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Hi Folks...thanks for dropping by. This is the business end
of my new SSTC (solid-state Tesla coil) putting out a brush
discharge (about 6.5" max) at about 100 W in. The power
supply is just the mains voltage, half-wave rectified and
filtered with several small electrolytic caps. For testing, I run
the thing from a variac. At this setting I am putting in about
90 VAC. The topload is a pan lid, the secondary is wound
with 30 AWG magnet wire and has 8.5 " of winding on a
~4.4" sewer / drain PVC pipe.
Here is the FET and accessories in the drive circuit. The two
black devices bolted to the aluminum heatsink are both
FETs, although only the one at the top is connected in the
circuit. This is an NTP12N50 (12 A, 500 V) from ON
Semiconductor. The other one is an IRF740 (10 A, 400 V).
Both offer nearly identical performance, except I have lost
a couple 740's in this circuit (hence my attempt to
upgrade to the beefier part.) The two diodes above and to
the left of the FETs are 3 A, 40 V schottkys in parallel
(I wasn't sure if their peak current ratings were good
enough by themselves). Directly above the NTP12N50, and
partly obstructed by a reddish capacitor, is a fast diode
(U1560 from ON) which I believe is rated for 15 A / 500 V. A
strip of Cu-clad board behind the FETs is a ground, and the
one behind that is the B+ (high voltage) rail. As you can
see, I put a lot of various capacitors across B+ and ground.
All are cut out of old equipment. Not seen are a few
electrolytic caps and the drive circuit itself, which is just a
little piece of perfboard with a 556 timer, a FET driver chip,
some pots and whatnot. One of the 555s in the 556 acts as
a "staccato" interrupter, enabling the other timer which is
the RF oscillator. The gate driver chip sharpens the pulse
edges up and directly drives the FET. The small reddish
capacitor on the above circuit that goes across the fast
diode is a 0.001 microfarad that I put there to short out very
fast spikes. Its worth in the circuit is dubious. Let me
know what you think.
Here are the 'scope traces of gate drive (the
square wave) and the voltage level at the FET drain.
The gate voltage was measured directly with the probe of
channel 1 on the gate. The drain voltage was divided by a
94 kOHM resistor and a 6800 OHM resistor, in series,
going from drain to ground, and the 'scope probe for
channel 2 was connected across the 6800 OHM. DC
supply voltage at this time was 50 volts, so with the sharp
peaks of the drain at about 7 V on the 'scope, this means
that the drain is seeing about 100 V at this time, or twice
the DC input. Let me know if you think this is BAD (I do!)
You'll also note that the duty cycle is not 50%. I have been
told this is bad, but the coil resonates with this type of feed.
The "on" pulses are 2 microsec. long, the "off" pulses are
only ~750 nsec or so. Comments?
My e-mail is email@example.com or just make a post
on the pupman list.
Here is another 'scope shot looking at the drain
voltage. I have always thought that the drain voltage should be a nice square wave ideally...although
the drain current may vary sinusoidally (in the ideal case anyway). The FET is off when the
drain voltage is high in the above traces.