Bleeder Resistors and the Zen of Monitor Discharge

        Hi everyone! Here are some notes relating to this thread (on the
Vectorlist):

        If you can, check out some of your local electronic surplus stores. I've
always used a 'Monitor HV Discharge Tool', and thought everyone else was in
the same practice. It's a requirement for working on old monitors without
integral bleeders. I've picked them up in the past for a song at the
surplus stores - usually only a couple bucks each - and have given them to
others who work on monitors.

        There are a couple things to remember -- if you're using a discharge tool,
take a little extra time holding it under the anode cap. It "slowly"
discharges, taking several seconds to discharge the tube fully. At least
with this, internal components will be spared.

        An instantaneous discharge of potential, although the CRT is fed mere
milliamps of current, by definition is a discharge nearing infinite
amperage. I know it sounds odd, but by definition an instantaneous change
from one potential to another (taking "NO" time) involves the release of
infinite amperage. Now, since this is impossible (everything takes time -
the speed of light is finite), a 'virtually instantaneous' release therefore
approaches this amount. By using a bleeder resistor, you extend the
discharge over several seconds, minimizing the amperage released. It also
keeps the flyback from causing a huge backlash of current as the field
within the transformer collapses.

        If you use resistors in a home-made bleeder, make sure of two things. One
is that you're very well insulated from the potential, and the other is that
the resistors are capable of holding this resistance at the high potentials
involved without arcing. You don't need to worry about the power handling
capability (no need for a 5-watter at 4, 6, or whatever-megohm where a
1-watt resistor will work fine) since you're not trying to carry a load upon
the potential and amperage involved - you're just discharging it. Just be
sure it won't arc from one lead to another while you're discharging the CRT
or your efforts will be for naught.

        24-gauge silicone insulated HV cable will work beautifully. Don't use Zip
cord - that's overkill in the gauge, and underkill in insulating qualities
(600V is a heck of a lot lower than 25kv). Always discharge to the chassis!
A while ago I saw a site where some fool used heavy cables attached to the
grounding pole on a home outlet, while shoving a screwdriver under the cap.
I think there was even a thread about this site on the lists. Needless to
say, that was a site showing exactly how NOT to discharge a CRT.

        If you hear a loud 'snap' when you discharge, that's bad. <grin> Reverse
current is your enemy. It should be inaudible or a very slight 'click' when
you discharge the anode.

        Hope this helps!

        John

---------------------------------------------------------------------------
** To UNSUBSCRIBE from vectorlist, send a message with "UNSUBSCRIBE" in the
** message body to vectorlist-request@synthcom.com. Please direct other
** questions, comments, or problems to neil@synthcom.com.
Received on Sun Apr 23 14:59:17 2000