On 2013-07-29, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...
On 2013-07-29, Jim Wilkins wrote:
Surprisingly I needed only a space frame box with wide-open sides
to
fully shield it for the test and calibration fixture. The circuit
rejected 60 Hz and above very well and the seemingly gaping access
holes were too small for lower frequencies.
Electrostatic fields only -- not electromagnetic?
There was no inductance sensitive to it. We all knew to minimize the
physical size of the summing junction node to avoid such parasitics.
O.K.
However the circuit I built to measure the dielectric absorption
current decay rate picked up ~40KHz audio from the security sensors.
The project manager, a brilliant physicist, calculated that the outer
panels were vibrating about a micron to cause the coupling we saw. I
cured it by creasing them on a brake.
Simple cure -- if you know to apply it.
From the mistakes I've seen on commercial products it seems that PC
board layout designers who understand electronics are rare. I could do
a high-speed digital board with dramatically less noise that the usual
products of commercial PC board companies. I debugged one failing
example that had 3V of ground bounce between two points on the same
plane, because it had been notched into a star configuration that was
wrong for the 74AS logic on it.
:-)
Hmm ... I remember at Transitron (early 1960s) one item on the
production line (long before computer controlled tests) was a
reverse
voltage leakage test for high voltage diode assemblies. Think about
the
size of a billy club made of a spiral of diodes potted in black
epoxy,
and perhaps 12" long with an anode cap (like from a tube) on both
ends.
AFAICT exotica like voltage multipliers, dielectric absorption,
magnetic amplifiers and ferroresonant transformers aren't covered well
in EE courses, but must be learned through job experience.
While working for Night Vision Labs (US Army lab) I got lots of
experience with multipliers. Little oscillators run from mercury cells
(now unobtainum) produced about 1 KV P-P, and went through a large
number of stages to produce up to 15 KV for the small ones, and 45 KV
for the large ones -- to run cascaded image intensifier tubes. All
potted in about a 120 degree segment of a thick-walled cylinder with
taps out one long edge to feed the voltages to the intensifiers.
Then add the high-voltage pulse generators intended to turn the
intensifiers on and off to allow selecting distance with a pulsed NdYAG
laser as an illuminator. :-)
I picked up
a lot of subtleties that new EEs didn't know. One of them insisted
that an open collector transistor driven from a gate didn't need a
series base resistor because the transistor model he had learned had a
current source in the base lead. I couldn't convince him that the base
was just a diode to ground.
Ouch! All models and no real life experience. How many did he
fry? :-) Even a SPICE model of the circuit should tell him how wrong he
was, if he bothered to read the currents -- even though the SPICE
models for transistors don't bother to model cooked junctions or
vaporized bonding wires. :-)
He didn't stay long, the rumor was that he touched a large exposed
electrolytic with 480VAC while putzing with the three phase
connections to a power supply. I was working on a machine next to it
but out sick that day, and came in to find the floor and the side of
my machine blackened from the explosion.
Ouch! Good day to be out sick or otherwise out of range.
Touched it with *what*? Something metal to vaporize -- or was
the black from carbonized employee?
Reminds me of a story from a friend who worked for AT&T Long
Lines. One new employee was given the job of opening each of the knife
switches (while power was being supplied from the other battery bank)
and greasing the mating surfaces, and then re-engaging them.
He was not very strong (at least not strong enough), and those
knife switches had a *lot* of grip, so he got a crowbar, and used it to
pry open the switches by the insulating crossbar which joined the
switches for both sides of the battery (and the handle, of course).
Only -- when the switch let go, he slipped, and one end touched the hot
side fo the switch, and the other end hit the metal doors on the racks
behind him. Well ... given the size of the battery banks, it did not
take long at all for it to slice through the door, spraying molten steel
all around.
When he got out of the hospital, he was allowed to do nothing
but push a broom until he got the idea and quit.
We never worked with *that* much power -- most things were man
portable -- but we did have a 28V lead-acid storage battery for testing
a DC-powered laser pump, and one of the other employees came back into
the room carrying it and commenting on how good a battery it was. He
had taken it out to start his car because the battery had died from the
cold and age. He was impressed with how fast it cranked the engine. (I
asked him if he had turned on any lights while it was connected, and if
so, he should check to see whether they were burned out. he had not
noticed the twelve filler caps on it instead of the usual six -- or did
not realize the significance. At least this was before computers in
control of the car's electronics. :-)
Enjoy,
DoN.
--
Remove oil spill source from e-mail
Email: | (KV4PH) Voice (all times): (703) 938-4564
(too) near Washington D.C. |
http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---