Diodes with the wrong markings create extra work for engineers and serve as a reminder that one should never assume anything when troubleshooting design problems
Years ago I ran a small electronics lab for the Zimbabwe Electricity Supply Commission (the power utility in Zimbabwe). We mainly designed and produced specialised test equipment for various requirements, but one of the projects I inherited when I started there was an annunciator panel. Real simple stuff. It was connected to various alarm circuits in a substation and lit up LEDs to show what alarms were active. It had a few diodes, relays and pushbuttons in it to latch and reset some of the alarms.
The previous incumbent of the position had done the design and the PCB design (this was in the days of the venerable Bishops Graphics and acetate films). All l I had to do was get the PCBs made up and assemble the units (around 50 were needed).
I initially got two PCBs made up and had an apprentice assemble one of them. They worked off 24 volts, and we had a 24 volt lab supply which we set to a reasonable current limit and fired up the first board. The relay coils had 1N4004 diodes across them to catch the back EMF when they switched. Initially, all looked good, but when we tried one of the inputs, the 24V supply current limited and the voltage went down to almost zero.
There was only the relay coil and diode in circuit between the 24V supply and the input connector, which went to a clean ground contact. There was a latching contact in circuit as well, but it wouldn’t have been causing the problem as the relay was not even operating. So it must have been a short on the board, or a diode the wrong way round acting as a short circuit. I asked the apprentice to check both possibilities, as I was working on another design.
He unsoldered the diode, tested it with a meter and pronounced it good. In those days the meters were still mainly analogue – we only had one DMM and that was MINE! Remember that when you test a diode with a DMM it conducts when the positive lead is on the anode. With most analogue meters it’s the other way round, which can be confusing.
Puzzled, my apprentice resoldered the diode to the PCB but the same problem showed up. Again he unsoldered it from the board. “Here, let me test it,” I said. I put it on my DMM – positive to anode – and it showed open circuit. On a whim, I reversed it and the DMM read between 0.6 and 0.7 – the usual forward voltage drop for a diode. But it was the wrong way round.
“Give me another diode,” I told the apprentice. He handed me one from the bag of 1,000 we had received from our local supplier. I tested it. And it also showed the usual reading – but this time as it should be, with the positive lead on the anode. I repeated my tests. And the inescapable result was that our board had a good diode, but wrongly marked!
We changed the diode and the board worked fine, so we got more boards made up and the apprentice got to work assembling and testing some more units. When he tested them, he found a couple more of the “surprise diodes” in the other units. So we made up a simple diode tester with a couple of strips of copper, a resistor, an LED and two batteries. We could quickly test each diode before we soldered it onto the board. And in our 1000 diodes we found around 25 that were wrongly marked.
I went back to our supplier and showed him, and he was as surprised as I was. He offered to replace the diodes, but I had more than enough good ones, and being of a mischievous turn of mind I could see uses for these diodes in causing consternation among my electronic-minded friends. So I hung onto them.
I did catch a couple of people nicely with them but stopped handing them out when one caused a friend of mine’s 60W power amp to blow a couple of expensive power transistors. For years in my spares box I had a little bag labelled “Surprise Diodes”. They got lost in a move, alas. But there’s been times recently I wish I still had them!
I'd be interested to know whether other engineers have had run-ins with mismarked components? It seems like the one kind of quality issue that would be difficult to screw up, but then again, I keep surprising myself at the sorts of things that do go wrong.
About Author David Ashton: "I’m not sure what I am….. I was born in London, UK, raised and trained and worked in Rhodesia, then Zimbabwe, and I now live in Australia. (So I’m a Pom-Rhodie-Zimbo-Aussie??) Work wise it's much the same. I have run electronics labs and managed telecoms centres, run my own comms business, and am now working as a telecoms specialist keeping a large comms network going. I’m a jack of all trades, and yes, admit I’m master of none, but I kinda like it that way. It makes it difficult to get bored."