analogZONE First Annual "War Stories" Competition

In a recent Editorial "Lab Disasters And Other Rites of Passage," Section Editor Lee Goldberg bared his soul and detailed a few of the technical mishaps that have punctuated his own professional life. We at analogZONE were just sure you had some, too, so we asked you to share your favorite foul-ups and FUBARs with us and your fellow analogZONE readers.

The proud winner of our "War Stories" competition is Alex, who works at an instrumentation manufacturer in the UK.

Here are some of his stories -- we particularly like the Plug-In Board story, which is one of those things that could drive any manufacturing line to distraction:

LCD Panel
A distributor lent us a 20 inch LCD panel to evaluate. This was a few years ago when they had only just come out and were quite expensive. It needed a 12-V supply, and was conveniently fitted with two flying leads, one red and one black. We hooked up a bench supply capable of delivering 2 A, and it was current-limited. What should you do next? Well, recently we had borrowed a camera which worked but needed a lot more current to start up then it needed to run. So we thought, "here we go again," and we hooked up the most powerful 12-V supply we had. You won't be surprised to learn that we blew up the LCD drive electronics: the flying leads had been wired so red was negative and black was positive! How embarrassing for both us and the distributor.

Plug-In Board
We designed a piece of equipment which was controlled by a processor on a small plug-in board. One of the functions of this board was to check the frequency of a pulse, buffered by an emitter-follower on the main board, and fed to the processor board at a level of 4V or so. Unfortunately the follower ran with a 12V supply and I had stupidly not put any protection resistors either in series with the pulse to the processor board, or in series with the collector of the transistor.

Everything was fine until one day the emitter-follower on one production board went short-circuit. This applied 12V to the input on the processor board which promptly blew up. The test department found that this main board plus processor board combination wasn't working, so they naturally swapped in another processor board to see if this cured the problem: it didn't. They then put the original processor board onto another main board: still no joy. What they didn't realize was that a blown-up main board would blow up the processor board, and a blown-up processor board would overload the emitter follower and blow up the main board. So it was a bit like a virus -- by the time I came in and identified the problem, we had a pile of broken main boards and a pile of broken processor boards.

Needless to say the design now includes a protection resistor.

Loudspeaker Abuse
When I was a teenager I was at a friend's house and we were connecting up a cassette recorder to an amplifier and a loudspeaker. The loudspeaker drive unit had been removed from an old radio and it was a magnificent 10 inch unit with a massive magnet, which my friend had built into a cabinet. When we had finished the wiring we plugged into the mains (line voltage) and switched on. There was an earsplitting bang. Unfortunately we had used mains cable to wire up the loudspeaker, and when I thought I was wiring a mains plug onto the amplifier mains lead, I was really wiring it onto the loudspeaker lead. Whoops. My friend was not too pleased.

(If you want to work out how much power we put into that poor 4 Ohm speaker, remember that we have a proper 240V mains here and not a puny 110V. Also our mains wiring is designed so we can plug in 3-kW electric heaters without significant voltage drop, so the source impedance can't be more than half an Ohm.)

Our thanks and congratulations to all who entered -- Alex's prize for his winning entry is on its way!