# Cathode Ray Tube question

Discussion in 'General Electronics Chat' started by Rolland B. Heiss, Apr 14, 2015.

1. ### Rolland B. Heiss Thread Starter Member

Feb 4, 2015
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I just picked up a nice 28 inch sanyo crt tv someone near me was giving away that I plan on using in my bedroom in place of another nice but smaller Sony crt I got elsewhere. Anyway, after I returned home I remembered that there is a high amount of voltage these tubes can hold for a very long time even after being unplugged. Now I realize voltage is one thing but does anyone know the amperage they may contain? I'm sure it would vary by the size of the tube. The reason I ask is because I thought it might be an interesting experiment trying to make something in order to tap into this energy when the tv is turned off or when it has been unplugged for an extended period of time. Something that might power some bright LEDs for a decent amount of time as a night light or should the power go out or whatever else the power might be useful for. Just a stray thought that hit me a bit ago. Input always appreciated.

2. ### Papabravo Expert

Feb 24, 2006
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Sounds like a neat idea, but check out the threads on "super capacitors". As soon as you start to draw current from them, the voltage drops like a brick in the rain. The more current you try to draw the faster the voltage declines. Lot of effort with very little payoff.

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3. ### WBahn Moderator

Mar 31, 2012
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Keep it a stray thought.

The high voltage cap inside a CRT is basically the CRT itself. For a 28" screen you are probably talking something along the lines of 25kV to 30kV (I don't know for sure, I just recall a rule of thumb of 1kV/diagonal inch) and perhaps 5nF to 10nF (another WAG). So that would be as much as perhaps 5J of energy stored. I think a typical stun gun is about the same voltage but about one-tenth the energy. So it is getting into a range that is probably potentially lethal and likely to make you really regret getting tangled with it.

Any circuitry that you add to tap into this energy has to be able to withstand these kinds of voltages AND you have to do it such that you don't mess up the normal operation of the CRT.

And how long could 5J of energy power a typical LED (not even a "bright" one)? Well, 20mA at 2V is 40mJ/s. So assuming 100% efficiency, you could power it for about two minutes.

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4. ### Rolland B. Heiss Thread Starter Member

Feb 4, 2015
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Thanks for the input y'all. I'll take WBahn up on his advice and keep it a stray thought.

5. ### crutschow Expert

Mar 14, 2008
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I remember someone years ago stating that they were working on a TV picture tube replacement and accidentally touched the HV anode while trying to remove the anode lead. As I recall, the next thing he remembered was being half-way across the room with the HV lead wrapped around his hand they he had ripped from the TV and taken with him on his journey.

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6. ### dl324 Distinguished Member

Mar 30, 2015
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Before I knew better, I worked on a BW TV and accidentally discharged the CRT through my forearm. I've rarely ventured near the high voltage section of a TV since then. Only 8KV and not fun.

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7. ### AnalogKid Distinguished Member

Aug 1, 2013
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Did a lot of TV repair back in high school, got zapped a few times. But the worst was a shock I got from the boosted B+ (not the CRT anode) in an old, fat RCA color set. 900VDC drilled a small hole clean through the tip of my thumb. Self-cauterizing, and sorta kinda hurt (!!!).

ak

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8. ### #12 Expert

Nov 30, 2010
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Same here. Silvertone TVs had all their screen control pots on the rear edge of the chassis...with the legs pointing up!

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Jan 15, 2015
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Not a good idea. Could result in a shocking experience. Things like sleeping dogs and the wire on the side of the CRT are best left alone.

Ron

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10. ### Rolland B. Heiss Thread Starter Member

Feb 4, 2015
236
40
Interesting you should mention Silvertone. Someone has an old 1950's Silvertone for free up on Craigslist today in my area and I've been thinking about picking it up. Problem is it needs a LOT OF WORK! But it brings back memories and if nobody wants it they are going to throw it away. Somehow that seems like a shame.

11. ### sheldons Well-Known Member

Oct 26, 2011
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You need to treat the final anode of your crt with a lot of respect, as even over time there can still be quite a high potential present with enough current available to give you a good tickle shock wise.....most crts of that size 25kv-30kv was normal....will certainly make your eyes sparkle

12. ### alfacliff Well-Known Member

Dec 13, 2013
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the horizontal out put tube was another hazard, it used that boosted b+ and had high voltage rf on it too. you could take an insulated screwdriver and draw an arc from the plate cap wuite a distance. the horizontal output stage was the most useage a tesla coil ever got commercially.

13. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
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FWIW: A CRT, within the confines of this discussion, may be regarded as a glass dielectric capacitor featuring graphite ('Aquadag') 'plates' -- In essence, a variant of the 'Leyden jar' q.v.

Please be advised that the electrical hazard, while significant, both augments, and is eclipsed by, liability to injury corollary to implosion --- glass + force + electrical 'gotchas' are a recipe for disaster!

If you must experiment with 'high-tension' capacitors - small Leyden Jars or ceramic 'skate wheel' caps are superior choices from a safety standpoint...

Best regards
HP

Last edited: Apr 17, 2015
14. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
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Indeed! Moreover, interestingly enough, instead of flyback topology, some of the older sets featured a series-resonant HVPS --- Hence true TC topology!

Best regards
HP

15. ### ian field Distinguished Member

Oct 27, 2012
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Any attempt to tap into that stored charge is likely to compromise the insulation - TV EHT can crack over a few inches when the flyback is running, if the other end of the arc finds the main PCB - its toast!

Although the glass of the CRT flare has good dielectric properies, the conductive coatings inside and out don't create a huge capacitance - you can get a lively crack off it because its charged to such high voltage. With the set off - that charge can leak away in minutes when humidity is high.

Jul 18, 2013
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I have had my share of CRT HV zaps and agree, not pleasant, but in the case of a non-powered CRT it is more of a unpleasant experience than a real danger, I always thought there was a little too much hyperbole on this and this sort of confirms it.
Max.

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17. ### alfacliff Well-Known Member

Dec 13, 2013
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some of the video game crt's had a prewtty good wallop too, especially if the power supply failed. it used a rectifier off the ac line regulated down to 130 volts or so for the horizontal stage which came up with the lower voltages for the monitor. an zray protect circuit shut down the hroiz stage if the voltgae got too high, but as the xray rpotect shut it down, the voltage would go up to high enough to give a really impressive (and painfull) spark.

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18. ### ian field Distinguished Member

Oct 27, 2012
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Since the advent of transistor flybacks - you could probably do as much damage to the TV as yourself by shunting the anode cap with your hand.

With valves you could probably make the horizontal valve's anode glow red - but you probably wouldn't be taking much notice!

The very early B&W TVs had mains derived EHT - it was only about 6kV, but similar lethality to a microwave oven transformer.

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19. ### ian field Distinguished Member

Oct 27, 2012
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The early SVGA monitors had a buck regulator for the line stage B+ - the longer the scan period; the lower the B+ had to be, or the core saturated.

These used to arrive on my bench one after the other with shorted buck MOSFET and ditto the horizontal transistor.

Later models used only about 60V HT from the main SMPSU, boosted as required by a flyback regulator. If the flyback booster failed S/C it just tripped the overload protection in the main PSU.

That latest development came just berely a year before CRT monitors went the way of the dodo.

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20. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
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While I tend to agree that the shock hazard (in and of itself) is negligible to healthy individuals, the real danger, IMO, owes to (corollary) involuntary motion with attendant liability to dropping, or otherwise disrupting, the tube...

Best Regards
HP