Eject the core!

April 5, 2006 10:41 PM

How did I manage to blow up a computer power supply when I tried to connect audio out from a VCR to the line in on the computer soundcard.

The setup is like this:
Mitsubishi TV, with 3 component in, video coming from gfx card in PC (old Celeron 400, which has been a reliable media PC for... fn ages), sound coming from soundcard line-out. New video capture card in PC with a single component line to the video out on the VCR, and 2x component to 3.5mm stereo line-in of the sound card.

I borrowed a video capture card from a friend to play with decrypting pay-tv signals - and incredibly it worked! But when I went to connect the sound from the VCR to the line-in on my soundcard (sb16 or something, I'm not using onboard sound) the tip of the stereo plug grazed the metal around the socket, made a little spark noise and the computer shut down.

I tried pushing the button, as you would, and nothing happen. Unplugged the power cable, plugged it in again and pushed the button and then there was a big flash from inside the (open sided) case. I had a look at the fuse in the power supply and it had completely blown - there was actually a hole, either blown or burned, through the glass!

Today I went and got another ATX power supply, but before I plug it in, I'd like to understand why this happened, and what I can do to avoid it in future. Is it a case problem? Am I plugging too many different devices together?

posted by The Monkey to computers & internet (12 comments total)

Why didn't you just replace the fuse instead of the whole power supply?

Also, were the VCR and computer and TV plugged into the same outlet, or different ones? If they're on different outlets, it's possible that your house wiring is incorrect (possibly swapped hot/cold/ground wires), so there was a current flowing where there shouldn't have been. Get a cheap outlet tester and test your outlets before trying again!
posted by xil at 10:59 PM on April 5, 2006

Make sure the computer and the TV are plugged into the same power outlet. The grounds can have a voltage between them if they are on different circuits, especially if the house wiring isn't done right.
posted by StickyCarpet at 10:59 PM on April 5, 2006

xil, the fuse was soldered onto a PCB in the PSU, not a push in type. Plus, the replacement PSU was only $20. Maybe if it happens again I'll break out the ol' soldering iron and replace the fuse, but to be honest I'm a little afraid - I know some components can hold a big charge, and I've shorted 230v through my body 3 or 4 times in my life already, and don't want to push my luck.

I'll make sure everything is plugged into the same outlet, but I'm pretty sure they're all just on a normal multiboard at the moment. (It's all behind a cabinet, so I could be wrong.)
posted by The Monkey at 11:03 PM on April 5, 2006

the tip of the stereo plug grazed the metal around the socket,

Metal around Socket = -
Tip of plug = +

(+)+(-) = Short

Remember, the voltage level for the audio is low, but the (amplified) current can be quite high.
posted by IronLizard at 11:34 PM on April 5, 2006

Are you certain you have the right connector from the vcr?
posted by IronLizard at 11:37 PM on April 5, 2006

IronLizard, I'm as sure as I can be. The video connector was right, so I can't have been plugging video into audio, I suppose it's not entirely impossible that I had my audio channels crossed (but I don't think so, they're colour coded, after all) and what could that do, anyway?

Surely the plug is likely to come into contact with the edge of the socket (and even the metal plate the socket is mounted in) from time to time, isn't this something that should be designed around? Was it an unfortunate confluence of events, or something I'm just very lucky I haven't encountered before?
posted by The Monkey at 11:56 PM on April 5, 2006

To answer your question:

"Plus, the replacement PSU was only $20."

This is probably why it blew up. You're buying crappy no-name PSUs with marginal components. You can't possibly be purchasing a new ATX PSU of any reliability for less than $40 or so, and if you're buying retail it's likely you'd be paying closer to $60 or $70.

Build quality, QC, and component testing cost money and the maker of a $20 PSU simply can't be doing a decent job of it while turning a profit. I'm not suggesting you run out and buy a $180 PSU, but it looks like you're buying below the line of respectability.

Just be glad a fuse blew rather than knocking out the entire machine!
posted by majick at 12:55 AM on April 6, 2006

For what it's worth, I've had plenty of $20 power supplies and they never have been a problem until they got so gunked with dust (that I neglected to clean regularly) and then subsequently overheated.

Don't waste too much money on something which is subject to such finicky blowouts such as the one you had.

Disclaimer: I'm no PS expert, just a guy who builds his own computers and studies how they work.
posted by zhivota at 4:27 AM on April 6, 2006

StickyCarpet writes "Make sure the computer and the TV are plugged into the same power outlet."

Also if you've got this stuff plugged into a suicide strip the grounds might be floating because of bad connections in the bar. You can check this with a meter and a PC power cord.
posted by Mitheral at 8:29 AM on April 6, 2006

I'm having trouble piecing together exactly how your problem happened, but here is some very interesting background on what happens when you mix double insulated AV equipment (two prong plug) with grounded computer equipment (three prong plug): I'm Shocked - Why am I Getting a Tingle?

The introduction of an earthed item into a system made up of double-insulated gear is not unlikely, but it is likely to be the start of some "interesting" problems. Let's say you have a DVD player, two vcr's, a tv/monitor, a CD player, a cassette deck, a graphic equaliser, ...you get the idea. All these things are double-insulated and they are all connected to each other either directly or through one of the others. It would not be unreasonable to expect 0.1 to 1 mA (or even more!) of leakage from the combined equipment because the leakage currents simply add together. If you were to connect a digital multimeter between this pile of gear and the chassis of your new item, let's say it's a big brute of a surround sound amplifier which just happens to be earthed, you will measure a very significant ac voltage, probably at least half the mains supply or even higher. (Don't try to measure the current, you might blow up your multimeter if there's a bad fault).

Now let's say you have the metal shell of an RCA on an audio cable which is connected to an output on the unearthed gear in one hand and you steady the amplifier with your other hand as you start to plug it into an input. As soon as you complete the circuit between them you will get a mild electric shock. Depending on how moist your skin is and how high the leakage is this might be unnoticeable or it might cause you to say a bad word! Unless you have a bad heart it is unlikely to hurt you physically, but it can be quite unpleasant, especially when you don't expect it. Once the first connection is made and the leakage is flowing to ground, there will be no more tingles.

...

Ever wondered why they always say in the instruction manuals that you should disconnect the power when making or breaking connections to anything? Well now you know! It's to avoid shocks, huge hums, loud crackles and damage caused by high voltages being connected to sensitive inputs before the common/earth/ground side manages to short out the dreaded mains leakage (or your own static electricity build-up). It might be a nuisance, but it does avoid all these problems, and could save your life (or at least that of your other gear) if one of your double-insulated 'whatevers' happens to have a serious electrical fault or a bit more leakage than your other equipment can tolerate.

I believe I blew out an input on my VCR in this way.. Nonetheless, I have little experience with this particular issue.

It sounds to me like the composite video cable from the VCR was already connected to the PC so, the grounds on the two devices should have been connected already.. Having this problem crop up while adding a second connection is a little confusing. Perhaps there is a continuity problem in a cable. The cable might work for high frequency signals due to coupling capacitance, but the actual DC electrical connection could be broken.

I believe the reason for the two stage failure is easier to explain. ATX power supplies normally stay on even when your computer is off. When the output is shorted they will shut down in a special way, and you have to unplug them to reset them. If one of the main current carrying transistors failed due to over-voltage when the initial problem occurred, the power supply might have been able to go into short circuit protection mode properly. During start-up the short circuit protection might work differently so, when you powered it up the next time instead of switching off electronically it switched off mechanically.

Normally a power supply failure would not effect other PC components. In this case there is a chance more damage was done.
posted by Chuckles at 8:46 AM on April 6, 2006

Thanks for posting that Chuckles, it sounds like that is probably on the right track. (And now I know why some VCRs and so forth have given me little buzzes in the past.)

I installed the new power supply last night, as far as I can see everything is up and running as per normal - but who can say if there was some slight memory damage or something like that which will lead to long term instability.

And yeah, I plugged the sound in before turning the computer on... Once bitten and all that.

Mitheral, I've never heard the term, but yes, everything is connected to a single multiboard/suicide strip.

I'm not sure how to go about checking for a floating ground with my multimeter, and I'm a bit scared of it to be honest - I wasn't kidding when I said I'd had 230v a few times already, and I really don't like how it makes me feel. First the shock, with associated surprise/pain/numbing, but then the fear that my post-shock hyperactivity is actually a symptom that my heart has been damaged and is about to burn out its bearings.
posted by The Monkey at 5:20 PM on April 6, 2006

Oh, and Majick, it's even worse than you think - when I say dollars, I mean New Zealand Dollars, and when I say $20, I really mean $17+tax. So we're actually talking about a US$10 PSU.
posted by The Monkey at 5:25 PM on April 6, 2006