DIYbanter - View Single Post - Does the CRT deflection coil hold enough charge to zap somebody?

Arfa Daily

"hr(bob) " wrote in message
...
On Jul 16, 4:55 pm, Bill wrote:
Franc Zabkar wrote:
On Wed, 16 Jul 2008 08:48:29 -0700, "Nicole Bischoff"
put finger to keyboard and composed:

Richard the St00pid Bullis says so on 24hoursupport.helpdesk.

A coil holds no charge. A capacitor does. However a coil's back emf
may belt you if you interrupt its current flow.

- Franc Zabkar

If the set is on and you are playing near the CRT you will get zapped
from the CRT and not the coil. If it is off you can get nailed by the 25
KV of the anode capacitance which can be there for days. I got knocked
across the room once by a hand to foot discharge, some people get killed
by it, and I now put my elbow on the chassis so only my arm gets hit, a
big ouch but not dangerous except to your arm and finger nerves.
Always discharge high voltage. I got nailed by a 2,000 volt capacitor on
a microwave that had been off for 3 days, less voltage but a big uF cap.
Bill Baka

Don't just discharge the anode one time, do it once, wait about 20
seconds and do a second discharge. There is a phenomenon that I can't
recall where the charge is stored partially in the glass envelope and
the voltage can rebuild up to a few kV after the initial discharge
unless the initial short lasts for several seconds.

Bob Hofmann

I was taught that it is because the area of the effective capacitor formed
by the internal anode coating, the glass, and the external coating, is so
large compared to the single small discharge point, that much of the
residual charge sort of 'rushes away' as far as it can to try to re-eqaulise
itself into an evenly distributed high voltage. Then, over a couple of
minutes, it spreads out again over the whole internal surface of the tube.
If you discharge over a couple of minutes with a resistive probe, you don't
get a problem.

Back in the days, I had an old anode lead cut from a scrap FBT, that fed
into a small plastic box that had a number of 10k resistors in it, in series
to up the overall voltage rating of them. Coming out of the other end, was a
lead with a croc clip on it. You used to just hook this to the external
coating grounding spring, and then plug the cavity connector into the tube.
Then go for a cup of coffee. When you came back, the tube could then be
dealt with in complete safety, including removing it for replacement.

Arfa