by Klaas Robers » Wed Aug 29, 2018 12:22 am
Pete, Fly Mario, to give you some overview in CRT's:
1. CRT's are in two variaties:
a. electro static deflection, with deflection plates inside the tube,
b. magnetic deflection, with deflection coils outside the tube, on the neck.
2. Type a. is always used in oscilloscopes, where you want to have a precise and high frequency (fast) deflection,
3. Type b. is mostly used in TV applications, where the deflections are sawtooth wise.
4. This is because it is difficult to have fast changes in the currents in coils.
5. the saw teeth for TV are relatively slow, and always the same frequency (speed).
6. A CRT needs a high voltage between the cathode (filament) and the screen,
7. the higher the voltage, the brighter the trace on the screen,
8. with the same very low current in the beam (generally 0.1 to 1 mA).
9. For (TV) picture tubes the cathode is generally at almost ground potential (0...100 V)
10. and the screen (anode) at a high positive voltage (5kV ... 30kV).
11. This is generally at a separate clip in the side of the picture tube, near the viewing screen.
12. These very high voltages cannot be transported via wires in the neck of the tube.
13. For oscilloscope tubes the final anode (screen) is generally at a voltage of about +600V
14. and the cathode (filament) at a negative voltage of about -1400 to -3000 V.
15. This still gives 2 kV to 3.5 kV over the tube, but for oscilloscopes you need less brightness of the spot.
16. The defelection plates should have a voltage slightly lower than the final anode (screen),
17. These voltages change according to the deflection of the spot that you like to have.
18. Voltages of 400 to 600 volts are just manageable with HV transistors or radio tubes.
The very high voltages are in principle dangerous, but if you design your HV power supplies such that the can only handle low currents (1 mA), you are relatively safe. The danger is in the current, not in the voltage. But any way, when you experiment with high voltages, higher than say 150 volt, only LOOK at your circuits when they are operating, and when really needed, do things with ONE HAND, keeping the other surely on your back. The bad habbit of working at a circuit while it is operating, should absolutely be abandoned. Don't be affraid, but think over before you do something.
In radio transmitters there are high voltages present (2 .... 10 kV) loaded with high currents (say 1A). They are absolutely lethal, but if you know what you are doing, you can have a long and a very pleasant life. Car driving, flying, swiming and diving can also be lethal,..... if you do the wrong things.