General > General Technical Chat
How dangerous or lethal is HV in CRT devices?
woody:
I still remember rather vividly how I found out that a CRT is basically a big HV capacitor. It was the eighties, I was young and eager and worked in an outfit that made, sold and repaired slot machines and video games.
One such game had a problem with its monitor. A colleague of mine had removed the CRT from its electronics and placed it on a chair. For the time being. A couple of minutes later someone wanted a seat. I said, oh, take that one, let me remove the CRT from it. I placed one hand on the frame, the other on the little hole in the side where the HV connection goes and learned a valuable lesson. These things hold their charge. For a loooong time.
Ah well, I lived. But it made me weary of high voltages from then on :)
dmills:
By FAR the bigger danger was that most tellies of that generation were live chassis.
The final anode supply could only generate pretty negligible current and even the energy stored in the CRT capacitance was mainly dangerous for the reaction (Dropped at least one big CRT because of an unexpected belt from the anode cap), but if the mains was on and you twitched and touched something you shouldn't have (Tube base and yoke were both likely lads here), a very low current affair could turn into something with the mains behind it.
Regards, Dan.
MathWizard:
Ok so mostly survivable but painful. I think I've shocked myself with a peizo BBQ lighter before, but nothing like an electric flyswatter, or taser, electric fence/etc. Holding a cheap ebay plasma globe once (by the outside of plastic globe) I did not like the tingle I felt, it was like I was getting a bunch of static shocks from my carpet or the bed??.
What about the HV for CCFL in LCD TV's ? I guess it's pretty low current, I have an old LCD TV power board I want to look at the PFC and buck sections on the scope. I was thinking of just disabling the CCFL chip, but I bet utube is full of videos of cool looking things to try with anything HV-low current.
BrokenYugo:
Probing around the hot side of a SMPS, even with all the recommended precautions (differential probe, GFCI, etc.) is way more likely to be life ending than the CCFL inverters. If you're afraid of the latter you should be terrified of the former. I've been bit by a backlight inverter before, IIRC it's more that it quickly burns a hole in the skin than the electric nature of it, fairly high frequency.
As everybody else mentioned, the big risk with the CRT HV is the tube envelope forms the HV filter cap in most CRT televisions, and it holds enough charge to really make you jump and do who knows what damage to yourself. Lower frequency analog scopes that do all the acceleration in the gun (no anode connection near the screen) tend to have a resistor network across the HV supply to derive the focus and screen voltages, so they're pretty safe once powered off, and the power supplies for the final amplifiers and whatnot are probably more lethal energy wise anyway, much like the TV.
CatalinaWOW:
This is one of those loaded questions. There is no question that those CRTs and their drive voltages were potentially lethal. But perhaps the best analogy is tall buildings. Any building over two stories is potentially lethal. You could even roughly equate one story with 24 V. And convince yourself that buildings over 20 stories are just incredibly dangerous. But large numbers of people use and occupy such buildings and the deaths from those buildings are dominated by suicides.
Designers, manufacturers, technicians and hobby people worked on CRTs very frequently, and only very infrequently were injured or killed. Just as with tall buildings the potential is there, but a combination of widely used safety features and widely known usage precautions results in relatively little mayhem.
For a personal anecdote, I generally used the one hand in a pocket rule, attempted to safely discharge capacitors and probed with extreme caution, but still had a couple of oops moments. The reason I am still able to type this in spite of those potentially deadly incidents is a combination of things including relatively high resistance in the ground path from non-conducting floors and relatively non-conducting shoes, high skin resistance because of dry air and personal genetics, good heart health, relatively low energy in the circuits because of intelligent design and a number of other factors.
The high voltage in the CRT is only one of the conditions that needs to occur for a lethal event.
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