Hi All,

I have been working on a version of John Freau's sync gap dwell controller.  
This is pretty much what I have decided on:

http://hot-streamer.com/TeslaCoils/Misc/SRSG-Dwell-Controller.gif

It is based on John's design at:

http://members.aol.com/futuret/page3.html

Some things I am doing differently are:

F1 - I put in a 5 amp slow blow fuse to protect everything and break the circuit 
if the variac is in a bad spot when the power is applied (starter windings stay 
on too long) or if no motor is connected (L1-C1 60Hz resonance).  The fuse value 
may vary depending on the varaic, cap, and motor, but this is what worked for 
me.  I am using this fuse in place of the capacitor fuse in the original design 
now.

K1 - I am using a 25 amp 480VAC zero crossing solid state relay to control power 
to the circuit.  This eliminates big current spikes at turn on due to the cap 
suddenly charging.  I could get close to 150 amp spikes without it if I played 
with the power switch.  That causes bad arcing at the variac and switch.  The 
relay also stops voltage spikes at turn off since it waits for the current to 
drop back to zero.  the relay only needs a very small heat sink at most.  Turn 
on current now looks like this as the motor spins up and then the starter 
windings fall out:

http://hot-streamer.com/temp/030607-01.gif

The motor starting current is pretty high at about 25 amps RMS but that does not 
hurt anything.  The relay needs to be rated at 240VAC+ since the cap may keep 
the voltage high during turnoff allowing twice line voltage across the relay. 
Unfortunately, these relays run about $55 new :-p  So one may have to use just a 
big high current 240VAC switch if you can't spend that much. 

Z1 - This is just an 1800V MOV that keeps the voltage across the variac at a 
controlled level if the motor is started with the variac turned up a bit.  I 
blew my 27XT meter up figuring this one out :-(  The high current at start up 
can drop a lot of voltage across a few winding of the variac and auto 
transformer action can transform that voltage way up at the far end of the 
variac.

R1, Z2 - If the fuse blows due to no motor connected, C1 can hold a very high 
charge.  R1 is a big power resistor that will drop the voltage in three seconds 
no mater what.  Z2 limits the voltage on the cap and clamps it if the L1 - C1 
goes into 60Hz resonance.  It also helps the fuse to blow in that case.

C1 - I found that the value of C1 seems to depend on the varaic.  Both my motors 
(1/4HP 1800RPM) run with the same cap value even though they have pretty 
different current draw.  As in John's design, the cap value should be selected 
so that the output voltage stays pretty stable throughout the variac's 
adjustment range.

Right now, the actual circuit is a little rough ;-)

http://hot-streamer.com/temp/030607-02.jpg

But all seems to be working well and things stay "tame".  Resonant circuits like 
this can be pretty goofy, but I "think" I have it under control ;-))

Cheers,

	Terry