What is your dumbest design mistake?

After 29 years of test and repair of embedded processor boards, I've seen and done quite a few:

- I worked for a company that spec'ed 12V tantalytic capacitors in their design. For a year the distributor kept suppying 25V versions, so everyone was happy. Then, we actually built a run of 250 boards with 12V versions. The first three I powered up had these caps go up like match heads. A later check of the supplies at power up showed a "spike" of 16 Volts for a split second. This was enough to torch the caps. We respec'ed to 25V. After that, only the occasional backwards cap flared up.

- I worked on ATE equipment that used a card cage and board set from Zilog. Surprisingly, the card connectors were installed dead center on the cage mother boards. It wasn't unusual for the new assembly women to plug cards in backwards for initial tests. I got real good at replacing the same three ICs on our floppy controller boards.

- An engineer showed me the "quick discharge" method for repairing small copper-whisker shorts on circuit boards. You charge up a sizable capacitor with 5 to 9Volts, then hook the wires across the two known shorted points. The whisker vaporizes, usually, and the tech avoids a long hunt around the board.
A friend stopped by at work just as I came across a board with a short. I told him I was going to use the QD system to fix it, but the power supply I used to charge the cap was in use, so we went out to my truck. I popped the hood and hooked two alligator clips with heavy wire across the truck's battery, then connected them to the board's shorted points. By high coincidence, this short was a major copper short between the internal power planes. The battery gave it everything it asked for, and the wires in my hands smoked near instantly, then burst into flame. But not before the board heated inside enough to suffer delamination. My employer was highly unamused, but cooled down when the engineer pointed out the massive internal short couldn't be fixed. And, no doubt, earned him several free drinks as he recounted his tale about the idiot tech who...

- Plugged in more chips backwards then I care to think about. Some actually made that crispy little noise as the substrates heated, just before the case cracked and blew smoke, and I sat there going, "any time, now, stupid board..."

- And, of course, the usual collection of solder guns grasped at the wrong end; iron cords snagging in chair arms as I swung about to get off the chair and have the iron whip out of the holder, onto my pants; 220V equipment plugged into 110V, and vice versa; low voltage parts installed in high voltage circuits; Panel cut outs done on the wrong side, producing "mirrored" results; fuming at the failure of the o-scope to take a long-term sample, only to discover the wrong probe was used; etc.

You know, the usual stuff...
kenjj

I have made a lot of mistakes during my empirical electrical / electronics "career".

1) 22 years old (a month ago). UV meter with LM3915. A big one, 1.50 meters tall, 15 cm of separation between each LED. When I finished, I realized the distance was too high, and the bar effect cannot be apreciated. Waste of wire.

2) 10 yrs. old: Lamp switch in series with mains (accidentaly!). When I turned it on, the switch exploded, the fire started to eat the wire from that point, like a path of powder burning. Tons of somke, breaker never tripped, so the fire only stopped at the plastic outlet. Nice burnt path on the carpet.

3) I guess, 13 yr. old: High Voltage capacitor, from a 12V strobe light - tested if it was still charged, with the tongue Don't remember too much from that, very nasty. I guess it wasn't fully charged, because I can still taste.

3) 20 yr old: beautiful PIR sensor, fell to the carpet, never saw it until crushed it with one of the chair's wheels

4) I guess, 19? : When powering a device with a 24 VDC supply, changed to 12 VDC ATX power supply. I was sleepy, and forgot to disconnect 24 VDC first. Seconds later, a huge bang inside the ATX, and a black mushroom of smoke slowly exited through the vents. Nasty burnt oil odor

I was underneath my house running new telephone wires.

Left my wire strippers outside....decided to strip the wires with my teeth.

The ringer voltage almost knocked me out cold....I saw stars!

The first 'big' mistake I remember from my youth was 'installing' 230V in the basement workshop and cutting the cable live. Both the fuses and my wire cutter went to heaven, at least half of the cutter.

The funniest mistakes I've been involved in was many years ago, the company I worked for as a designer had delivered a lot of special built equipment to an installer in Germany. It was to be used in one of the broadcasting studios. I was sent there because 'nothing worked'. We had done a FAT, both on software and hardware and knew the euipment worked perfect at dispatch time.
When I came to the studio I found that the installer had recased some of the displayboards, and cut of some milimeters of the pcb, without thinking of the wiring on the pcb. I had to repair all the boards, and luckily they worked in the end, after rearranging the sequence of connectors. I don't know if this is common, but the tech had counted 1-2-3 from top left, AND bottom left, seen from the underside of the connector rack. I had to swap 1 and 3 to get it right.
The last error in the system was a RS232 cable, where we had specified a crossed cable, 2-3 and 3-2. This was of course connected straight, and no communication. I told the manager of the installing company that it was just a swap of 2 leads, while I made the change. He then got a bright light in his eyes, and said, 'Ahh, and then we have to do the same in the other end!'. I tried to hold a straight face when I told him, no, there is no need for that. There where others in the room that had more problems with keeping the laughter away.

Those were the days!

TOK

I haven't made my dumbest design mistake yet. It's not because I'm not stupid enough as I am dripping with ignorance. It's not for lack of negligence because most of the time I don't give a f..... My lack of effort is substantial and robust. I have all I need to be my dumbest. I seem to be a natural idiot at times, possibly even an anti-genius. I can guesstimate a fuse size for a circuit I designed and be farther off the mark than any ten common morons totaled that have never even seen the thing. I can mislabel a 480 vac wire as a 5 volt logic input with world class carelessness. I can pay no attention whatsoever to critical conversations and apply that ignorance with disastrous results. I can fail to advise others of changes I have made at the speed of a rat on fire. I can, with no regard to any kind of reality, specify stuffing ninety 14 ga. wires through one stick of ten foot long, 3/8" conduit over an 23 ft distance. I can msilepl amlsot ervey wrod on imoprtant peojrct insturcitons wtih splelchcek on. Yet I know deep in my heart I can always do much worse.

Me too! Especially the rat on fire thing.

GonzoEngineer said:

I was underneath my house running new telephone wires.

Left my wire strippers outside....decided to strip the wires with my teeth.

The ringer voltage almost knocked me out cold....I saw stars!

Click to expand...

I learned that lesson of what 90vac 20hz feels like when I placed my fingers across the 'low voltage' telephone terminals in a friend's garage when some idiot decided at that very moment to give him a ring

Lefty

Well my dumbest mistake has just happened recently!

I pulled the timer unit out of a kitchen oven, and put it in a plastic case to use as a clock for my shop. I bolted the circuit board to the front using some metal spacers. I noticed that one of the spacers touched a solder joint, but didn't really care too much about it... this was a year ago i made this... and it works flawlessly...

but the other day i was setting up two nice new LCD monitors for my shop - doing some cabling behind them - and grabbed the clock to prevent it from falling... just then i felt a really bad sensation, which i knew from experience was strait 120 ac... it turns out that that trace that spacer was touching was actually live 120 strait from the wall, and i was grounding myself through the aluminum monitor base so i just wrapped some electrical tape around it and called it good

mramos1 said:

When I worked in R&D, we generally checked, re-checked, and past the design around. Now we did play with the thermatron (temp cycler) in ways not covered in the manual.

Click to expand...

Our Thermatrons went up to 450F. We talked about cooking a pizza for lunch but we didn't.

An engineer from Vietnam used the Thermotron to do environmental testing on a computer under power with a program executing. He forgot to purge the moist air. As the temperature cycled down, it rained inside the thermotron; all over the computer. Not only that, but he set the temperature to cycle up to 100C. The small flat plastic internal speaker melted. So did the CD player. hehe The speaker mounting bosses drooped downwards and the speaker looked like processed cheese on a hamburger! Other than that, the PC survived the torture.

There's your environmental test for you!

I was testing RTDs in an oven to 100C, but forgot to take the rubber mat out of the oven (normally used to keep PCBs from shorting out against the metal insides of the oven...yeah the mat can't withstand 100C. It started to smoke...but we shouldn't have opened up the oven because then the smoke got into the fire smoke detectors and we weren't able to stop the fire department from coming.

dknguyen said:

There's your environmental test for you!

I was testing RTDs in an oven to 100C, but forgot to take the rubber mat out of the oven (normally used to keep PCBs from shorting out against the metal insides of the oven...yeah the mat can't withstand 100C. It started to smoke...but we shouldn't have opened up the oven because then the smoke got into the fire smoke detectors and we weren't able to stop the fire department from coming.

Click to expand...

Bob and DK,

We had a guy leave pizza in ours and forgot. It was a furry thing days later as he ran off and forgot about it. But on the fire department, our building had over $1000 in fines last year (and they give you 3 free ones) and 90% of it was over the toaster oven and bagels/toast. None caused by me, I use the microwave for my warming.

I "almost" did this recently.. a 500VA hand-wound toroid power transformer and was bolted onto a chassis with a conductive path right through the middle of it making a giant shorted turn. Changing a portion of it to nylon hardware was an easy fix.