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]

...
The amperage of the cable is not really an issue. The thickness and condition of the insulation is.

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

[Locator]

I never measured the voltage, just went by what we were told. We discharged them two ways. One was a big “discharge tool”, that had a rubber handle with a resister inside and a needle probe. We clamped the alligator clip to ground on the chassis.

The other was using a jumper with alligator clips. We clipped one end to ground and the other to a screwdriver. I can tell you the wire was no where near 10ga wire, as the aforementioned video suggested.

To this day, I’ve never found out approximately how may amps are in the discharge when discharging the monitors. 2a-5a? 10a-20a? I have no idea.

[T3sl4co1l]

I never measured the voltage, just went by what we were told. We discharged them two ways. One was a big “discharge tool”, that had a rubber handle with a resister inside and a needle probe. We clamped the alligator clip to ground on the chassis.

The other was using a jumper with alligator clips. We clipped one end to ground and the other to a screwdriver. I can tell you the wire was no where near 10ga wire, as the aforementioned video suggested.

To this day, I’ve never found out approximately how may amps are in the discharge when discharging the monitors. 2a-5a? 10a-20a? I have no idea.

See above.  A series resistor above Zo limits current further, slowing discharge and moving the curve towards an RC discharge.  Even a few kohm will discharge it in less than a blink, while limiting current to a thin spark.

It's probably a good idea to limit current, as you may not know where all that current is really going, with respect to the surrounding circuit -- or what voltages you're inducing therein, with such peak currents through the jumper wire.

Similarly, shorting out capacitors to discharge them, isn't recommended; a typical snap-in type mains filter capacitor can sink kiloamperes for microseconds, potentially causing internal damage, putting wild voltage drops in the surrounding circuit, not to mention gouging craters out of your screwdriver.   Just a few ohms suffices to bring that within ratings, and tens of ohms still discharges even large capacitors quite quickly (seconds?).

Tim

[Locator]

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

I was posting my last message at the same time as yours…

I’m not an EE, so am not completely sure of everything you mention. Are you saying these may discharge up to 1000 amps? A 20-30 Kv CRT?

[T3sl4co1l]

I’m not an EE, so am not completely sure of everything you mention. Are you saying these may discharge up to 1000 amps? A 20-30 Kv CRT?

For ten-millionths of a second -- indeed.

For perspective, an ESD spark might be 10s of A peak, with a duration of roughly 100ns.  It doesn't feel like much (I mean, considering the peak power in the 100kW ballpark), but it's way more than enough to cause microscopic damage, hence why semiconductors (with structures on the order of even 10µm being "large") are so sensitive to it, and precautions must be taken to handle it.

For another perspective, a wire might handle its rated current for a temp rise of 10°C or something like that.  For short durations, a second say, the fusing current (meaning, enough to raise it to melting point) might be 100 times higher current.  And so on.  At very short durations, current rating kind of stops mattering, and it's just whether the resistance (or inductance) is enough, given the circuit it's in.

At extremely short time scales, actually just the outer layer of the wire might heat up, and thus heat capacity is less: with nanosecond pulses at high enough power, a wire can be physically stripped, vaporized from the outside; this happens with such shock and ferocity that it's used to ignite secondary explosives ("electronic bridgewire detonator").

Tim

[wasedadoc]

Even after being discharged with a shorting wire, the glass can "relax" and a significant voltage reappear.  As I know from personal experience.  I was exchanging the tube in a flying spot scanner for televising 35 mm slides.  (See image at http://lampes-et-tubes.info/cr/cr149.php?l=e) I removed the old one without issue.  The replacement had seen some previous use but had been in its protective box with shaped internal packaging for years.

Replacement tube box standing upright on the floor, faceplate of tube at the top pointing at the ceiling.  To withdraw the tube I reach down into the box, one hand each side on the cone and receive a very unexpected and unwelcome belt.  I let go the tube which fell back harmlessly into the box.

More than 45 years ago but a lesson I never forgot.

[nali]

I served my apprenticeship on flying-spot telecines. 36kV if I remember correctly... troubleshooting the PSUs was one of the least popular jobs for some reason which is why us apprentices ended up doing it.

Never got bitten though, even though as I recall no-one had any specialised HV equipment back then

[CaptDon]

Did you guys forget the one obvious discharge that was safe and recommended, The Pomona High Voltage Meter / Probe. It had the pointy tip to push under the rubber cup and had a little square meter 0 to 30KV built in to the probe. It drew maybe 1/2 of a milliamp at 25KV and was also a good way to discharge the CRT when the set was off. You could watch the voltage bleed down. And yes, it was common for some charge to build up again after the discharge on the older sets. The newer sets had focus dividers and that function discharged the CRT when the set was off.

[coppercone2]

thick cable is harder to break if its crimped right so there is less of a chance of broken strands under the insulation


even if 30AWG can handle it, I would feel better with 14

the durability as compared to wear and tear from human handling goes way down with higher gauge

something as simple as trying to untangle the wire, after the 100th time, can break it

and what you want is higher voltage insulation, like 3kV rated or better

[Locator]

thick cable is harder to break if its crimped right so there is less of a chance of broken strands under the insulation


even if 30AWG can handle it, I would feel better with 14

the durability as compared to wear and tear from human handling goes way down with higher gauge

something as simple as trying to untangle the wire, after the 100th time, can break it

and what you want is higher voltage insulation, like 3kV rated or better

Good points! I was going to make a couple of jumpers and was trying to find 10ga, silicone-insulated wire. I didn’t think about the insulation voltage rating. I have no idea where to find 10ga wire with 3kV insulation.

Any recommendations on quality wire vendors or brands?

[inse]

If you place the resistor at the tip, all your wire is at outer shield level, considered GND and does not need high insulation.
Look at the wiring inside the TV set…

[jzx]

But use a resistor rated for 30kV.

[jzx]

The replacement had seen some previous use but had been in its protective box with shaped internal packaging for years.

Could it be charged by rubbing against the packaging?

[CaptDon]

There is no point in the wire being insulated to kilovolt or more levels. Worthless advice. First off, the cable SHALL be grounded to chassis or the factory designated point when discharging a CRT of a set that was recently in operation. Secondly, what good does 3KV insulated wire do when you would have 25KV on it in the event you were stupid enough to not connect the ground clip? Any off-the-shelf stranded 300 volt rated insulated wire between 14 and 20 gauge will be absolutely fine. A discharge without a current limiting resistor could possibly damage the connection between the aquadag and the cup. I have never heard of any damage and to my knowledge in tens of thousands of intentional discharges at my father's T.V. repair shop I don't know of any short or long term CRT failures caused by hooking a ground lead to a long thin screw driver and discharging the CRT without current limiting. Most often however, we preferred to discharge with the Pomona high voltage meter which by nature of its physically long internal high voltage resistor limited the discharge current and any potential damage from a high current arc. Some manufacturers specified where the return lead MUST be connected for discharging and I believe it was to the ground straps wrapped around the outer aquadag of the CRT.

[andy2000]

The highest voltage I've heard of on a home TV is 31.5kV, on large 35"+ CRTs.  Most are closer to 25-27kV.  The same goes for typical projection TVs.  They usually run at about the same HV as a color TV, which is a lot for a 7" monochrome CRT.  While I have heard of projectors using 40kV and more, I believe those were all really early (1960's) professional systems, not something you would find in the home.

I'm not sure where this concern about damaging the CRT with the discharge current came from.  I've been working on CRTs for many years, and read many books, and service manuals.  None have ever said one word about potentially damaging the CRT.  There just isn't enough energy there to cause that kind of damage.  What they do caution you about it zapping the electronics with the HV discharge. 

The safest place to ground the HV to is the grounding strap on the outside of the CRT since that directly connects to the outer plate of the capacitor formed by the coatings on the CRT.  If you have a HV probe, by all means use it to gently discharge the CRT, but if you don't, I guarantee you won't hurt the CRT by using a jumper wire.  Buying a HV probe just to discharge one CRT would be a pure waste of money. 

I'm not sure why people obsess over CRT discharging.  Yes, you need to do it if you're going to unhook the HV connection, but it's not a big deal.  It's not even close to the most dangerous thing in a CRT TV.  I would be more worried about the mains, and B+ voltages in the power supply, dealing with sets that have a hot chassis, flying glass if you break the CRT, or back injuries from lifting a heavy TV. 

[wasedadoc]

The replacement had seen some previous use but had been in its protective box with shaped internal packaging for years.

Could it be charged by rubbing against the packaging?

No.

[Locator]

There is no point in the wire being insulated to kilovolt or more levels. Worthless advice. First off, the cable SHALL be grounded to chassis or the factory designated point when discharging a CRT of a set that was recently in operation. Secondly, what good does 3KV insulated wire do when you would have 25KV on it in the event you were stupid enough to not connect the ground clip? Any off-the-shelf stranded 300 volt rated insulated wire between 14 and 20 gauge will be absolutely fine. A discharge without a current limiting resistor could possibly damage the connection between the aquadag and the cup. I have never heard of any damage and to my knowledge in tens of thousands of intentional discharges at my father's T.V. repair shop I don't know of any short or long term CRT failures caused by hooking a ground lead to a long thin screw driver and discharging the CRT without current limiting. Most often however, we preferred to discharge with the Pomona high voltage meter which by nature of its physically long internal high voltage resistor limited the discharge current and any potential damage from a high current arc. Some manufacturers specified where the return lead MUST be connected for discharging and I believe it was to the ground straps wrapped around the outer aquadag of the CRT.

Good points. This jives with my experience in the shop, years ago. I couldn’t count how many CRTs - monochrome and color, that we discharged with just a jumper and screwdriver. I can tell you, that jumper wire was probably no larger than about 16 ga.

My curiosity about how much current is on the wire the moment it’s discharged is simply due to wondering if I should make a jumper with a 10ga wire, after viewing that video. What you’ve said coincides with my experience, though and we never melted the wire, insulation, nor heated it up.