coupled to the ADC will throw a breaker at programmed current thresholds. My question is this, can the 2 current transformers be connected in reverse polarity in parallel directly, or should there be a buffer circuit between them?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    What have you found when you pulled apart a commercially available GFI.
     
  3. tcmtech

    Distinguished Member

    Nov 4, 2013
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    Just run both wires through the same current transformer so that their current flows are in a normally oppong flow pattern.

    You will get the same normal current flow cancellation effect you are looking for while using less parts.
     
    JohnInTX likes this.
  4. E_Weisbard

    Thread Starter New Member

    Oct 17, 2017
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    I know that what you are saying should work, but it does not seem to.
     
  5. E_Weisbard

    Thread Starter New Member

    Oct 17, 2017
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    I haven't officially taken one apart, but Google says it is two opposing current transformers powering two relay windings
     
  6. E_Weisbard

    Thread Starter New Member

    Oct 17, 2017
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    I haven't officially taken one apart, but Google says it is two opposing current transformers relay windings.
     
  7. E_Weisbard

    Thread Starter New Member

    Oct 17, 2017
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    My current design with a single wire through one transformer yields 250 mv/ma after amplification. The micro controller runs a digital display working just like an ammeter, so you can easily see the output. My next baby step was powering a light bulb on 12VAC (approx .42 amps) running both wires through one transformer. The resultant output after amplification was 5 VAC, far from canceling out.This threw me for a loop. I am amplifying something that shouldn't be there.
     
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