Give it a rest!
Thank you,
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Landman
Sent: Monday, August 19, 2013 3:55 PM
To: [log in to unmask]
Subject: Re: [TN] Refreshment
Henning has replied (I knew he'd have the answer :-))...
-----Original Message-----
From: Leidecker, Henning W. (GSFC-5600)
[mailto:[log in to unmask]]
Sent: Monday, August 19, 2013 2:13 PM
To: Bob Landman; TechNet@IPC
Subject: RE: [TN] Refreshment
The tubes have a glass dielectric between conductive surfaces, and the glass develops a persistent polarization when left "on" for along time. So, shorting the conductive surfaces dumps the charge there, achieving V = 0 as measured between the two conductive surfaces, but the stored polarization in the glass remains for a long time (can be months or more), and this polarization attracts charge back onto the conductive surfaces, preparing a "surprise" for someone who places body parts across the two conductive surfaces. This discharge-recharge process can happen a number of times. Something like this is a standard experiment in electrostatics labs.
This also happens to capacitors. People have been killed this way. If the capacitor has been charged to V, and then discharged, one should suppose that it could recharge back to V again -- the time depends on surface films that conduct the charge to the plates. If the capacity is large enough to source a milliamp thru a person, when the capacitor is charged to V, then "beware!" One should connect a shorting wire between the terminals, and check on a schedule that this shorting wire is still present. (But remove that wire before returning the cap to use.)
I took a large TV tube, wrapped it in a blanket, and heaved a brick at the front. A wonderful window-womping thump!
Best,
Henning
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Landman
Sent: Monday, August 19, 2013 1:24 PM
To: [log in to unmask]
Subject: Re: [TN] Refreshment
James,
Great story! And what fun!!
Did you discover (the hard way of course) that CRTs can have a residual charge after you discharge them by jumper wire?
My company has a product that still uses b/w CRTs. We custom modify them so have to take them apart. When reassembling we re-ground the anode terminal to the ground band (which is making the electrical connection to the conductive outer coating of the glass envelope.
I don't have a scientific explanation as to why there can be a charge left. You'd think the arc created when you initially discharged the tube would dissipate the energy.
This is a perfect question for my former physics professor and renowned tin whiskers expert Henning Leidecker at NASA Goddard SFC so I'm cc'ing him :-))
Bob
Sent from my iPhone
On Aug 19, 2013, at 11:19 AM, James Head <[log in to unmask]> wrote:
> When I used to work for Toshiba at the TV plant in Plymouth we were meant to break the tube necks before disposing of CRTs but we thought it was more fun to put them in the crusher unbroken and watch the old crusher shudder. We once had a high-end special 44 inch tube. We wheeled it out and very carefully lifted it up to place it gently face down in the crusher on its own, pushed the green button and hurried away. There was a low thumping bang and the whole cage around the crusher rattled. Crushing flat panels was not as exciting.
>
> We used two large screw drivers to discharge the tube before working on a set. Holding both screw drivers like sheers so they touched each other, we touched one screw driver on the ground side of the tube and wriggled the other under the anode cap. I had a few 35 kV shocks though when I forgot to only hold the screwdriver by the handle (doh!). Fortunately I remembered to hold them correctly the one time when I tried to discharge a tube where the TV was still switched on - although we used 1:1 isolation transformers I suspect that it still could have been very painful.
>
> James
>
> James Head BEng CID+ MIIE MIET
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