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This unusual plug interfaces an electric hair dryer to the 120V 60Hz mains. The thing that makes it peculiar is that when it is tipped back and forth, a small 'clunk' can be felt, as something inside shifts in response to being tilted.

It is a small boxy thing, about a 1.75" x 1.25" x 1", has no ground lug, but is polarized. The wire coming out of it almost certainly carries 120VAC. There is a molded-in warning not to replace, open or immerse the plug. There are no openings of any kind, nor any obvious ways to disassemble it non-destructively.

What would clunk back and forth inside there? It probably isn't an isolation transformer, not in a box that size, for 1800W. I doubt it's a tilt sensor, but can't rule that out. A replacement fuse might be about the right mass, but wouldn't be much use in an box that can't be opened. Anyone know what this is, before Mr. Plug is introduced to Mr. Hacksaw?

Mr. Plug's mugshot ..

strange plug that goes clunk

No buttons of any kind. That metallic looking spot where a screw might ordinarily be is absolutely flat, and doesn't move. There are two not-screws like that, the other one is not visible in the picture, being hidden behind the right-hand prong.

follow up ..

After removal of the cover, a circuit was revealed:

hats off

Turns out that the the thing that went clunk was that inductor in the foreground. Interestingly the connection on the right side failed, so the loose inductor was just wagging up and down. You can just see the hole in the wiring board where the lead was meant to connect.

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  • \$\begingroup\$ What unusual plug? I don't understand what you are describing. A picture would help a lot (now why wasn't that obvious!?). \$\endgroup\$ – Olin Lathrop Dec 28 '11 at 19:36
  • \$\begingroup\$ @OlinLathrop - it's just a boxy little plug. A picture will show you a squarish little box, with an ordinary looking cord and two prongs you stick into an outlet. \$\endgroup\$ – JustJeff Dec 28 '11 at 19:38
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    \$\begingroup\$ Time to introduce Mr Plug to Mr Hammer and Miss Chisel \$\endgroup\$ – Majenko Dec 28 '11 at 19:51
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    \$\begingroup\$ The metal circle in the center may be some sort of anti-tamper screw or rivet that could be removed for a less completely destructive disassembly. For example, you may be able to cut a slot for a straight screwdriver, or file a single-use compatible bit from some piece of metal scrap. \$\endgroup\$ – Chris Stratton Dec 28 '11 at 21:50
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    \$\begingroup\$ Or let Mr Drill penetrate it with his long spinning shaft.... \$\endgroup\$ – Majenko Dec 28 '11 at 22:31
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I was inclined to say it was a ground fault current interrupter. However, those that I am familiar with have manual test and reset buttons.

I did a search to see if an auto-reset version exists, and apparently it does.

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  • \$\begingroup\$ hm, for some reason i believed they needed ground wires, but upon investigation (and thinking), they don't. so this seems much more likely. \$\endgroup\$ – Jay Kominek Dec 28 '11 at 21:11
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    \$\begingroup\$ @JayKominek - I started to think that too, but remembered I have a physical example without. I believe the trigger is difference in current on the hot vs. the return. Measuring current on the safety ground (when present) wouldn't work anyway, since there's no guarantee that the fault current would be taking that path, and indeed the worrisome case might be when the fault current is flowing through the water to the plumbing drain. \$\endgroup\$ – Chris Stratton Dec 28 '11 at 21:22
  • \$\begingroup\$ There's enough of a circuit that you are almost certainly correct. \$\endgroup\$ – JustJeff Jan 1 '12 at 21:38
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No buttons on it? Frequently there is a little breaker in those boxes, and they have an exposed reset button on them. I'm going to guess self resetting thermal breaker inside, with some springy or loose fitting mechanical elements that are moving around.

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  • \$\begingroup\$ no buttons at all. whatever's in there seems really loose, not springy at all. \$\endgroup\$ – JustJeff Dec 28 '11 at 20:09

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