# [AMPS] Resistor in B+ lead

David Kirkby davek@medphys.ucl.ac.uk
Thu, 09 Oct 1997 11:21:25 +0100

```I've been pondering the following question, and would apprecaite any
thoughts others have.

Whats the best value of resistor in the B+ lead to protect a power tube in
the event of an arc ??

Obviously, if you make it too large, you dissipate a lot of power, but
ignoring the power dissipation for a moment, is a high value resistor more
likely to protect a tube in the event of an arc than a low one ?? My initail
thoughts were that the higher the resistance, the lower the arc current, and
so the better. However, I'm not now so convinced this is true.

If you make the assumption (possibly  not 100% true) that the arc stops when
a fuse opens, then if you make the resistor bigger, the current is less, but
the fuse takes longer to blow. The parameter i^2 t (current squared times
time), is approximately constant for a fuse. Hence doubling the resistor,
halves the current, but the fuse takes roughly 4x as long to open. Since the
energy in an arc is V*i*t, where V is fixed at about 80~100V across an arc,
I arrive at the conclusion that increasing the resistor actually results in
more energy being dumped into the tube in the event of an arc!! This seems
counter intuitave.

Interestingly, an Eimac application note says use 25 Ohms/ kV of HT, yet on
the Eimac 3CX5000A7 or YC156  data sheets, which both run at typically 5 kV,
they suggest using a 10 Ohm resistor. Theres quite a difference between 10
and 125 Ohms, but I'm now wondering what would be better for arc protecton ??

I apprecaite the current will not remain fixed throughout the period of an
arc if the power supply voltage drops.(which it will in practiice), so this
will modify the conclusions.

G8WRB

David Kirkby,
Research Fellow,
Dept. of Medical Physics,
University College London,
11-20 Capper St,
London,
WC1E 6JA.
Tel (direct line):             0171-209 6406 (+44 171 209 6406)
Tel (department office): 0171-209 6262 (+44 171 209 6262)
Fax:                              0171-209-6269 (+44 171 209 6269)

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