Discharging CRT anode and capacitors on a VT52 DECscope

From: Paul Koning <pkoning_at_equallogic.com>
Date: Fri May 28 08:01:59 2004

>>>>> "Tony" == Tony Duell <ard_at_p850ug1.demon.co.uk> writes:

 Tony> Use a limiting resistor!

But not a 100 ohm one -- that might as well be a short. A megaohm is
probably better. Voltage rating may be an issue.

>> In this process, be sure you keep your fingertips WELL away from
>> the metal shaft of the screwdriver. The most natural position for
>> the

 Tony> Considering the shaft is connected to ground through a very low
 Tony> resistance in your set-up, what's the harm in touching it ?

That's true IF the wire hasn't popped loose, and IF the limiting
resistor has a much smaller resistance than your body.

>> There are special "safety" screwdrivers (designed/approved for
>> work in live mains power systems). Those have shafts that are
>> insulated right up to the tip.

 Tony> And are often only rated to 1000V. That's enough for working on
 Tony> live mains (which is where I use them), but CRT anodes run at
 Tony> 10-25 times that voltage.

True.

>> You could give yourself a little extra safety by wrapping a few
>> layers of electrician's tape around the top 2-3 inches of the
>> screwdriver

 Tony> I am not sure what the breakdown voltage of insulting tape is
 Tony> (yes, I always call it that!), but it's not that high. I doubt
 Tony> that a layer would stand 10kV (which is what you're asking for
 Tony> here, at least on a colour CRT).

True again. So I guess "keep your fingers at the top of the handle"
is the better answer.

   paul
Received on Fri May 28 2004 - 08:01:59 BST