HV Corotron monitors diesel emissions
We were asked by a lab at the Oldsmobile division of General Motors to design a 12,000 volt power supply to be used with a "corotron" – which was going to be used to monitor diesel engine exhaust emissions for concentrations of soot. Simplistically, the corotrons looked like this:
Figure 1 Basic structure of a corotron
How these things were installed in the vehicles, I never knew, but I designed the power supply and a couple of units were shipped. A few days later we heard back.
When the corotrons were energized and there was soot detection, the corotrons broke down – as in, going BANG! The engineers at Oldsmobile called us and apologized for not having properly defined their needs. Rather than a 12 kV voltage source, they actually needed a power supply that would operate as a current source, with compliance capability of 12,000 volts.
What we learned was that corotrons require current-source drive, not voltage-source drive.
The lab's budget was tight, so I decided that the quickest and cheapest way to fix this problem was to take the power supply's schematic and mark it up to change the feedback control scheme from voltage mode to current mode, then, send that to the Oldsmobile lab along with modification instructions, like “Cut R12 away from IC1 pin 3 and reconnect it to the base of transistor Q5..., etc.”
The lab staff at Oldsmobile followed my instructions and lo and behold, everything worked!
If you've ever driven behind a diesel-powered vehicle, you know about soot. The Oldsmobile folks were looking to measure their soot in hopes of finding ways to reduce it. We helped. Then, perhaps a year or so later, the Pontiac division called us up to order a couple of those same power supplies for their own efforts toward that same goal.
We told them what had happened with Oldsmobile, and they said, “Okay, send us the voltage-mode power supplies so that you don't have to redo your design files, and when they get here, we'll make the same modifications the Oldsmobile people made.”
We did. They did. Everything worked the same way. It was success all around.
Another year or so went by and we got a letter from the Oldsmobile lab, thanking us for our efforts on their behalf. They were especially grateful for our being able to assist them in getting past the technical hurdle they'd encountered without undue extra costs.
Reading that letter made me feel good.
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—John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).