EM field detection, vinyl flooring and other observations (fwd)

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Original poster: <sroys@xxxxxxxxxxxxxxxxx>

[First...I've had problems logging in, so the HV list has note been
working.  Things have been resolved (obviously).  SRR]

---------- Forwarded message ----------
Date: Sun, 04 Sep 2005 20:07:11 +0930
From: Matthew Smith <matt@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: EM field detection, vinyl flooring and other observations

Greetings all

Thanks to those who responded on my "radar flyback" post; I tried one of
them out today on a new controller I have just built - worked fine, but
further testing for another day.

I built the new controller for the "Green Monster"; a large, hefty
voltage multiplier that I bought on eBay, attached to a (non-working)
controller board and mini-flyback.  I have attempted to get this working
before using various mosfets driven by a 555 circuit; I have never
managed to get any significant runtime (like a minute) without something
cooking.  The "green monster" was labelled as 90kV (24V DC input to the
board as far as I could make out), although I am a bit dubious, despite
the ten-inch fibreglass tube that shrounds the output terminal. (The
label was a paper one, stuck onto the multiplier box with Sellotape; no
manufacturers details that made sense apart from being made by Ero in
Germany.)

Last week, I received a pair of stw8nb100 mosfets from a US eBay
auction.  Rather than fuss around with snubbers and multiple mosfets, I
thought I'd just throw a large lump of silicon at the problem.  Rated at
1000V/7.4A, I am driving the stw8nb100 with a TL494, the output
transistors of which are wired in totem-pole configuration.

So far, so good; I've tested the system with a power resistor as load, a
radar flyback with a small spark gap, the bare mini-flyback that came
with the "Green Monster", and finally that flyback with the "Green
Monster" connected.  Nothing got more than vaguely warm and the whole
system seemed much more stable than my previous attempts with 555s.
There's a spot of ringing on the gate when the load is inductive, but
nothing I'm going to fret about.

I have yet to be brave and go to more than 30% duty cycle or 12V supply.

Here are some observations:

1) My Fluke Volt Alert (pocket device for seeing if mains wiring is
live) came on when testing; moving it around, I was able to map where
the EM field was (up to a foot away) around the rig, with the unloaded
flyback.  With the device operating at greater than audible frequency (I
can't stand the whine!), this is a good way of spotting whether the
system is "hot".

2) It is possible to solder SOT23 SMD devices to matrix board; not easy,
but possible.  I have a part-reel of 6.2V Zener diodes (eBay bargain)
and wired four in series to protect the mosfet gate (it's good to 30+V),
lacking any other value Zeners.  Wasn't the easiest task, but it worked.

3) Operating the "Green Monster" as it is, not a lot happens.  However,
poking some copper wire down the fibreglass shroud of the output
terminal, so that half an inch protrudes from the end makes life
interesting.  I wondered where there sparks were that I could hear
ticking away and worked out that they were going from the (grounded)
frame of the rig to the industrial vinyl flooring I have in my office.
The circuit was completing to a steel tubular framed chair a couple of
foot away.  I'm guessing that the chair was charging up enough (the wire
from the output was pointing at it) to discharge through the flooring.

4) Moving the "Green Monster" to within about four inches of the chair
frame, I didn't get the arc I was expecting; instead, the chair made a
rapid, ticking discharge to a machine vice on the floor, less than an
inch from the chair.

I wish I knew how much this beastie puts out; a brief test on an older
(now dead) rig a few months ago gave me a surprise - there was that much
charge coming out of the discharge wire that my hair stood on end (from
about four feet away) and bits of plastic swarf and dust went a-shootin'
around my bench.

I'll post a picture of the new rig, complete with "Green Monster", once
I can find a backdrop that can compete with Terry Fritz's famous carpet ;-)

Cheers

M

-- 
Matthew Smith
Kadina Business Consultancy, South Australia
Work: <http://www.kbc.net.au> Personal: <http://www.mss.cx>