High Voltage Low Current

Discussion in 'General Electronics Chat' started by SoundGuy, Mar 31, 2005.

  1. SoundGuy

    Thread Starter New Member

    Mar 31, 2005
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    I had always heard when I was growing up that a television tube can store a high voltage and the discharge from it can be fatal even if the tv is unplugged. I asked an electronic service technician about this and he said, "it really isn't that dangerous because even though the voltage is high, the current is low." Since Ohm's Law states that I = V/R, current is proportional to voltage meaning that the higher the voltage is the higher the current will be as long as the resistance remains constant. I have also heard of other devices that administer a high voltage shock that is non-lethal because it is low current. Examples are electric fences, Tazers, electrostatic discharges, etc. How is this possible? Is there something besides the person being shocked that is limiting the current? Is the current of such short duration that it doesn't cause a disabling response? I once got a hand-to-hand shock from a bank of capacitors in an unplugged tube amplifier. It definitely got my attention! I actually felt the current pass from my right hand to my left hand, but I could tell it was travelling on the surface of my skin rather than through my muscles. I had no residual pain and even though I didn't let go of the conductors, the duration of the current flow seemed to last only a fraction of a second. The capacitors were charged in excess of 400V. Did I dodge a bullet because the current could not be sustained by the capacitors long enough to cause damage or because it took an outer path rather than an inner path? What is the best way to discharge devices that can store a high voltage electrical charge?
     
  2. Brandon

    Senior Member

    Dec 14, 2004
    306
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    Voltage is like raising something up into the air. The greater the difference between the heioght of the object and the ground, the more energy that the object can have. Notice I say CAN have. The amount of actual energy it along the same ideas as current. Voltage is only a measurement dealing with the difference of charge between 2 places.

    For this example, lets just say that every 10% difference in electrons = 1 volt. This isn;t true, just for this. So we have 90 electrons on 1 side and 100 on the other, theres 1 volt, and only 5 of those electrons are going to travel because as soon as both sides are at 95, there is no difference any longer so nothing flows.

    Now lets look at 10000 on 1 side and 9000 on the other, still only 10% diffeence so 1 volt, but now we have 500 electrons that are going to flow from 1 side to the other. i.e. 500x more power than the first example.

    If you raise 1 atom a mile up and drop it, its a HUGE difference, but nothing to it. Little maxx so little energy, but we raise a bowling ball to the same height, same difference, but now 1000x the force.

    I've gotten nailed with 100kV before, but no charge to it. Quick little zap and that was it and I've had 12 Vdc knock me on my ass, hello car battery, lol.

    When current flows 2 things are going on at the same time. First the charge flows over the path of least resistance. Whole reason insulators and conductors work the way they do. Secondly, electrons repel each other being negativly chagred so that move to the farthest point away from each other along the path of least resistance. In a wire, the current flows down the edge of iit right on the surface. The higher the frequency the closer to the surafce it flows until it nearly hovers over it. With our bodies, our blood vessels offer the path of least resistance but to get to them it must pass through skin which is difficult without enough voltage. The next path of least resistance on our bodies is our skin due to our sweating. Wether you feel it or not, our bodies constantly sweat and leaves a residue with salt in it over our body which provised a nice conducting surface. Thats why you felt it on your skin. A contact like that takes tons of current to really being to hurt you because its flowing around you and not through you basically.

    Now, if you were to say insert 2 probes into a blood vessel and try to pass current through it, you could kill yourself very easily since it will cause your blood to warm just like a component in a circuit. You can actually boil the blood right in your body with half an amp I believe. I know its not much, but you can get hit with 10s of Amps at 120 volts and it just throw your butt and give you one hell of a shock.

    If you rub your feet on the carpet and you get a shock from a door knob, thats usually somewhere in the vicinity of 1000 volts and up to get the charge to cause air to ionize and permit current flow, but you being a very poor conducting device can't store enough charge to actually hurt you from this.

    The best way to discharge caps is to use a shorting resistor. Get a good resistor that can handle say 1-5 watts of heat at about 100 ohms. You just short the resistor across the capacitor and the cap will discharge through the resistor. If you notice the resistor gets hot from this, go for a bigger one with a higher power dissipation.

    Hope I've helped you out.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    About you're not-so-smart technician friend - beam current in the CRT is low, some tens of microamps. But the voltage across the tube is up around 25 KV. If you make yourself the conduction path for that voltage, the current will (briefly) be somewhat higher. CRT's do carry lethal charges. Like Leyden jars, the charge is stored as a strain in the glass, and so may be maintained for long periods of time.

    If you need to mess with any CRT, get a long, skinny screwdriver and a clip lead. Attach the lead to the metal frame and to the screwdrive near the handle. Slide the tip under the boot on the side of the CRT where the HV goes in. Wiggle it in until you have contacted the metal HV leads, and then leave it for a couple of minutes.

    With tube sets, the CRT wasn't the fun part. The terminal strip where the damper diode was really bad news. Touching it gave you a nice RF burn. There would be a little black hole, a line of char under the skin, and another black hole where the current exited. You only stuck one hand at a time into a working tv chassis.
     
  4. Nomad

    Active Member

    Oct 21, 2007
    43
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    and the reason for only one hand? because that current, even a very small current, going from one arm out the other will cross your chest and can/will stop your heart.
     
  5. Nomad

    Active Member

    Oct 21, 2007
    43
    0
    oh and devices like stun guns etc run from a 9v battery. which has limited output. and theres a trade off when coverting. if you raise voltage you lose current, power remaining the same. there's also internal resistances to account for. it will drop voltage as soon as a completed circuit is made thus limiting the output current.
     
  6. erlend

    Member

    Feb 8, 2008
    18
    0
    What if you store charge up in a millifarad capacitor using just a 9V batter? When that discharges, you can get a high current and a high voltage surely?

    For example, put the 9v battery through a transformer to bump it up to 1000v, but with a tiny current. That would arguably be safe-ish. But then store that in a large capacitor... when that discharges the instantaneous current could potentially be an amp or more - wouldn't this be dangerous?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    When I was on active duty in the Marines, we would charge up capacitors and leave them lying around for nosy victims to pick up.

    Said nosy victims learned that they shouldn't mess with our stuff. ;)

    (((BZZZZZT!!!)))

    It was funny stuff back then.
     
  8. erlend

    Member

    Feb 8, 2008
    18
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    What size of capacitor? Surely you'd have to charge the capacitor using a high-voltage to get anything of a shock from it?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, I don't recall what size the caps were. They were from the HV section of the radar system that was used in F-4J Phantoms. The Klystron transmitter tubes used 18.5kv.
     
  10. OscarChad

    New Member

    Oct 1, 2008
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    The Klystron transmitter tubes used and I managed to fit them with approval on my learners insurance 18.5kv...
     
    Last edited: Oct 3, 2008
  11. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    It's funny how we are so much more immortal when we're younger. Some of the kids in my high school would toss a charged cap at an unsuspecting victim, when the teacher wasn't looking. Of course, I never did anything like that. :rolleyes:

    Something that hasn't been mentioned yet, and needs to be. If a person has a pre existing problem even a mild normally non lethal shock can trigger it. I keep hearing stories where people die a few hours later from a police taser shock, I suspect heart problems are more common than we realize (and the victims don't even know themselves).
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
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    At NAS Corpus Christi,1946, the projectel of choice was a .1 uF 600V cap charged to 300V, 'never caught one.
     
    Last edited: Oct 31, 2009
  13. ziouranio

    Member

    Jul 23, 2009
    20
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    That's one of the additional reasons tube amplifiers had "bleeders" resistors on the supply rails.... other than having a better stabilizazion of the voltage by imposing a certain bias current (as far as I remeber the only stabilization was done by discharge tubes) they were going to discharge filter capacitors when the power had been removed.... thus saving hands of the ham-radio specialist and eventually their lives....
     
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