Hard learned lessons

Thread Starter

wayneh

Joined Sep 9, 2010
17,498
We've all made stupid mistakes. Let's come clean and list them here. Maybe someone will be spared from repeating our stupid mistakes.

I suggest a format of the take-home lesson as a heading, followed by any explanation or story to go along with the lesson. Here by example is my most recent lesson:

Don't put your multimeter away with the probe still plugged into the 10A socket.

I use my meter >90% of the time with the probes plugged into the 'normal' sockets, for measuring voltages, ohms, etc. Not long ago I measured the current draw of something using the 10A socket and put the meter away when I was done. Then yesterday I went to probe for mains voltages inside an appliance and POW, blew the tip off my probe. Vaporized it in a flash. It gets worse. I though I must have bumped the probes together, shrugged it off, and proceeded to touch them again to the mains. This time it cooked the probe wire off the probe, which at least prevented a 3rd touch. It had never dawned on me to check first where the probes were plugged in. Once I spotted it, it was a forehead slap.

A good rule is to always check your probes and where they're plugged in, but I think it's also a good habit to not put the meter away with the probes shorted together by the 10A shunt.​
 
Last edited:

DickCappels

Joined Aug 21, 2008
10,167
I suspect that if we all posted our "lessons learned" with respect to electronics this would be one of the longest threads in the forum.
 

joeyd999

Joined Jun 6, 2011
5,269
I learned early on to be really, really careful around electricity -- and in many aspects of life -- and I learned it the hard way.

In high school, we had a very nicely set up electronics classroom. Each bench had its own built in voltage/current regulated power supply. The supplies had rotary switches for D.C. voltage range -- 3V, 30V, 300V -- and a course and fine adjust for voltage output within the selected range. Same on the current side.

On one very memorable day, I was fooling around with a couple of other students, and I said, "Hey! Why don't we try to blow up a transistor?" Others, "Cool! Let's!"

So I turned the voltage selector switch to 3V, adjustment knob to 0V, and current cranked all the way up. Held the pointy probes on the transistor leads and proceeded to slowly increase the voltage.

Nothing.

Reset the adjustment knob to 0V, switched to 30V range, and repeated.

Nothing.

Dammit, I said to my friend, let's try the 300V range. He turned the selector.

Anyone ever feel 300VDC hand-to-hand? All I can say was my world was abuzz. All my muscles locked up and held on tight to the leads. I couldn't let go.

Danced around a bit, and knock a TV (under repair) off one of the benches. Either someone disconnected me, or I pulled far enough away from the bench to pull the leads out of my hands -- never knew -- but if you look closely, I still have scars on my hands from that day.

Anyhow, I've made few really atrocious mistakes since then. It's amazing how certain experiences can stay with you and guide you in various -- even unrelated -- endeavors.
 

MrChips

Joined Oct 2, 2009
30,781
The biggest mistake I made was trying to check for airflow of a muffin fan on a DEC PDP-15 computer. I could not see the fan from where I was standing so I stuck my finger into the fan.

Finger was ok. I broke the fan blade.
 

joeyd999

Joined Jun 6, 2011
5,269
The biggest mistake I made was trying to check for airflow of a muffin fan on a DEC PDP-15 computer. I could not see the fan from where I was standing so I stuck my finger into the fan.

Finger was ok. I broke the fan blade.
As a teenager, I got my hand caught in one of these while I was cleaning it:



Luckily, the drive belts were old and worn, so they slipped before the machine had a chance to rip my arm off.
 

Robin Mitchell

Joined Oct 25, 2009
819
Lesson - Never get cocky with a drill press when drilling PCBs

I got into a flow of Drill - Raise - Use finger to wipe swarf away. One time, I went Drill - Raise - Finger AND drill. You can imagine the result
 

OBW0549

Joined Mar 2, 2015
3,566
Here are a few things I've neglected to do at one time or another, much to my sorrow:
  • When building a circuit, ALWAYS work from a printed schematic-- NEVER from a design "in your head."
  • Use a Hi-Liter to color in each connection on the schematic, as you make them.
  • Before powering up a circuit you've built, double-check all connections.
  • If possible, simulate a design before building it. Do the results match your design calculations?
  • RTFM.
  • RTFDS. Especially the part titled "Absolute Maximum Ratings."
  • With variable-output power supplies, turn the voltage down to minimum before turning them off.
  • Set scopes and meters to minimum sensitivity before turning them off.
  • Don't leave charged-up electrolytic caps laying around the lab. Discharge them before leaving them.
  • Beware of static electricity-- it can damage even non-CMOS components.
 

spinnaker

Joined Oct 29, 2009
7,830
Always file your parts away as soon as the shipment arrives. I ordered some 32,768 crystals lost them, ordered some more, found the originals, when the "replacements" came in I did not file them away then. Now I have lost both orders. :(
 

GopherT

Joined Nov 23, 2012
8,009
Care is proportional to the cost of the component (and time is money if the replacement part has a long lead time).

That being said, I’ve made a mess of a lot of inexpensive things.

Price is also relative. I’ve been very careful with my $10,000 motorcycle but treat an old Suzuki worth $800 like it was disposable.
At the same time, I treat a $100 power supply and $80 scope like they are made of gold.

That being said, I don’t even bother sorting/re-using resistors from prototyping/breadboarding work.
 

WBahn

Joined Mar 31, 2012
30,012
DO disconnect the battery when performing significant work on a car's electrical system.

Back in about 1984, while still in my teens, I was replacing the alternator in my '75 Bronco and left the battery connected so that I could have the radio play while I worked, something I had done a few times without incident. The positive lead to the alternator was easiest to remove once the alternator had been dismounted, but this time as the alternator came loose it rocked back and shorted the post to the engine block -- and welded itself there so that I couldn't just pull the alternator up. Taking the time to disconnect the battery at this point wasn't a viable option as the wire was already starting to smoke. Fortunately, I had a set of heavy duty dikes at hand and so I just cut the alternator lead -- burning my hand in the process since I had to pull the wire up to get the dikes on it.

The only added cost turned out to be the replacement alternator lead, so I lucked out. Could have been SO much worse.
 

Sinus23

Joined Sep 7, 2013
248
When something doesn't work like you want it to do (latest example programming a PLC) Take a break before you start messing with the things that actually worked trying to fix the things that didn't until you just don't know anymore(It was a test so the adrenaline was running higher than usual) ...

Like Richard said I could write a book.
 

cmartinez

Joined Jan 17, 2007
8,250
DO disconnect the battery when performing significant work on a car's electrical system.

Back in about 1984, while still in my teens, I was replacing the alternator in my '75 Bronco and left the battery connected so that I could have the radio play while I worked, something I had done a few times without incident. The positive lead to the alternator was easiest to remove once the alternator had been dismounted, but this time as the alternator came loose it rocked back and shorted the post to the engine block -- and welded itself there so that I couldn't just pull the alternator up. Taking the time to disconnect the battery at this point wasn't a viable option as the wire was already starting to smoke. Fortunately, I had a set of heavy duty dikes at hand and so I just cut the alternator lead -- burning my hand in the process since I had to pull the wire up to get the dikes on it.

The only added cost turned out to be the replacement alternator lead, so I lucked out. Could have been SO much worse.
... an MP3 player would've come in handy for you back then... But no. What was available in those days? I believe it was the sony walkman cassete player... too bulky, too delicate, and too expensive to be used during such mundane tasks... and battery life was a big no-no...

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spinnaker

Joined Oct 29, 2009
7,830
Document your process for producing PCBs with all of the software you use. I have a fairly complicated process that is a hybrid / toner transfer process. This seems to give me my best results. Anyway it is fairly complicated. To make a long story short, I produce a gcode file for my CNC where drilling is output mirrored but my toner transfer is printed not mirror. Plus there are a lot of other settings for the drilling process. Last I did anything was at least 6 months ago. I just picked the project back up and forgot the whole process I spent a day figure it all out.

I also updated my PCB design since I produced the original board (needed to make another one). The new board was not matching the old one as I was perplexed as to why. I had completely forgotten I updated the board. so it might be a good idea to include a version number on the board.
 

WBahn

Joined Mar 31, 2012
30,012
... an MP3 player would've come in handy for you back then... But no. What was available in those days? I believe it was the sony walkman cassete player... too bulky, too delicate, and too expensive to be used during such mundane tasks... and battery life was a big no-no...

Yeah, the Sony Walkman was about it. I got one about six-months later -- at a cost of $199 (in 1984 dollars!). By today's standards, it was a rugged brute. But by the standards back then it was meant to be babied. The way it ate batteries is probably the biggest issue I had with it -- and it resulted in me not getting anywhere near the use out of it that I had envisioned. But batteries lasted forever in it compared to the first digital movie camera I bought -- I took it back and got a refund because it ate a set of batteries before getting to the end of the tape!
 
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