One item we made was packaged high voltage, low current power supplies. They ranged from maybe 1000 VDC @ 5 MA, to the Big Honkin' Mutha, a 100 KV, 100 MA job, built in a can of mineral oil to keep 'em from arcing over internally.

These things were BIG... the big one had to be moved around the Quality Control Lab with a fork lift truck.

I tried repeatedly to find out what they used these grotesque things for, but I never got a good answer. Just the same tho, we had one customer who bought maybe ten of them per year, and returned the old ones (blown) for rebuilding. I was vaguely told this outfit used them for powering industrial X-ray tubes for some purpose.

Testing something like this is a major undertaking.

For one thing, we had to constantly monitor the weather. A wet bulb / dry bulb thermometer was used to check the humidity (the plant wasn't air conditioned). If humidity was over about 50 - 60 percent, high voltage testing was put off that day; if you tried it, you'd get an arc from the 18" high insulator that held the output corona ball, and it would zap into plumbing, building structural steel, etc. I've seen these supplies throw a six foot spark to find ground.

We didn't have a high voltage cage big enough to fit these monsters into... so, we'd put it in the center of the floor in the basement, and rope off a danger area, ten feet around it.

We obviously didn't wanna get anywhere near these things when they were in operation. The output voltage meter was something built in house; somewhere, they found a meter movement that was about 12 by 16 inches, and could be easily read thru binoculars! We had a meter multiplier resistor bank built on a sheet of plywood, and the resistors inside of glass tubes filled with mineral oil to keep them from flashing over.

Primary power was applied through a Variac with a LONG cord!

One day the humidity was low enough, meaning that we could probably get thru it with maybe a half dozen tries before we got there without an arc popping the breaker, requiring us to start over!

We set up the rope around the danger area, and I put the Variac behind a heft steel beam that was holding the building up. I sat behind it, reasoning that an arc would have to take out the beam before it could get ME!

It had been a marginal morning... my nerves were shot; I'd get the supply to 60 or 70 KV and then we'd get an arc to ground that sounded like a rifle shot.

It happened to be a Friday... Pay Day.

I was hunched behind the beam, slowly inching up the Variac, and I almost jumped out of my skin... someone had come up behind me and touched my shoulder.

It was the boss's secretary. It was her habit on pay day to walk around the plant, handing out the pay checks in person to each employee. It was always accompanied by a smile on her plain, chubby face, and a soft "Thank You".

She handed me my check, and smiled as I nervously stuffed it into my pocket and returned my attention to cranking more AC into the supply. She wandered away.

I was almost to 90 KV when I near wet my pants... somehow, one of the danger zone ropes had been removed, and the secretary was walking straight toward the power supply!

I yelled a warning, but the noise level was pretty high and she didn't hear. I slammed my hand down on the breaker, dumping the AC to the power supply... but it contained filter caps. It would take a while for the bleeder resistors to discharge those caps!

She did a left turn about 3 feet from the supply, headed toward another employee.

The electrostatic charge around the beast caused the hair on the right side of her head to stand up almost straight!

She never noticed the danger she was in... she kept walking, and raised her hand to smooth her hair down... and she walked away from the power supply.

It never touched her!!! She never knew how close to disaster she came.

Thoroughly shaken, me and the other techs put the ropes back up, and started powering up again.

This time, we got less than halfway there... it threw about a 5 foot arc at about 50 KV!

WHY it didn't zap her I'll never know...

Mr. T., W9LBB