Voltage detector circuit

Discussion in 'General Electronics Chat' started by mentaaal, May 24, 2007.

  1. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Hey guys, could anyone please explain to me how a voltage detector screwdriver works? i mean say you touch a conductor with the screw driver and there is current flowing through it and the led or whatever it is inside the screwdriver lights up... how does this occur as you arent completing a circuit as such are you?
     
  2. electronics wiz

    Active Member

    Mar 15, 2007
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    The one I have has 2 conductors one for ground and one for the input.
     
  3. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    i have had the same question,
    once i tried touching the cap of tester at different angle so as to not put pressure on it (i though may be we complete an internal circuit by small pressure) but it still lit up,
    i tried a liitle few more tricks (dont remember exactly) and came to an inference which might be wrong, but i think we do comlpete the circuit by becoming a part of it that is a small current passing thru us may be the cause dont know for sure.
     
  4. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    It works through the capacitance of your body to surrounding/ground. The neon lamp only needs small amount of current to lit up.
     
  5. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    The bulb in your screwdriver is actually a neon bulb.

    The voltage in the house outlets are relative to ground, since neutral is virtually grounded (I'm not confusing neutral with ground (the ground is different since it is pulled close to the houses), but tri-phase systems don't need a neutral return wire going all the way back, the earth is used to balance instead, and only three wires are used to transport current). Since neutral is grounded, when you touch the hot pole of your outlet with the screwdriver, the current flows through the screwdriver making the neon bulb lit, and also through your body, to the earth (and subsequently to the ground). Of course you are protected by the neon bulb and a series resistor.

    I insist on this, because in a floating electrical system, where none of the "voltage source" terminals are grounded, you need to touch both terminals to "conduct", independently even if it is a 110VAC system (this is why isolating transformers are used).
     
  6. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    thanks very much for all the replies guys... makes a lot more sence now.. not sure about the whole virtual ground thing but thats just becuase i have never really been bothered about looking into house wiring...

    incedently enough i was looking at my immersion heater switch the other day as i thought it had a problem.. i was using my trusty multimeter to measure the resistance between the live and neutral input terminals (with the mains power turned off of course) and with the switch on the off position the resistance was in the megaohms but when i turned the switch on the resistance fell to only a few ohms???? is this possible or am i on drugs or something??
     
  7. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    That naturally happened, because you forced more current into the multimeter. You shouldn't measure the resistance of anything live. You may trash permanently the multimeter (or the ohmmeter).
    Remember that an ohmmeter (or multimeter reading ohms) reads resistance by establishing a constant voltage between the leads. The current passing through the component being measured is then evaluated and the resistance is calculated according to R = V/I. When measuring something live, you are forcing more current going through the leads and ohmmeter, thus evaluating a much bigger current and leading to a much smaller resistance being red. This situation will cause erroneous reading and may cause permanent damage to the ohmmeter.
     
  8. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
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    no no as i had said in my previous message, i had turned the mains power off so the circuit being measured had no power flowing though it whatsoever.. i wouldn't trust myself just yet to measure a circuit handling full220vac and potentially kill myself in the process. so the question still stands, how can the switch reduce the resistance to a mere few ohms between the live and neutral in terminals?
     
  9. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Ok, I didn't noticed. I don't know about heater resistances in the US, but in Europe, they might be in the order of 52.9Ohm per kilowatt. I measured myself but you can confirm using calculations.
     
  10. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    oh... well thank you very much i just immediately didnt believe the resistance and thought i was doing something wrong. wow that would explain the high power consumption. thank you!! so now i dont have to think about it constantly! excellent!

    ps i am in europe, ireland actually so sounds very much in accordance with the measurement i got.
     
  11. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Sorry, I gave you an incorrection. Should be 52.9Ohm times Kilowatt, or 52.9Ohm.KW.
     
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