Re: Australian Source of Resistors?

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

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Date: 01 January 2001 02:55
Subject: Re: Australian Source of Resistors?


Hi Simon, Hans et al!


>Original poster: "Hans Scholze by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <hanscs-at-eagle.ptialaska-dot-net>
>
>Simon
>
>How about an "MMR"?  I am planning on building Terry's
>filter with eight (four per side) cheaper, smaller 25W 220
>ohm power resistors, the kind that look like little white
>blocks of ceramic.  I don't see wht this wouldn't work, as
>the power dissipation and voltage would both be spread out.
>They should be much cheaper and easier to find that the
100W
>ones, even in Australia.
>
>Hans
>


This is the route I went for my 500VA coil 3 years ago -
used 10 x 1k 11W resistors from RS Components
http://rswww-dot-com who quite possibly have an outpost Down
Under.  If I recall your setup, it's a 15kV 30mA unit, so
450VA.  You'd want to lose around 25W in the filter, so I'd
calculate it thus.  For 25W, 30mA current requires a
resistance of ca. 28k ohms (i^2 R) so stick ca. 15k ohms in
each of the EHT leads, which to me sounds like three five
watt rated resistors of 5k each in each of the two lines, so
you need six 5k 5 watt resistors, or equivalent, in total.
For the sake of some headroom, I'd use a rating above five
watts, say around ten watts for reasons below.  The total
volts drop at 30mA current draw and 30k ohms will be 0,03 x
30000 = 900V.  If you use six resistors, that's only 150V
per resistor which should be fine, well within ratings.

When running with the main gap closed down for tuning you'll
find those resistors dissipate **a lot** of extra heat, so
having some headroom on the watts rating is highly
advisable.  For example, if the main gap is firing at 5kV,
then a whole 10kV has to be dropped across the resistors
unless you have a variac to turn the wick down (another good
reason for using a variac), which would dissipate 300W in
the resistor chain!!!  If when tuning you just give the
thing a brief "squirt" for no more than a second or two at a
time it probably won't object much (mine doesn't but YMMV)
but this should tell you one reason why your streamer length
hasn't been too good when running with only part of your
main gap in service - the power is cooking the resistors
instead of the air ;-)

Hope this helps!

Dunckx