High Voltage, Low Current?

Discussion in 'General Electronics Chat' started by speedster239, Jul 4, 2010.

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  1. speedster239

    Thread Starter New Member

    Nov 24, 2009
    This is a really simple question for the gurus, but still puzzles me. As far as I've read in the online textbook, Current = Voltage/Resistance. If this is so, why doesn't a shock from a taser or a piezoelectric lighter which can apparently reach over 3000 volts kill a man? The higher the voltage with constant resistance means higher current flow, and surely 2 amps (3000/1500ohms of resistance) will easily kill somebody. So, why does this not occur?
  2. sceadwian

    New Member

    Jun 1, 2009
    Because the ENERGY is what kills not the current, or voltage, and the energy is limited because the voltage is a pulsation not a steady constant 3kv.

    High voltage is a different field than common electronics when you get right down to it.

    A piezoelectric igniter supplies large voltages 10+kv, but the total energy is so low it's only enough to spasm the nerves that the discharge occurs in, even directly across the heart the energy is so low all it will cause is a missed heartbeat in a healthy human.

    The current output of a taser is on the order of a few ma at MOST. The total energy is limited by careful control of the circuit that delivers the voltage/current

    Tasers can be dangerous, especially if misused, however a person that is likely to get tased is also likely to be a healthy adult that is a danger to the officer using the taser.
  3. bertus


    Apr 5, 2008

    The taser has much more power and is much more dangerous as the piezo electric lighter.
    It is also the time of the shock that is very important.
    A piezo electric lighter gives a very short high voltage pulse that will hurt you but not really kill you.
    Tasers sometimes have a repetition mechanism that makes the really dangerous.

    They are NOT to play with.

  4. speedster239

    Thread Starter New Member

    Nov 24, 2009
    Alright, but how is the current limited. I mean, with 3000 volts there is a possibility (assuming there is 1500 ohms of resistance in the body) for 2 amps to flow. That would kill easily. So what limits the current flow?

    Is the piezoelectric lighter physically incapable of transferring that much current or electrons, but it still has the potential (voltage) to do so and would do so if it had that many electrons? If so, that makes a lot of sense to me.

    Ie in the piezoelectric lighter there is 100 electrons on the positive terminal and 90 on the other (%10 difference, lets say every %10 is 1 volt).

    In the same instance we have a power outlet on the wall. There is 100,000 electrons on the positive terminal and 90,000 on the other (%10 difference, so 1 volt also), but if given the opportunity much much much more electrons can flow--5,000 as opposed to 5. The potential difference is the same but the energy as sceadwin said doesn't exist.
    Last edited: Jul 4, 2010
  5. retched

    AAC Fanatic!

    Dec 5, 2009
    The current is limited over time. A taser uses an oscillator. Kind of like PWM.

    It gives high voltages at very short pulses that repeat quickly.

    This frequency change is important to the operation of a taser.

    Think about the power supply. Typically a 9v transistor battery.

    These have such a small amount of energy density, you cant make 10,000 volts with 30 amps. It doesnt contain that much.

    You only have so much power. if you have 10 watts, you can (in an ideal world) turn that 10watts into 10v at 1 amp or 10,000v at .001a, or 100v at .1a

    It is still 10 watts.
  6. speedster239

    Thread Starter New Member

    Nov 24, 2009
    So, like I said above there simply isn't enough energy to flow. There is a great potential difference, but the actual amount of energy is different and thus Current = Voltage/Resistance applies, but if the resistance is very low and the source of electricity is a 9V battery, for example, the formula may work it out that lets say 90 amps should be flowing, but it simply won't be because there isn't enough electrons to actually do so?
  7. count_volta

    Active Member

    Feb 4, 2009
    I think it has more to do with actual time. Power is not only Voltage * current, power is also joules/second. Its the amount of energy spent each second.

    I heard people on this forum say, and I think I seen it in real life too, "if the max rating of your device is 100V and you accidentally apply 150V for 2 seconds, most devices will work perfectly fine". I guess by that logic you can connect 30,000 volts to yourself for 0.5 seconds and live. ;)

    "Kids please don't try it at home, I am very probably wrong anyway." :D
  8. retched

    AAC Fanatic!

    Dec 5, 2009
    Most of the over voltage specs have to do with how much heat a package can handle.

    So the overvoltage for a short time wont melt the internals(like a few uC voltage spike), but, like a light bulb, it takes a few nS before it gets to temp.

    BUT 30,000v at .5 seconds WILL cause damage. Half a second is an eternity when talking electricity/electronics. Especially with a fragile human.

    Now .5 seconds of 30kv pulses at %1 duty cycle may not cook you, but it will probably get you a one way ticket to dead, or a permanent hospital stay that includes drooling on yourself for the rest of your days.

    The overhead powerlines dont usually carry 30kV. But if you touch them for 1mS, your missing body parts.
    Last edited: Jul 8, 2010
  9. Wendy


    Mar 24, 2008
    DerStrom8 likes this.
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