You did What?

Got a good "goof up" you'd like to share? Tell us what you did wrong, what happened, and what you learned. We'll send a PCB clipboard to contributors whose stories we publish. E-mail us at goofs@tmworld.com.

Paul Packebush, Austin, TX

Just before a Christmas holiday, I tested a new calibration system. The system worked fine and improved measurements tenfold. A few weeks later, I needed to update the system, but it now produced data that bounced around erratically. Although I installed new hardware and software, the measurements still didn't make sense. No matter what I tried, I got bad data.

So, I convinced a coworker to let me use his computer. After installing the hardware and software, I powered the system on his bench and got almost perfect data. Hoping the problem had magically disappeared, I hurried back to my computer and rebuilt the system just as I had it on my colleague's PC. I still saw bad data.

As I voiced my frustration, an intern looked up and said, "Why don't you turn off that plasma lamp you got for Christmas?" (A plasma lamp comprises a globe filled with inert gas and a flyback converter that ionizes the gas and provides flashes of pretty colors.) After I switched off the lamp, my measurements settled back to normal. That lamp generated a heck of a lot of EMI! We quickly forget how the electrical environment can affect measurements.

Pat Sheets, Wichita, KS

A technician and I worked with an engineer to find the ambient temperature that would cause thermal runaway in a prototype spectrum analyzer. The engineer placed the $20,000 prototype in a small, well-insulated environmental chamber that included a small oven. The engineer told us to run tests at 5° increments until a specific transistor would not stabilize. He was adamant about preventing thermal runaway, which could destroy the prototype.

At the end of the day, we were still running tests. The engineer didn't want to waste time getting the prototype back up to temperature the next morning, so he had us turn off the oven but leave the prototype running. When I arrived the next morning, I smelled the unmistakable odor of burned PCB material. Overnight the prototype had gone into thermal runaway because the prototype's 60-W power supply generated enough heat to keep increasing the temperature in the small oven.