I want to make my next amp with -35-0-35 rails and I'm wondering how dangerous a 70VDC shock would be. I read somewhere that 60hz voltage is over 5 times more dangerous than DC to humans. I really don't want to get into using lethal voltages in my projects. The AC component of the amp's power supply would be sealed off but I'd be using the DC side pretty frequently. Do you guys ever get shocks from your rails?
70VDC is definately potentially lethal if you get a belt directly across the supply capacitors. I've been zapped by ~300VDC almost directly across the chest, it was incredibly painful and in hindsight i'm probably lucky to have survived (fortunately it was only a 120uF capacitor). On the other hand, if you get shocked by 35VDC the worst your likely to get is a fairly small burn that might require a few days of attention.
Solid Snake said:
Dunno about the five times figure, but an AC voltage is more lethal because the body's impedance is lower under AC conditions. A little like leaky capacitor. At DC, the impedance is quite high. But under AC conditions, the impedance drops depending on frequency. The higher the frequency, the lower the impedance and the more current that will flow through the body.
Suffice to say that it would take a quite unusual situation to find yourself connected between the two supply rails. Touching either rail and ground would only come to 35 volts. So you'd have to have something like one hand on one rail and the other hand on the other.
Shocked? Hey, yer talkin' to a professional here!
se
Working on high output R.F tube amps, I hate to admit one or two hits at 800 volts D.C. and one real memorable hit from 4,800 volts D.C. at about 1.5 amps from a real tube rig running 4 4-1000's in a grounded grid config. For some reason I thaught I'd check the D.C. + at the top of 2 6000 volt D.C. oil filled caps with my good meter. Don't remember much after the blinding ark. Burned a good mark in my knuckle and out my right foot to the concrete. Meter was fine but I was not.
I seriously doubt the human body has impedence. Doesn't impedence require some kind of magnetic property? Anyway, I read that the higher the frequency past 60hz, the lower the danger. It has to do with the rapidly contracting and expanding muscles in your body. The safe limit of DC through your body is 2mA, while the limit at 60hz is only 0.5 mA. The whole frequency thing has to do with what frequencies your muscles will respond to, 60hz just happens to be one of the most deadly frequencies that can go through your body. The bottom line is how many mA go through you. 2ma @ 60hz will be painful while 2ma at 1Mhz probably won't even be noticed.
I'm working with -17-0-17 rails now and I can't even feel anything across 34vdc, let alone getting burned. Although supposedly people have died from as low as 25vac.
Ok, I should have better explained my comments with regard to 35VDC. If both points of the circuit touch your body within about 1 foot, you'll certainly feel a tingle. Within, about 6" and it'll hurt. Within 2" and it'll probably leave a burn. If you talking across both arms and chest then 70VDC is right on the edge where given the right conditions, it could be fatal (or do serious injury) or if your lucky, you'll be ok.
Solid Snake said:
Not sure what you mean by magnetic property.
Impedance is basically a frequency-dependent resistance brought about by inductive or capacitive reactance.
At DC and low frequency AC, the body behaves as a simple resistance. But as frequency increases, the body behaves more capacitively. And like any other capacitance, as frequency increases, impedance decreases, allowing for greater flow of current.
Dunno. Haven't heard anything along those lines. I just know that the body's impedance decreases with increasing frequency. There may be some cutoff point where for whatever reason the increase in current becomes less dangerous.
Yeah, but try putting your fingers across the binding posts when your amp's swinging a +/- 17 volt sinewave.
se
Lucky for me, the load brings it down a bit
Ok, impedence has absolutely nothing to do with this. Aside from bodily resistance/capacitance/reactance or whatever, 10mA @ 60hz will be much more painful than 2mA @ 1Mhz. Notice, current is exactly the same, voltage is irrelevent. If you have the exact same amount of current flowing through you, 60hz hurts more than a higher frequency or even flat out DC.
Solid Snake said:Lucky for me, the load brings it down a bit
Unlucky for you, you ARE the load.
Ok, I'm having a hard time making sense of this.
You say 10mA at 60 Hz and 2 mA at 1 MHz, and then you say to notice that current is exactly the same. How is 10mA at 60 Hz the same current as 2 mA at 1 MHz?
se
Odds are you'll probably survive a zap at 70V -- but why bother -- just be careful, don't get shocked in the first place.
It'll hurt, is unnecessary, and under the right circumstances could be fatal.
FWIW -- I dunno about 60 Hz hurting more than DC. I had a zap right out of a 300V DC cap once, and I remember that being particularly awful.
I've been zapped by 120V plenty of times, and it doesn't even begin to compare to that cap.
If you want to build higher power amps, you're going to have to deal with dangerous voltages.
If you're careful, take your time, and most importantly, know what you're dealing with, you'll be fine.
I have 90-0-90 for the rails of the last amp I built with 66,000uF on each of the rails.
I'm super careful when dealing with the insides of that thing.
Just as a general safety note -- if you start dealing with higher voltages, an isolation transformer is a good investment.
It'll hurt, is unnecessary, and under the right circumstances could be fatal.
FWIW -- I dunno about 60 Hz hurting more than DC. I had a zap right out of a 300V DC cap once, and I remember that being particularly awful.
I've been zapped by 120V plenty of times, and it doesn't even begin to compare to that cap.
If you want to build higher power amps, you're going to have to deal with dangerous voltages.
If you're careful, take your time, and most importantly, know what you're dealing with, you'll be fine.
I have 90-0-90 for the rails of the last amp I built with 66,000uF on each of the rails.
I'm super careful when dealing with the insides of that thing.
Just as a general safety note -- if you start dealing with higher voltages, an isolation transformer is a good investment.
AudioFreak said:
I think you all guys forget that it's not the voltage which is dangerous, it's the current and where it goes in the body!
DC is dangerous because it creates elektrolys (don't hev the english word) in the body and you can die later, not right away as with AC.
I'm not 100% sure but 50 V is limit for safe voltage, 50V DC or 50V AC top value. By experience, I can say that above 100 V you can easily feel the voltage and 230 V as in Europe, you definitely wake up!
As I said the current is dangerous, 5-10 mA is not healthy, and > 20 mA is directly lethal. < 1 mA is hardly recognizable.
if you start dealing with higher voltages, an isolation transformer is a good investment.
uh, unless you have a dangerous auto-transformer PSU , your power rails are isolated from the mains in the first place.
the problem isn't the connection to the mains ( thats a slightly different argument ) , it's simply that high voltages will try to impart a significant current to flow through your body.
stick one hand on each rail and no amount of isolation transformers will help you.
I've managed to get a tickle off a 24v supply ( yes it was isolated ) , no biggie. try the same trick with 50-0-50 , or 70-0-70 rails and you'll be risking a whole world of hurt.
One trick from TV servicing is "keep one hand behind your back" when you are messing with HV. at least it directs any current to flow DOWN through your body instead of across your chest, where it's much more likely to be 'goodnight vienna'
Ray
If you are lucky it'll hurt like hell, unlucky and you wont feel a thing
uh, unless you have a dangerous auto-transformer PSU , your power rails are isolated from the mains in the first place.
the problem isn't the connection to the mains ( thats a slightly different argument ) , it's simply that high voltages will try to impart a significant current to flow through your body.
stick one hand on each rail and no amount of isolation transformers will help you.
I've managed to get a tickle off a 24v supply ( yes it was isolated ) , no biggie. try the same trick with 50-0-50 , or 70-0-70 rails and you'll be risking a whole world of hurt.
One trick from TV servicing is "keep one hand behind your back" when you are messing with HV. at least it directs any current to flow DOWN through your body instead of across your chest, where it's much more likely to be 'goodnight vienna'
Ray
If you are lucky it'll hurt like hell, unlucky and you wont feel a thing
SolidSnake,
I was taught in tech school that DC can be more dangerous than AC because it can cause muscles to contract and hold more readily than AC.
Lower voltages can tingle and higher voltages can hurt, and higher voltages can kill, according to the current capability of the supply concerned, especially when the current path is through the cardiac area.
An easy mistake when working on large amplifiers is to ground a part of the hand when touching transistors in order to check temperatures - this can hurt but ought not to harm you.
The best advice is to wear insulating shoes and keep one hand/arm away from the work area so that no current path throught the chest area is possible.
Also be wary of touching part of your chair when working on high voltages - best to stand up when working with high voltages.
Eric.
I was taught in tech school that DC can be more dangerous than AC because it can cause muscles to contract and hold more readily than AC.
Lower voltages can tingle and higher voltages can hurt, and higher voltages can kill, according to the current capability of the supply concerned, especially when the current path is through the cardiac area.
An easy mistake when working on large amplifiers is to ground a part of the hand when touching transistors in order to check temperatures - this can hurt but ought not to harm you.
The best advice is to wear insulating shoes and keep one hand/arm away from the work area so that no current path throught the chest area is possible.
Also be wary of touching part of your chair when working on high voltages - best to stand up when working with high voltages.
Eric.
Actually, I'm surprised no one mentioned that across a range of current there is generally a rather specific amount that will kill you. Go under it, or over it (to a reasonable extent), and you live.
The primary cause of death in the critical range is uncontrolled fribrillation of the heart.
http://www.physics.ohio-state.edu/~p616/safety/fatal_current.html
The primary cause of death in the critical range is uncontrolled fribrillation of the heart.
http://www.physics.ohio-state.edu/~p616/safety/fatal_current.html
Solid Snake said:
Lucky for me, the load brings it down a bit
Ok, impedence has absolutely nothing to do with this. Aside from bodily resistance/capacitance/reactance or whatever, 10mA @ 60hz will be much more painful than 2mA @ 1Mhz. Notice, current is exactly the same, voltage is irrelevent. If you have the exact same amount of current flowing through you, 60hz hurts more than a higher frequency or even flat out DC.
Just as usual it seems the standard common mistake. The impedance is almost the ONLY thing that matters as it is what sets the current and the current is what kills or wounds. I myself am very dry-skinned (more detail than most like to have I assume) and have been grabbing, not touching but GRABBING hard, wires and things carrying 230 VAC live mains on more than one occasion. In most cases I have without too much trouble been able to remove my hand and disconnect even when the connecting parts have been left and right hand (not a good selection unless you really want to kill yourself). If I had sweaty hands or were a girl (I seem to recall that females have lower skin resistance the men from somewhere and that it is why they are more vulnerable to electric shock than men) the current would have been much higher and most likely killed me. There is as mentioned also a hugh difference between DC and AC and the effect varies greatly with frequency. 70 VDC I can not even feel and I don't even think it would be possible to get a burn as I don't even get a burn from 230 VAC just a shock. However telephone lines in Sweden carry 48 VDC at idle and that I can not feel (although there are ways of telling which are left out for SinBin pervention). However when the call signal comes in that is a 50 VAC signal or similar which is REALLY easy to notice but still gives no burn. My worst case of electricution was when I was fixing camera flashes as a teenager and unintentionally rested my arms on a discharge tube causing the flash to trigger and giving me 5000 V (no-load) pulse in my left arm. That was not a lot of fun and my heart was arythmic for about 15 minutes.
As for the common mistake from people unaquainted with Ohm's law you often hear that they "survived" an electric shock from a 115 or 230 VAC 20 A or 100 A (or whatever) line. The thing is of course that it does not matter what the maximum current is from the line you touch. If it is a 6 A or 1000 A fused 230 VAC line does not make a bit of difference. Your body impedance is so high that you will pull only a few mA (current equals voltage divided by impedance...) and get nowhere near those currents.
SinBin prevention disclaimer:
PLEASE understand and respect electricity. Always take proper precautions and handle electricity carefully and remember that it is potentially lethal. Never do what I have done and never try what I have. IT IS REALLY DANGEROUS. REALLY. I mean it.
I can verify it will hurt ...
Solid Snake,
I think you would be unlucky to die with a 70V dc shock but it's not impossible by any means !!! I would not cosider this voltage a particular hazard but I would still take precautions, in addition to those already prescribed :-
[1] Check the mains really is isolated first, for all equipment, no matter what the internal voltage. This is particularly important in europe with 240V ac.
[2] When you are going to touch something conductive don't grab it in the palm of you hand - it could be real difficult to let go if it is live
And yes, I can verify 240V ac hurts plenty, but not as much as 450V dc. I recall these experiences just before I start working on something live (not to mention remembering the after effects of someone who managed to momentarily short out a 500MW generator with an aluminum ladder) ...
Effects
BTW
elektrolys = Electrolysis (I think)
Dave / takes care now
Solid Snake,
I think you would be unlucky to die with a 70V dc shock but it's not impossible by any means !!! I would not cosider this voltage a particular hazard but I would still take precautions, in addition to those already prescribed :-
[1] Check the mains really is isolated first, for all equipment, no matter what the internal voltage. This is particularly important in europe with 240V ac.
[2] When you are going to touch something conductive don't grab it in the palm of you hand - it could be real difficult to let go if it is live
And yes, I can verify 240V ac hurts plenty, but not as much as 450V dc. I recall these experiences just before I start working on something live (not to mention remembering the after effects of someone who managed to momentarily short out a 500MW generator with an aluminum ladder) ...
Effects
BTW
elektrolys = Electrolysis (I think)
Dave / takes care now
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