Big mistakes we or someone else did with electronics. - Page 1
Forum section: electronics / general

author: firewalker
author_rank: Super Contributor
timestamp: June 05, 2011, 08:04:16 am
content: So.... What mistakes/stupid things with electronics do you vividly remember? Equipment destruction e.t.c.Once upon a time I was teaching in the uni's electronics lab. I was fooling around with a screw driver I was holding (it was one of my first teachings and I was nervous). I noticed a capacitor array on one of the benches (a presentation for a small Telsa coil). I don't really know exactly why but I thought it was discharged (and it should be!). I sorted it with the screwdriver. God... The bang was loud... As I can recall it was about 2000 joules.I really wouldbn't like to be in the shoes of the guy that programed Mars Global Surveyor or Ariane 5 (although it wasn't a single persons mistake).Alexander.

author: Psi
author_rank: Super Contributor
timestamp: June 05, 2011, 10:50:57 am
content: Quote from: firewalker on June 05, 2011, 08:04:16 amIf you hadn't discharged the cap bank someone might have touched it and died.So i wouldn't really call it a mistake.Call it a safety check that proved very informativeit wasn't safe at allOnly big mistake i can think of that i've done was this one time i was testing lightbulbs and i couldn't work out why they would always burn out when i powered them up. Definitely had the right voltage, it was written on the side. I couldn't get any of them to work, tried 20 or so.It was later on that i was told they were expensive magnesium flash bulbs

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 05, 2011, 11:23:15 am
content: The most common mistake that burns your money mostly ,Is when you operate as hobbyist one FM broadcast transmitter with double MRF-317 as output transistors,and you connect to it as antenna, your own home build (good for nothing) solution.Big mistakes its not the accidentals events.But mostly the mistakes that you do,when you are trying to cut corners in situations that normally you cannot.

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: June 05, 2011, 05:59:06 pm
content: Quote from: firewalker on June 05, 2011, 08:04:16 amThat's what everyone always thinks, and it's not an unreasonable assumption. But that's also why you ALWAYS store high-value caps like that shorted. If it has screw terminals, you should store it with a short, period.A physics teacher in college showed a similar demo (something like 'this mass dropping from 1m stores 100j, the same energy in this capacitor. You can see how it'd crush your head and kill you, but it's not as obvious that the cap can'). While putting it away, he specifically mentioned that it should be stored shorted for just that reason.

author: reflowman
author_rank: Contributor
timestamp: June 06, 2011, 12:46:55 am
content: Way, way back (TMS1000 was just making an appearance) my then boss had spent months developing a big TTL project to sweep away all the old analogue stuff our company depended on and pave the way for a bright new digital future.At the crucial presentation to the head honcho (a rather unsympathetic chap), my boss had total brain failure and connected the beefy power supplies the wrong way round.Every chip on the board (there were 100's of them) blew out in quick succession; it sounded like a machine gun. A deathly silence ensued as a blue pall of smoke rose in the air; I effected a shamefully rapid exit and my boss left the company not long afterwards. Shame, he was a very bright chap actually. Happy days though!Bob

author: Rufus
author_rank: Super Contributor
timestamp: June 06, 2011, 02:19:42 am
content: While commissioning a new control system install had problems with a circuit breaker tripping out.Eventually managed to narrow it down to a fault in this cable

author: Joshua
author_rank: Regular Contributor
timestamp: June 06, 2011, 02:46:19 am
content: That connector kinda looks like the connectors the military use to connect the tail lights of a trailer to their humvees...

author: Ferroto
author_rank: Frequent Contributor
timestamp: June 06, 2011, 02:55:03 am
content: I once accidently reversed the polarity on a cmos 4001 series quad 2 in nor gate.I hooked up a battery to it and nothing happened. I checked the voltage and it was fine. I then left it for awhile then when I got back to it I hooked it up to the +5v rail of an old ATX power supply with 30 amps on it.There are now char marks on my breadboard. It was at this point that I realized I was reading the wrong data-sheet.

author: copiertalk
author_rank: Contributor
timestamp: June 06, 2011, 03:32:36 am
content: What big mistake?I watched a guy one day bend over a color unit while working on it and his necklace came out of his shirt and blew an expensive DC-Controller.We were working on a monster old 208v unit and I watched a guy brush against a noise filter with his wedding ring.I no longer wear jewelry.Over the years I have blown expensive boards, melted screwdrivers and gotten knocked out being thrown back while working on things.

author: Lance
author_rank: Frequent Contributor
timestamp: June 06, 2011, 06:49:25 am
content: I once plugged a circuit with some 555 timers in it that was supposed to take 12 volts into the mains. That was somewhat entertaining.

author: Psi
author_rank: Super Contributor
timestamp: June 06, 2011, 10:02:17 am
content: Dad once told me a story of a guy at the airport many years ago (~40) who went to pick up some sort of large unit off a trolley (it was some sort of radar unit i think, about the size of a small tv).But the unit was powered and had ~400VDCon the back.When he grabbed it his muscles contracted and he threw the thing over his head and it landed behind him ~10meters away.

author: flolic
author_rank: Frequent Contributor
timestamp: June 06, 2011, 10:35:01 am
content: Quote from: Kiriakos-GR on June 05, 2011, 11:23:15 amYou should build SWR meter first

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 06, 2011, 10:58:56 am
content: Quote from: flolic on June 06, 2011, 10:35:01 amIf you found any that measures and the true impedance of the antenna it self,send me a telegraph.

author: tekfan
author_rank: Frequent Contributor
timestamp: June 06, 2011, 12:36:28 pm
content: A very nice collection of just such incidents:http://www.edn.com/channel/Tales_From_The_Cube.phpNot a mistake, just age:http://www.diyaudio.com/forums/solid-state/154884-bad-day-capacitor.html

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: June 06, 2011, 01:06:42 pm
content: Hardware: I once reached inside a 5kW HF transmitter without realising it was on and got a really painful burn in my thumb. I used to be a single operator and worked it in CW mode - it sort of makes you complacent after awhile. At that moment the transmitter was auto adjusting the antenna tuner box (size of a small fridge) at full power. RF burns are very different from mains AC or DC power. It creates a sort of burnt pinhole through the skin and heals very slowly.Firmware: After a programming glitch in a PLC firmware 6 station press (each printing cylinder weighs about 2 tonnes) suddenly started to go backwards.

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 06, 2011, 01:21:12 pm
content: I was working near 480V. Some idiot on a fork lift backed into me and almost crushed me against the wall. In trying not to die, I inadvertently got 480V across my pinky finger. It cooked pretty well. In about at the knuckle and out near the nail. I have no ill effects other than a good scar at my knuckle. But it was a while for my body to deal with the killed skin and muscle tissues.

author: Lance
author_rank: Frequent Contributor
timestamp: June 06, 2011, 03:21:15 pm
content: Quote from: sacherjj on June 06, 2011, 01:21:12 pmI don't understand how people who can't operate machinery like that manage to survive. I've been fairly lucky in that I've only ever gotten a second or two of line current.

author: A-sic Enginerd
author_rank: Regular Contributor
timestamp: June 06, 2011, 05:15:34 pm
content: Most difficult to deal with: Senior project in college. Team of 5. One of the bench power supplies we were using was a hand-me-down that came for free. Had old school analog readouts. Also had a range switch that gave you either 6V max, or 12v max. One of our team mates was working on bringing up his board for his part of the project. He was focused on having various probes and connections all correct and reached up to flip the power switch on the supply. When nothing happened he looked up and flipped it again. Well, the first switch he flipped turned out to be the range switch. Blew some nice craters in our hard to come by FIFO memory chips.Most entertaining: Same project. On a different board, a different team mate was bringing up a newly tossed together backup board we had to try and verify an issue we were seeing. For this case, the power connections were just a couple pins we would clip onto with alligator leads. He got them crossed. Blew traces off the board.Most expensive: One of the chips I worked on some years ago. We were using the ARM IP core from the chip vendor. (we design the chip, they handle having it fabricated, and we use some of their generic IP). Well, turns out both the person on our end that was responsible for dropping the ARM into our chip, as well as everyone on the vendor side that was supposed to check all this kind of stuff - missed the fact the enable signal for the JTAG port was strapped to disabled state. Only possible fix is to spin the chip. At that time, a spin - just for the mask generation alone, ran a few hundred grand. That doesn't account for any engineering overhead, actual fab process time, etc.Fortunately, I've never had a life threatening case. Although, I've been bit a few times by automotive and small engine ignition systems. Yeah, that smarts.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: June 06, 2011, 05:28:12 pm
content: Many years ago whilst servicing a chain of restaurants, the illustrious general manager was worried about the rising costs of incandescent light bulbs. So he cottoned on to a large quantity of light bulbs that were apparently intended for use in trains. The problem, I believe, was the bulbs were rated 40 volts not 40 watts.I don't remember the exact specifics unfortunately, but I do remember seeing a slew of new bulbs being installed. Man it was so bright in the room for a few seconds(240v).. then it went dark.

author: jahonen
author_rank: Super Contributor
timestamp: June 06, 2011, 05:32:34 pm
content: Quote from: Kiriakos-GR on June 06, 2011, 10:58:56 amHow about this one (well, not a SWR meter exactly)? (sorry for poor picture and off-topic). A VNA can measure the matching of an antenna from transmitter perspective, in terms of impedance or return loss, whatever. So antenna can be tuned precisely before the real transmitter is used.Regards,Janne

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: June 06, 2011, 05:35:40 pm
content: I actually have a couple of good ones:Personal: We had a big beefy ATX power supply that we had bought to test some high-current motors. We calculated that, for the worst test case, we might need 100A, so we got a Corsair with one single 100A 12v rail.Trying to make a clean job of breaking out the wires, I took everything apart and wired up a terminal block screwed into the outside of the case such that, with no leads screwed in, you could install the PSU into a computer and use it as normal. Problem was, the inside was packed tighter than a duck's butt. Some fine engineering in that thing, for sure.So somehow, one of the screws I was using to fix the terminal block to the case went in a bit too far and shorted the case to the +12v rail. Of course, the outputs are all floating so that was fine, ish. The problem came when I was trying to track down some noise in our linear regulator. I hooked a scope up to +- 12v (ground to negative), and I'm sure you can guess what came next. Connecting ground to the -12v line (currently 12v below building ground) created a loop through the chassis of the scope.Of course, the power supply had no problem driving a half ohm load at 12v. For about half a second we wondered why the lead sparked when we connected it. Then the 22 gauge test lead caught fire.Took a while to figure out where the short was, but after cutting the bolts flush to the required length, everything worked just dandy. Didn't damage any equipment either, only dignity.Work: We made an LDPC error corrector. Everything was going great, so we spun version two with some tweaks. Got it back a few weeks later, had the test guy wire it up in the lab and it just wouldn't correct anything. The part worked fine for any checkable metric - configuration worked fine, power wasn't a problem, every electrical check was great but it just output nonsense. it took about 4 weeks before someone finally figured out that nobody told the test guy that version two used a new parity check matrix. The code generation software was totally incorrect for the part.

author: Neilm
author_rank: Super Contributor
timestamp: June 06, 2011, 06:44:27 pm
content: I was working with a guy who was showing me how to take a motor control unit to bits as there was a problem with some of the other circuits in it. It had a large capacitor bank at 100V. The guy I was with decided not to power it down and discharge the caps before undoing some of the screws. One of them fell onto the cap bank.One mistake I did was on a servo control system. (in my defence it was about 3:00AM). I had to make a minor change to some control logic. One of my collegues suggested that I disable the servo while I do this but it was such a minor change I felt I didn't have to bother. (it ment opening up another unit). Logic re-programmed, I enabled the servos and BANG the unit moved at full demand into the end-stop. The result was a dent in the roof and me having to live with my comment just before I pulled the servo control enable. "I know what I'm doing".Neil

author: ivan747
author_rank: Super Contributor
timestamp: June 06, 2011, 09:29:32 pm
content: I almost failed the final project of a course in lineal electronics I took a couple of years ago. (well, I lost 5% of the points) It was an audio amplifier around the TDA2040. I had everything working inside the case of an old ATX power supply. It sounded great and was powerful enough to drive the speakers easily.There where some challenges building my project: we had to do it in one day because my colleague didn't live near me, it was all done in a veroboard and we got the board done, but some things where still missing like the enclosure and some other details I can't remember. I have my lab in my room so I couldn't let anyone in (to messy and weird). We didn't have enough light and the soldering iron was (and now is even more) terrible. To make things worse, the +/- 12V transformer was outputting 15V, this is what happens when you decide to go for the cheap things. That, combined with the decoupling caps, got as high as 20V, which is the absolute maximum rating for the TDA2040. We had to find a way to lower the voltage. I first paralleled some 7815 and 7915 regulators and it went VERY wrong. At the moment I didn't know why but now I realize I got the pinout for the 7915 regulators totally wrong and didn't have the load resistors in place. I ended up with the regulators heating up. With 3 days left I had to do something. I googled for some high current diodes and put them in series, effectively droping the voltage to an acceptable range.It was the best project on the class (as long as you didn't figure out what where the diodes for) except for the physical construction quality of an 120V square wave inverter some guys did. They work building inverters so they already had the transformer, the case and the fancy connectors, so I think that mine was the best one built from mostly junk I had around.It was a bad day for me at the end: I had attached the amplifier IC to a heatsink, but it turned out it wasn't attached properly. After 5 minutes of playing music the sound started to distort and something exploded inside the case. I was afraid it fried my recently bough laptop I was using to play the music. I quickly disconnected everything and when we opened the case the IC was literally split in two. I think I still have the board around.Some guys where assigned to do the same amp, but they took the test circuit from the datasheet, rather than the actual amplifier circuit. It had no series cap attached so the speaker. I knew that amps need a capacitor in series but I thought that could be a feature of the IC or something. They had hooked up like 3 speakers and they blew all of them. Then someone though that the problem could be that all the speakers where small, so they used mine, and naturally, it was blown after a few seconds.If you are curious about the third project that was done, it was a small dual rail bench supply with a LM317 and a LM337. I think it would have been more useful for me to do that one.Ivan

author: ivan747
author_rank: Super Contributor
timestamp: June 06, 2011, 09:47:36 pm
content: Quote from: reflowman on June 06, 2011, 12:46:55 amSince then the suppliers noticed an increasing demand in 1N4001's

author: Time
author_rank: Frequent Contributor
timestamp: June 06, 2011, 10:19:08 pm
content: I witnessed this mistake/failure:I guess its more like a failure since the mistake wasn't really user error related.There was a marx generator bank submerged in an oil tank. The output voltage of the bank was a little over 500 kV and the total energy was nearly a mega-joule. The switches in the marx bank were gas discharge switches which were submerged in the oil tank as well but with gas lines running through some poly flo out to pressure regulators and nitrogen tanks.The cap bank was charged up and the operator pressed the fire button to trigger the shot. There was a loud explosion followed by a huge fire ball/mushroom cloud like burst of flames and heat over the oil tank (this was in a large bay area with a few stories of head room thank god). After investigating we found a partially empty oil tank with the melted and charred gas lines. The arcing formed in the gas lines which caused a burst in pressure that sent oil shooting up into the air. The oil flying through the air created a good fuel/air mixture ratio and was thus ignited creating a fire ball.At least no one had to clean up a huge oil mess since it was all ignited.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 07, 2011, 01:45:14 am
content: At work, I once got a breakout board (sent back from a customer) that had a spider web of wires on the back. After a little examination, it turns out that the pinout was accidentally mirrored. While I was thinking the board designer wasn't paying attention, I later looked through a stack of paperwork from the customer (the manufacturer of the module that was meant to go on the board), and on the last page was a pinout diagram with corrections scribbled in pen. (The customer wasn't right that time!) The board was returned because it was performing very poorly, not surprising because the rework job ruined all the nice, controlled paths. That board ended up as a paperweight.And then there's the time when someone put a chip into the socket the wrong way around and ended up melting the socket...

author: McPete
author_rank: Regular Contributor
timestamp: June 07, 2011, 09:37:08 am
content: I spent a few hours one day trying to make a small adjustable DC supply regulate properly- it was built by electrical fitter mechanic apprentices in the 80s, so no diagrams, and some pretty average construction... But it had "sentimental value".I had a rough idea of what the circuit was doing, but I couldn't for the life of me explain why the ammeter wasn't working, and the voltage regulation was playing silly buggers with me. I took careful note of a mark on the ammeter "I max" at 750mA, and was careful not to exceed that, measuring the output with my DMM.I leant back, and asked a question to a colleague... Who walked over, grabbed the current regulation knob and cranked it out. Immediately, a large resistor caught fire, as 10A flowed from the output... The resistor had burnt a hole clean through the board, and the main transistor (an MJE3055, I think?) had gone short circuit... Needless to say, the customer was not thrilled with the report "technician error- unrepairable"...

author: qno
author_rank: Frequent Contributor
timestamp: June 07, 2011, 10:37:00 am
content: need an antennae tuner ??http://www.mfjenterprises.com/Product.php?productid=MFJ-266

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 07, 2011, 01:15:53 pm
content: Quote from: Lance on June 06, 2011, 03:21:15 pmHe was fired that day. He backed into a full pallet of very light equipment and didn't see me behind the pallet. I was real luck there.One of the coolest screw ups I had as a kid learning electronics, many decades ago, was hooking up a 555 wrong. I believe it was either an off by one error on a pin on the breadboard or a rotated chip. I don't remember which. But the whole setup got really hot, really fast. I was using bare wire for short jumpers and I pulled out a jumper and left a jumpered shaped burn between my finger and thumb. The chip melted my breadboard really well underneath it. The holes would now only take DIP width pins, as they were molded to the chip leads. I remember being amazed that the old 555 took the abuse and still worked fine. And I cooked the crap out of it.