When a power supply explodes, an engineer dredges up his childhood days spent playing with Heathkits to save the day
Many years ago, at the start of my career, I was involved in the testing of a three-axis, cryo-cooled IR tracking system. The Navy was our client. The thing looked like a long, cylindrical missile, with a round nose and a golden lens. We had to bring the tracking system and its control electronics to a GE test center in Philadelphia, Pennsylvania, where we would bolt it up to a hydraulically driven three-axis gimbal system. Our tracker would be bolted into the innermost gimbal, and we would be able to test its ability to track a stationary point while its reference frame oscillated underneath it.
We bolted the system to the intermediate support structure—an aluminum weldment made of 2-inch- thick plates and now looking an awful lot (in hindsight) like Darth Vader’s TIE fighter. We strapped this assembly to the middle of the otherwise empty floor of a rental van, and I drove it from NY to PA. I figured if anyone stopped me along the way, they would have assumed I was carrying a nuclear weapon across state lines, and I might have a few problems.
All went well, and I arrived at the test center without incident. We unstrapped the assembly, put it on a dolly, and wheeled it into the huge lab.
As I walked into the lab, I came across two gigantic consoles. I remember them as standing over 10-12 feet tall, and at least as wide. There were two such consoles, standing side by side. A desk-like area jutted out at waist height, and above that was what looked like a giant breadboard. I was intrigued. I had played with Heathkits when I was a kid (“101 Electronic Projects”!!). I asked our host, “Is this a giant analogue computer?” He answered that it was. The year was 1984.
The three axis gimbal was probably no more than 50 feet away from the analogue computers (this becomes important later in my story).
Now, since the tracking system was eventually to mount up to a Navy fighter jet, it needed to run on 440 V, 400 Hz, 1 phase. The electrical engineers with us (I must admit, I am a mechanical engineer, but I’ve gotten into and out of plenty of trouble pretending to know something about electronics) had this cute little aluminum block that they said was a DC to 400 Hz converter. It was about 2 × 3 × 4 inches in size, had an orange stripe around it, pigtail contacts on the bottom, and a warning label. I forget, now, exactly what the warning label said, but I do recall quite specifically (even to this day) that it warned the user that the converter HAD TO be mounted up to an aluminum heat sink, and went as far as to specify the dimensions, and provide a few tapped holes on the bottom to make such mounting simple and straightforward.
But we didn’t have a heat sink. I asked the lead electrical engineer if he had brought a heat sink with him, but he said that it really didn’t need one. I pointed out the warning label, but he insisted that no heat sink was required. So be it.
We did need a DC power supply, however. It turned out we needed a DC power supply with a pretty good current capacity. The lab did have a DC power supply, over on the other side of the room, on the bottom of a 19-inch rack, but we needed such a high current that we had to bypass the front, fused contacts and tie directly into the output of the supply. I’ll say “we” here, to be polite, but I didn’t do the tie-in.
Maybe it was at that point that I asked about the heat sink, I can’t exactly remember.
As it turned out, no one at the facility knew how to run the three-axis gimbal. There were no instructions. What I had was a 19-inch, rack-mount function generator that belonged to the facility, and a couple of patch cords. The control panel for the test gimbal had a three-way switch, a multi-turn pot, and a place to plug in the patch cords. The three-way said something like “POT/NULL/INPUT.” I figured that if I spun the multi-turn roughly to center, and switched to “POT,” the gimbal (one gimbal at a time, mind you) would jump to center, and I could waggle the gimbal when I waggled the pot. I then fired up the function generator, selected the SINE wave, set it to a low frequency, turned the amplitude to zero, and selected “INPUT” on the test gimbal control. Bringing up the amplitude would start the gimbal swinging back and forth in a controlled manner. All was good.
Then the “human hurricane” swung through. No one knew who he was, but he worked at the test facility, and claimed to know the test equipment we were using. He insisted that it wasn’t really necessary to center the multi-turn pot before switching to it, and promptly commandeered the rack and proceeded to prove himself completely wrong. With the gimbal in a center position, and the selector set to “NULL,” he spun the pot all the way to one end, and selected “POT.” The gimbal (with our tracking system bolted to it) swung hard over, and the building screamed. It wasn’t actually the building—it was actually the hydraulic power unit, located somewhere remote from the test area. While the GE guys figured out where it was and how to reset it, we examined our tracking unit. One of the controlling push-rods had snapped from the shock, and we had to jury-rig a repair to be able to show our clients the unit when they came in later that afternoon. But that wasn’t the worst of it.
Once we got the push-rod fixed, and restored the test gimbal back to health, we could start running our initial tests. The tests were all going well when, without warning, a huge cloud of very loud white smoke erupted from the DC power supply at the far end of the room. Remember the scene in the original Star Trek when they would fire a phaser at some poor unsuspecting computer, and it would erupt in sparks and white smoke? Who knew that that’s how computers (well, DC power supplies) really explode.
But it wasn’t the fault of the DC power supply. It was the fault of the DC to 400 Hz converter. Remember the orange stripe? You probably realized right off that it was a thermal paint stripe. It was whatever color it was supposed to change to when you exceeded the allowable temperature…it had changed to that color. So, our 400 Hz converter was toast, as was the facility’s DC power supply. And the client—the Navy—was due in a few hours.
No one else knew what to do. I did. I had played with Heathkits as a child, and the term “Multistable Multivibrator” was stuck in my head. I asked our host if anyone knew how to run the analogue computers. We got lucky, someone did.
I asked their expert to make me two circuits. The first circuit needed to be a 400 Hz signal generator. The second circuit needed to be a power amplifier. He hooked up his wires and blocks and connectors, and turned what were probably a few million dollars worth of cutting edge computers (well, what might once have been cutting edge) into a Navy power supply.
He tied an oscilloscope to the terminals to check his work, and we were good. I think we had a little clipping at the top, but it was good enough to run our equipment for the test and eventual demo later that afternoon.
I learned a few things that day: Read the instructions, don’t necessarily trust the “experts,” and don’t be afraid to think outside the box. And fuses are your friends.
Stephen Sywak is currently the Chef Mechanical Engineer at McLaren Engineering Group, and a Professional Engineer in NY and Nevada. He says: “I started out with the best intentions--I was originally a EE/CS major at Washington University in St. Louis, but I guess you would say I lost my way! Still, my knowledge of electrical engineering, computer programming, and some servo design has always helped me design some truly fascinating pieces, ranging from a force-compliant-control chip-assembly robot, accurate to better than 0.0004", all the way up to the 40 ton Sand Cliff Deck and its 110 ton movable base for Cirque du Soleil's KA at the MGM Grand Theater in Las Vegas, Nevada.”