TRS-80 Model 12 crt substitution

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu Mar 4 18:56:25 2004

> If you're discharging a recently-used tube, don't use a simple
> resistor. The EHT on a CRT could be anything from 8kV to several tens

Indeed. COnsider several resistors in series -- 50 - 100 1M in series
would seem to be OK. A recomended way of mounting them is to 'wind' the
chain round a plastic strip. That way, points at very different votlages
are kept apart.

> of kV in a fancy colour tube, and most common resistors have a
> breakdown voltage of only 500-1000V. Besides, think about the power
> dissipation if you discharge a 25kV tube through a 10M resistor: 2.5
> milliamps, but 62 watts. Sure, that 62W won't last long, but neither
> the tube nor your resistor will like it!
>

Hmm.. I would be suprised if the capacitance of the average CRT was more
than 110pF (I've never measured it, though). Which, using E=1/2*C*V^2,
would give the stored energy as 31.25mJ at 25kV EHT. That is not going to
damage any normal resistor.

-tony
Received on Thu Mar 04 2004 - 18:56:25 GMT