Be careful with those teardowns, kids! - Page 1
Forum section: electronics / general

author: TerminalJack505
author_rank: Super Contributor
timestamp: October 10, 2012, 02:07:26 am
content: Teen electrocuted while working on unplugged computerQuoteApparently this happened when he took the PC's power supply apart. I'm guessing there was still a lethal charge on the mains cap(s).

author: Monkeh
author_rank: Super Contributor
timestamp: October 10, 2012, 02:12:36 am
content: Darwin at work.More wildly inaccurate bullshit from the supposed experts and the media, too.

author: FenderBender
author_rank: Super Contributor
timestamp: October 10, 2012, 02:38:14 am
content: Quote from: Monkeh on October 10, 2012, 02:12:36 amWhat did you expect?

author: bullet308
author_rank: Frequent Contributor
timestamp: October 10, 2012, 04:18:54 am
content: :-/Guess I need to post a sign over my bench... "When Working on Mains Stuff, Sit On Your Left Hand!"I am gathering that it is unlikely to get a fatal zap from a big cap with one hand involved.

author: Monkeh
author_rank: Super Contributor
timestamp: October 10, 2012, 04:27:01 am
content: Quote from: FenderBender on October 10, 2012, 02:38:14 amNothing less.

author: MrPlacid
author_rank: Frequent Contributor
timestamp: October 10, 2012, 05:29:23 am
content: Dave need to do a tear down of a computer powersupply immediately. Dave, give your opinion and advice on how to properly tear down power supplies.

author: IanB
author_rank: Super Contributor
timestamp: October 10, 2012, 05:48:37 am
content: Quote from: MrPlacid on October 10, 2012, 05:29:23 amWell see, there are these big round things that might have something written on the side of them like "300 uF 500 V". They might bite you if you touch their pins. It's a good idea to make sure you discharge them with a screwdriver or something before you go near them. (And watch out for the bang if you do that. It might startle you if it takes you by surprise.)

author: Bored@Work
author_rank: Super Contributor
timestamp: October 10, 2012, 06:05:02 am
content: Quote from: Monkeh on October 10, 2012, 02:12:36 amActually not. For a tech report in the media they were more accurate than usual. The website got it rightQuoteYep, "fancy themselves" is exactly the point. "fancy themselves" because they can download a ringtone to their cellphone, install some memory in a computer, manage to spell Arduino, or manage turn on the microwave. While in reality they have no clue.But the worst, they are ineducable. Just look here. Almost every week some idiot shows up, with an idea for an non-islolated power supply, with an idea to convert a computer SMPS into a spaceship, idiots promoting unsafe meters, idiots being proud that they got zapped at least 30 times. You name it, we got it here.And dare you you tell them that something is not safe and they should keep their hands off. Ha! No, they are entitled to it, because the "fancy themselves as technology experts".The videohttp://www.kctv5.com/story/19767067/teen-electrocuted-while? Yes, inaccuracies like "power storage box" and an expert who is just a box shifter. But the basics are still right, electricity is dangerous, and nature doesn't care if you feel entitled.

author: Dago
author_rank: Frequent Contributor
timestamp: October 10, 2012, 06:21:48 am
content: Having played a lot with electrolytic caps I'm fairly sure touching the mains filtering caps on a computer PSU would NOT result in "electrocution burns". Causing tissue damage requires a LOT of energy (cause humans are mainly water). I'd be willing to bet it was plugged on...

author: westfw
author_rank: Super Contributor
timestamp: October 10, 2012, 06:52:36 am
content: I'm under the impression that a typical computer power supply will rather quickly discharge its main capacitors when unplugged, and it takes something special (like a picture tube or strobe circuit) to retain a lethal charge for more than a few moments. True, or not?That's not to say that you can't get unlucky with relatively low voltages.I don't know what "electrocution burns" as seen in an autopsy might be. Most (US) mains-caused electrocutions are going to be less than visible burn-causing events, but they could leave behind something that medical professionals might call "burns."And it's not to say that I've never done anything stupid. I especially remember the AC cord I sliced through with a knife, that turned out NOT to be the cord I had just carefully unplugged. (It ruined a good kitchen knife!)

author: ThievingSix
author_rank: Contributor
timestamp: October 10, 2012, 07:02:31 am
content: QuoteSounds like you fancy yourself to be an expert at safety?, i bet you didn't pick up all that knowledge from a textbook. But of course, dare i not tell you such a thing.QuoteMaybe "some idiot" is posting that design so he can get feedback and learn about what's wrong with it and how to improve it. Why in the world would he post a perfectly engineered design that meets all safety standards? Oh that's right, I forgot the standards book costs over $400, and we're not all professionally trained engineers supported by the company credit card. If "idiots" such as Michael Faraday, Nikola Tesla, Thomas Edison and J.J. Thomson, to name a few, hadn't been "playing" around with potentially dangerous stuff we wouldn't even know as much as we do about electromagnetism today. If anything we learn more from our mistakes than our failures, and ultimately every safety precaution should be taken, but at the end of the day, if you don't take risks you don't learn anything.To add, i thought i'd do a little investigative work, and i found that there were no records of death by police on the day mentioned "August 16th" and the news article cites KCTV as the source, since when did a news station become a reliable source??QuoteSo essentially the police said what was actually quoted, and the reporter made up the rest. To add to that it sounds like they've simply dug up an old story to fill "news time" with a little fear mongering. They mentioned nothing on the boy himself, his family, any other circumstances surrounding his death, or even the death report. Hmnn i smell a fishy news story.http://gsh.cityofshawnee.org/WEB/PoliceCMS.nsf/vwContent/PoliceDailyActivity?OpenDocument&Start=1&Count=610&Expand=2

author: MrPlacid
author_rank: Frequent Contributor
timestamp: October 10, 2012, 07:06:39 am
content: IanB, I was thinking Dave should follow the news so he can stand out more. That is what those guys who appeared on the news over and over again do. I think people would start scouring youtube for how to open computer power supplies videos, so it would be a good time to make such a video. And Fast.BTW, is a 300uF 500V the actually value of capacitors in a computer power supply? I would thought it would be much larger.

author: MrPlacid
author_rank: Frequent Contributor
timestamp: October 10, 2012, 07:10:57 am
content: Quote from: Dago on October 10, 2012, 06:21:48 amI am with you. My thought is that the boy had it plugged in and electrocuted himself. Guardian disconnected it and rather claim that it was unplugged. If they don't they'll probably be charged with child endangerment or something.

author: retiredcaps
author_rank: Super Contributor
timestamp: October 10, 2012, 07:12:33 am
content: Quote from: westfw on October 10, 2012, 06:52:36 amA few months ago I worked on a LG 2234 monitor. It was made in 2008 so it is fairly "modern" SMPS lcd power supply.The big cap retained its 165V DC charge overnight after I unplugged the lcd from the power bar before going to sleep. I had to discharge it manually. Now maybe this is a one off, but I always check the large filter capacitor with a multimeter before working on anything now.A couple of ATX power supplies that I have worked on takes several minutes to discharge. Most, but not all, SMPS power supplies that I have worked on in lcd monitors discharge the big cap in usually less than 1 minute.

author: sorin
author_rank: Frequent Contributor
timestamp: October 10, 2012, 07:14:20 am
content: QuoteI dont thing that 300V 500uF capacitor can do this...

author: grumpydoc
author_rank: Super Contributor
timestamp: October 10, 2012, 07:33:46 am
content: QuoteEnergy = 0.5 CV2so 22.5J. Water (we're mostly water) has a heat capacity of 4.2J per g per °C so, no, that much energy would be incapable of producing much in the way of significant deep burns - maybe some superficial ones.That said electrical burns often look much less serious on the surface because all you see is the relatively small entry and exit points for the current - the current path spreads out in the body and considerable heating and damage to tissue can take place.The stored energyisenough to interfere with the heart rhythm though if he was unlucky enough to manage to get the current path to flow across the heart - that's really the theory/folklore behind the "sit on your left hand" rule.As has been observed these caps can hold high voltages for some time, especially if the power supply is faulty and there is no load to drain the charge. In this case it seems likely that the PSU was faulty as it was being "stripped for parts".

author: vk6zgo
author_rank: Super Contributor
timestamp: October 10, 2012, 07:35:00 am
content: Quote from: westfw on October 10, 2012, 06:52:36 amActually picture tubes are highly overrated when it comes to danger from them remaining charged.The EHT voltage is quite high,& they do retain charge for quite a while,but their capacitance is too low to supply a lethal shock.It would be quite easy to see how much charge is available in the caps after the mains rectifier in a switchmode power supply.I would have thought,nasty,maybe life-threatening,but electrical burns seems a 'bridge too far".

author: tom66
author_rank: Super Contributor
timestamp: October 10, 2012, 07:43:44 am
content: Typical capacitor in a genuine 300W power supply without PFC: about 1200uF 200V, times two. Caps are in series to make a 600uF 400V cap, the centre tap is used as part of the half-bridge converter (I think.)Typical capacitor with PFC: around 220uF (low 250W Dell supply) to 1000uF (1200W beast - IBM ThinkCentre RAID server PSU), at 420-450V.The caps would typically discharge within about 2 minutes due to the constant ~0.5W lost in the standby circuit and other parts of the PSU such as bleeder resistors.Both would cause a bad shock if touched: if you had a weak heart, maybe even fatal... but it wouldn't cause burns.@BoredAtWork: You seem bitter at people? I started off in electronics as your typical computer nerd. Now I'm doing a MEng.

author: David_AVD
author_rank: Super Contributor
timestamp: October 10, 2012, 08:16:03 am
content: A faulty switch-mode power supply (that didn't start) can certainly hold a charge for some time on the main filter capacitor. Some units have a bleed resistor, but not all.

author: amyk
author_rank: Super Contributor
timestamp: October 10, 2012, 08:31:29 am
content: My bet is on it being plugged in, or he was particularly lucky to have bridged the cap across his heart with both hands. Probably the former, as the amount of stored energy isn't what would cause the burns described.Quote from: vk6zgoThe danger of those is not from the shock itself, but from the surprise making you do something else, like drop the set or smash your hand against the neck. I've gotten static shocks on dry days, those can be tens of kV and the secondary effects are what really makes you do something stupid, not the shock itself.

author: Bored@Work
author_rank: Super Contributor
timestamp: October 10, 2012, 08:33:37 am
content: Quote from: tom66 on October 10, 2012, 07:43:44 amNo, I am not bitter at people. I am annoyed by these fruitcakes who think they are an engineer just because then can rip off the latest Lady Gaga abomination and load it as a cell phone ring tone, or who glue Arduino to everything. And I am annoyed by those companies like Make who encourage them and rip them off.Just read this thread. It didn't take long and you have a bunch of fruitcakes coming out of the closest telling you that the primary cap in a SMPS can't kill you. Because, wait for it, they survived it. Hint, those who didn't survive it can't talk any more. Hint 2, if you get zapped you failed, nothing to be proud of. They should sit in a corner, being ashamed of themselves, think about how to avoid it in the future, instead of proudly announcing that this is all harmless. And it does not matter if the boy in the current case was killed by it or something else. That doesn't change the fact that it is dangerous.

author: G7PSK
author_rank: Super Contributor
timestamp: October 10, 2012, 09:04:35 am
content: At one time many large high voltage caps had a built in discharge resistor,on others there was one across the terminals, what has happened to these.

author: LoyalServant
author_rank: Regular Contributor
timestamp: October 10, 2012, 09:04:40 am
content: I have to agree with @boredatwork...I have tried to show a few young fellas a thing or two and basic safety like keeping one hand behind your back.They won't do it.. they are invincible.I guess in a way they win a Darwin award?Some of these articles from supposed experts are painful to read too....

author: T4P
author_rank: Super Contributor
timestamp: October 10, 2012, 09:45:10 am
content: the last ATX PSU i take apart has massive HV warnings everywhere ...And even a "no user serviceable parts inside" on the outside ... Yeah sure ...Give them 15mins max, it won't keep much voltage much longer, unless it's so broken ... Well, always use a capacitor discharging pen to be sure (I used to have giant HV stickers then i dumped them)As it's a a PSU for a PC it's not going to keep much of the voltage unlike what you see with a monitor ... they can run on the standby for like 5mins after flipping the switch (I'm talking about a Samsung 4 series 165Watt 22" TV not Monitor!)

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 10, 2012, 12:05:38 pm
content: I'm skeptical too of the facts presented in this article.As others have pointed out you're not going to die of burns from these caps.Maybe you can die from heart failure from these caps but you probably would have more chance of being struck by lightning.Actually I think the whole electrical safety thing is a bit overrated, as far as I know the most dangerous things are cars, rich food and alcohol, but I'm not about to do the risk analysis.Remember:Don't turn it on, take it apart.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 10, 2012, 12:08:51 pm
content: Quote from: amyk on October 10, 2012, 08:31:29 amBack in the very early days of picture tubes,they were quite dangerous if dropped,but almost anything from the early 1960s on is very unlikely to implode catastrophically.If you smash your hand against the neck,you may break that part of the tube & give yourself a dangerous cut,but it won't implode.Back when I used to fix TV Picture Monitors & TVs,if we junked a CRT,we were supposed to knock the end off the neck to render them "safe".We were also supposed to stand back & do this when the dumped tube was already in the dumpster.It was incredibly hard to do,even with direct hits on the neck with lengths of galvanised pipe or star pickets.No implosion if you did,just a gentle sigh.On one occasion ,we had some tubes from very old 1950's 12" monitors--theydidgo bang when we chucked them in the dumpster.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 10, 2012, 12:32:03 pm
content: Quote from: Bored@Work on October 10, 2012, 08:33:37 amI would pretty much say "Bravo" to the sentiments in both your postings.I'm just a bit concerned I might be included among the "fruitcakes",as I was a bit doubtful about the lethality of such a capacitor.I did "hedge my bets" though,as I said:-"I would have thought,nasty,maybe life-threatening,but electrical burns seems a 'bridge too far".There are documented cases of fatalities by getting across the big oil filled caps in Broadcast Transmitters.Many years back,I got a nasty belt from one of these.Luckily,it wasn't fully charged,& the"zap"was through a big filter choke,the reactance of which limited the current.Obviously,those things have lower capacitance,but are charged to a lot higher voltage than a switchmode primary cap.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 10, 2012, 12:39:29 pm
content: i smell religion thread.... people died while they are asleep... sleeping is dangerous!

author: N2IXK
author_rank: Frequent Contributor
timestamp: October 10, 2012, 12:42:51 pm
content: QuoteYou heard the "expert"--don't try to FIX anything, or take the risk of LEARNING ---your duty is to CONSUME!

author: David_AVD
author_rank: Super Contributor
timestamp: October 10, 2012, 12:47:56 pm
content: Quote from: vk6zgo on October 10, 2012, 12:08:51 pmWhen I was a kid (about 1975 I think), I pulled apart a couple of dead TVs that I got from the local appliance retailer. I had stored the CRTs underneath a table in a downstairs room.One evening, we heard an almighty bang and went down to find glass spread all over the room. One of the tubes had imploded (I guess) but the other was still perfectly intact! What a mess to clean up.

author: tom66
author_rank: Super Contributor
timestamp: October 10, 2012, 12:50:36 pm
content: Quote from: Bored@Work on October 10, 2012, 08:33:37 amI wouldn't call those people engineers either. Still, I have no problem with makers/hackers/tweakers/what-ever-else'ers, as long as they keep it to themselves. Creativity is good. About half the students in my ELEC1130 class have never touched a soldering iron. They don't know ohms law or how to use an opamp. I wonder how long they'll last. They are MME students though - music multimedia electronics - not sure what horrible combination that is!I wouldn't call a primary cap safe by any means. I have been stupid before. Shocked myself on a cap on a big LCD TV. I had assumed, after the fuse had quite spectacularly blown across the room (which in itself was a danger) that the cap would be discharged, so when one hour later I went to remove it, it would be safe. Nope! Got a 160V shock - turns out the TV discharges it down to 160V then dies - and it sits like that for a very long time. Maybe I got lucky - it was only 160V. It definitely hurt. (It also turned out that it had a separate standby bridge rectifier, and that part shorting out is what made the fuse go but it didn't discharge the main cap.)However, you learn from your mistakes. I learnt my lesson. Maybe I'm lucky. I think they're mostly survivable, but it's kind of like a car accident: sure, you can survive it if you're lucky (and if it's not too violent), but you'll do you best to avoid it. Yet, we still don't do the #1 thing that avoids it: not driving a car, because it brings so much advantage for the risk of being seriously injured or even killd.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 10, 2012, 12:58:02 pm
content: There is a lot of bogative stuff in this story. A $20 psu? A pic in the orig quoted article of a Corsair 650. I own a few of them and I know I didn't pay $20 for them.Coincidently, I started buying the corsair psu's because I got tired of the twenty dollar cheapys with crap noisey fans that rattle. Added up the cost of putting in jaycar replacement fans a few times and I could have bought the better psu with a nice, quiet fan.I've now got an army of these monsters. I don't think about psu's anymore.The kid just pharked up. No one's fault. They should just say that. I can understand that it is good for selling newspapers though...

author: SeanB
author_rank: Super Contributor
timestamp: October 10, 2012, 03:23:27 pm
content: Built in UPS? If he touched one lead while at the same time had the other hand on the heatsink connected to the other terminal then bye. It might leave a burn mark on the finger that touched the cap, and you find that discharge resistors in power supplies often quietly fail open circuit.

author: G7PSK
author_rank: Super Contributor
timestamp: October 10, 2012, 03:41:14 pm
content: The most dangerous place to be has to be the cemetery. There are more dead people there than any where else, ergo it must be hazardous to one health.I do know of one kid that was killed by a TV tube imploding, it was in the mid sixty's he climbed into the yard of the local TV. repair man (whose name was Collin Crisp and ran the radio club at the school I was attending) and started to throw bricks at the old tubes stacked there, one of them imploded and a piece of glass cut his femoral artery, the kid bled to death before any one could help him.

author: free_electron
author_rank: Super Contributor
timestamp: October 10, 2012, 05:40:13 pm
content: Quote from: bullet308 on October 10, 2012, 04:18:54 amadd this to it :- lift feet off ground- make sure no other part of body comes in contact with any object that can close an electric loop with the right hand

author: tom66
author_rank: Super Contributor
timestamp: October 10, 2012, 05:43:54 pm
content: Installing a solid RCD is also a very good idea.Many household installations now require it, but older houses don't always have them.It's a lifesaving bit of kit, but you must understand its limitations.

author: free_electron
author_rank: Super Contributor
timestamp: October 10, 2012, 05:44:21 pm
content: Quote from: Mechatrommer on October 10, 2012, 12:39:29 pmLife is defined as a medical condition to which 100% of the patients finally succumb...

author: Monkeh
author_rank: Super Contributor
timestamp: October 10, 2012, 05:51:15 pm
content: Quote from: tom66 on October 10, 2012, 05:43:54 pmFixed that for you.

author: XynxNet
author_rank: Regular Contributor
timestamp: October 10, 2012, 06:14:21 pm
content: The kid needn't be electrocuted. Going into shock or having a heart attack because of the unexpected electric discharge can be enough.

author: tom66
author_rank: Super Contributor
timestamp: October 10, 2012, 07:02:04 pm
content: Quote from: Monkeh on October 10, 2012, 05:51:15 pmIn the UK, sure - but it's not necessarily a requirement in other countries.

author: Pentium100
author_rank: Frequent Contributor
timestamp: October 10, 2012, 07:35:53 pm
content: Caps can store the voltage for a long time. My tube headphone amp has two big caps (470uF and 330uF) for the anode supply and they are big enough so that when I switch off the amp, the tubes cool down faster than the caps discharge. That is why I put a bleeder resistor and also a little neon lamp - when the lamp turns off there is less than 50V on the cap. When I need to repair it or whatever, I always wait for the lamp to turn off before doing anything (and even then I measure the voltage with a meter, the lamp is just a convenience so I don't have to continually measure the voltage waiting for the cap to discharge).Somebody should have taught the kid about safety - charge a small capacitor (so the shock is not lethal, but still pretty painful) and let him touch it - after that he would have learned to not touch capacitors that could be charged.

author: Mediarocker
author_rank: Contributor
timestamp: October 10, 2012, 07:47:19 pm
content: Honestly I was kinda expecting it. Kids these days DON'T respect power. They aren't taught to respect and be careful around electricity.I mean, you gotta start somewhere... but I gotta call darwinism on this. I was 7 years old and did the same thing. How am I still alive? I made sure my tools didn't touch the caps. I used insulated tools, and when I started tinkering with the PSU I discharged the caps beforehand using a resistor.

author: tom66
author_rank: Super Contributor
timestamp: October 10, 2012, 07:51:24 pm
content: That reminds me of one very nice feature Panasonic include in their plasma TVs. Plasma TVs use 200V to make the panel light up with typically over 2000µF on the bus for this voltage (50A peak currents at 200kHz... and older sets had at least double, if not more.) There's a little LED tied in series with the voltage detect divider. It stays lit until the caps discharge below about 30V, at which point you risk only a minor shock if anything.Repairing stuff is good. Just have a safe respect for electricity. It can kill you. Maybe you'll get lucky. Maybe that's why we're only allowed to use 30V in the labs, max (though the teachers don't realise the power supplies have a button for selecting series configuration. I accidentally had the output at ~50V into a logic IC, and I smoked it - I was running it at the maximum 20V but had the other supply set to 30V when I bumped the switch which is very sensitive.)

author: asbokid
author_rank: Regular Contributor
timestamp: October 10, 2012, 09:15:51 pm
content: Quote from: TerminalJack505 on October 10, 2012, 02:07:26 amProbably just made up. The meeja at its worst. Or if the poor sod really did die, what probably killed him was some deadly heart disorder that he knew nothing about.

author: grumpydoc
author_rank: Super Contributor
timestamp: October 10, 2012, 09:32:38 pm
content: QuoteIt's always possible - some will be obvious at Post Mortem eg HOCM (hypertrophic obstructive cardiomyopathy) others don't show up on gross pathology eg Brugada.However the "electrical burns" bit still doesn't _quite_ add up.It's all speculation and guesswork though - the fact remains that anything with a high voltage capacitor can yield a nasty surprise even with the power off and anything connected to the mains can readily supply a fatal shock. If you don't realise this you're playing with your life.

author: LaurenceW
author_rank: Frequent Contributor
timestamp: October 10, 2012, 09:56:17 pm
content: I have had three THUMPING GOOD electric shocks in my life - all from direct AC mains and all when I was much younger. I musta learned, 'cos I'm still alive and no shocks for a couple of decades. Except... I did have the minor embarrassment of posing as a discharge path to a large cap in a cheap (no bleed resistor) electronic flashgun, a few weeks back. THAT woke me up.But - could a kid (even a standard issue, 21st century stupid kid) REALLY kill themselves just from the energy stored in a partial charged PSU cap?? I'd put money on "no"It isjustpossible, I suppose, that the mee-dee-yah have cocked up their facts.Anyone with medical experience care to suggest how much current (and importantly, for how long) it really takes to STOP a heart? Once we've got that the maths should be easy enough.$20 dollar PSU?? Remind me not to buy spare parts fromTHATguy!

author: asbokid
author_rank: Regular Contributor
timestamp: October 10, 2012, 09:59:14 pm
content: Quote from: grumpydoc on October 10, 2012, 09:32:38 pmSure. I got a bolt from a PC PSU weeks after its last use. Certainly woke me up! no need for a perm that weekcheers, a

author: TerminalJack505
author_rank: Super Contributor
timestamp: October 10, 2012, 10:37:41 pm
content: If this individual did, in fact, have an outstanding health issue then it could be a big factor in his death. Supposedly (and I don't necessarily subscribe to this) outstanding health problems are frequently the cause of death in taser-related fatalities.

author: meanpc
author_rank: Contributor
timestamp: October 10, 2012, 10:52:02 pm
content: Fruitcake checking in. I'll strap an Arduino to almost anything.

author: Psi
author_rank: Super Contributor
timestamp: October 11, 2012, 01:02:58 am
content: It's perfectly plausible that he went to short out the cap with a screwdriver but touched both sides of the cap as he did. The current flowed through his heart for a fraction of a second, killing him. Then the screwdriver made contact with both sides of the cap which covered his fingers in carbon and metal from the vaporized screwdriver tip.Anyone viewing the body would see he died while working on something electrical and would notice charring on his fingers and assume they're electrical burns.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 11, 2012, 01:17:22 am
content: It sounds like something for Mythbusters to test. The largest caps I have seen in a PC power supply were in an old HP (back when they were good stuff), and those were a pair of 330uF, 450V. (The PC I'm using right now has even bigger ones, but it's definitely not stock...) I'm sure they can kill if fully charged, but it would take a few orders of magnitude more energy to burn more than a little spot. Maybe someone (photonicinduction?) can try discharging capacitors of various sizes and voltages into a piece of scrap meat to see what it would take to make a significant amount of it char.Chances are, I don't think it's likely for the caps to stay charged for long since the standby circuit and voltage dividers would discharge them pretty quickly. (All bets are off if the power supply is bad, however!) The bleeder resistors are not used much nowadays in order to increase the standby efficiency.I suspect the power supply was plugged in at some point (maybe he wanted to test it?) and maybe he even figured out the green to black trick to start it up. (I knew that at about age 14 or so, but I was smart enough to check high voltage capacitors before working and I especially kept a distance from exposed mains connections.) Then if he touched the primary heatsink, he'll get both a deadly shock from the mains and severe RF burns from the high frequency AC.EDIT:QuoteThen they do the autopsy and notice that it's only a "surface" burn.

author: asbokid
author_rank: Regular Contributor
timestamp: October 11, 2012, 01:33:34 am
content: Sheesh! Bit of a morbid thread is this!"It's being so cheerful as keeps me going!"

author: amyk
author_rank: Super Contributor
timestamp: October 11, 2012, 07:47:48 am
content: Quote from: LaurenceW on October 10, 2012, 09:56:17 pmMore than it takes to cause fibrillation, which is what causes death. Defibrillators give a controlled shock to get all the muscles to start beating in sync again, and they put out a lot more energy (few hundred J) than two PSU caps.

author: westfw
author_rank: Super Contributor
timestamp: October 11, 2012, 08:35:01 am
content: QuoteAh! Yes, I didn't think of that. Very good point.QuoteMoore ("Electrostativs") said 10J for electrostatic discharges (ie anything high voltage enough that the discharge is "short.") I'm not sure if mains voltage is quite high enough for that (but it's pretty close.)The rule of thumb is "20mA" across the body.You can be unlucky; Pacemakers run years on a relatively small battery, and having one get messed up can surely kill you. Actual nerve impulses are very tiny.I've always been more worried about slicing an artery on sharp sheet metal.

author: Pentium100
author_rank: Frequent Contributor
timestamp: October 11, 2012, 09:48:22 am
content: It just so happens that I am currently repairing two PC power supplies - a Q-tec 350W (cheap) and Coba 400W (a bit better) - both have bad caps (Coba needs new HV (1000uF/200V x2) caps as the current ones get quite warm) and the Q-tec has a bad fan and a shorted diode (probably because of the fan). Now that I replaced the caps (and the diode with a more powerful one, since I could not get the exact part quickly), the Q-tec will probably be better than new. It's still cheaper than buying new PSUs.

author: Mediarocker
author_rank: Contributor
timestamp: October 11, 2012, 10:01:49 am
content: Quote from: amyk on October 11, 2012, 07:47:48 amI call bullshit. Sorry but a typical switched mode PSU primary cap contains enough voltage and current in it to stop your heart. It only takes a couple of milliamps to cause fibrillation.http://www.physics.ohio-state.edu/~p616/safety/fatal_current.htmlhttp://van.physics.illinois.edu/qa/listing.php?id=6793http://van.physics.uiuc.edu/qa/listing.php?id=1339It takes about 100 mA of current to kill you. Even currents under 75mA your breathing can stop completely, which can kill you.Provided certain conditions are met, an input cap on a PSU has more than enough current to kill you.A basic US household switching power supply has over 350VDC in it's primary. 350VDC WILL go through your skin which (when dry) has a resistance of 1,000 to 100,000 ohms depending on thickness and if it's burned or open. Burnt skin allows for an open wound which will readily conduct more current. Granted that the internal resistance of the human body varies depending on muscle content and other factors... Considering this was a kid I'm willing to think:A. He has no calluses on his hands and the skin on his hand is fairly thin.B. I'm not assuming he has much muscle as most teenagers aren't very musclebound... most certainly those inclined to work on their family computer.So assuming those factors we'll take his total resistance:Rtotal = Rskin(in) + Rinternal + Rskin(out)Perfect condition:Rskin(in)= 1000ohm (This is the "Input" resistance)Rinternal= 100ohm (being generous here as more muscle = more resistance)Rskin(out)= 1000ohm (This is the "output" resistance)I=V/R so I=350VDC/2300ohm350VDC/500ohm=.15ABest case Scenario (Older gentleman with lots of muscle mass and calluses):Rskin(in)=100,000ohmRinternal=1000ohmRskin(out)=100,000ohm340VDC/201,000ohm= 0.001APlausible scenario:Rskin(in)=1,500ohmRinternal=400ohmRskin(out)=1,500ohm340VDC/3,400ohm=.1ALikely scenario:Rskin(in)=2,000ohmRinternal=500ohmRskin(out)=2,000ohm340VDC/4500ohm= .0755 repeatingEnough to stop breathing.Depending on the person, it'smore than enough to kill you.Regardless we don't have his skin resistance measurements, we don't know if he had any open wounds which would have readily conducted the current, and we don't know anything other than what they released in the news. Saying a cap CAN'T kill you is negligent and voiding respect for electricity.ANYTHING. ANY CURRENT can kill you. Period. It just depends on all the factors and if the moon is lined up.People survive lightning strikes... but capacitors can kill you. Just depends if it's your day or not.

author: grumpydoc
author_rank: Super Contributor
timestamp: October 11, 2012, 10:09:26 am
content: QuoteSort of - the idea with a defibrillator is to give a massive enough shock to stun all the heart cells, then hopefully the heart's normal pacemaker can take over once more to produce co-ordinated activity. That's why you continue CPR after giving the shock - because it takes a while for electrical activity to recommence.So looking at the energy supplied by a defib (360J normally) and saying "well a PSU cap is much less than that" isn't valid - the defib is designed to overwhelm.If you hit the 200ms or so "relative refractory period" from the peak of the T wave to the start of the next cycle you have a very high risk of causing ventricular fibrillation with even a quite small shock.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 11, 2012, 10:50:13 am
content: I don't know if this is more accurate than the other report but it says here that the official cause of death was electrocution. Doesn't mention burns.http://digitallife.today.com/_news/2012/10/09/14321140-teen-electrocuted-while-taking-apart-unplugged-computer

author: westfw
author_rank: Super Contributor
timestamp: October 11, 2012, 04:09:48 pm
content: QuoteHow do you figure? I get 120V * 1.414 = 170V, which explains the 200V caps I see in most US-only supplies. (Most computer supplies are "Universal input", designed for up to 240V input.)

author: Pentium100
author_rank: Frequent Contributor
timestamp: October 11, 2012, 07:04:31 pm
content: Quote from: westfw on October 11, 2012, 04:09:48 pmOld PSUs (the ones that allow you to select between 120V and 230V and have no PFC) configure the caps as a voltage doubler when set in 120V mode. In 230V mode the caps are in series so 200V per cap is enough. Regardless of the input voltage, the primary will see the same voltage, about 330V.PSUs that have active PFC can automatically adjust to the input voltage, new PSUs with passive PFC are usually 230V only (or I guess in the US they would be 120V only but probably still have the voltage doubler).

author: SeanB
author_rank: Super Contributor
timestamp: October 11, 2012, 07:16:44 pm
content: Active PFC does mains voltage compensation, so can wotk on 90-260VAC. The capacitor then is normally always charged to 390V if the PFC is working, irrespective of the mains input voltage.

author: tom66
author_rank: Super Contributor
timestamp: October 11, 2012, 07:29:56 pm
content: One other possibility is that while it was live, he touched some switching portion of the power supply.At 300Vp-p at 30-60 kHz hurts a lot more than 50/60 Hz or DC.

author: grumpydoc
author_rank: Super Contributor
timestamp: October 11, 2012, 09:59:50 pm
content: QuoteInteresting, I'm not about to try but I wonder if the "surface effect" is strong enough to avoid passage of current through the heart at these frequencies.

author: tom66
author_rank: Super Contributor
timestamp: October 11, 2012, 10:24:18 pm
content: Quote from: grumpydoc on October 11, 2012, 09:59:50 pmVOLUNTEERS WANTED FOR STUDY ON SAFETY.FREE CAKE FOR ALL APPLICANTS. TAKE ONE.Another thing to note is many switching power supplies have transients on the switch node. Especially flybacks for the standby supply, upwards of ~700V for many designs (hence why you need a good 800V FET and flybacks aren't used over about 50-60W.)

author: necroscope
author_rank: Regular Contributor
timestamp: October 11, 2012, 10:46:25 pm
content: Could it have possibly have been an old crt monitor and he never discharged the flyback befire trying to pop the anode out with a screwdriver,That could surely Kill and would be an easy mistake for a lot of adults let alone a child.

author: N2IXK
author_rank: Frequent Contributor
timestamp: October 11, 2012, 11:38:55 pm
content: The charge stored on the anode circuit of a CRT monitor isn't going to kill you. The voltage is quite high (25-30 kV), but the capacitance involved is a few hundred pF at most, and the stored energy is correspondingly low. There is no actual filter capacitor in most cases, just the capacitance between the internal and external conductive coatings on the CRT itself. Yeah, you will get a zap like a really bad carpet shock, or a spark plug wire. The involuntary muscle contraction can cause problems if you jerk your hand away and hit a more dangerous voltage, smash the CRT, or scrape your hand on the cabinet and cut yourself, etc.The low voltage power supply in a monitor or TV is far more dangerous than the HV supply to the CRT. 150V or so of regulated DC, capable of an amp or so of current, usually derived from a non-isolated "hot chassis" type mains rectifier arrangement. Similar to the primary side of a switchmode power supply.

author: necroscope
author_rank: Regular Contributor
timestamp: October 12, 2012, 12:51:26 am
content: Quote from: N2IXK on October 11, 2012, 11:38:55 pmThanks i never knew that,i have always been told by arcade machine techs the flyback can store a charge big enough to kill you even after being switched off for a day,I guess they were exagerating,still i wouldnt like to cop a hit from one,Still if the young lad had a weak heart maybe a hit like that could be enough to stop the ticker.

author: Mediarocker
author_rank: Contributor
timestamp: October 12, 2012, 01:45:03 am
content: Quote from: Pentium100 on October 11, 2012, 07:04:31 pmThank you for pointing that outAlso, There is something else to point out... what if the kid had really sweaty hands? The high salinity of the sweat would conduct current more readily and possibly grant the "perfect" condition I listed. So as I mentioned before, this is most certainly possible.

author: grumpydoc
author_rank: Super Contributor
timestamp: October 12, 2012, 07:06:17 am
content: QuoteLOL, I liked that.Lest it not be clear I'm not advocating anyone try it. Also in case anyone is unclear about my own views I'm in the "this shit is dangerouseven ifyou know what you're doing" camp.ButI do just wonder whether those incautious enough to be in the "I've had loads of whacks from SMPS's and I'm still breathing so itcan'tbe all that bad" group might have been saved by a degree of surface effect.Medical diathermy/electrocautery units rely on the surface effect to avoid killing the patient while allowing local current density high enough to heat tissue. In fact they vaporize tissue when used in cutting mode. Having seen them used countless times as a student I can attest that the patient does not have an immediate cardiac arrest when they are used. They use AFAIK AC in the range 300kHz - a few MHz at (I think) a couple of hundred V. As an aside this is one of the things that makes ECG (or EKG, if you're that way inclined) machines expensive - their inputs must be totally electrically isolated and be able to record microvolt signals but ignore (and be protected against) 10's or maybe 100's of volts of RF current or defibrillator current.But I digress....Does anyone know or have a reference on the surface effect in human bodies and at what sort of frequencies it becomes significant. Idle curiosity you understand - no experiments please

author: Psi
author_rank: Super Contributor
timestamp: October 12, 2012, 07:42:51 am
content: Quote from: NiHaoMike on October 11, 2012, 01:17:22 amThere hadn't been any autopsy at the time the article about burns was written.The whole "burns" thing was the initial report from the scene.

author: G7PSK
author_rank: Super Contributor
timestamp: October 12, 2012, 07:57:00 am
content: I have had a belt from the fly back of a 26 inch colour TV that was on. I was adjusting the picture at the time when my hand got to near the tube connection, the charge threw me across the room but did not leave any burn marks.Now the output from a magneto on a paraffin tractor that leaves a nice black burn mark but does not kill.I don't think that it is just voltage or current that is required for lethal effect but duration as well.I have collected a few shocks from power plant of many KW output but due to never having closed my hand around leads and always wearing insulating foot wear I have managed to survive and become ever more cautious.The best way I know of to get shock while working with electrical equipment is to be in abject terror of it, you have to be ever vigilant of the risks but fear will lead to as many mistakes as over confidence.The only way that a capacitor in a piece of computer equipment will kill is if the person has a dodgy heart or it is still connected to the mains and the person grabs something in such a manner that they cannot release it.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 12, 2012, 08:05:51 am
content: I have been just trying to get a handle on (quantify) the dangers of the non Mains parts of the SMPS's.I looked around for some coroners cases of people dying from sub 400v dc electrical shocks. I couldn't really find details of any, but often the coroners don't have a clue whether the cause of the shock is AC or DC anyway, so they don't state it.I did find that sub 70v electrocutions in the US were almost non existent.this is old but gives some detailshttp://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&ved=0CEMQFjAF&url=http%3A%2F%2Fwww.lookupandlive.org%2Fmaterials%2FNIOSH%2520Electrocution%2520Study%252098-131.pdf&ei=GM53UJupHcmViAfzgIHwCw&usg=AFQjCNEr9JWYKpyF0XmucOJdz7tADDw7IA

author: notsob
author_rank: Frequent Contributor
timestamp: October 12, 2012, 08:10:47 am
content: Here's two examples of shit going wrong that I know of,1. A motor mechanic was repairing a tractor and shorted the metal wrist band of his watch across the rear of the starter motor whilst tightening up a nut/bolt with a spanner, the watch band was melted on his wrist in quick time (12V at the starter can deliver several hundred amps). Lesson - remove all metal from your hands/wrists.2. A TV repair tech, (this was back in the 60's) was working on a B&W TV when his tie contacted the EHT (yes they wore ties long ago), unfortunately the fashion at that time was for gold coloured stripes in ties, and they were metal stripes, he had a lot of small circular burn marks around his neck as a result.

author: G7PSK
author_rank: Super Contributor
timestamp: October 12, 2012, 08:20:13 am
content: I have just been trying to find the regulations regarding electric fences which some years ago when I was involved with them were limited by law to not more than 15 joules output. Now I cant find any regulations but health and safety recommend not more than 5J but units of 30 J are available in the UK but they work in a different way from the old type units the charge is stored and only released when grounded by an animal rather than a constant pulse.But from the old regulations I have always assumed that it took more than 15 joules of energy to kill a normal person which is why the legal limit was there.

author: Psi
author_rank: Super Contributor
timestamp: October 12, 2012, 08:50:35 am
content: Quote from: G7PSK on October 12, 2012, 08:20:13 amThere is no real "level" because every person is different.Not just in skin/flesh resistance but with regard to how strong their heart is and how much current it can take before giving up.There's also the physical shock of the event (as opposed to electrical).It's possible to scare a person to death so for anyone who's never experienced an electrical shock before the physical shock of getting a serious electrical shock can be pretty extreme.It would be interesting to see statistics on deaths from electric shocks and look for any correlation between people who had past experience and knew what electric shocks felt like vs people who didnt.

author: nukie
author_rank: Frequent Contributor
timestamp: October 12, 2012, 01:50:39 pm
content: It's really sad to have such cases happening, then the regulators would make stupid laws and the DIY would be in difficult times.If it was me, I probably be on the news for at least 7 times. I am not boasting I cannot be killed by electricity but as a matter of fact my brain cells are probably half fried. I haven't been zapped for many years due to extra care, but the last time was a few weeks ago. I was fixing a Samsung LCD TV surprise surprise it's not due to bad cap. I took all the precaution, bleeding off the main cap as it was 330uf 400v. I did it over and over again as I try to diagnose the problem. Then due to fatigue I reach out to feel the heatsink if the diodes are heating up... That was the most painful zap I ever experienced. The pain lasted for at least 5 minutes my whole arm felt sore perhaps due to muscle reaction. That was as close experience as commit suicide, it really scared me properly.When I was young I was mains electricity curious. I trip the fuse often, as a result my butt always get rewarded with a cane. I hate christmas lights, they give varying intensity of electrocution. The closer you are to the plug the stronger the zap. Once when I was a kid, I stood on a table to plug in my NiCD charger, I got zapped and it threw me off the table and I was lucky I landed on my butt not my head. But I was unconscious for a few seconds luckily there was no one around to administrator the cane. I learned to keep my tiny fingers away from those unshielded pins. Nowadays it's kids friendly they are shielded pins on the mains plug.When I was a teenager, I manage to get a job in a TV repair shop. There I learned a lot from the seniors regarding mains safety. Of the 3 years I worked there as a part time apprentice, never once I saw someone got zapped.This kid misfortune probably due to his weaker heart. I have seen a lot of cheap power supply but most of them have cheap input caps and they have quite high leakage so they self discharge quickly. You can measure the voltage drop as soon as it is unplugged. He's probably operating a name brand supply with a good input cap but without a bleeder resistor? I don't think he's stupid, he's just as curious as everyone here. If he was trained and killed then he is stupid and unlucky.

author: SeanB
author_rank: Super Contributor
timestamp: October 12, 2012, 03:30:00 pm
content: A big CRt can have an anode capacitance of quite a few nanofarads, which is pretty high energy when charged to 30kV.

author: G7PSK
author_rank: Super Contributor
timestamp: October 12, 2012, 04:04:04 pm
content: I have just found this, which is very useful information on electric shock, it would apear that under certain conditions a higher current is more survivable than lower current.http://www.physics.ohio-state.edu/~p616/safety/fatal_current.html