RE: Terry's DRSSTC - All Ready!!

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

>Great.  You still have zero margin (spec wise)

I have been bitten before by this "LM339 does crazy things when inputs
pulled below ground" bug. You can see how I worked round it in the OLTC II:

http://www.scopeboy.com/tesla/oltcdriver_1_print.gif

R13 and R21 haul the input up a little so when D3 clamps, the LM339 input is
still within spec. The circuit is a single supply zero crossing detector
straight out of the LM339 application notes, so it wasn't really _me_ who
worked around it :-/


In my new DRSSTC driver I figured the CT signal must be symmetrical (it's
got to be right? a CT has no response at DC, and the DRSSTC primary
capacitor blocks DC anyway) so I just AC coupled it and dangled the whole
current limit comparator mess halfway between the rails. I didn't use a
centre tapped CT or rectifier, I decided to go with a window comparator
working on the plain AC signal instead.

http://www.scopeboy.com/tesla/drsstc/dr3_2.gif

I guess now I think about it, the LM339 inputs in this circuit could be
pulled down to -0.7v if the CT drove it really hard. It shouldn't affect
operation since the current limit should have kicked in before then, so the
primary current should never get that high. But then "never" is a 5 letter
word and maybe it actually damages the LM339? I'll check it out.

I also took on board (literally- I'm laying out a PCB) what Terry said about
putting the burden resistor back on the driver board and transferring the CT
signal as a current. I think I'll include that as an option on the board.

Here is the other sheet of my driver fwiw. I have since added a UVLO and
some safety interlock and EMI filtering stuff, which is not shown here. By
messing with the many controls on the PLL it can operate on either pole at
will with true zero crossing switching. The first two cycles always look a
bit hairy, it seems to take that long to lock in, but from then on
everything seems OK.

http://www.scopeboy.com/tesla/drsstc/dr3_1.gif

If anyone has any comments apart from "Does it work" (cause I'm not sure
myself yet- I have tested it with a mock-up of two coupled resonators but
not a real Tesla coil) please go ahead.

Steve C.