Re: High power resistor

["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>

Hi Bob!


Hmmm, looks like there's no easy way round this one if you're using
the resistor for current limiting / smoothing / rectifier protection
in the dc supply.  You do obviously need a resistor.  A choke coil
will aid smoothing, but evidently won't limit dc.

However, a bit of thinking shows a 100W lightbulb will pass around the
same current (100W / 240V = 420mA vs 440mA) at your maximum 1kW / 5k
rating.  For a mere 250mA, a 60W bulb is perfect (240V x 250mA = 60W).
Thus I suggest a string of 60W or a string of 100W bulbs with the
advantages:

1) cheap.
2) simple to upgrade to 100W if 420mA needed (or higher, e.g. 150W).
3) automatic damping - as current drawn increases, so does the
resistance, tending to counteract the increase in current.

For 5k you would need 5x 60W bulbs, resistance per bulb is V^2  / 60 =
960 ohms for 240V bulbs.  5x 960 = 4800 ohms.  This will pass up to
250mA before anything blows.  But perhaps a bit close to the limit for
comfort.  The string will drop 4800 x 0,25 = 1200V at maximum current
draw (before it goes phutt).

If you went to 100W bulbs, resistance per bulb is V^2  / 100 = 576
ohms for 240V bulbs and you will need nine in series for 5184 ohms (or
maybe ten or more if you are running a fair bit under 420mA).  This
would take up to 420mA.  Voltage drop = 5184 x 0,42 = 2200V at max
rating.

The above refers to hot resistance.  Cold resistance may be ten -
twenty times less than hot resistance, FWIW reflector bulbs tend to
have a higher cold resistance than plain bulbs (just been and measured
some :-).

Water resistors sound fine for pulse use or short runs, but I have
nasty visions of boiling resistors if you try using one for a
continuous dissipation of 1kW, especially given the lengthy runs you
do in the museum.

Still, if you use something innocuous e.g. Epsom salts to get
conductivity you could always make a cup of tea with it afterwards ;-)
Assuming the dissolving electrodes don't poison you . . .

Gaffer: "Eeeee, Stanley, put Tesla coil on for a cuppa!  Toroidal
Tetley teabags in your mug . . . thousands of tiny perforations made
by 'igh voltage sparks let flavour flood out . . ." etc etc.

Apologies to non-UK readers (except those in the Boston area ;-)

Dunckx