GFCI double toroid design.

[Dreamer1]

Hi guys, first post, so sorry if i'm breaking rules or something of the like.

Anyway, I pretty much recently graduated, and my first duty was to design a GFCI for one of our inverter designs.

First thing I did is open an existing GFCI circuit too see how they do it. I quickly noticed the double toroid design, this confused me greatly! I'm confused about sensing the ground-neutral fault condition.

you can see examples of this design here, and here

http://www.fairchildsemi.com/ds/RV/RV4141A.pdf
http://www.ti.com/lit/ds/symlink/lm1851.pdf

(go to typical application circuit)


Ok, so here is how I think the double toroid circuit works. the ground-nuetral sense transformer (one with 200 windings in both drawings) has one primary connected to the AC line voltage. It also has to secondaries, hot and neutral. this transformer imparts a voltage of (max ac)/200 on both hot and neutral wires. if neutral is connected to ground, this induces a current, which is picked up by the current sense transformer. if neutral is not connected to ground, no current is induced.

this site explains it pretty well http://www.rhtubs.com/GFCI/GFCI.htm.

Is my guess (and the websites explanation) of how the double torioid sysytem work correct?

further more, if a ground-neutral fault is large enough, it SHOULD be picked up by the primary sense transformer no? the second transformer (neutral transformer) is for extra safety? In my mind, the second ground-sense transformer _increases_ the differential current to be sensed, but without it, in the case of a ground-neutral fault, this differential current would still exist?

Thanks for the help guys and gals!

[Andy Watson]

Can't get to the third link! But here's my best guess. Ground and neutral are nominally at the same potential. If there is a ground-neutral fault it is difficult to tell which return path the current will use - it depends on the relative resistance on each path. It is easy to imagine scenarios in which a ground-neutral fault exists but does not trip the detector. What is happening with the extra toroid is that they are effectively forcing the issue. i.e. a signal is being imposed on what should be an isolated piece of equipment - obviously no fault current will flow if the equipment is truly isolated. The first datasheet RV4141A cunningly creates an oscillator from the fault loop. The second chip, lm1851 is using double the line frequency to impose a signal on the isolated equipment. In both case they are imposing a non-line frequency signal and detecting when a fault closes the loop.

[Dreamer1]

Do you have any idea why it's TWICE the line frequency? What will that accomplish?

[Andy Watson]

Do you have any idea why it's TWICE the line frequency? What will that accomplish?

IIRC It's driven from the output of the bridge rectifier - two pulses per cycle of the line input.