MadSci Network: Engineering

Re: Re: How can a 'voltage tick' meter still work when no load is present?

Date: Tue Jun 6 22:10:04 2006
Posted By: Zack Gainsforth,
Area of science: Engineering
ID: 1148564711.Eg

A voltage tick meter is basically a device that forms a capacitor between
one electrode in the meter, and the wire you wish to test.  The insulator
between the "plates" of the capacitor is the insulator of the wire.  If
there is an AC voltage in the wire, then some AC current will pass to the
other side of the capacitor, i.e. the meter.  The current is usually very
small since the capacitance is puny.  A voltmeter connected to the
electrode in the meter registers any voltage, it then generates a ticking
noise (some models light a little bulb) if it finds any.

Whether or not a load is present doesn't alter whether or not an AC current
is flowing into the capacitor.  This is because capacitors depend on the
electric field and presence of charge.  The presence or absence of a
current doesn't change the fact that an electric field is formed, and hence
that an electric potential appears on the far terminal.

Looking at it another way, when you have an AC current, there is in fact a
load: the meter.

You must have a VERY sensitive voltmeter, so it can detect this small
voltage.  An excellent and thorough explanation of the electronic
principles can be found at:

You can actually make your own sensor using an ordinary VMM.  You must have
a fairly good model, as it must have a sensitive AC voltmeter (millivolts).
 Set it on it's most sensitive setting, grab one electrode, and then touch
the other electrode to the outside insulator on a power cable.  You should
see a change in the voltage registered.  Pull the second electrode away,
and the voltage should return to roughly it's previous value.  Do this
several times until you are convinced you are really reading a voltage
present in the cable.  Then you can unplug the cable and see that it
doesn't now occur.  Doing this myself, I got a change from about 50 mV (not
near cable) to 100 mV (near cable).  Of course, if you were building a
professional version, you would make a highly sensitive voltmeter, a better
electrode, etc.


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