Electronics > Beginners
How did CRT TV sweep down 1 row at a time ?
basinstreetdesign:
--- Quote from: lordvader88 on July 28, 2018, 08:17:47 am ---... but some roommate took it from the basement. Time to find another 1.
--- End quote ---
Time to get another roommate, too. A more honest one.
strawberry:
http://obsoletetellyemuseum.blogspot.com/2010/06/cathode-ray-tube-displays-case-study.html
They are not dangerous, simply don't poke around with both hands. CRT tube is like large capacitor, and glass surface(dirt , moist..) can conduct electricity about 20..~50mm apart from anode plate
Vertical deflection signal is sinus like shape. Horizontal deflection have sawtooth shape, at the end there is beam roll back signal....
BrianHG:
I didn't realize that the OP was at the absolute basic level on CRT tech and CRT color tech. I forgive that my earlier response was intended for someone who was way more advanced and wanted to know about sophisticated super high end computer CRT tech.
Here, watch these videos and you will see everything you need to know about CRT driving, that there are no pixels and why color CRTs also truly have no color pixels...
If you need more, let me know...
IanMacdonald:
The deflection was magnetic, not electrostatic because of the wide deflection angle needed.
The vertical (field) deflection was done with something much like an audio amp driven with a sawtooth signal, except that the feedback was taken from a current sensing resistor in series with the deflection yoke. At this low frequency the coils behaved much like a speaker, having mostly resistance but some reactance. Hence the current in the coils was an accurate sawtooth, but the voltage across them was a somewhat distorted sawtooth due to the inductance.
The horizontal (line) scan at 15kHz could not be done that way because the coils had too large a reactance, in fact they behaved like a near-perfect inductor with minimal resistive loss. To achieve flyback in a reasonable time called for a large voltage, typically 1200v, to be applied for a few microseconds.
The way this was achieved was similar to IC engine ignition - the primary of the flyback transformer (or choke, as far as the scan circuit was concerned) was connected across the DC supply by a special high voltage transistor such as a BU508, the current allowed to build up, then the circuit abruptly opened. The sudden collapse of the magnetic field in the transformer created a large voltage spike, the flyback voltage. This was applied to the scan coils via a capacitor, and caused the spot to rapidly return to the LHS of the screen.
The forward scan drive required nothing more than a fixed voltage to be applied to the coils. Remember they behave as a near perfect inductor, hence with a fixed voltage applied the current, and hence deflection, would rise at a near linear rate. This forward voltage was provided by the charge induced on the series capacitor during flyback.
Consequently the scan waveform looked like a half-sine pulse a few microseconds long, of ~1200v amplitude at the end of each scan. You needed a x100 probe to scope it - a x10 probe could fail as they are typically not rated for that voltage. The DC scan voltage was not greatly noticeable due to its much smaller value.
-and no, the BU508 was *NOT* driven with a sawtooth waveform. It was driven with a square wave of slightly more than 50% mark to space ratio.
Why >50%? Because it also served as a return path for the scan current on the second half of the scan. On the first half the current was in a negative direction and was carried by the reversed C-B junction. Or, by a separate diode. The second, positive, half of the scan current required the transistor to be already on when it commenced.
As with SMPS practice today, the BU508 had to have a fully saturated 'on' interval and a clean, sharp switch-off otherwise it would dissipate substantial amounts of heat due to resistive loss. When the transistor failed repeatedly (in spite of no obvious picture symptoms) it was usually because of a driver stage fault giving weak base drive.
The scan voltage was the one to really watch out for, safetywise. 25kV EHT could give you a painful zap, but it was usually just that. A kilovolt at a substantial current could terminate your existence. Period.
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