Electronics > Repair
Current from Discharging 50Kv Color CRT
Locator:
Hello,
I discharged a good many color CRTs years ago in a shop. I was told the ones we worked on were between 20Kv-50Kv. Does anyone know how many amps color CRTs may have? I have no idea. These were computer monitors and varied in size, from 14” to about 24”.
I saw a video of a gentleman making a CRT discharge jumper and he said he liked to use 25amp cable! Surely, it’s no where near that?
By the way, this was all with 120v USA voltage.
Thanks for any advice.
Gyro:
You'd never get 50kV on a normal domestic CRT, 24kV max (an exception might be high intensity projection CRTs).
The capacitance between the inner and outer coatings of a CRT is in the few nF range, a discharge current through a solid conductor would be relatively large, but vanishingly short in wire thermal time-constant terms. In practice, there would be series of smaller discharges as the wire approaches the cap, with most of the energy dissipated over the reducing air gap [Edit: as a series of sparks]. A direct shorting wire, although common practice among TV repairmen, was never recommended as it could degrade the connection between the anode cap and the painted on internal aquadag conductive coating - some resistance in the circuit was advised.
As you suspect, it's a case of little knowledge making for a more 'dramatic' video.
wasedadoc:
120 or 240 volts is irrelevant.
I don't think domestic direct view CRTs ever went above 25kV.
The amperage of the cable is not really an issue. The thickness and condition of the insulation is.
Gyro:
--- Quote from: wasedadoc on September 03, 2024, 06:14:44 pm ---...
The amperage of the cable is not really an issue. The thickness and condition of the insulation is.
--- End quote ---
Even then, only if you don't trust the chassis connection. Your average TV repairman would just tie or clip one end of a bit of wire to the chassis, wrap the other end around the shaft of an insulated screwdriver and then slide it up under the lip of the rubber anode cap connector.
T3sl4co1l:
Maybe a kiloampere?
Just for a few hundred nanoseconds though, nothing even a thin wire can't handle.
The actual DC supply is a few mA, the trick is the glass tube's capacitance has considerable peak current capacity if you provide it a low-impedance path. A typical jumper wire might be a microhenry, giving a resonant frequency of a few MHz, and a resonant impedance Zo of 10s ohms. That is, starting from a worst-case say 30kV peak, the inductance is charged to a peak current around Ipk = Vpk / Zo or maybe 1kA.
The two most important equations you need for working with LC networks:
Fo = 1 / (2 pi sqrt(L C))
Zo = sqrt(L/C)
A real set will discharge some after turn-off, as the gun doesn't cool down instantly but keeps conducting for some seconds, and the focus circuit likely draws some current from the 2nd anode (internal to the flyback transformer, which is more than just a transformer, but also rectifiers and voltage dividers, often a coupling capacitor too for dynamic focus (Trinitron) sets). A few kV is still the makings of a nasty bite, of course!
Projection CRTs I believe went to 50kV, maybe even a bit more, and had glycol-cooled faces; not to mention more xray risk (but hey, projection, easily solved). But obviously, those aren't "normal domestic".
Tim
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