RE: Power Resistor - first light!

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

Original poster: "David Dean by way of Terry Fritz <twftesla-at-qwest-dot-net>" <deano-at-corridor-dot-net>

Hi Steve,

When the SG fires, if it is across the transformer in series with that
resistor, will appear as a short, and the full 10KV will be dropped across
the resistor. ( .045A X 10,000V = 450W ) So as the cap starts to charge, a
lot of energy is dissipated in the resistor and the spark gap. As the
voltage across the tank cap rises, the voltage across the resister and gap
are less, and when the voltage across the cap reaches the voltage of the
source (less the drop across the resistor and gap) the gap goes out. This
all happens pretty fast, and the total time the resistor is dissipating a
lot of power is very short (per total "bang" time) so the average power
dissipated may be well in the range of the resistor, however if the peak
power is very much higher than the rated power, the device may not be able
to conduct the peak heat out fast enough to dissipate it efficiently. This
may lead to elevated operating temperatures in short order. Higher break
rates would compound the problem.

I could see this as an application for a charging choke with a dampening
resistor across it. Of course if you use a variable break rate, then you
would have to make it tunable. Resistors sure are simple though.

later

deano

> -----Original Message-----
> From: Tesla list [mailto:tesla-at-pupman-dot-com]
> Sent: Wednesday, July 11, 2001 8:29 AM
> To: tesla-at-pupman-dot-com
> Subject: Power Resistor - first light!
>
>
> Original poster: "S & J Young by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <youngs-at-konnections-dot-net>
>
> Folks,
>
> Here is a strange, unexpected result of current limiting with a power
> resistor.  I put a 55 watt 1500 ohm power resistor in series with my 10 KV
> DC supply and my SPDT RSG.  Purpose was to limit tank cap charging current
> so I wouldn't pop microwave oven diodes in the power supply.
>
> As I was running my twin TC for the first time, I happened to glance down
> and noticed the power resistor was glowing red-orange!  Not the sort of
> first light I wanted!  But the current was only about 45 ma
> through it.  The
> power dissapation should have only been 3 watts, but it must have been at
> least 100 watts to make it red-orange hot!
>
> So, what's going on?  Has to be that the inductance of the wire wound
> resistor was making it act like a much higher impedance to the
> high current
> pulses going through it.
>
> Putting a 6 mfd cap across the feed to the RSG & tank circuit stopped the
> pretty glow - power resistor was cool to the touch as it was now
> just having
> a fairly constant 45 ma flow through it.
>
> Moral of the story - wire wound power resistors are also
> inductors, and can
> cause weird, unexpected effects.
>
> --Steve
>
>
>
>
>
>