How tester works

Discussion in 'General Electronics Chat' started by aamirali, Feb 3, 2012.

  1. aamirali

    Thread Starter Member

    Feb 2, 2012
    415
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    Testers which are used for testing voltage at some plug (more like a screw driver with led & external metal tail), when you insert it in a plug & touch external metal point LED glows giving indication about voltage present. If you don't touch metal it won't glow even if it is in plug having voltage.

    I want to know how led glow when I touch the metal.How does the circuit gets completed. Even if sit on plastic or wooden chair, it still works same way?

    I don't understand how circuit gets completed.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    In these voltage testers there will be a neon bulb in stead of a led.
    The neon bulb only needs a very low current to glow.
    Your body will form a capacitor for the AC surroundings.

    Bertus
     
  3. aamirali

    Thread Starter Member

    Feb 2, 2012
    415
    1
    Hi,

    Appreciate your inputs.
    1. Can you help me to understand it more with some circuit.
    2. How does body forms a capacitor? I think even for capacitor to flow current it needs complete circuit.
    3. Does that these tester can't be use to check dc voltage. Never checked it.
     
  4. BJT_user

    Member

    Oct 9, 2011
    35
    8
    Let me see if I can help you, aamirali. The simple answer is... conductance.

    The human body conducts electricity, albeit with a lot of resistance, like tens to hundreds of K-ohms. Even so, we do conduct. So when you touch that external metal point on your voltage tester, your body is completing the circuit.

    To put this in perspective, let's examine a scenario that almost all of us have experienced at some point in our lives. You have an electrical appliance with a metal case, and you plug it in. Then you go to operate the appliance and when you touch the metal case, you get anywhere from a little tingle, to a major shock. YOWCH!

    So you jerk/jump away from the appliance and wonder for a moment, what the heck? But it doesn't take long for you to realize that the appliance has a fault in it and the case has somehow become shorted to the hot side of your house voltage. They key here is that you feel the shock only because the electricity is conducting through your body. But how?

    How does it conduct if all you are doing is making a single point of contact with the appliance? The answer is... ground. The low side of the 120 volt power coming out of our AC outlets is at common to ground. At some point, the low side of our home AC power is wired to earth ground, be it at the junction box outside our house where the power comes in, or further back on a power pole feeding our home.

    So knowing that there is another point of contact, which is ground, we understand that ground is the second point of contact we are providing that completes the circuit, through our bodies. Even through clothes, shoes, carpet, wooden floors, etc… we still feel a shock because all of those things conduct electricity to some degree at high voltage, and thus create in us a conductive path to ground at almost all times.

    So when you touch the metal end of your voltage tester, you are the one completing the needed voltage path to ground, which will illuminate the LED in the tester. Hope this helps.
     
    Last edited: Feb 3, 2012
  5. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    If indeed you do have one of the NEON type testers you will see two rods inside a glass bulb. With AC, both will tend to glow. Actually they are taking turns glowing at either 50 or 60 times per second, depending upon where in the world you live. For DC, only one electrode will glow. It will indicate the NEGATIVE lead. These units usually require somewhere around 55 to 60 volts to light up. Inside the housing, there is a resistor to limit the amount of current the bulb can draw.
     
  6. aamirali

    Thread Starter Member

    Feb 2, 2012
    415
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    Like you said above it even conducts some current via wooden chair. But they are very high insulators. Moreover wooden chair is placed on tile flooring & do floor also conduct a bit at high voltage?

    One funny thing I have done, is jumping in air & then making contact with metal. Air is very high insulator. But tester still works?

    // Jumping in air test is dangerous & tested under proper measures. Don't try this at home. ;) Just an empty devil's mind at work.
     
  7. BillB3857

    Senior Member

    Feb 28, 2009
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    Think of your body as a large bucket. When you are touching one lead of your tester and the other lead is connected to line power, for 8ms (assume 60Hz) electrons (not many) flow into your body, the next 8ms, electrons flow out of your body. Since they are not filing your body, nothing happens except the light glows. Now, shuffle your feed across a carpet. Electrons flow to your body from the carpet, but don't flow back. After a while, your body has a lot of extra electrons. Touch a grounded device and you get a spark. Your body is getting rid of the extra electrons. That is static electricity. Now I may have it exactly backwards, but the concept is the same.
     
  8. bretm

    Member

    Feb 6, 2012
    152
    24
    That's just it -- it doesn't. Look at the circuit symbol for a capacitor. It looks like a break in the wire. And in fact it is. The electrons do not actually pass all the way through the capacitor.
     
  9. aamirali

    Thread Starter Member

    Feb 2, 2012
    415
    1
    1. Some say its an capacitor. However to charge a cap I needs a return path.
    But as "bretm" replied above capacitor is break in circuit. So to confirm this I connected +ve leg of cap with +ve dc power supply & left -ve leg of capacitor unconnected. cap don't get charged. that means even for cap to charge it need a complete ckt but in tester case no ground is provided as I am standing on wooden or plastic surface.

    2. A comment above was charge get on your body for 8ms & next 8ms it gets off. Then I had tested it on Dc supply also. It works. How to explain this.

    3. I have connected a ammeter in between tester & my body. Current was 17-18uA ac for 220Vac. I have tested it on Dc supply voltage of 100V dc also. Although I neon lamp was dim. Current in ammeter was 0uA dc. Might be due to meters's precision
    problem.

    4. One comment was body is cap, but as I said above in point 1, cap needs return path

    5. i think one probable answer is static electricity or human body model. but don't know how it works & not sure its a reason?
     
  10. BillB3857

    Senior Member

    Feb 28, 2009
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    You would have to wait a very long time for your body to loose the ability to accept or supply electrons before the light goes out. Place a capacitor in series with your test light and your body, then connect to DC. The light will flash and then go out. This is because the capacitor will charge to the point that it will not allow any more electron flow. For a fun experiment with your light, look up "neon relaxation oscillator".
     
  11. aamirali

    Thread Starter Member

    Feb 2, 2012
    415
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    How does charge gets stored on the body. Is it static electricity.? Can charge be stored without GND also.
     
  12. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    Have you ever touched something, like a door knob with no electrical connection, and received a small shock? That is because the charge on your body and the charge on the door knob are different. The small shock results from them becoming equal by transfer of electrons. If the charge difference is large enough, it will make a spark. Another way to generate small static charges is to run a comb through dry hair. Tear up small bits of paper and place on a table top. After combing your hair with a plastic comb, place it near the paper. It will pick the small pieces up. Sometimes, the paper pieces will jump to the comb, then jump away. Can you predict why this may happen?
     
  13. @android

    Member

    Dec 15, 2011
    178
    9
    It is static electricity only. Don't forget the basic(Ω). GND isn't universal constant. See basically how does the current flows?? If there is potential difference between two points then to balance the potential electrons flow from higher potential point to lower potential point. The lower potential point may not be having '0' potential. :cool:
     
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