back to article 'The capacitors exploded, showering the lab in flaming confetti'

Welcome to the very first edition of “Who, me?” a new Reg column we hope will prove as entertaining as our Friday On-Call tales of tech support gigs gone wrong. In Who, me? we’ll celebrate the times techies stuffed up, the lessons learned and the career consequences. To kick things off, meet “Alvin”, who “In the early '90s …

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  1. Anonymous Coward
    Anonymous Coward

    Isolation

    Like coding, where you need to plan for unexpected inputs, so you should with hardware.

    After toasting a ZX Spectrum whilst building some custom breadboard hardware plugged into the expansion bus, I too learnt this lesson.

    Don't plug prototyped electronics into expensive computers without building a very robust isolation circuit between the two, so only voltage limited data signals can ever get thru no matter how bad you screw up your build.

    1. Sorry that handle is already taken. Silver badge

      Re: Isolation

      And don't give your undergrads your most important and irreplaceable piece of office equipment for use as a "prototyping" tool!

      1. Niall Mac Caughey
        Mushroom

        Re: Isolation

        Don't give your PC to undergraduates? Undergraduates I can handle!

        In days of old I ran a lab which had a recently-acquired and very expensive robot arm. It was controlled by a very large and complex PCB covered in ICs, all of them soldered in place. It also had external I/O, with optocouplers for the I and relays for the O. The software ran on an Apple II and the pair were connected via a ribbon cable.

        I foolishly took a day off and returned to find that a senior lecturer had demonstrated the I/O features to his class by connecting 230V AC to one of the optocouplers.

        Suffice it to say that it took me a very long time to fix the Apple and the robot itself became a decorative paperweight due to the prohibitive cost of the replacement board.

        1. Anonymous Coward
          Anonymous Coward

          Re: Isolation

          "a senior lecturer had demonstrated the I/O features to his class by connecting 230V AC to one of the optocouplers."

          Hence my suggestion "without building a very robust isolation circuit between the two", where 'very' means however large you think that isolation should be to protect against it, think user stupidity, and then multiply that value by 20!

          1. DropBear Silver badge

            Re: Isolation

            ...except the extension connector on the back of the Spectrum was gold-plated PCB traces ("fingers") of the CPU's bus, as-is, and building something that would properly galvanically isolate it while still doing bidirectional I/O at 3.5MHz, while not impossible, was at the time not a practical option for 99.999999% of its owners. Anything less would only have been buffering, hopefully protecting against back-driving the bus but not against any "funny" voltages. So what you actually did instead was design stuff carefully and properly the first time*!

            * let's not get into risk multipliers caused by the appropriate PCB-edge connector being unavailable and fabricating your own from two other pieces of PCB comb-slit into flexible-ish individual contacts for each finger, with a bit of wire soldered onto for better-than-PCB-on-PCB contact. Yes, I did get a Kempston joystick interface out of it and yes, I was definitely wearing my brown pants when I first powered the thing up...

          2. Stuart Halliday

            Re: Isolation

            Did have a twat hand over his son's car remote control unit to me.

            He wanted it repaired.

            Every metal transistor in it was exploded out. Every capacitor/resistor cracked in half, etc.

            He thought attaching the pp3 9v battery clip to the live and neutral of the Mains would substitute for a missing battery.

            1. Lotaresco Silver badge

              Re: Isolation

              "son's car remote control unit ... He thought attaching the pp3 9v battery clip to the live and neutral of the Mains would substitute for a missing battery."

              I take it that the Darwin award still applies if, instead of removing oneself from the gene pool, one removes one's progeny. Darwin-by-proxy?

  2. Shadow Systems Silver badge

    "Who Me?" should be the RSS feed title as well.

    The RSS feeds for this & the On Call posts should be titled as much to make them easier to find among the regular news.

    I know if it's from "Simon" then it may be one I'm particularly interested in reading, but if it actually described themselves as "On Call", "Who Me?", or (in the case of Mister Dabbs) "For the weekend Sir?" then we could write an email rule to flag them, play the air raid warning siren, & advise all those around us to leave us alone while we got our weekly infusion of silliness. "Don't talk to me, I'm reading Who Me? again." should be a valid defense for when the H&S folks come running to find out why you've strangled another coworker.

    *Cough*

    I mean, ummmm... for when you've just come up with a BOFH worthy form of retribution to enact when that nosey coworker three desks over comes trundling past to stick his honker where it doesn't belong.

    *Shifty eyed nervous glances around*

    I mean, ummm, that I would appreciate it if the RSS feeds could begin with the fact that they contain an On Call, Who Me?, or similar recurring article content, the better for me to know that such an article is available for me to eagerly consume.

    *Pure, sweet, & innocent smile*

    "I'm sorry Your Honour, I have no idea where that lake came from & I'm not responsable for the crater in which it formed."

    /runs away as those nice young men in their clean white coats chase after me screaming for me to take my frog pills...

    1. lesession

      Re: "Who Me?" should be the RSS feed title as well.

      And whilst you're at it ...

      Those of us who get the daily email digest (well, me, anyway) would greatly appreciate it if 'On Call' and 'Who, Me?' could be included in the digest articles titles, for much the same reasons as @Shadow Systems highlights ...

    2. Anonymous South African Coward Silver badge

      Re: "Who Me?" should be the RSS feed title as well.

      Upvoted for RSS feeds.

      Or do El Reg need eyeballs rather?

    3. collinsl

      Re: "Who Me?" should be the RSS feed title as well.

      I hope those frog pills are appropriately dryed...

    4. allthecoolshortnamesweretaken Silver badge

      Re: "Who Me?" should be the RSS feed title as well.

      "... a valid defense for when the H&S folks come running to find out why you've strangled another coworker.

      Despite possible boni for reducing headcount, you want to watch that. It's habit forming.

      1. Anonymous Custard Silver badge
        Headmaster

        Re: "Who Me?" should be the RSS feed title as well.

        Or better yet, how about a specific feed and/or email with just BoFH, On Call, SFTW and now Who Me? So it can be specifically marked as high importance, must read immediately and a possible solution to get me through until beer o'clock on Friday?

  3. Anonymous Coward
    Anonymous Coward

    All this talk of breadboards...

    I feel like I've wandered into a Talkie Toaster convention!

    Mmmmmmm... toast.

    1. Anonymous Coward
      Anonymous Coward

      Re: All this talk of breadboards...

      Ahh so you're a waffle man!

      1. Headley_Grange Silver badge

        Re: All this talk of breadboards...

        Vero funny.

        1. choleric

          Re: All this talk of breadboards...

          I'm board now.

      2. The Central Scrutinizer

        Re: All this talk of breadboards...

        And I definitely don't want any smegging baps, baguettes or bagels!

      3. Anonymous Coward
        Anonymous Coward

        Re: All this talk of breadboards...

        A waffle man? Perhaps yes, perhaps no. I can't make up my mind.

  4. DainB Bronze badge

    Improbable

    There's nothing in PC, modern or 30 years old, that could have caused this.

    1. Mine's a pint

      Re: Improbable

      Agreed, although we are assuming that the PC was in a sturdy case as used at those times. Still.....

    2. malle-herbert Silver badge
      Coat

      Re: Improbable

      Really ?

      I've seen some of those 30 year old switch-mode power supply's that could put out more than 20 amps at 12 volts that DIDN'T have over-current protection built -in like those more modern versions !

      1. DainB Bronze badge

        Re: Improbable

        Power socket on your wall can deliver much more and yet on some reason it does not burn down your house every time you plug something into it.

        1. Evil Auditor Silver badge

          Re: Improbable

          DainB, I don't know where you live but power sockets around here are limited to 10, 13 amps. If it's a 400V socket it may deliver up to 16 amps. (For higher currents we are talking about serious hardware.)

          The power supply in my PC could deliver 70 amps @ 12V - enough to roast a few things...

          1. Omgwtfbbqtime Silver badge

            Re: Improbable @EvilAuditor

            I have a 32A supply in the garage. Comes with SWMBO having one of those new-fangled electrikery cars. I assume its 240v as we only have the one line coming in to the meter.

            1. Evil Auditor Silver badge

              Re: Improbable @Omgwtfbbqtime

              In my garage too, I have a 32A supply (@400V), which I could even upgrade to about 70A if needed. But it's rare to find those in office spaces, powering some PCs.

              1. StargateSg7 Bronze badge

                Re: Improbable @Omgwtfbbqtime

                "....n my garage too, I have a 32A supply (@400V), which I could even upgrade to about 70A if needed. But it's rare to find those in office spaces, powering some PCs. ..."

                WHAT THE BLARNEY are you powering?

                That's 12 800 Watts (32 amp) and 28 000 Watts (70 amps) !!! That's the type of energy we use to power our plasma-arc welding systems or our mainframe busses! What exactly is the sort of computer system are you using to need over 12 THOUSAND WATTS+ of power in a small office?

                An old VAX 780 or IBM Zxx series mainframe?

          2. DainB Bronze badge

            Re: Improbable

            And ? How is that contradicts what I said ?

            Hint: your power supply might be able deliver 70A but those 2 mm wide and few microns thick power lines on PCB won't be able to pass them without melting. And that where your roasting typically ends.

            1. HieronymusBloggs Silver badge

              Re: Improbable

              "those 2 mm wide and few microns thick power lines on PCB won't be able to pass them without melting"

              Having seen many prototype PCBs go up in flames (despite being made of flame-retardent material) I would say that is not necessarily the end of the problem. As has been pointed out, old switch mode power supplies didn't always have reliable current limiting.

              As for 5V being too low to cause a current surge, have you ever used an arc welder?

              1. DainB Bronze badge

                Re: Improbable

                "As for 5V being too low to cause a current surge, have you ever used an arc welder?"

                On a PCI bus ?

                No.

                1. Missing Semicolon Silver badge
                  Happy

                  Re: Improbable

                  You forget two things:

                  1) 30 years ago, the science of Switched-Mode power supply design was less advanced than now. Plus, really cheap "offshore" PSU's were often a bit marginal even when not abused.

                  2) Cascading failure. Imagine you connect +12V (or -5V) to +5V, you would fry a lot of components powered from the +5V rail on the motherboard . If they then fail short, the PSU can then go into overload. Or even the PSU itself might not tolerate +12V on the +5V rail. Once caps start failing in the PSU, the high fault currents will start killing semiconductors, which could overvolt or reverse polarity the remaining caps. While there may not necessarily be actual flames, I could well imagine much smoke, and an acrid smell.

                  1. Symon Silver badge
                    Pint

                    Re: Improbable

                    "30 years ago, the science of Switched-Mode power supply design was less advanced than now"

                    More or less when Linear Technology introduced the LT1070, which made it a whole lot easier. Not easy, just easier. I've posted this link before, the 'troubleshooting hints' are worth a read, even today. Hint no.12 is a favourite.

                    http://cds.linear.com/docs/en/application-note/an19fc.pdf

                    1. Phil O'Sophical Silver badge

                      Re: Improbable

                      Linear Technology introduced the LT1070, which made it a whole lot easier. Not easy, just easier.

                      I like the TSR 1 and friends, not cheap but really easy. Drop-in replacement for a 7805, no heatsink required.

                  2. PNGuinn Silver badge
                    Coat

                    Re: "Plus, really cheap "offshore" PSU's were often a bit marginal even when not abused"

                    Y'mean theyr'e not now?

                    I always buy a cheap psu for my computers. One from a reliable manufacturer with decent specs. One that's unlikely to give me weird data errors due to spikes and noise and one with proper protection so that its unlikely to blow up random bits of the machine.

                    Unfortunately, they tend to be somewhat more expensive. Cheap at the price.

                    I can still get a new case complete with psu delivered to my address from halfway round the world by UK distie for around 25 squid. Apart from the lacerated arteries assembling the machine, what else do you think I'd get?

                    >> Thanks - it's the one with the unused first aid kit in the pocket.

                2. HieronymusBloggs Silver badge

                  Re: Improbable

                  ""As for 5V being too low to cause a current surge, have you ever used an arc welder?"

                  On a PCI bus ?

                  No."

                  DainB, no matter the rated current of a PSU, if there are large filter capacitors on the output it can supply many times the rated current while those capacitors discharge, even after any upstream short circuit protection has triggered. That is essentially how a spot welder works, so I'll stick to my welder analogy.

                  You have clearly been very lucky with all of your circuit prototyping. My experience has been quite different.

          3. Symon Silver badge
            Facepalm

            Re: Improbable

            DainB, the old PCI connector has +12 and -12V on it. https://en.wikipedia.org/wiki/Conventional_PCI#Connector_pinout

            My guess is that the tosspot shorted one of those, probably the 12V rail (it's got the highest power rating) to either the 5V or the 3.3V rail. I guess the 5V rail, as that could happen if he counted from the wrong end when wiring up the 5V, and getting the 12V pin instead. Either that or one of his 'mates' added an extra wire to the breadboard. That shoved 12V back down either the 5V rail, or down all the logic signals through the 74138's protection diodes, or maybe both, with hilarious consequences.

            I've connected loads of stuff directly to PC's ISA, PCMCIA and PCI busses. Never broke anything. However, I can count. Twice. Chipmunk's on the wrong course.

            1. DainB Bronze badge

              Re: Improbable

              Or he might as well have been testing his 8-segment indicator driver with 220V AC, or as many idiotic scenarios as you can imagine trying to prove your point, but none of them change the fact that it is improbable that short circuit on PCI bus can turn PC into "either glowing, or smoking, or both."

            2. Robert 22

              Re: Improbable

              That would seem to be a plausible scenario. If 12V ended up on a lower voltage rail, I could imagine the capacitors on the lower voltage rail going. Alternatively, if the computer was old enough, there would have been a negative supply that one could short a positive rail to.

              This reminds me of an experience I had in the early 1980s. My group had acquired a memory expansion board for a Floating Point Systems array processor. Unknown to us, there were slightly different versions of this exact model that differed in the backplane power pin assignments. On being told by the sales guy that "you just plug it in - what's hard about that?" we did just that with seemingly catastrophic results - smoke and, on further investigation, melted PCB traces. Miraculously, we were able to revive it after improvising repairs for the burned traces - it seems that the large current flow that resulted when the negative bias supply on the memory chips became forward biased was distributed over a sufficiently large number of chips to avoid destroying them.

          4. Ian Johnston Silver badge

            Re: Improbable

            The sockets in my house will all do 30 or 40A, for a while, anyway. It's the plugs which limit things to 13A if you haven't had the foresight to replace the fuse with a short length of copper bar.

            1. PNGuinn Silver badge
              Flame

              Re: Improbable

              Yeah, had to do that to prevent a meaty arc welder consuming 13 A fuses. (Bit 'o fuse wire soldered across the 13 A fuse to give it a bit more "delay".)

              Worked fine until the welder developed a shorted turn. Took out the 30 A fuse for the ring in the main fuseboard rather than the one in the plug.

              Remember, there's (almost) always something meater further upstream ... £30 A in the consumer unit, 80A or 100 A main service, 400A in the substation ...

              As my first governor used to say "So it steams a bit."

              Reminds me of back in those days in a test gear department ... One of the Test Room boys brought in a bit of kit that "didn't work". I whipped off the plug cover to discover that one of his coworkers had replaced the fuse with a 1 inch length of pot spindle - nylon pot spindle.

          5. Gordon JC Pearce

            Re: Improbable

            > I don't know where you live but power sockets around here are limited to 10, 13 amps.

            No, your sockets are fused at 32A. They are limited by the total resistance of the wiring and impedance of the transformer that feeds your house, so it's safe to say they're limited to a few thousand amps.

            1. terrythetech

              Re: Improbable

              Hmm, electricity suppliers aren't keen on you taking thousands of amps without being warned. That is why they stick a fuse on the feed to the house. Back in the day when I lived in a bedsit we regularly blew the 60A electricity board fuse every winter as we tried to get warm with bar fires. Annoying as it is a call out job to get it replaced :(

              1. psychonaut

                Re: Improbable

                completely possible. i had a machine in not so long ago. it had a powercool 550 watt psu in it.

                the sata power cables were melted, it absolutely stank, the inside of the case had black smoke / scorch marks on it, the sata connectors for the dvd-rw and SSD were burnt to a crisp, they fell to bits in an avalanche of disgusting, smelly, acrid burnt plastic.

                i think what had happened was a layer of dust and crud had built up on one of the sata power connectors over time. its one of the ones that isnt fully moulded, it had little blanks removed on the back of the connector. (for testing the voltage i suppose?). the sata connector was really tight up against the chassis, a mm ish away.

                i think the whole thing shorted through the crud buildup against the case.

                I didn't believe the customer when they told me what had happened, but after seeing it, i was converted. quite spectacular by all accounts when it happened. I'm not sure about massive flames and flying debris across a whole room, but enough to scare the shit out of you. and 30 years ago, PSU's were indeed very different. (for different read "absolute dog shit"), so yeah, why not.

                i now make sure all the psu's i buy dont have those little blanks taken out own the back of the power connectors..... corsair cx 550m's these days.

          6. JeffyPoooh Silver badge
            Pint

            Re: Improbable

            EA noted, "...power sockets around here are limited to 10, 13 amps."

            "Limited" isn't quite the right word in this context. A current "limiter" typically means a circuit with effectively instantaneous current limiting action. If you have a current limiter that limits current to 10 amps, then you'll probably be unable to extract 10.5 amps. This doesn't apply to household wiring.

            Fuses are not bad, speed-wise, but can still take SECONDS to open. Magnetically-assisted circuit breakers (Square-D QO series for example) are pretty quick with huge overloads, but can still allow significant overload to remain connected for many seconds. The older thermal breakers are hopelessly inept, taking a Very Very Long Time™ to open. They protect the wiring from catching fire, but will sit there stupidly warming up slowly while you draw 50% over current for perhaps a full minute (YMMV).

            1. StargateSg7 Bronze badge

              Re: Improbable

              May I suggest a Varistor (aka Voltage Dependent Resistor) in-between your vital circuits.

              for the bigger cicuits, an entire regulated power supply system with BIG VARISTORS need to be installed.

              I think the SurgeX/Ametek company now sells such beasts. Get the biggest one you can afford. The surge gets shunted into a heat-producing/taking resistor that will survive MULTIPLE big hits!

              There are SMALLER or LARGER lightning fast varistor setups that have microsecond and less switching/shunting speeds but you will pay a FORTUNE for them!

              1. Danny 14 Silver badge

                Re: Improbable

                our UPS tell us we get between 240 and 250, usually 244. the site gets 3 phase.

          7. Jeffrey Nonken Silver badge

            Re: Improbable

            P=EI. When you down-convert the voltage, you get a proportional increase in amperage to maintain conservation. Minus conversion inefficiencies, ofc.

          8. swm

            Re: Improbable

            I once designed and built a rack of TTL logic powered by a 5v 100 amp (or was it 500 amp - I forget) switching power supply wired with welding cable. I once hot swapped one of the cards without turning off the power and there was a noise like a firecracker - POW! followed by a tinkling sound. Investigation showed that one of the chips had totally exploded down to the pins and sent shrapnel cascading down the rack. Replacing the chip fixed it but I never did figure out why the chip exploded.

            Another time I pulled a large prototype card plugged into an extension card plugged into a NOVA computer without powering down the computer. This succeeded in making the NOVA unusable. I noticed that the -15v power run on the back plane was next to a system bus line so I spend a day replacing all chips in the NOVA that touched that line.

            I was awarded an "ataboy award" for that one: "you collect three ataboy awards and it makes you a leader of man with super powers, the ability to walk on water etc." One "aw shit" wipes out all of the ataboy awards though.

        2. Korev Silver badge

          Re: Improbable

          Taking the UK as an example, 230V * 13A = 2990W. Almost three KW verses a part not designed for it isn't a fair contest.

          1. Symon Silver badge
            Alert

            Re: Improbable

            @Korev, athough the UK nominal mains voltage is 230V, it's still 240V really. They simply changed the tolerances specification to match up with that Europe. Shove a voltmeter into the wall socket if you don't believe me!

            http://www.twothirtyvolts.org.uk/pdfs/site-info/Explanation_230Volts.pdf

            "Because of the change in 1995 in nominal supply voltage (U0) from 240 V ±6% to 230 V +10% to - 6%, calculating load current from kW or kVA rating is made more difficult. Whilst the nominal voltage was officially changed, the actual range stayed much the same and the actual distributed voltage to homes and other premises remained unchanged. Manufacturers may state equipment ratings at 240 V. In determining current demand from kW or kVA demand, the voltage at which the demand is calculated may be used and this may be 240 V and not 230 V."

            p.s. Don't do things that random strangers on the internet tell you to do.

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