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Re: Oscilloscope question - Inverter Powered DC TCs



Original poster: "Kevin Ottalini by way of Terry Fritz <twftesla-at-qwest-dot-net>" <ottalini-at-mindspring-dot-com>

Darin:
        This is a fairly common "inverter" problem, especially with
unprotected Royer
(two transistor self-resonant) push-pull designs.  The problem is the
discharge
transient will often turn off both transistors (or saturate the regulator if
there is one)
at the same time so the inverter just stops working.

There are several possible fixes for this, but it really depends on the
transient
return path (not that putting in several would hurt).

For DC-driven Tesla coils, you don't need to know the frequency.  The output
of the Cockroft is already rectified to DC, and the frequency is only
important
from an efficiency standpoint ... anything above 15kHz is probably fine
(just
best if it isn't audible).  Inverters like this are useless for AC driven
TCs, so
again knowing the frequency isn't all that useful.

First, you need to get the inverter and Cockroft running reliably. Start
with resistive
loads (no sparks!) and make sure your supply can deliver the rated power.
If
things are running well, the output power should be running between 80%
and 90% of the input power.  If the % efficiency is too low, there is likely
to be
something overheating inside.

For the output voltage you want, let's say 2X your 3kv, 40ma  AC input.
With 6kv
output you should be able to drive 20ma output or ~120watts.  That really
means that you should be able to drive a 300K ohm load reliably.  Remember,
you have to dissipate 120watts!  So, 6000vdc / 300,000 ohms = 20ma.

Next, use this 300Kohm resistor on the output of the Cockroft in series to
charge
up the capacitor.  What you have now is a limiter that makes sure you can't
overload your inverter, and an attenuator that will limit any transients
that ring
back from discharging the capacitor.  You can also add a capacitor to the
output of the Cockroft to ground before the limiter resistor, but it needs
to be
rated for at least 2X the operating DC voltage.

Next, you need to run the system without your secondary in place.  This is
to
make sure that the primary system is solid.  I.E., run the system with the
cap,
spark gap and primary.  This will be quite noisy and bright and obnoxious,
but must work reliably.  If the inverter locks up at this stage, we will
need to
investigate the inverter itself in detail to fix it.

This step works well for DC supplies but should not be used with AC
supplies unless great care is taken.

The next few steps are tuning steps, since an un-tuned disruptive TC
generates a lot of offensive electrical noise that will have an exaggerated
effect on the inverter.

Next, add the secondary.  Use a totally isolated ground (like from a copper
water line under the sink) for this test.  I'm assuming that the inverter
output
ground, the Cockroft Ground and the Tank Cap Ground and Primary Ground
are all attached to the standard AC wall ground.  You should have a bleeder
resistor across the capacitor, but an analog meter (not a DMM) is better so
you can see what is happening.  In any case, use a discharge stick every
time you turn the system off to protect yourself.

Make sure the grounds are nice and solid.  Use a good solid piece of
metal to join them all and don't just use clipleads.

With your topload, you should have a rough idea of what tap on your primary
is close.  If not, go to www.pupman-dot-com for links to one of the many great
spreadsheets and calculators.  This is very important, since small coils
have very high resonant frequencies and are tricky to get working without
the right cap size and primary settings.

Assuming then that you know approximate settings, start with a breakout
spike on the topload.  This will help identify your rough tune point as well
as somewhat limit the amount of power needed to get a spark from the
secondary.  A small fluorescent tube can be a big help too since it will
light up if there is a working field even without the sparks.

At this point, even without any sparks coming out of the secondary, if you
have latchup problems with the inverter, than the inverter design is the
problem and you need to add specific local protection (assuming it is
even possible).

If things are still working, then so far, so good!  Carefully move the
primary
tap (first turning off the power and discharging the tank cap!) and record
the approximate AC input voltage needed to get a discharge from the
breakout point.  You can then try +/- 1/2 turn around the best tap point and
see if there is any improvement.

Next, assuming things are still operating, remove the breakout point and
run up the voltage and record the minimum input to get spontaneous
breakout.  This will be about the same if you have a rough or "edgy"
topload, but you may not get a breakout at all with a very smooth topload.

I usually try moving the primary tap +/- again and re-record minimum
input voltages since without a breakout point it is more sensitive to
the primary tap position.

(I found that I prefer a bend-able breakout for my small coils so I can
direct the sparks up and away from the primary and the rest of the
electronics).

OK, if the inverter fails somewhere short of operating, what can you do?

The next step is to make sure that the inverter is protected.  If it has a
metal case, is the case grounded?  If not, add shielding around the
electronics and ground it.  Probably a good idea regardless.

If it is grounded, and adding ground shielding doesn't fix the problem,
then chances are very good that it is an internal problem.

Next, you can try adding ferrite cores on the leads.  Clamp-on types
are easy, but not as good as several turns around a ferrite ring.  Use
one ring for each HV lead.  This is to dampen any high-frequency
ringing (MHZ) that might be affecting the inverter.  If you don't have a
ferrite handy, take a spool and wind it with 10 or 20 turns of the HV
wire (AC line to Cockroft) and see if that helps.

At this point, if none of the fixes has worked, then the main difficulty
probably resides inside the inverter itself which is probably potted.

If it was open, it is unlikely that a schematic is available so one would
need to be drawn.  There may still not be a fix available depending on
exactly what part or parts are sensitive to the transients.

You may need to face the possibility that there is a good chance that it
isn't possible to get this particular inverter to work with disruptive TCs.

I'm sure other members of the list will have suggestions, so work
with it a little and let me know.

Best,
        Kevin Ottalini

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, November 15, 2001 3:51 PM
Subject: Oscilloscope question


> Original poster: "Darin Willson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <darin.willson-at-voyager-dot-net>
>
> I have a 3kv 40ma inverter for a neon sign licence
> plate. I would like to use it to make a VERY
> small coil. I need to find out the frequency
> of its output but dont have access to an
> oscilloscope. I've tried to make a Cockroft-Walton
> voltage multiplier to make a DC coil but it
> appears that this wont work. Every time I
> discharge the circuit it makes the inverter
> stop working and I have to disconnect the
> power and reconnect it. Any info as to why
> this happens would be great and I wont have
> to try to figure out what Im going to ask next.
>
> I have found an oscilloscope program that uses
> a PC's sound card mic input for the signal input.
> Does anyone know how I could safely test
> the inverter using this? Im not to crazy about
> puting 3kv into my pc, but if there is some
> simple circuit I could put between the two
> to effectivly step down the output with out
> altering the signal that would be awsome.
>
>
> Thanks,
> Darin
>
>
>
>
>