Re: Power Resistor - first light!

["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>

Hi Steve,

I'm building one of those DC outfits. SPDT RSG and all. NST bank almost
completed, 15Kv unrectified. Will be shooting for 150ma. The microwave
diodes I've selected, not purchased yet, are rated 16Kv -at- 750ma. Building a
full wave bridge, 2 diodes in series per leg, filtered with a 30Kv cap -at-
1.0uf.

Your addition of a limiting resistor interests me. The tank cap will be up
to .1uf in .01 steps for adjustment, the ones that have been tested and sold
here. I'm hoping the filter cap will reduce surge through the supply. I have
10 Dale 50K ohm 50W power resistors I could parallel for 5K for limiting.
This would result in a full charge time of  2.5 ms with the .1uf tank cap.
(5 time constants). These are wire wound also. So your post caught me eye.

About that 6mf cap you used. Did you connect it as a "filter" cap for the DC
output? And I'd be interested in hearing about that SPDT RSG also like how
is it constucted?

Thanks!

Rick Williams
Salt Lake City

----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, July 11, 2001 7:29 AM
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
>
>
>
>
>
>