Off-Line Tesla coils (OLTC)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi All,

Today I substantially rethought the front end of my off-line Tesla coil,
again...  With a great clue from Jim Lux ;-)), I checked out Richie's
site...  The "present" idea is at:

http://hot-streamer-dot-com/temp/IGBTcoil01.gif

I did away with the big storage caps and the about $400 worth of control
equipment (many big contactors) it needed.  It just was not working well.
The big line caps were starting to need a lot of safety critical items and
boring stuff like that which would make it hard for the average fellow to
make and run safely.  The power resistors in the old design were cooking
off a tremendous amount of heat and I could not get around it easily.  The
source power vs. load power thing...  You can get to very high power the
old way with resonant charging, but things start to get expensive and hard.
 I am going for the easiest natural way to make an off-line Tesla coil, and
leaving it to the future to find great improvements.  Just have to get a
nice first working one in the beginning.

This design looks very promising!  It filters the line and even has some
power factor correction through C4.  The firing voltage is only about 540
volts but worth the vast reduction in complexity (nice range for cheap
IGBTs!)  When you get down into this "conventional" voltage range, a giant
arsenal of devices becomes available!!  All the voltages and currents look
good but timing and such is really important.  Not a big deal since it is
fully electronically controlled anyway.  No big power storage elements and
no heat dissipaters!  Should easily be the most efficient Tesla coil out
there.  Nothing should get hot (or even warm) except the streamers ;-)))

Basically, a bridge rectifier made from some big power diodes (probably in
range of a typical bridge rectifier now).  C4 acts to help filter the line
and power factor.  The value is fairly arbitrary.  L1, L2 and C3 are
resonant tuned to 60Hz to get the voltage up to about 530 volts on a single
cycle (it is running at 120BPS).

The current through L1 and L2 is about 6 amps peak and sort of sinusoidal.
Probably a simple to make pair of inductors.  The value is important.  Lots
of modeling work is still needed especially regarding "when things go bad..."


I looked into IGBTs today.  The big single brick ones run about $700!!
However, if we get "cheap" IGBTs from DigiKey, we can parallel up the
equivalent for about $100 (15 X 200 amp peak TO-247s) .  The cheap IGBTs
have more loss and such, but they are 1/3 the cost of the best ones and
they should work perfectly fine.  A "MMIGBT", (sorry :-)))


I think I can make the primary loop less than 1uH now bit still with about
a 0.2 coupling factor.  The caps and IGBTs are actually part of the
inductor's loop!  This would allow higher frequencies and/or greater power.
 I think I would like higher frequency at this point.  Maybe close to Fo =
30kHz...  Have to watch out for induced voltage in the gates of the IGBTs,
but those nice driver chips are good at that.

The coil's power can be estimated.  A 50uF primary cap firing at 120 BPS is:

1/2 x 50E-6 x 540^2 = 7.29 Joules per bang.  

At 120 BPS, we get a primary power of 7.29 x 120 = 875 watts.

Assuming 90% of that makes it to the streamers, the streamer power is 790
watts.

If a normal coil is 60% efficient, that would be equivalent to a 1300 watt
coil.  Using John's formula, that implies a streamer length of 60 inches.
Not bad at all and hopefully it will be fairly simple to make.  It's ALL a
bit "imaginary" right now, but time will tell...

BTW - Without MicroSim, the Internet, and friends on the list, I don't
think a design like this would be possible!  There is zero "prior art".
MicroSim can crunch through idea after idea for only pennies in
electricity, but it can find subtle fatal flaws in a design instantly!  It
would take weeks and a lot of cash to prove any one of these designs was
worthless without it!  With the Internet, I can suck up data sheets,
component values, who has it in stock, and for how much money in a few
minutes.  This stuff relies an a lot of little details I could not find
"fast" without the Internet.  Of course, many thanks to all 800+ of you for
little hints and tips!  Jim's tips have been especially helpful!  Nothing
like having a few hundred experts helping with a project like this ;-))

Hopefully, it will all work out well and we will soon have a brand-new,
high-tech, class of Tesla coil to play with ;-))

Cheers,

	Terry