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Thread: When does DC voltage become dangerous?

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    Flashaholic* BatteryCharger's Avatar
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    Default When does DC voltage become dangerous?

    Ok, theoretically in the right place you could probably be killed with 1ma @ 1v. More realistically speaking, when does DC voltage become dangerous to touch with your bare hands? 36v? 48? 75? How high does it need to be before you should be afraid of it like 120v sockets?

    Just curious...
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    Default Re: When does DC voltage become dangerous?

    It varies, you usually start feeling it at about 50Volts, which also is about the limit in safety regulations. And as with any powersource, not the Voltage is dangerous but the current.
    I may check this in more detail when I'm back at home.

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    Default Re: When does DC voltage become dangerous?

    IF you drop down 1 metre you could die
    if you drop down 25 you could survive !!

    same goes for voltage ,,don't be afraid for sockets They hurt but, normaly you wont die unless you put some wet steel rods in them and grab them with your hands for a long time !! ,,,, has somthing to do with how easy the current goes through You.

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    Default Re: When does DC voltage become dangerous?

    also you can touch 10000v electrical fenses , but they only hurt like **** and won't kill you same reason: not enough current !!

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    Default Re: When does DC voltage become dangerous?

    Probably not going to find an exact number people can agree on, but I'd say 60+ VDC.

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    Default Re: When does DC voltage become dangerous?

    36-48 is where you might start feeling it. It depends on varying conditions. 50v is where you start to have to aboe3y different wiring standards per the National Electric Code.

    You must remeber that dc is much more hazardous than ac of the same voltage. Dc will sustain an arc at much lower levels than ac. If you get shocked by dc you will likely not let go of what you touch. Ac will trow you away.

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    Default Re: When does DC voltage become dangerous?

    It depends how sensitive you are, whether you have sweaty hands, and so on. I can certainly feel a tingle from 24 V, and in the wrong circumstances 12 V could be enough to kill you.

    If you always assume electricity is dangerous you won't go far wrong.

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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by jhellwig View Post
    You must remeber that dc is much more hazardous than ac of the same voltage. Dc will sustain an arc at much lower levels than ac. If you get shocked by dc you will likely not let go of what you touch. Ac will trow you away.
    Conventional wisdom in electrical engineering is exactly the opposite. The most common form of electrocution death is from stopping the heart. 60 Hz AC is much more likely to cause heart stoppage than DC current of the same voltage.

    AC voltage is quite good at causing muscle contractions and causing you to "clamp on" to an exposed wire as well.

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    Default Re: When does DC voltage become dangerous?

    The position of the shock, and distance between entry and exit points is critical. A voltage of 60V per cm across your body will cause tissue damage. If the entry and exit points are somewhere like your leg, you can survive even the frighteningly high voltages from lightning. If the voltage goes across your heart, even 100VDC could be fatal and stop it.
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    Flashaholic Apollo Cree's Avatar
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    Default Re: When does DC voltage become dangerous?

    Here's some info from the CDC.

    http://www.cdc.gov/niosh/docs/98-131/overview.html

    It lists around 16 mA as a current level where you may not be able to let go.

    Around 20 mA can paralyze your respiratory muscles and stop breathing.

    100 mA can cause ventricular fibrillation and stop the heart. This is particularly dangerous because even if you get disconnected from the current, your heart won't necessarily restart.

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    Flashaholic Magic Matt's Avatar
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    Default Re: When does DC voltage become dangerous?

    Good info... just a word of caution though... the figures you've quoted are at ~600VAC 60Hz though, not DC.
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    Default Re: When does DC voltage become dangerous?

    I heard somewhere that low DC voltages that you may not notice can do electrolysis damage to cells. Supposedly, a person was killed while working on a vehicle with a 24 volt battery and some how manged to get pinned with part of his body in contact with one of the terminals and another to the body (ground) of the vehicle. He was trapped that way for a while (hours?) and died later due to the damage to his cells from the current.

    I can't attest to the validity of this story, but it sounds plausible as electric currents passing through chemicals can break down the molecules or form new ones.

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    Default Re: When does DC voltage become dangerous?

    The problem with such a question is that the answer is so complicated. It's mostly the amount of current flowing into a vital organ that causes death.

    If you run a current between two fingers on the same hand, only a small percent of the current will flow through, for instance, your heart. If you run the current from your left hand to your right hand, a larger percentage of the current will flow through the heart. A medical patient with, for instance electrodes for a heart monitor could have a much lower current threshold if there's some sort of electrical problem because .

    Even though we say current is the problem, what we see most of the time is voltage. We are usually dealing with what we consider to be "constant voltage" sources. This would be something like a battery. It produces 1.5 Volts most of the time. If you have 0.01 mA flowing, it's 1.5 Volts. If you pull 500 mA out of it, it's still close to 1.5V.

    Now, assume you have an exposed voltage of 50 V somewhere. If you walk up and touch it and have on shoes with rubber or plastic soles, the electrical resistance of your shoes is so high that you might only get a few micro amps. Change to leather soles, you still probably don't get much current. Now, assume you're touching a metal piece of furniture with one hand and touch the 50V circuit with the other. You get considerably more current. Now, consider if you have sweaty hands and are making really good contact with a grounded metal table. Now, assume you're standing in a decorative fountain with wet hands working on the water pump and you don't realize that the 50V DC power supply isn't turned off.

    The threshold of "safe" voltage varies widely in these different situations because the electrical resistance varies so widely.

    In the electrical engineer safety discussions, a "nightmare" scenario was something like "A technician is working on a piece of low voltage electronic equipment. The equipment has energized components with sharp edges, for instance voltage test pins. The technician slips and manages to spear a finger on each hand with a pin and pierce the skin. What's a safe voltage level here?" The answer was that there probably was no safe level.

    With all that said, you mostly worry about voltages above 50V. You understand that lower voltages can still be dangerous in certain conditions. You become more concerned in wet conditions, or any kind of medical situation. Me, I start getting nervous above 12V, and ratchet up the nervousness as voltage gets higher.

    You also understand that a lower voltage high current situation can cause thermal burn problems. For instance, shorting out a car battery with your class ring can cause a really nasty burn.

    By the way, telephone wiring is around 48 Volts when the phone is on the hook, and we normally don't worry about that too much. However, if you're messing with some phone line connection and thoughtlessly decide to hold the connector in your mouth to free both hands, it can be an unpleasant or even fatal surprise. Especially if the phone rings, when the voltage jumps to around 90V.

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    Flashaholic Apollo Cree's Avatar
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by Magic Matt View Post
    Good info... just a word of caution though... the figures you've quoted are at ~600VAC 60Hz though, not DC.
    In terms of the hazards listed, the voltage is irrelevant. 100 mA will tend to cause fibrillation no mater whether it's 50 VAC with wet hands or 500VAC with dry hands.

    AC vs. DC is mostly relevant to the ventricular fibrillation.

    Normally, you "play it safe" with AC vs. DC and just assume they're equally dangerous. e.g. DC is less likely to cause fibrillation, but you treat it as if it were equally dangerous.



    By the way, I think we tend to think of AC as more dangerous than DC because the common AC power sources we are familiar with are higher voltage than DC power sources.

    House wiring is 120 VAC or higher. Electrical distribution lines on power poles are 7000VAC or higher. Car batteries are usually 12V. Batteries are commonly 1.5V or 9V. Electronic circuitry in consumer electronics is commonly under 5V.
    Last edited by Apollo Cree; 02-11-2010 at 08:40 AM.

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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by JohnR66 View Post
    I heard somewhere that low DC voltages that you may not notice can do electrolysis damage to cells. Supposedly, a person was killed while working on a vehicle with a 24 volt battery and some how manged to get pinned with part of his body in contact with one of the terminals and another to the body (ground) of the vehicle. He was trapped that way for a while (hours?) and died later due to the damage to his cells from the current.

    I can't attest to the validity of this story, but it sounds plausible as electric currents passing through chemicals can break down the molecules or form new ones.
    Entirely plausible. I'll bet the 24V current was still painful and he would have done something to stop it if he wasn't pinned down.

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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by Apollo Cree View Post
    In the electrical engineer safety discussions, a "nightmare" scenario was something like "A technician is working on a piece of low voltage electronic equipment. The equipment has energized components with sharp edges, for instance voltage test pins. The technician slips and manages to spear a finger on each hand with a pin and pierce the skin. What's a safe voltage level here?" The answer was that there probably was no safe level.
    At some voltage, there'd presumably be an equal risk of damage from other factors (the technician jumping back after stabbing their fingers (from pain rather than electric shock), and tripping over something on the floor, etc), and at some lower voltage, even if the electrical risk was nonzero, it would be so relatively small compared to other factors as to be beyond worrying about.

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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by Apollo Cree View Post
    The technician slips and manages to spear a finger on each hand with a pin and pierce the skin. What's a safe voltage level here?" The answer was that there probably was no safe level.
    That is what I heard back in my Engineering days. They referred to an Everready cell killing one. Needing only 10+ ma to do it. Imagine one Eneloop and above stiuation and 210 lb technician being brought down. Gives me the willies.

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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by uk_caver View Post
    At some voltage, there'd presumably be an equal risk of damage from other factors (the technician jumping back after stabbing their fingers (from pain rather than electric shock), and tripping over something on the floor, etc), and at some lower voltage, even if the electrical risk was nonzero, it would be so relatively small compared to other factors as to be beyond worrying about.
    The point wasn't necessarily that the exact scenario was likely. It was more of a reminder to not make any assumptions that a particular voltage is safe in all conditions. There's one urban legend about some teacher demonstrating to his students that a car battery wouldn't electrocute you. Bare foot in a bucket of salty water, and a hand in another bucket of salty water. Oops, bring in a new teacher and counseling for the kids.

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    Default Re: When does DC voltage become dangerous?

    A friend of mine (many years ago) was changing a battery on a truck ... He made two mistakes ... Firstly, he left his wedding ring on ... secondly, he used a non-insulated spanner.

    As he tightened the live battery terminal, the spanner was in contact with his wedding ring ... The ring unfortunately came into contact with the bodywork of the truck.

    The ring suddenly became extremely hot and luckily did not weld itself to the spanner and the bodywork ... He was left with a ring shape burn on his finger and the ring had to be cut off due to the swelling of the finger.

    Where you have an almost infinite ammount of current (as in this case), it can be very dangerous.

    I worked for many years on 48 volt and 110 volt DC systems and you actually get used to these voltages ... I found that the back-EMF, off highly inductive devices (such as electro-magnetic relays), really makes you jump though ... You never really get used to that.
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by JohnR66 View Post
    I heard somewhere that low DC voltages that you may not notice can do electrolysis damage to cells. Supposedly, a person was killed while working on a vehicle with a 24 volt battery and some how manged to get pinned with part of his body in contact with one of the terminals and another to the body (ground) of the vehicle. He was trapped that way for a while (hours?) and died later due to the damage to his cells from the current.
    Well, they use the principal of electrolysis to kill hair cells, so there is some validity here. I think what happens is the electrolytic reaction produces lye, which in turn kills the hair cell. Something similar probably happens to other types of cells subjected to low but continuous current.

    Also, everyone here is talking about voltage and current, but it's total energy in joules from the charge which actually kills. I think you need around 100 joules to stop the heart, give or take, although I've heard figures as high as 200. Accidentally discharging a 10,000 F capacitor connected to a full-wave bridge across a 120 VAC would be enough ( stored energy = CV = ( 0.01 ) ( 169 ) = 142.8 joules ). However, accidentally discharging a 1000 F cap wouldn't be, although it might cause some really nasty burns, plus a heck of a jolt. Notice however the squared term with voltage. A seemingly small value capacitor at a high enough voltage could be deadly. If you're working on CRT TVs, for example, all it takes is a cap of 0.3 F charged to 25 kV to kill you. I think the capacitance of the picture tube is more than this. On the flip side, you would need HUGE capacitances at 12 VDC to kill you, roughly on the order of 1.5 Farads.

    Obviously almost any battery can store well in excess of 100 joules ( a AA Eneloop stores roughly 9,000 joules, for example ), so given a low impedance path through the heart, even a 12 VDC battery can kill.

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    Default Re: When does DC voltage become dangerous?

    IIRC your kidney can also fail from current in your body. There has been occurence where someone sustained an electric shock, was well minutes after but would be found dead the next morning because the kidney were damaged during the process.

    After any electrical incident where you suspect current has gone through your body you should seek medical assistance. Now, I agree it's not easy to define in what case the incident was "significant" enough to decide whether to have a check-up or not

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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by jtr1962 View Post
    Also, everyone here is talking about voltage and current, but it's total energy in joules from the charge which actually kills.
    I think joules is only going to matter for burns or tissue damage. A short shock without many joules can easily stop the heart and it may not restart on its own. A long shock below the dangerous current levels will tend to not do any damage except in extreme situations, even though it accumulates a lot of joules.

    The amount of current and length of time actually flowing through the various organs is usually the most important factor.
    Last edited by Apollo Cree; 02-12-2010 at 02:57 PM.
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    Default Re: When does DC voltage become dangerous?

    While it's true the lethal amount of current across the heart is very low, it takes a whole lot of voltage to allow that amount of current to flow through your heart.

    Also joules is a measure of energy per time, the longer the time the more joules that flow, shorter time, less joules.

    DC reacts to resistance more than AC, it takes a fair bit more DC volts to generate lethal current across the heart than it does AC. 24VAC will cause muscle contractions and some pretty bad feelings. It takes around 50VDC to feel the current flowing through your body.

    You're more likely to die from higher voltages than low because they will travel farther through your body.

    The actual chances of being electrocuted from normal circumstances of the average person is very low. It's hard to generate a short across your heart with current electrical codes. Sure you could burn yourself very badly, or cause muscle damage and soft tissue damage, but death is unlikely.
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    Default Re: When does DC voltage become dangerous?

    It's worth bearing in mind that the electronic muscle stimulators with sticky pads that are sold to increase muscle tone use a pretty low voltage of about 12 V. If that voltage can cause muscle contractions around your body, it is not difficult to imagine it doing the same kind of thing to your heart muscle.

    There is always a statistical variation where people are concerned. Just because 99 healthy people are unharmed by an electric shock, it does not prove that 1 person might not have an undiagnosed heart condition or genetic predisposition that causes a fatality.

    To be safe, never get glib with electricity.

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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by 65535 View Post
    While it's true the lethal amount of current across the heart is very low, it takes a whole lot of voltage to allow that amount of current to flow through your heart.
    Not necessarily. Depends on where the voltage is applied, sweaty hands holding a metal wrench, in the bathtub using a telephone, patients connected to medical equipment, standing in the rain working on a "low voltage" piece of electrical equipment you though was turned off, accidentally cutting your finger on an electrical connector, etc.

    Quote Originally Posted by 65535 View Post
    Also joules is a measure of energy per time, the longer the time the more joules that flow, shorter time, less joules.
    Joules is a measure of energy. Power is energy per unit time.

    Quote Originally Posted by 65535 View Post
    DC reacts to resistance more than AC
    No. Period. End of story. See Ohm's Law.

    Quote Originally Posted by 65535 View Post
    it takes a fair bit more DC volts to generate lethal current across the heart than it does AC. 24VAC will cause muscle contractions and some pretty bad feelings. It takes around 50VDC to feel the current flowing through your body.
    Current flows through things. Voltage is across things. AC current is more efficient than DC at causing ventricular fibrillation. The other potentially lethal effects of electrical current aren't necessarily more potent with AC than with DC. Long term low levels of DC current may have some effects that AC doesn't due to the effects of electrolysis, according to some posts in this thread.

    Quote Originally Posted by 65535 View Post
    You're more likely to die from higher voltages than low because they will travel farther through your body.
    No. Current always flows all the way from one contact point through your body to the other contact point on your body. For instance from your hand touching a live circuit to your feet touching the ground. Higher voltage simply increases the amount of the current. (Except where the voltage is high enough to arc through the air and electrocute you from a conductor you're not touching.)

    Quote Originally Posted by 65535 View Post
    The actual chances of being electrocuted from normal circumstances of the average person is very low. It's hard to generate a short across your heart with current electrical codes. Sure you could burn yourself very badly, or cause muscle damage and soft tissue damage, but death is unlikely.
    Yes, chances of electrocution are low. 500 or so per year in the use. Thank goodness.

    No, it's not hard to get electrocuted. Work on a live house wiring circuit, and it's easy to electrocute yourself. The circuit breaker will not usually save you unless it's a GFCI breaker. Work on a consumer electronic device like a DVD player and it's easy to electrocute yourself. Once you remove the cover, there may well be exposed 115VAC terminals.

    It may be hard to electrocute yourself if you use all your devices the way they were designed to be used. Many CPF members are electrical tinkerers. Many of them aren't engineers or electricians or any other kind of trained electrical professional. Many of us do extreme electrical things with normally low-voltage equipment. Many of us will open things up and probe voltages with the power on. Want to bet you couldn't electrocute yourself if you open up a 35W HID spotlight even though the battery is only 12V?

    We should not get too callous about the dangers of electricity.

    I'm an electrical engineer. I worked for years on electrical safety for consumer electronic equipment, factory equipment, international, US, and internal company safety standards, including the US National Electric Code. Not only do I understand the rules, I understand the underlying risks involved.

    When I give advice, I do go overboard on pointing out even the unlikely scenarios because I know someone will go out and try it.
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    Flashaholic Apollo Cree's Avatar
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by Mr Happy View Post
    Just because 99 healthy people are unharmed by an electric shock, it does not prove that 1 person might not have an undiagnosed heart condition or genetic predisposition that causes a fatality.
    I suspect the the variation caused by how good a contact you make with the live circuit and ground makes more difference. Pressure, dampness, contact area, etc. makes an enormous difference. I've gotten just a glancing touch with household AC a number of time and gotten anything from just a tingle to a good kick.
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by Apollo Cree View Post
    I suspect the the variation caused by how good a contact you make with the live circuit and ground makes more difference. Pressure, dampness, contact area, etc. makes an enormous difference. I've gotten just a glancing touch with household AC a number of time and gotten anything from just a tingle to a good kick.
    Right. What I'm trying to get across, though, is the problem with saying, "I once crossed a busy road safely without getting run over, therefore there is no danger involved in crossing a busy road."

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    Default Re: When does DC voltage become dangerous?

    I just saw this somewhere on TV - the reason 110v AC is so dangerous isn't the voltage or current, it's the frequency. The heart beats at the frequency the brain tells it to with electrical impulses. When house current goes through your heart it makes it try to beat at 60 hertz which is impossible, so it stops. Same thing with DC. Your heart matches the frequency and stops.

    Screwing around with house current is stupid - period. Turn off the breakers or make sure you offer no path to ground for the current no matter how little.

    I'm an amateur radio operator and tinkering with radios while they are operating is absolutely necessary for alignment or repair depending on the problem. The first thing I learned was to only have one hand in the radio at a time especially if it is a tube radio and high voltages are present. That applies even with solid state devices and insulated instruments.

    A broadcast engineer friend of mine has an internal scar in his right index finger. He got it while aligning a CB for a friend at his home. His wife called his name for something and it distracted him. His finger wasn't visibly burnt on the outside, but it was fried inside. That happened 20 years ago, and he still feels the weather change from the internal scar in his finger.

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    Default Re: When does DC voltage become dangerous?

    With my electric bikes, even soaking wet in the rain, I can touch the 48v battery sections indivdually and feel nothing more than a tickle. Doesn't feel much different than welding in the rain.

    I try to avoid touching the dual 48v batteries when in series. 96vdc has never caused me any noticed physical damage, but it does trigger muscle action (at least in my body, doesn't seem to bother a buddy of mine).

    These batteries are 20Ah Lithium Polymer packs with 25C discharge ability, meaning they have the potental to continously dump 500amps. Fortunately, our skin (even wet) is fantastic protection from shocks at the lower voltages like <100v.

    Go under the skin, and cause the current path to include the heart, and almost any amount of voltage or current can kill you.
    Last edited by liveforphysics; 02-12-2010 at 07:28 PM.

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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by Apollo Cree View Post
    Electronic circuitry in consumer electronics is commonly under 5V.
    That's a bad thing to assume...

    Most large flat panel monitors aren't lit by EL panels. I was surprised to draw an arc across two terminals in one, then found it was lit by two long fluorescent tubes with several hundred volts at 50kHz. Even EL panels run at 100 volts or more. The only true low voltage displays are LED displays, or ones backlit with LEDs like my modified TV at home.

    I once worked on laser bar code scanners years ago - they ran on 10kV at 1ma. Yikes! Now they use 3 volt diodes...
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