Non-contact voltage tester

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I thought someone wrote that is was a Capacitor that your seeing flash'n and beep'n, I did not search old threads...

" The dielectric dictates what kind of capacitor it is and for what it is best suited. Depending on the size and type of dielectric, some capacitors are better for high frequency uses, while some are better for high voltage applications. Capacitors can be manufactured to serve any purpose, from the smallest plastic capacitor in your calculator, to an ultra capacitor that can power a commuter bus."

From how things work
 
We call them a chip stick. While I will trust it to show presents of voltage, I'll NEVER trust it to show no voltage. Mine got me one time and that was enough! Showed outlet to be dead. Was in a flower bed.. I grabbed the outlet... IT WAS NOT DEAD! ouch
 
090829-0825 EST

chris:

Capacitance.

A capacitor is formed by two conductors separated by an insulator. The capacitance increases with an increase in the dielectric constant of the insulating material and an increase in the conductor area. Capacitance decreases as the distance between the conductors increases.

With a cord and my LC meter probe (about 1" of #24 wire) close to the cord (within 0.05") I read about 0.1 pfd. With a dime as the probe plate the capacitance was about 0.3 pfd.

Capacitors in series form a voltage divider. In the voltage probe there is capacitance from the circuit being tested to the probe of maybe less than 0.1 pfd, input capacitance to the probe of maybe 5 pfd, capacitance from the probe handle to your hand, guess at 50 pfd, and from you to surrounding ground of maybe 50 pfd.

Thus, simplify this to a series circuit of 0.1 pfd and 5 pfd. 1/50 of the input voltage appears at the probe detector. Not difficult to make a threshold detector of 1 V. Thus with these parameters you could detect 50 V on a circuit.

I do not know the specific parameters of one of these probes, but the above gives you a rough idea of how these work.

Also note that the voltage division across a series capacitor string is independent of frequency.


nakulak:

Any grounded conductor has near zero voltage on it relative to earth and therefore the source voltage to the test probe is near zero.

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090829-0946 EST

My capacitance to ground when standing on my basement floor was about 40 pfd, and on a first floor (wood) away from a lot of metal surfaces it was about 25 pfd.

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ohmhead said:
http://www.techni-tool.com/content/resources/articles/Fluke-Understandingcapacitivevoltagesensor.pdf
Click to expand...

The capacitive voltage sensor works because when you
hold the barrel in your hand and place the tip near a live
conductor, you are inserting the high impedance sensing
element into a capacitively coupled series circuit. As in
the previous example your hand and body form a
relatively large capacitor coupled to the floor.
Click to expand...

Mine works without me holding it. I use it as a down and dirty way to locate which breaker circuits are on.
 
You can get a ticker to ring on a neutral or ground by putting your other hand around the insulation of a hot wire (don't grab any live bare copper ;) ). While holding on to the insulation, the ticker's sensitivity gets reversed. While testing for neutral and ground with a ticker doesn't do you much good, at least you know it'll work :D
 
K8,
I had a general class Ham license when I was 14,
and I was good.

"""Mine works without me holding it."""

Not this Great, however! :)

I do like the idea you introduced
about affixing the tester to a Receptical
and listening for the signal from around the corner.
I do like that idea, maybe with some amplification.
Since I always work alone,
this is idea has immediate application.
Maybe I could put my spare cell-phone at the receptical,
within ear-shot of the ticker,
and and have it transmit a signal to my first cell-phone.
:)
 
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