Flash, bang, fizz.

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Boustrophedon
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Flash, bang, fizz.

Post by Boustrophedon » Sun Sep 26, 2021 8:27 pm

Long ago I bought a one third sheet electric sander for £10 from B&Q for just one job. Today, 25 Years later that sander died, it was going really slow and noisily, so I brought it in to dismantle, not with any real intention to fix but just in a spirit of enquiry. I plugged it in one last time to the extension lead by my desk, pulled the trigger switch and there was a spectacular pop, a flash and a fizzing, followed by the magic smoke escaping from the vicinity of the plug. Then all the lights went out and the computer screen went blank.

Turns out the flash the smoke wasn't from the plug connected to the sander but from the phone charger plugged in next to it. I popped the charger open and inside it was toast. I reset the trip in the fuse box.

Now given that the sander ground to a halt and died at exactly the pop/smoke instant, it seems reasonable to assume that the two events are linked.

But what fault up in the sander could possibly lead to the destruction of a phone charger? Or is there a possible fault inside the dual socket extension?
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basementer
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Re: Flash, bang, fizz.

Post by basementer » Sun Sep 26, 2021 8:45 pm

Motors present an inductive load to the mains and they can produce quite a voltage spike as they switch on and off. Someone will be able to explain this better than me.
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Re: Flash, bang, fizz.

Post by Boustrophedon » Sun Sep 26, 2021 9:06 pm

Here is the inside of the phone charger, the fluffy thing on the right of the circuit board is the remains of a electrolytic capacitor.
Splody Charger (Medium).JPG
Splody Charger (Medium).JPG (209.02 KiB) Viewed 430 times
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Re: Flash, bang, fizz.

Post by science_fox » Sun Sep 26, 2021 9:42 pm

Do you know what protection you've got on the various circuits? What sort of trip in the fuse box?

If the sander had mechanically the failed the motor would try to draw max current to turn. This might have spiked/shorted enough to blow the considerably lower rated phone charger, especially if it was on a 13A fuse. You may also want to inspect the whole length of the extension cable as they often aren't rated for large currents either.
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Re: Flash, bang, fizz.

Post by Boustrophedon » Sun Sep 26, 2021 9:57 pm

science_fox wrote:
Sun Sep 26, 2021 9:42 pm
Do you know what protection you've got on the various circuits? What sort of trip in the fuse box?

If the sander had mechanically the failed the motor would try to draw max current to turn. This might have spiked/shorted enough to blow the considerably lower rated phone charger, especially if it was on a 13A fuse. You may also want to inspect the whole length of the extension cable as they often aren't rated for large currents either.
It blew neither the 5A fuse in the sander's plug nor the 13A fuse in the extension plug. It did trip the ring main over current trip, but I can't see what that is rated at. The cable looked OK and passed a Megger test. Yes, I have an old hand cranked Megger.

The sander was still free to turn so I suspect it had short circuited in one of the windings.
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Re: Flash, bang, fizz.

Post by shpalman » Mon Sep 27, 2021 11:40 am

Explodey capacitors usually means overvoltage. I don't really see the relevance of talking about current because it's not like the sander was being powered by the phone charger (and anyway the fuses didn't blow, although fuses are slower than circuit breakers which is why we have circuit breakers).

Two things I can think of which can happen with motors. The first is that the coils of a motor draw much more current at the beginning in order to get going (and continue to draw more current if the motor is somehow prevented from turning). The second is that when you stop supplying current to a coil, the magnetic field collapses and this induces a voltage spike in the coil. Of course an AC motor has the current continually (but smoothly) changing but you may have been unlucky and done it at the moment when the 50 Hz cycle was at a maximum.
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Re: Flash, bang, fizz.

Post by Martin Y » Mon Sep 27, 2021 12:22 pm

I think it must have been something like that - a big overvoltage spike which broke down the insulation in the capacitor because the charger was connected so close to the sander. Devices elsewhere wouldn't get quite the same jolt as the spike would dissipate through the mains wiring. Once the capacitor was turned into a low value resistor and was still being supplied with power it would quickly have overheated and boiled its electrolyte until it popped.

It probably wasn't the spike itself that made teh cap asplode, it would have been the current pouring through it afterwards.

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Re: Flash, bang, fizz.

Post by Boustrophedon » Mon Sep 27, 2021 1:17 pm

The sander was 25+ years old. The "soft feel" coating it can with progressed through soft feel, to sticky and gummy, to hard and crazed (bit like my life then.) It wasn't nice. So it was an excuse to buy a Makita.

The phone charger was annoying though.
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Re: Flash, bang, fizz.

Post by Millennie Al » Tue Sep 28, 2021 1:18 am

My guess is that if the motor suddenly stopped drawing current, that would have created a voltage spike at the motor. The wiring acts as an inductor, inhibiting sudden changes, but the adapter was sufficiently close to the motor that the spike killed the capacitor, and when it exploded, the debris allowed a small amount of current to flow from live to earth (it'a all open inside the plug, so the capacitor debris could bridge the terminals), which triggered your RCD.
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Re: Flash, bang, fizz.

Post by nekomatic » Wed Sep 29, 2021 10:07 am

Millennie Al wrote:
Tue Sep 28, 2021 1:18 am
the adapter was sufficiently close to the motor that the spike killed the capacitor, and when it exploded, the debris allowed a small amount of current to flow from live to earth (it'a all open inside the plug, so the capacitor debris could bridge the terminals), which triggered your RCD.
I’m thinking that the motor drew excess current, which tripped the ring main circuit breaker (before the fuse had a chance to blow), and the collapsing current through the motor winding then created the voltage spike that killed the charger.

Don, can you show us the other side of the charger circuit board?
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Re: Flash, bang, fizz.

Post by Boustrophedon » Wed Sep 29, 2021 12:18 pm

nekomatic wrote:
Wed Sep 29, 2021 10:07 am
Millennie Al wrote:
Tue Sep 28, 2021 1:18 am
the adapter was sufficiently close to the motor that the spike killed the capacitor, and when it exploded, the debris allowed a small amount of current to flow from live to earth (it'a all open inside the plug, so the capacitor debris could bridge the terminals), which triggered your RCD.
I’m thinking that the motor drew excess current, which tripped the ring main circuit breaker (before the fuse had a chance to blow), and the collapsing current through the motor winding then created the voltage spike that killed the charger.

Don, can you show us the other side of the charger circuit board?
Sorry, it's gone now. Wasn't interesting, nowt to see. I think your explanation is very plausible.
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Re: Flash, bang, fizz.

Post by nekomatic » Wed Sep 29, 2021 5:54 pm

I was just curious to see how closely it resembled any of the worst offenders in these teardowns, that’s all.
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Re: Flash, bang, fizz.

Post by Boustrophedon » Wed Sep 29, 2021 8:20 pm

“This wasn't just plain terrible, this was fancy terrible. This was terrible with raisins in it."
Dorothy Parker.

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