Re: Need help with mystery

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Steve,

On 22 Jul 01, at 13:34, Tesla list wrote:

> 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

I would say for the same reason that a current waveform which has 
peaks a number of times the RMS value and gives circuit breakers, 
switches and any less than perfect wallplug contacts a hard time. 
I^2.R heating. The resistor can't get rid of heat at the same rate it 
aquires it. I expect comp resistors will suffer the same way. I don;t 
think the self-inductance caused reactance of wirewounds to be 
significant at charging frequencies.

Regards,
Malcolm