Can you help explain how this electric neon tester works (or doesn’t)?

Ajinkya D May 5, 2014
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Hi guys,

I want to know about the workings of an electric neon tester. For example when I insert the tester in one terminal of the AC socket, the light glows but when I insert into another terminal it doesn’t – why?

As taught to me in college, for the first half-cycle one terminal (say T1) of the AC socket is positive while the other (say T2) is negative and for the second half cycle terminal T2 is +ve and T1 is -ve and our body offers resistance as well as capacitance to complete the circuit and cause the light to glow.

Then according to this logic the tester should glow when inserted in any of the two terminals but it doesn’t,why?

  1. Jan F
    May 5, 2014 at 2:12 pm

    It would go beyond the extend of this topic to explain this in detail so let me try to make this short and simple.

    Those single-phase testers you appear to be using do not care about positive or negative. They don't care about high or low voltage either. They care about potential differences.

    The live phase will always result in the light going on as it has a different potential.
    The neutral phase however, is supposed to have (near) same potential as the grounding and since this is your testers second "measure" (via your body) it will not light up on the neutral phase.

    If you happen to have a radiator nearby you could try putting the tester into the neutral phase and touching the grounded radiator. In that case the difference might be noticeable enough for the lamp to go on.

    Anyway, unless you are skilled enough in electrics you should stay away from all of this, especially the phase tester as they put you in direct contact with the phase and offer no protection whatsoever. Your only measure should be the breaker circuit being on or off.

  2. Oron J
    May 5, 2014 at 11:15 am

    What you were taught in college is inaccurate. There two terminals are actually Live and Neutral. The live goes through the whole cycle (positive, then negative), while the negative always stays at zero. The tester uses your body as an earth-terminal (i.e. somthing to "soak up" the current). Therefore, when you plug the tester into the T1 you get a light, and when you plug it into T2, you get nothing.

    • Ajinkya D
      May 5, 2014 at 12:22 pm

      Please can explain in more detail...
      And if T2 terminal remains neutral all the time then how does the phase reversal circuit(part of centre tapped full wave rectifier) work??

    • Oron J
      May 5, 2014 at 1:15 pm

      I don't really see the problem. Electrics/electronics is premised on difference in energy potential, not absolute values. If the T2 is 0v and T1 is, say, -110v, then the difference in potential is 110v (in the oposite direction to what it would be if T1 was +110v).

      In a properly wired circuit, you can touch the neutral terminal and not receive an electric shock. Do not try this at home! There is no guarantee that your circuits are 100% compliant, but if you connect a lightbulb to T2 and to Earthing terminal then the light should not go on.