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My wife has a food processor, bought a few weeks ago, powered directly from the A.C. mains, i.e. not via a "wall wart" power supply, and, after it has been switched off at the wall and fully unplugged, she experiences a slight, but significant shock -- more than a tingle, if she touches the pins on the plug as she packs it away.

Please, how does this happen? It seems almost as though a capacitor has remained charged, but I do not understand how this can occur on the A.C. input pins. Also, is it a fault with the appliance that means we should return it to the retailer as dangerous?

Following up from the accepted answer, thank you, could I or should I, and I know enough to do this safely, connect a high ohms value resistor across the line to neutral to discharge the capacitor?

Harry Weston
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    I don't think this is off topic, and I'd return it immediately. Better be safe than sorry, we can speculate about what's happening later, when both you and your wife are safe. – Vladimir Cravero Jul 12 '14 at 22:14
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    I wouldn't bother discharging the motor; as specified below, the charge is not dangerous, and will probably self-discharge eventually anyway. If you want to, a 1M (one megohm) resistor would probably be what you are looking for. Just touch it across the terminals (without touching the leads) and hold it there for a few seconds. – felixphew Jul 12 '14 at 22:31

1 Answers1

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First, this is not a fault, but perfectly normal and safe (if slightly unpleasant!).

The large brushed motor in a food processor can generate small sparks and electrical noise (interference to radio signals) when it is running, so a small capacitor is connected across the AC mains to suppress this noise.

If you unplug the appliance when the AC voltage is at its peak, the capacitor is charged, and can hold its charge until you touch the pins.

It is a low value capacitor (typically 0.1 uF) so the charge stored should not be dangerous. (It is also a special construction - usually "class X2" so that if the capacitor breaks down it will fail safe instead of creating a short circuit)