Re: Power Resistor - first light!

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Steve,

You may want to check the power resistor's resistance with an ohmmeter.  It
would not be the first time one of these things was miss marked.  Jon had a
similar problem and found his resistor was on order of magnitude different
that the marked resistance.

Cheers,

	Terry


At 10:56 PM 7/10/2001 -0600, you wrote:
>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
>
>
>
>