moroney@jon.DEC (Mike Moroney) (05/30/85)
>Sun workstations (ANY CRT for that matter) *is* a negative ion generator. >Therefore it is unlikely you need to protect it from other ion generators. >When I touch with my finger the screen of my Sun 100U workstation, there >is a crackle of discharge of static electricity between my finger and the >screen. The reason for this is that every CRT works on the principle of >beaming electrons from the cathode at the back of the CRT to the phosphor >inside the screen, which means that the negatively charged electrons tend >to charge the screen negatively, hence the screen becomes a generator of >negative ions unless it is thoroughly grounded. Most CRT's have the >cathode at a very high negative potential and the phosphor grounded to >minimize this effect (as you wouldn't want to get a serious shock from the >alternative arrangement of putting the cathode at ground potential and >raising the screen to a large positive potential). The higher performance Not quite true. Almost all CRT's have the cathode at about ground and a very large positive charge on the anode, since the alternative requires some tricky electronics. You don't get a shock from touching your screen since glass is a very good insulator. I don't know myself whether the static charge that accumulates on the screen is from the negative electrons hitting the screen or stray positive charge from the anode, since glass isn't a perfect insulator. "There's a madness to my method." Mike Moroney ..decwrl!rhea!jon!moroney
mike@amdcad.UUCP (Mike Parker) (05/30/85)
> >cathode at a very high negative potential and the phosphor grounded to > >minimize this effect (as you wouldn't want to get a serious shock from the > >alternative arrangement of putting the cathode at ground potential and > >raising the screen to a large positive potential). > > Not quite true. Almost all CRT's have the cathode at about ground and a > very large positive charge on the anode, since the alternative requires some > tricky electronics. You don't get a shock from touching your screen since > glass is a very good insulator. I don't know myself whether the static charge > that accumulates on the screen is from the negative electrons hitting the > screen or stray positive charge from the anode, since glass isn't a perfect > insulator. > > "There's a madness to my method." Mike Moroney > ..decwrl!rhea!jon!moroney I worked in a tv shop as a youngster, I didn't understand much of what we did at the time, but I remember a few things that might shed some light on the subject. First, the static is caused by the electron beam from the cathode. When we got a set with absolutely nothing on the screen, not even static, we would feel the screen. No static meant no drive to the cathode, bad amplifier. Static meant that there was no drive to the anode, bad high voltage power supply. Second, I know that the cathode is not at ground. We used to measure cathode voltage with a high voltage probe using the chassis ( ground ) as a reference. Old B+W sets measure over 15,000 volts, new color sets measure as high as 35,000. For those who question my terminology, the cathode voltage is measured at one of the pins on the back of the neck, the anode is under the suction cup. The anode is also not at ground, it's the part that Dad always said would kill you if you didn't know what you were doing. So, let's move the discussion to net.analog and see if someone can explain the facts. Mike