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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
even 
more sensitive - it only takes a few microamps to trigger one - back in
the 
early 70s, I used to do alarm installations in buildings - a military
comany 
wanted a special alarm that would latch an indicator when a door, etc
was 
opened and only turn off the indicator when the door was closed.  I
decided 
that SCRs would be the right way since they automatically latch once on
until 
the current reverses.  I built the circuit and tested it at home, then
by 
wrapping quite a length of wire around a fluorescent fixture (and
discovered 
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
3 
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
into 
the gates of anything with SCRs or MOSFETs in them and they won't work
as 
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
from 
tesla stuff.

I believe a bipolar transistor circuit used as a motor speed control
would 
work fine for your gap motor controller because the bipolar transistors
don't 
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
controller
(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
the
motor goes full speed.  Turn off the coil and the motor controller
returns
to normal.

Chip

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