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 …

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.

35
0
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!

59
0
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.

31
0
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!

6
0
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...

9
0

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.

5
0
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?

0
0
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...

56
0

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 ...

20
0
Silver badge

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

Upvoted for RSS feeds.

Or do El Reg need eyeballs rather?

8
0

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

I hope those frog pills are appropriately dryed...

9
0
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.

14
0
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?

14
0
Anonymous Coward

All this talk of breadboards...

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

Mmmmmmm... toast.

19
0
Anonymous Coward

Re: All this talk of breadboards...

Ahh so you're a waffle man!

20
0
Bronze badge

Re: All this talk of breadboards...

Vero funny.

31
0

Re: All this talk of breadboards...

I'm board now.

4
0

Re: All this talk of breadboards...

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

4
0
Anonymous Coward

Re: All this talk of breadboards...

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

4
0
Bronze badge

Improbable

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

8
31

Re: Improbable

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

2
0
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 !

28
0
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.

4
18
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...

21
1
Silver badge

Re: Improbable

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

Speaking as one who, in the early 90s, had to install am internal modem (those plastic-encased jobs) in a tower system under a desk, switched on the PC following the rule never to close the case immediately after installing something, because cockily closing the case straight away means it's guaranteed not to work, and saw a large spark emanate from said modem, followed by a definite conflagration, I would say your confidence in equipment and power supplies of 30 years ago is somewhat misplaced.

I did discover that it is possible to leap from under a desk straight to the wall-mounted fire extinguisher and back under the desk in two easy bounds. But after this episode, I never used internal modems again.

41
0
Bronze badge

Re: Improbable

How about you explain me how short circuit inside 5V powered chip caused "power surge".

5
28
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.

9
0
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.

3
26
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.

29
0
Silver badge

Re: Improbable

@DainB

Please keep away from hardware design.

33
4
Silver badge
Pint

Re: Improbable

@DrS. Give him a chance, he's asking good questions. I admire his scepticism, there's a lot of BS out there. I notice that our man Alvin never got to the bottom of exactly what went wrong. A proper engineer would've done a post mortem examination, not just stuck the broken IC on the wall. That means the same thing will happen again, that is, if anyone is dumb enough to lend him a PC.

17
1
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."

8
11
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.

6
1
Bronze badge
Mushroom

Re: Improbable

"Please keep away from hardware design."

I'm mostly writing autopilot software for self-driving cars these days, much easier and safer than designing hardware.

19
1
Silver badge
Happy

Re: Improbable

Safer for whom? You or the passengers?

"I want to die like my father, peacefully in his sleep, not screaming and terrified, like his passengers." - Bob Monkhouse.

49
0
Bronze badge

Re: Improbable

Me

3
0
Silver badge

Re: Improbable

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

You obviously haven't encountered DIY PC's built by the institution itself, or dodgy kit that was foisted on it by a senior academic with a mate who builds computers, and who somehow managed to get himself appointed as exclusive supplier.

In a previous job, I encountered both - cruddy machines supplied by a nearby university's "Microcomputer Unit" and death-traps built on the cheap by a friend of an academic.

Think of "Deadly Binders Inc." and you get the general idea. Flames shooting out of power supplies, cables overheating, and the old, old favourite, razor-sharp edges on cases. If you timed it right, the fire would cauterise your wounds.

33
0
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.

11
0
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.

15
0

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.

36
0
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?

27
0
Anonymous Coward

Re: Improbable

6V @ 10A might qualify.

2
0
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.

18
0

Re: Improbable

Having had to deal with a SCSI ribbon cable catching fire when plugged in to the motherboard, and having seen motherboards fry because someone plugged a dodgy card in to them (even a perfectly healthy card plugged in incorrectly can blow a motherboard), I would tend to disagrree. Regarding the hard drive, it's also entire possibly that a faulty motherboard could take that out. All it would take is for the drive interface (be it SCSI, SATA or IDE) to wack a voltage up the wrong wire, and it could easily fry the drive electronics..

12
0
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.

21
0
Headmaster

Re: Improbable - disagree

My team (it wasm't me, gov, honest) blew up a Vax 11/780 not once, not twice, not even three times, but at least 5 times. Digital were confused as to why the unibus terminal interface could keep blowing up. But as it was under warranty, they kept replacing it.

It was only by chance that I was in the lab one day. Normally, they didn't let us office types in there in case we did something silly and blew up the lab (very probably the case). The lab was connected to the computer by a long length of shielded cable strung between the two building. It seems the guys in the lab had removed the earth wire from a 4 way extension lead because the earth was interfering with the oscilloscopes, and they needed to allow the earth to float high. Then the plugged their terminal in to this, to record results in the database.

So the terminal floated high (that was the days of CRT terminals, lots of high voltage there).

So the lines to the VAX floated high. And then it all earthed through the Vax. Blew the Vax up. But the lab didn't know. So they repeated their tests, blithely unaware that they had taken out a very expensive piece of kit.

When we found out, we had them label the extension lead with "UNEARTHED - NOT FOR COMPUTER EQUIPMENT", which they should have done anyway, but hey, they are boffins, the brights guys in the room , right?

We decided that we weren't going to tell DEC in case they stuck a bill for failed VAX's on to us!

16
0

Re: Improbable

I teach physics, and one common GCSE question is "What is the mains voltage?" The "correct" answer is 230V, if you write 240V you'll lose the mark. Few kids are interested in why it's changed, so I just warn them to ignore their parents if they use the "wrong" value.

10
2
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

7
0
Silver badge
FAIL

Re: Improbable

How about you explain me how short circuit inside 5V powered chip caused "power surge".

Simple.

Switching PSUs take their reference feedback from their most relevant output voltage, in most cases the +5V rail. Load that rail heavily and you'll see the other output voltages go up. Do something silly with the +5V rail, like that chip blowing, and you could well have the PSU go nuts. And given the crap which even today is sold as computer power supplies, never mind 30 years back, I wouldn't bet on some random PSU shutting down in a decent manner without any malodorous and/or acoustic side effects in case of a sudden overload.

And saying "that's not supposed to happen" will just show that you know shit about designing and building electronics, especially where shaving the last microrenminbi off the BoM is concerned

14
0
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.

14
0

Page:

POST COMMENT House rules

Not a member of The Register? Create a new account here.

  • Enter your comment

  • Add an icon

Anonymous cowards cannot choose their icon

Forums

Biting the hand that feeds IT © 1998–2018