GFI Ground Fault Interrupter Circuit

I always wondered what was inside one of these clever devices, so I found one in my junk box and popped the lid—it was from a defunct blow hair dryer—never throw one of these away because the GFI unit long outlives the hair dryer and has many experimental uses. To get it apart, I had to make a special screw driver bit to remove the tamper resistant screws. Here’s the figure of the design circuit;

You will notice that the parallel power leads make a single turn through the primary of the current transformer (CT). The flux field of the source lead is cancelled by the flux field in the return lead so the net result is zero and the CT sees no primary current. Should these currents ever become unequal (as in a ground fault condition), the CT senses this difference and induces current into the 1000 turn secondary. The secondary current is low, but the load resistance is 1M, so it develops significant voltage. This voltage is sufficient to exceed the comparator threshold voltage of the IC and fire the SCR. When the SCR fires, it energizes the solenoid coil and jerks an iron slug toward the center of the coil. Attached to this iron slug is a stainless steel pin that actuates the mechanical release for the electrical contacts. When the contacts are open, the ground fault current is interrupted and the appliance is off-line. It remains off until the mechanical reset button is pressed.