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Re: Tube coil capacitors (implosion hazard)
>
> Original Poster: "Robert W. Stephens" <rwstephens-at-headwaters-dot-com>
>
> Tesla List wrote:
> >
> >
> DANGER !!! I would not advise employing a large CRT as a capacitor
> unless the vacuum inside has been replaced with air. Either puncture the
> thin metal at the bottom of the HV connector well carefully and gently
> with a sharp pointed awl, or snip off the glass evacuation stem at the
> e-gun end with wire snips or pliers. During the operation as a
> capacitor the glass of the tube could puncture and possibly cause the
> tube to implode with potentially catastrophic effects. The high speed
> glass projectiles from a crt implosion can do a lot more damage than
> just take out your eyes! Be ever careful!
>
Robert's (and other's) advice to bleed the vacuum is well taken. However,
as a practical matter, it is quite difficult to get a modern CRT to implode
catastrophically.
In my former employment as a special effects person, we destroyed many,
many CRT's of all sorts, kinds and sizes, usually by dropping them on cue.
Not one imploded violently. (some did throw shards, but that was because of
the explosives we used to make them break when they hit the ground, or when
the actors foot hit the screen, etc.see #1, below).
Some empirical data:
1) To reliably get the faceplate to break you need shaped explosive charges
or a fairly high energy projectile (for instance, a .22 LR bullet will
bounce off).
2) Second, most, if not all, modern CRT's have some sort of plastic film
over the outside presumably to prevent shards of glass from flying if you
do break it.
3) Dropping a TV generally doesn't break the crt. If it does, it breaks at
the end of the neck (near the connector), which bleeds the vacuum down
fairly quickly.
4) Small holes in CRTs don't cause them to catatstrophically fail. Here's
my guess why: Glass is expensive and heavy, tube mfrs don't want to spend
and more, or have their tubes weigh any more (probably the more important
reason), so they design the envelopes so that they aren't critically
stressed. Without stress concentrations, punching a hole and relieving the
stress in one place, doesn't transfer much stress elsewhere. The tubes also
are obviously not tempered glass and appear to be annealed.(which has a lot
of internal stress in it, just so it breaks into tiny pieces when broken).
5) Look at it from a stored energy standpoint. The volume isn't all that
great inside the tube (say a liter or so) and the pressure is low (1 atm),
so there isn't that many joules of stored energy to move the particles,
barring a freak accident where all the energy happens to go into one
particle.
This said, I wouldn't be stupid and line up a bunch of CRTs on my bench and
start plinking them with a bb gun. However, the danger from imploding CRT's
is probably overstated. Throw a blanket over them and you have nothing to
worry about.
<<<< this is off topic, but since it is safety related I let it go... -
Terry >>>>