While getting dinner tables ready today for the coming Christmas Dinner the boss (Millie) asked me to get some wrinkles out of a plastic table cloth. After retrieving my shop hair dryer I began to heat the plastic cloth to remove the wrinkles. After I had finished I noticed the little GFI Plug, on the end of the cord, plugged into the wall outlet. I thought to myself, you know I have never tested the GFI,and so I pushed the test button. The GFI however did not click and open but rather began to buzz that nasty 60 cycle buzz that means something is wrong and before I could pull the plug from the wall it let loose with a plume of smoke.
Opportunity had knocked as I have never investigated a plug based GFI before. Several minutes later I had it disassembled and off the cord. Since I was doing it anyway I decided to document the exploration so you too could see what was inside.
After removing the covers I found the inner mechanism to be a nice mix of the electronic and the mechanical.
The manual red Reset button engages a mechanism that holds the two mains switches closed and one smaller switch that leads to the Test Circuit. A small solenoid (Yellow Center) latches the mechanism in place. If the solenoid is energized the latch is pulled out of the way allowing the mechanism to drop and the three switches go open. A quick inspection and measurement with the ohmmeter showed that the solenoid coil had been the source of the smoke as it was now open. I removed the main transformer which had 2 common mode windings (the Large Red and Large Blue), one Test Winding ( Small Blue) for manually triggering the solenoid, and one Sense Winding (Small White) that led back to the electronics under the transformer.
This opened the board itself for better analysis.
The board was a pleasure to reverse engineer as all the components were well marked for identification and the single sided board with 14 components gave up its secrets quickly. Here is a schematic of the GFI device. If you click on the schematic it will open a larger view.
I found it fascinating that the designer put the coil of the solenoid in series with the line side of the bridge rectifier. The rectified side of the bridge was connected directly to the cathode and anode of SCR EC103. If the conditions in the circuit are such that the SCR is triggered it creates a short that applies sufficient current to the solenoid coil to pull the latch and open switches SW1, 2, and 3. The conditions for triggering the solenoid are #1, the Test button is pushed and sufficient current is induced in the Sense Winding of T1 by the Trip Winding. or #2 an imbalance in the current through Common Windings of T1 caused by Mains current leakage to ground rather than returning through Neutral also induces current in the Sense Winding.
My further analysis of the electronics is more guess than knowledge. Diode D2 should remove one side of the AC Current produced by the Sense Winding and the 5.6 Volt zener ZD1 probably adds a DC bias to the signal to put the gate of the SCR closer to its trigger point. The data sheet on the EC103 says that it is a sensitive gate SCR which makes sense as we would not want any more delay than necessary before energizing the solenoid. I hope someone corrects and improves on this analysis if they have time so I can understand it better. I have linked the data sheet in case it is helpful in better understanding the circuit.
http://datasheet.octopart.com/EC103M-Teccor-datasheet-5013.pdf
The hair dryer which is a shop tool no longer has a GFI plug and has been labeled for safety concerns. Unfortunately the way this GFI failed would not have protected the user in the case of a real emergency. When the solenoid coil burned out without tripping the mechanism it left the mains switches closed and the current still flowing to the hapless victim.
Thanks John
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