Re: Need help with mystery

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

Original poster: "Rick W by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rickwilliams404-at-earthlink-dot-net>

Hey Steve,

Once that 6mF cap charges up the transformer's current would then be
insignificant to the current the filter cap will provide. At the instant the
"charge" spark gap fires the tank cap looks like a temporary short. Though
the time would be very short, 67 * .000000019 = 1.273 uS and with 5 time
constants for a full charge,  6.4 uS, the current pulse would be very high
as you have stated.

The transformer outputs 12Kv but after full wave rectification and filtering
the DC value could be in the neighborhood of 16KVDC. (not sure of the exact
math) so there could be a pulse of 200+ amps for about 6uS. (16000 / 67 =
238amps).

A needle voltmeter won't read this pulse quick enough and will read MUCH
lower than the actual current pulse. A scope with a  HV probe capable of
that DC HV would help a lot to determine the exact current pulse but of
course not many of us have access to that type of equipment.

I don't know how to calculate wattage in such a pulse circuit though.

Rick Williams
Salt Lake City.


----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, July 22, 2001 1:34 PM
Subject: Need help with mystery


> Original poster: "S & J Young by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<youngs-at-konnections-dot-net>
>
> List,
>
> This is more puzzlement regarding a problem I mentioned in earlier posts.
I
> have a DC supply of about 12 KV which has a reservoir cap of about 6 mfd.
> It connects to a RSG which alternately charges the tank cap from the DC
> supply, then discharges it into the primary (SPDT switch action).
>
> If I directly connect the output of the reservoir cap to the RSG, the
> charging gap spark is quite bright and noisy (same as the discharging gap
> spark), and tends to erode the electrodes.  The charging gap sparks can be
> toned down by putting a power resistor between the reservoir cap and the
> RSG.  This lets the tank cap charge a bit more slowly and doesn't burn up
> the gap electrodes as much.  Charging gap spark is reduced to a much
> smaller, dimmer spark, and TC performance is still good.
>
> Here is the puzzle.  My most recent power resistor is 6 400 ohm 10 watt
> power resistors in parallel for 67 ohms at 60 watts.  Current going the
> resistors is high amperage pulses, but the average current is only 50
> milliamps.  Current is measured with an ordinary moving coil ma meter.
>
> Power dissapation for this series resistor is I-squared R = .05 x .05 x 67
> which is about 0.17 watt.  So the resistors will stay cold, right?
Wrong!!
> When the TC runs for a minute, the resistors get literally smoking hot!  I
> am guessing the power dissapated is well over 100 watts!!  This means the
> resistor impedance is at least 40K??
>
> Another clue is that the voltage pulse across this power resistor is on
the
> order of 5 KV as it will jump across a 1/4 inch gap.  If the power
resistor
> is a pure resistor, this would indicate current pulses of about 75 amps
> flowing into the 19 nF tank cap.
>
> What in the world is going on here??  My guess is the wire wound power
> resistors are also inductors - thus the higher impedance.  I suppose I
need
> to make a big bank of carbon resistors (non-inductive) to see if the same
> heating effect occurs.   (another alternative of internal arcing between
> turns of the power resistor doesn't solve the mystery.  This would not
> generate any more heat than pure resistive heating).
>
> So, list, why do my power resistors get smoking hot?
>
> I suppose the same effect should happen with a normal AC powered TC.  How
> about if some of you add a power resistor in series with your spark gap
and
> see what happens?  If you have a 60 ma NST, for example, the RMS current
> through the resistor should be 60 ma or so.  If it was 100 ma through a 50
> ohm power resistor, it should only dissapate a half a watt.  A big
resistor
> would barely get warm after a few minutes.  What do yours do?
>
> Thanks for any light you can shed on this mystery.
>
> --Steve
>
>
>
>