EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: MathWizard on January 28, 2022, 04:16:28 am
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I guess there have been lots of linesmen, utility workers, industrial workers that get hurt or killed working on high energy systems. What about repair technicians back when all TV's and things had CRT's in them ? How dangerous was it really and how much worse is it if the TV was on when u made a mistake or had an accident ?
I have a couple of old things with CRT's in them, they "only" have about 1000V or so tho. I plan to know those circuits backwards before I go looking at that stuff. I wish I had an old, all/mostly discrete TV to play with.
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As I remember it, those CRT anode potentials were indeed dangerous. For color TV they were up in the 25 kV range. It doesn't take much capacitance to store a lot of energy at those potentials. And energy is what can get you. Any voltage above around 100V will cause conduction of human tissue, enough to cause problems. As for electrocution, I believe the old electric chairs were around just a few kV.
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I guess there have been lots of linesmen, utility workers, industrial workers that get hurt or killed working on high energy systems. What about repair technicians back when all TV's and things had CRT's in them ? How dangerous was it really and how much worse is it if the TV was on when u made a mistake or had an accident ?
I have a couple of old things with CRT's in them, they "only" have about 1000V or so tho. I plan to know those circuits backwards before I go looking at that stuff. I wish I had an old, all/mostly discrete TV to play with.
A CRT television shock certainly could stop your heart if you were unlucky--and it did happen--but the majority of the time you just got a really hard wallop, like a very powerful electric fence. The total energy in an CRO without post-deflection acceleration is reasonably low, but not something I would willingly risk getting fried by. I don't know how much worse it would be if the device were on, but it certainly isn't better. With a capacitor, even if you can't let go it will finish discharging very quickly.
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EHT output from flyback transformers are typically around 25 kV and deliver very little current on their own, if disconnected from the tube. I re-purposed a few of those, using a custom driver circuit, and got zapped by those a countless times. The real danger comes from the tube itself, as conceptually operates like a high voltage capacitor, and an unplanned anode discharge through your body could be potentially lethal.
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A teenager like me, back in 1991, seems to have survived well, since I was trained well and didn’t make daft assumptions.
Back in ye olden days, back when people had brains that were generally fully functional, and I’d think that generally no mollycoddled and dumbed down fools were in charge of repairing equipment, when training manuals and service schematics were properly written, engineers were fully competent and aware of the high voltages present on the usual suspects. This fact has been pointed out by Paul Carlson (“Mr Carlson’s Lab” on YT) many a time - people didn’t need “protecting” with PVC boots and isolated guards, as they knew what they were doing.
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Then you've got mains frequency EHT transformers which were fairly common in vintage CROs. Those could supply an overload of tens of mA for several minutes before they'd burn out, so were absolutely lethal as they could continuously electrocute you, rather than the one big shock then typically 1mA a flyback EHT transformer could deliver, even though their EHT was typically lower than you'd find in a large screen CRT TV with a flyback EHT transformer.
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A CRT had a lot less HV capacitance than a mid sized van de Graaff. For an operating device it was probably less dangerous than mains, more likely to knock you off. On the other hand, it might knock you into a mains connection.
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As the old saying goes:
It's volts that jolts, but mils that kills!
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Best the ranges of voltages found in such devices are properly specified and there's normally 4 to be wary of.
Lowest is the CRT plate output supply of transistor based scopes but it can still be some 150V.
Next is the older instruments using hollow glass and these can top out ~350V.
EHT is next, typically negative for the CRT cathode, grid biasing and such and a wide range of voltages exist in various equipment, 1-3 KV is typical.
PDA for the CRT anode when used is typically derived from the EHT typically using triplers or quintuplers and in scopes can approach 10KV.
They all bite and one should use safe practices when playing with this stuff and take great care.
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As a kid I worked repairing all sorts of things to pay for college, color TVs were one of those. The CRTs could hold the 25KV charge for quite some time, you quickly learned to discharge the CRT with a screwdriver before poking around inside the TV :o
The biggest risk wasn't from the shock but the reaction to the shock, recall gashing my arm on some sheet metalwork inside the TV when I brushed across the HV CRT cable :P
Best,
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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 :)
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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.
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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.
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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.
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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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I've been zapped by the EHT on a 19" CRT before, it HURT, but I'm still here. It felt like a powerful static electricity zap. Technically it could mess up your heart, but it's unlikely in most cases. Even so, be very careful.
The ~180V B+ in many CRT displays is actually much more dangerous than the EHT, it can deliver a great deal more current.
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By far the most dangerous part of any CRT display is the mains supply and B+ voltage. The HV supply is well insulated, and not capable of supplying much current. A microwave oven is much more dangerous.
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... except as I stated above - mains derived EHT for a CRT in a really vintage CRO or valve TV is generally at least as dangerous and possibly more dangerous* than the HT in a microwave.
(https://www.oldtellys.co.uk/ltbeht1.jpg) (https://www.oldtellys.co.uk/otltbeht.html)
* Its typically several times the voltage of microwave HV so will jump further to bite you! OTOH the reservoir capacitor is typically a much lower value than a microwave one, so there *may* be less energy instantaneously available to blow chunks out of your fingers and stop your heart! :-\
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Bonjour a tous: Just notice this thread now, late to respond...For decades I had designed and manufactured special purpose magnetics, HV modules, CRT display drivers and HVPS.
1/ classes of HV, with different hazards:
Mains freq transformer raw PS with large filter caps
High freq (flyback) transformer with voltage mult or rect, small caps (most CRT)
DC HV at the CRT leads
2/ Shock damage is often collateral: Your hand touched the CRT Ultor anode connection, hand and arm are shocked, your sudden motion throws you to the floor or hand hits the CRT (see below)
Certainty shock effect its unpredictable as some are more susceptible (moist skin? Pacemaker?, Elderly? Standing with thick rubber shoes or barefoot on concrete that's wet?)).
3/ CRTs are large vacuum vessels designed to be in a TV or scope with safety shields and safety bezel.
In servicing we may have to defeat interlocks and remove such shields.
Striking a CRT can break the glass, causing a serious implosion.
Glass shards are thrown many meters as shrapnel.
Extreme danger to face, eyes, bleeding...VERY bad.
Risk of implosion damage is like the CRT dia exp 3. (volume enclosed)
(NOTE: in my experience I have had all sorts of shocks and at least two implosions of CRT or arc lamp)
4/ DC and mains freq is most dangerous, high frequency AC HV like a flyback or Tesla coil may no shock, but the RF energy may cause burns and arcs.
5/ Typical scope HV 2kV...15kV, color TV were up to 25 kV and projection TV to 35kV.
The Aquadag (conductive anode coating) to ground capacitance is typically between 50...3000 pF.
Calculate stored energy 1/2 C *Vexp2.
I strongly suggest the OP read the literature and books of CRT and HV safety before proceeding with his experiments.
Nikola Tesla (NOT THE CAR!) would always stand on rubber mats and keep one hand in his pocket!
Bon Chance!
Jon
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This is a very interesting that this has come up . How dangerous or lethal is HV in CRT.
Reminds me of an accident that happened in one my father's TV & Radio shops .. Way back in the 60s.
One of the techs was working on a very large TV . When he some how touched the HV wire to the tube . These old
sets had about 25 - 30KV . The The shock through him across the room with enough force to Crack his head open on the
Wall on the opposite side of the work shop . If I remember right, he had over 10 stitches.
Its not so much the voltage its more how your body mussels react to electricity .
And whats in your flight path when landing.
Golden Rule Discharge First . :-+
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trivia 4 u
I would play with old valve TV chassis as a kid, would get blue corona discharge off the CRT HT make pinwheel spin.
I can not say how much ozone I generatord. but never got zapped because i'm still here ;D
its unknown the level of x rays coming off the output valve .
but many power valves of that time did emit x rays from the gaps in the sides of the plates.
to make a corona discharge generator from an old TV one had to strip the TV cases of all non non essential electronics.
cut out all un-used electronics from the chassis
however this made the HT power supply voltage rise as valve TV cases did not have HT voltage regulation but relied on load & a resistor network.
did blow up a 300v electrolytic capacitor by hooking it up by mistake to 400v coming off the rectifier valve.
-discharge of smoke & spraying electrolyte across the room. :palm:
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Back in the 70’s in high school electronics class we had a term where we fixed TV’s instead of conventional lab work. The brightest kid in the class was super skinny and was nicknamed Chucky because he was the opposite of Charles Atlas. The bully antagonist was Horowitz, big and as stupid as they come would have married his sister if given the choice. Horowitz made it his mission to give Chucky a hard time. It started with Horowitz charging up a 100V capacitor, sneaking up behind Chucky while he was probing inside a TV and then touching the capacitor to Chucky to give him a jolt. This went on and on with ever larger capacitors and escalating voltage levels. One day Horowitz showed us this huge string of series capacitors that he was planning to use on Chucky, Horowitz then proceeded to charge up the contraption with the 25kV anode connection on one of the TV’s. We heard a good snap and Horowitz came flying out of the back side of the TV and crashed into everything in his way. We all had a good laugh and that was the last time Horowitz attempted to shock Chucky.
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CRT HV was mainly scary and risky when you had to go in the set to probe things. You can't always access the underside of a pcb or chassis to scope things.
Putting your arm inside the chassis, the electrostatic crackling and your arm/hand hair getting pulled up is scary, not for the faint-hearted. Because your arm's reflexes can jerk and you don't want to hit the CRT neck, it's like a Zen experience. Measuring the CRT HV with a HV probe was not nearly as scary as putting your arm in there. I still glow from 6BK4 radiation...
In the shop one tech would charge electrolytic capacitors to 450VDC and throw them, yelling "catch!". If you caught the cap you got nailed. I never did, just let them land on the ground. The Heathkit cap tester could charge to 600VDC. Probably could kill someone that much DC hand to hand :phew:
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In the shop one tech would charge electrolytic capacitors to 450VDC and throw them, yelling "catch!". If you caught the cap you got nailed. I never did, just let them land on the ground. The Heathkit cap tester could charge to 600VDC. Probably could kill someone that much DC hand to hand :phew:
Mechanic shops played that game too back when ignitions were all points based. Charge points suppression cap with ignition tester and play catch the ZAP !
New ones that didn't leak much just left charged on a workbench would catch out the unwary too ! >:D
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Games for true idiots.
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A teenager like me, back in 1991, [...]
Back in ye olden days, back when people had brains that were generally fully functional, and I’d think that generally no mollycoddled and dumbed down fools were in charge [...]
From teenager in 1991 to Statler & Waldorf just 30 years later -- that's what I call rapid progress... ::)
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from starting messing around with vale radios etc at an early age maybe 5 or 6yrs old,when i was about 10yrs old my folks tv went south,it was there first colour set (very expensive then circa 1974),they asked me too look at it,well it had been unplugged and turned off for about 5 mins,i went to unplug the anode cap next thing i landed about 7 feet away on the other side of the room,i have had many clouts over the years but that one i remember as being the worst!.
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Unless you have some heart disease, not really.
You won't get "stuck" like when touching mains AC, the current is just a few mA, the initial discharge will instantly trigger a nervous system reflex, before you even realise, just like when touching a hot plate.
If you ever get shocked by it, specially by the tube (Which is a capacitor itself), you will learn something, and never forget :-DD
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As others have mentioned, the real danger is with the violent reflex motion after receiving a shock.
When I was an EE student I had a part time job at a local TV shop.
One day aligning the convergence and purity of a color TV, which I had powered up while I was manipulating the back controls. For this procedure I was really focused watching the CRT image’s reflection on a mirror. I touched an HV point, don’t know exactly what.
I sprang backwards, lost my balance and hit a vacuum-tube amplifier chassis with the back of my head. Since I broke several tubes with my skull, glass shards lodged deep in the skin, which had to be painfully removed before stitching me up.
This accident left a scar which 47 years later I still have.
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Its an old thread,
but as others have said, not really lethal at all. The real danger is the reflex action in pulling your hand away and cutting yourself on the sharp chassis. I spent the first first half of my life as a TV engineer for a once national and now defunct company. Cut hands and lots of colourful metaphors is all I really got from it. The voltage in a colour tv was around 25kv but at a low current. It sparked and left a small burn point. Tv tubes make a fine high voltage capacitor, so it was essential to discharge it before changing the tube.
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Really? OMG. I remember opening on of these with about 10-12. Felt the shock all they way up to the shoulder!
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anything thats high voltage and is anywhere near a cap i view with suspicion and keep clear of!.
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I once touched the wrong place in my 37inch CRT TV while it was on and I was tuning a pot. A jolt went down my arm like I never experienced before. I had a numbing burning nerve pain along the length of my arm for an entire year. I'm lucky to be alive...
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The bigger the tube the more dangerous the charge is. 40" is around the largest that was ever in consumer use. Those weighed about 300 pounds, which meant they were treated like an appliance for transportation and installation.
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deffo wouldnt want to piss on its anode cap !.
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In my younger days one thing I always hated working on was TV sets. They all had CRTs back then. And I have been hit with the residual charge on the CRT a time or three. Not from a working flyback, but just the charge remaining on the tube after it was disconnected. It was wise to discharge it more than once. But those shocks were not too bad and I was OK in a few minutes if not immediately. The thing is, the consumer TVs were deliberately made with a limited current being available from the HV circuit. It only needed to energize the tube which was as much of an insulator for that HV as they could make it. Yes, they thought of consumer safety back in the 1950s.
BUT, one time I was working on a professional, broadcast quality TV video monitor. A color one. And it had about 35 or 40 KV on the tube, not the usually lower HVs of the consumer TVs. And probably capable of somewhat more current. In this incident it was ON and I was probing around the flyback circuit and got on the wrong pin. That full Voltage went through the plastic of the probe I was holding, in my hand, out my foot, and through my sock and shoe. I found the burned in, pin holes in both places. My foot, with sock and shoe was resting on a metal foot rest of the work bench, which was grounded. It must have passed through at least a half inch (2 cm) of insulating materials in addition to my body. I felt the effects of that one for three or four hours that day. I probably should have been checked out by medical personnel, but I was young and indestructible. Today, with my 79 year old heart, it would probably kill me.
So, dangerous? Yes, and no. For the most part no. But it could be dangerous or even fatal under the right circumstances. And definitely NO FUN!
Edit: So ...
Been there,
Done that,
And didn't get the tee shirt.
I wonder if I could get one printed; "I survived 40,000 Volts!"
Not many people can say that, can they?
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The real danger comes from the tube itself
+1 for this. The tube is a large capacitor, HV + Capacitor = danger.
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As I remember it, those CRT anode potentials were indeed dangerous. For color TV they were up in the 25 kV range. It doesn't take much capacitance to store a lot of energy at those potentials. And energy is what can get you. Any voltage above around 100V will cause conduction of human tissue, enough to cause problems. As for electrocution, I believe the old electric chairs were around just a few kV.
Hi
The upper "safe" limit is about 48VDC. You see this applied in battery packs etc. This reference https://burncenters.com/safety/the-short-term-and-long-term-effects-of-electric-shock-on-the-human-body/ (https://burncenters.com/safety/the-short-term-and-long-term-effects-of-electric-shock-on-the-human-body/) states even 50V can be lethal.
Lots of things affect a person's conductivity. One of them is fatigue, which chemically reduces body resistance and increases the risk of doing something stupid.
I am super careful with HV. Your first mistake could be your last. I connect probes etc with power off. If there is something I need to do with power on (eg probe something), I do a power off rehearsal of the actions first.
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Reminds me of this video. But seriously... "dangerous to lethal" sums it up perfectly.
https://www.youtube.com/watch?v=vNgMjfRglUk (https://www.youtube.com/watch?v=vNgMjfRglUk)
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A CRT had a lot less HV capacitance than a mid sized van de Graaff. For an operating device it was probably less dangerous than mains, more likely to knock you off. On the other hand, it might knock you into a mains connection.
I measured some Tektronix CRTs and the post deflection acceleration connection had 10s of picofarads, and discharged rapidly.
I do not know about color television CRTs; they are much larger.