High Voltage, Low Current?

Discussion in 'General Electronics Chat' started by jimmy101, Jan 18, 2014.

  1. jimmy101

    Thread Starter New Member

    Jun 23, 2012
    Moderator Comment:

    This was split from High Voltage, Low Current?, as it was both necroposting and hijacking of someone elses thread. A thread belongs to the original OP (original poster), and hijacking is not allowed. Now you have a thread of your very own.

    Nobody gave a decent answer to the original question.

    The issue is the impedance of the piezo as a source. A piezo (or most any very high voltage source) only generates the high voltage if it is attached to a load of infinite resistance. When someone says a piezo sparker generates 30KV that means it can build up a voltage potential across an air gap of that voltage. The gap is basically an infinite resistor so no current flows and no power is expressed. When the voltage across the spark gap exceeds the breakdown voltage of air the air ionizes and behaves like a very low value resistor, probably much less than an ohm. Once breakdown occurs current flows but because of the high impedance of the piezo the voltage drops to nearly zero basically instantly.

    For a stun gun you can't just take the (widely inflated) claims of the gun's output voltage and multiply that by the bodies resistance since that isn't what happens. That high voltage can only be generated by the gun when there is no load (or the load has extremely high resistance, say many gigaohms). As soon as there is any current flow the output voltage of the stun gun will drop to a low value. I would WAG the voltage at a few tens of volts when it actually in contact with a person.

    Then what's the point of a 30KV stungun? Why not just use a few tens of volts? You can touch your finger to a tens of volts power suply and not feel anything. But, connect a couple of sharply pointed wires to the same supply then jab those wires into your skin (which breaches the high resistance of the skin's surface and gets into the conductive salt water inside the body) and ZAP. Or, take a 9V battery and touch the contacts...nothing, then touch them to your tongue and you can "taste" the voltage. Same as with sharp wires, skin is very high resistance but your wet tongue is much lower resistance.

    But still, why 30KV for a stun gun? The 30KV replaces the sharp points you need to breach the skin, indeed you don't even have to actually touch the skin with 30KV. Get the gun's probes close (within a mm or so) to the skin and the HV will jump the gap, like a piezo BBQ sparker. Even if there is cloth between probes and the skin the spark will jump through that as well. The HV will also jump through the high resistance of the skin into the much lower resistance of the salt water inside the body and we get current conduction over to the other probe. Once that happens though the high output impedance of the stun gun seriously limits both the voltage and current that actually flows in the victims body. The actual voltage across the gun's probes when it is shocking someone is a lot closer to zero volts than it is to 30KV.
    Last edited by a moderator: Jan 19, 2014
    inwo likes this.
  2. inwo

    Well-Known Member

    Nov 7, 2013

    I agree......................

    It's not just the body resistance that limits current.
    The complete circuit includes the source impedance.
  3. Austin Clark

    Active Member

    Dec 28, 2011
    Two things.

    1) Once the tazer begins discharging, the high voltage across the terminals will suddenly drop due to the relatively high output impedance compared to the human body/spark gap. The high voltage is necessary to penetrate clothing and breakdown a small gap of air to get a good connection to the body. It's also necessary to make sure that the electrical resistance of the body (which is extremely variable) has little to do with how much actual current flows. After the air breaks down and a spark is created, a conductive path to the body is present, the voltage drops, and a very short burst of current is produced.

    2) The human body can't be modeled like a simple resistor. If you introduced, say, a perfect 1KV across your body, if the pulse is short enough, the amount of current won't be allowed to grow too high. Similar to an inductor, It takes time.
    Last edited: Jan 9, 2017
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    Bravo on the great post