In reply to a post by eckiedoo:

A colleague many years back, also reversed the connections to an electrolytic capacitor of the period, probably 16 to 32 MFds, 300V DC.

Probably bout 1.5 inches diameter by six inches tall, say 37.5 mm diameter by 150 mm tall.

Large enough to be potentially lethal.

It exploded directly in front of him, as he switched mains power on.

The contents, rolled aluminium foil, separators, corrosive alkaline electrolyte etc, headed upwards at high speed to splatter on the laboratory roof, about 25 feet (8 metres) above


The resulting damage, splatter etc was there for many years, being pointed out as a salutary warning, "tae mak siccer, afore switching on".


By sheer coincidence, his name was highly appropriate to the event!

I well remember an incident that happened to a pal and myself when we were attempting to grind away the rear forks of a cycle frame for some reason that escapes me fifty years later.
We were using his fathers home made grinding machine which comprised an unguarded 9" wheel mounted on a 3/4 HP single phase electric motor.
Mounted on the top of the motor in a sealed steel cylinder was a bi-polar electrolytic capacitor used for stating the motor. It was cut out of circuit by a centrifugal switch when the motor reached full revs.
I should mention that the grinder was located in a damp poorly lit cellar.
Unfortunately my mate applied a bit too much pressure to the wheel and the motor stalled.
Immediately there was a terrific flash and bang, followed milliseconds later by something whistling around the cellar bouncing off all the walls.
Somewhere along the way it took out the single light bulb and we were left in darkness terrified to move in case what seemed like miles of aluminium foil that we saw for a moment before the light went out, were still alive. Examination afterwards showed that the foils had disconnected when the thing blew up.
His father was not amused.

In reply to a post by Alex1M6:

That inductors core looks like it has been getting very hot as it looks pretty discoloured.

I noticed the discolouring, but guessed it might be normal wear and tear, given the heat that builds up in these things. Up close, it looks a little like a tiny amount of liquid has touched it and dried it. Paint melted off maybe? (I notice the others are green)

In reply to a post by Alex1M6:

Its a coupled choke that carrys the high current +12, +5 and +3.3v rails and is normally used to help smooth out the supply.

Yep, they reduce 'noise' or jitter on the rails. Or that was my interpretation. Frankly, I've never been inclined to poke around inside PSUs for the simple reason that you state later on

In reply to a post by Alex1M6:

Another Edit: Please be careful when poking around inside of the PSU. Its unlikely but there is a small chance that there is still a charge stored inside of capacitors on the rectified mains side, and those heatsinks can be electronically connected to various points in the circuit.

Yep, I know. Once switched off, I flicked the off switch on the PSU, left it for an hour or so then popped the mains cable.

In reply to a post by JimmyBoy:

Nice pictures!

Ta! Just got the camera for my crimbo. First use...PC parts.

In reply to a post by JimmyBoy:

That coil appears to have received a good 'burning'. It's probably in the 3V3, 5V or 12V output (whichever output has the highest current rating).
If you move the output cabling from the PSU chassis, you may find the culprit underneath the cabling - look for the smoke trail/two legs (with charcoal or an air gap between them) or a damaged electrolytic capacitor. Also check the underside of the circuit board.

I'll probably get round to it in a week or so. Want to let the new PSU get a few days of use, juts to make sure. Assuming it is the PSU, and I'm 99% sure it is now, I'll cut off all the good wiring and tie-wraps to see what we can see. More pics to follow!

In reply to a post by JimmyBoy:

EDIT: Picture 6 appears to have captured the damage - zoom in to the base of the coil/inductor - the windings appear to be scorched. The damage should be visible on the underside of the PCB.

You had me going for a second when I revisited that pic, but alas it is not what you think. See:

Pic 9
Pic 10
Pic 11

See the thinner wrap of copper around the coil? It appears that once said wire exits the coil it gets black sheathing. The black thing in the background is a sheathed capacitor. At this level of detail my zoom/flash/focus/handshake issues are getting noticeable. Not to mention the dust.

Good stories.

Was Googling for 'PSU electric shock' and came across a few funnies:

http://www.tomshardware.co.uk/forum/329620-10-shocke...

Got shocked by a PSU heatsink. Will I explode?

No you won't explode but 2 things can happen:
->the electric current will force cell division which can lead to some extra limbs OR
->your body will become one with the PSU and you will become....the chosen one

Due to the increased electrons in your body because of the shock you will most likely notice your head increasing in size. This is known as Bloaty Head syndrome. You will need to go to the hospital were the doctors can literally pop your swollen head and then pump it back up to the correct PSI.

So you were actually dumb enough to open a plugged in power supply and touch the inside?

Wow, there's one born every minute.

death by electrocution is instant (or near instant), so you are fine.

However, none of these tell me how long a PSU can retain a charge. I assumed an hour with the PSU switch off to be able to open the casing without serous risk, and as long as I didn't poke around inside with a wet finger I should be ok.

If The charged capacitor is completely isolated and has a very high inter-electrode resistance approaching infinity, then the charge could remain for years, rather like the aircraft crashed during WW2 in Greenland, when found relatively recently, with tyre pressures close to normal.

However in typical circuitry, the load such as the (rest of the) computer will tend to discharge those supply capacitors to a safe level quite quickly.

When dealing with EHT capacitors (25 KV typically), we generally used a "Brinkley stick" to discharge them.

This was a metal hook, mounted on an insulating handle, with a wire to chassis or earth.

Brinkley was the one-armed boatman who ferried early radar developers over the river short-cut to one of the early radar stations. His missing arm was replaced by a hook, "Long John Silver" style.

In reply to a post by eckiedoo:

If The charged capacitor is completely isolated and has a very high inter-electrode resistance approaching infinity, then the charge could remain for years, rather like the aircraft crashed during WW2 in Greenland, when found relatively recently, with tyre pressures close to normal.

However in typical circuitry, the load such as the (rest of the) computer will tend to discharge those supply capacitors to a safe level quite quickly.

When dealing with EHT capacitors (25 KV typically), we generally used a "Brinkley stick" to discharge them.

This was a metal hook, mounted on an insulating handle, with a wire to chassis or earth.

Brinkley was the one-armed boatman who ferried early radar developers over the river short-cut to one of the early radar stations. His missing arm was replaced by a hook, "Long John Silver" style.

I understand that most of the German WW2 bombs dropped on the UK were electrically detonated.
Paper capacitors within the fuse cartridges were charged immediately before release from the aircraft via a button on the fuse assembly.
Bomb disposal officers had to drill into the fuse and inject a liquid of a high specific resistivity to discharge the capacitors slowly but not low enough to cause the things to detonate.
Allied bombs used a chemical time delay utilising the action of acetone on acetate sheets holding back a striker pin.
I too used a "Brinkley Stick" although we didn't call it that, to discharge the high voltage capacitors within induction heating machines used to anneal the ends of small springs for clockwork timing mechanisms.
Many of these machines were built locally from surplus WW2 radar set componants.
They were potentially lethal with no safety interlocks on doors etc. They operated in the low Mhz region with powers of around 5kW (R.F.) and HT supplies of around 5kV.
One much more modern machine was found to be wiping out UHF television reception over a wide area when BBC2 started on UHF.
We were puzzled because the thing ran at 3.5 Mhz and we couldn't see how it could produce harmonics at a high enough frequency.
The machines either used a single large-water cooled or air-cooled triode valve or two valves in push-pull.
It turned out that the valve type was prone to producing spurious UHF radiation by the action of stray electrons going into very fast circular orbits within the internally finned valve anode. Rather like a klyston or magnatron.
The solution was to fasten a dirty great magnet to the outside of the valve anode to attract and keep the stray electrons, this meant extensive metal bashing to provide the necessary HV clearances.
Funny stuff RF (except when you got a burn from a work-coil).

I'd replace all the electrolytics as a matter of course.

A new year and the central heating was on the blink, fired up on hot water but not on heating, problem is actuator motor. Thank heavens for ScrewFix Direct.

http://www.screwfix.com/p/drayton-synchronous-motor/...

I decided to reduce the amount of clutter in the PSU case. Just a tad mind.

Pic 12


This is definitely the offending component. The coil in Pic 13 next to the bad coil was half an inch from it and has no acrid smell, while the big coil still does. Note also the discolouring of the big coil, as if some liquid has dried on it.

Also notice in Pic 14 the colour of the copper ^[1] on the coils versus the copper on the big one. Not the 'red' copper, but the orange copper. Also the red copper at the bottom of the big coil is slightly brighter than that of the red copper higher up. Heat difference I imagine.

I thought all that black gunk inside was an obvious sign, but it seems to be there on most of the coils, albeit to a lesser degree.

Lastly, a quick peek at the underside of the PCB in Pic 15. There's little obvious burning or melting, other than normal heat marks. Any damage seen is due to my determined use of pliers and cutters.

[1] - The 'copper' in all the coils and indeed in all of the wiring I removed is not copper. It is (probably) aluminium with a copper jacket. You can see the cut off ends in Pic 14. A nice, cheap way to avoid copper costs, but is the aluminium as conductive as copper? Certainly, the ,melting point of aluminium is far less than copper, but I'm pretty sure 600+ Celcius is way above what we would see in a PSU anyway.

I assume the copper jacket prevents corrosion. Is it a copper jacket, or just copper coloured (or copper based) paint? See the cut end on the yellow coil in pic 13. Looks like paint.

Btw, the coil wire is solid core, while the plastic jacketed wiring within the PSU was stranded (most likely for flexibility in the PSU cables).

So that's that! I hope my PSU saga been slightly interesting.

In reply to a post by camieabz:

A nice, cheap way to avoid copper costs, but is the aluminium as conductive as copper?

At high frequencies something called "skin effect" (Wiki) comes into play- most of the current flows in the outer layer of the conductor. So it's not too important what the "core" is made of, 'cos most of the current isn't flowing in it!

Even at mains frequencies it's enough to allow the high voltage cables on pylons to use a steel core for strength, with the current flowing in the outer layer of aluminium.

In reply to a post by eckiedoo:

A colleague many years back, also reversed the connections to an electrolytic capacitor of the period, probably 16 to 32 MFds, 300V DC.

Probably bout 1.5 inches diameter by six inches tall, say 37.5 mm diameter by 150 mm tall.

I doubt it ... 1F at 300V is diffucult, 16 MF - probably impossible.