I unplugged my fridge freezer, moved it across the room, and then after leaving it half an hour to settle, picked up the plug intending to plug it back in. And got a reasonable shock from the plug! Anyone got any idea why?
I can speculate about inductance in the motor, but surely that sort of thing should be isolated by the fridge's own on/off switch?
Ben
It was probably you that was at a high potential, rather than the fridge. Were you wearing rubber soled shoes?
Christian.
That occurred to me, but it was a proper jolt, not the sort of thing I normally get from car doors...
I was wearing socks, no shoes.
Ben
Large capacitor across the live/neutral. It's used to supress interference.
sPoNiX
I often get really quite severe static shocks, certainly up to "jolt" level. I'd still be quite surprised if a fault in a fridge could cause a high energy shock after 30 minutes of disconnection. Perhaps there is some sort of capacitor in there that got charged up, although I'd be sceptical. The static charge would have to be DC, so the inductive windings of the motor couldn't be the cause.
Christian.
Probably a capacitor left charged up as you disconnected.
The fridge has been off, has warmed up, so the thermostat switch will be on.
Did you actually touch the pins on the plugtop?
It's also legally required to have a large value resistor across it to avoid this problem. What make is the fridge ?
Does that really need an answer!? yes, and I had a patch on my finger where it touched the pin that almost felt burnt.
Ben
Sorry, but I thought it did need answering. The shock could have come through the casing on the appliance. Your shock has more than likely came from the IF capacitor which is fitted across the live and neutral supply to reduce radio interference from the compressor motor and switching relays. It is known that if the plug is pulled out of the socket before the socket is switched off, then the IF capacitor doesn't have a chance to drop its charge to earth, which only takes a second, and the charge will be held for the length of time the capacitor is disconnected.
One story that sticks in my mind from my apprentice days, is the bloke that lifted an old TV set out of his loft after 5 years. He thought he'd cut the plug off it before dumping the set in the skip. When he grabbed the plug the jolt through him out of his chair because of the charge held in the capacitors. We also used to have great fun charging capacitors and throwing them to each in the class room as well. So you can probably blame Faraday for this one.
sounds apocryphal to me... even if you could make a capacitor which would hold its charge for 5 years, it would be unlikely to be employed in a television set.
Oh right. Maybe I should have said it was an "old" TV set. Oh right. I think I did.
I have a simple LCD clock I made as an apprentice. It actually works on a large capacitor as its sole power source. In fact, the capacitor was taken from a very old television set with valves rather than transistors. It was last charged over two years ago and is still working perfectly now. I normally have to charge it every four and a bit years from a 12 Volts battery.
Where can I buy a capacitor that won't hold some kind of charge for the length of time it is not connected to anything?
Try this. Take an IF filter capacitor from an old washing machine. Charge it with a 12 Volts battery through a 100 ohms resistor. It doesn't take long. Place the capacitor in a sealed plastic box to keep it dry and some what insulated. After a year or so, remove it from the box and place both terminals on your tongue. Keep trying this experiment with increasing time delays between touching both terminals on your tongue. Then get back to me with the results please.
Fascinating - and that's exactly what I did. Almost tempted to go do that again to see if I get a shock again.
Ben
I can take 2200uF 35V caps from our storeroom, which have been in stock for at least a year and they have a significant charge in them still.
Dave
If it was a shock between the pins of the plug, then it almost certainly would have been from an interference suppression capacitor.
These days, suppression/filtering components are very often at the 'outer boundary' of the equipment, on the mains lead side of the switch.
Could be something left on a supressor capcacitor.
If the jolt was across two of the pins on the plug, rather than fromn a pint to earth.
The version I heard was of a picture tube that had been stored in an airing cupboard. The owner picked the tube up and put his finger on the anode connection.
Done that on a valve wirelss. - picked a cahssi with 300v still on the anode cap. Bloody heck!
EHT capacitrs generally not electrolytic, so quite loss free. We used to wire resoitsors in there to leak it all away for precisel that reason. However a Gigohm resistor that will take 15KV without arcing is summat expensive, so in production I guess they eliminate them.
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