RE: Variac vs. Fan speed control - a bit more on SCRs

Subject:  RE: Variac vs. Fan speed control - a bit more on SCRs
  Date:   Thu, 24 Apr 97 05:21:51 UT
  From:   "William Noble" <William_B_Noble-at-msn-dot-com>
    To:   "Tesla List" <tesla-at-pupman-dot-com>

SCRs and triacs are sensitive (very sensitive) to dV/dT, and the gate is
more sensitive - it only takes a few microamps to trigger one - back in
early 70s, I used to do alarm installations in buildings - a military
wanted a special alarm that would latch an indicator when a door, etc
opened and only turn off the indicator when the door was closed.  I
that SCRs would be the right way since they automatically latch once on
the current reverses.  I built the circuit and tested it at home, then
wrapping quite a length of wire around a fluorescent fixture (and
that I needed to add quite a bit of filtering to the gate circuit). 
When I 
thought I had it fixed, I installed it.  IT was horrible - it would trip 
randomly all the time - I never could get it to work right - after about
trips back to the plant to add more filtering on the gates I gave up and 
redesigned the thing with Xtal can relays and it worked perfectly - as
far as 
I know, it may be running still.

The relevance of all this to tesla work is that the E fields will couple
the gates of anything with SCRs or MOSFETs in them and they won't work
expected.  I now know how to sheild well enough to make it work,but it's
a big 
deal - the better answer is just to keep high impedance electronics away
tesla stuff.

I believe a bipolar transistor circuit used as a motor speed control
work fine for your gap motor controller because the bipolar transistors
have the same sensitivity.

[Bill]  snip > They are totally unsuitable for 
> voltage control, could be use to vary the speed of a SMALL ac motor
> running a rotary gap but not the low side of a drive transformer.

Not quite true.  They are totally unsuitable for the rotary gap too.  I
tried this a long time ago.  The controller was an actual motor
(not a light dimmer) that I got at an electric motor place.  The problem
is that once the coil starts firing, the controller loses its mind and
motor goes full speed.  Turn off the coil and the motor controller
to normal.


First Corollary of Taber's Second Law:
        Machines that perturb people often get murdered.
               -- Pat Taber