[SOC] FOC Attacks

[Ray Goff]

>> What's really fun is watching the lights flicker to the sound of my
voice...  The neighbors' lights, that is! :)

Hi,

My best experience of lights flickering occurred many years ago when I was a
hardware development engineer. It was the time that Dynamic RAM was just
starting to appear on the market and I was having all sorts of problems
getting my circuit stable.

One area we suspected was the power supply browning out and causing dropouts
on the 5 volt supply rail. Well the Engineering Manager discovered a piece
of equipment that you could hire which could be configured to dropped half
cycles of the mains supply to the equipment. If I remember correctly, you
could adjust the frequency at which the drop-outs occurred and the duration
in half cycles from one to two and a half cycles.

It was actually a useful piece of kit and allowed my to optimise my PSU
design. However, having an enquiring mind, I decided to see how the box
worked with one of the FerroResonant Transformers that were used to keep
computers going back in those days (mid 1970's). For those not familiar with
these transformers, they were effectively a one-to-one transformer at mains
voltage (in my case 230 Volts), but very heavy because the ferrite was
designed to resonate at the mains frequency and thus store energy.

Well, I dug one of these transformers out from a store cupboard, connected
my equipment to the output and the mains interrupter to the input. These
FerroResonant transformers usually hum quite loudly at the mains frequency
(50 Hz in the UK), but as soon as I turned on the interrupter, the
transformer let out a magnificent 'G-Dong' sort of noise each time the mains
was interrupted.

I discovered that these FerroResonant transformers were very effective at
supplying the power during the cycles removed by the interrupter, if
somewhat noisy in the process.

At this point, for reasons that I can no longer remember, I left the
interrupter running and went down the street to a shop to buy something - it
could easily have been a lunchtime snack. As I was queuing to pay for my
purchase, I suddenly realised that the lights in the shop were dimming
significantly at about the same frequency I had been running the interrupter
at - about twice a second!

I hurriedly paid for my purchase and checked out a couple of other shops on
the parade as I made my way back to the lab. In each case they were also
suffering from this light dimming effect.

I never did find out how far a field my interrupter was affecting the mains
supply, but I did rapidly finish my experiments once I got back to the lab!

Oh, the good old days!

73

Ray, G4FON / N4FON