| MadSci Network: Engineering |
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: http://www.ecmweb.com/mag/electric_know_capacitive_voltage/index.html 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. Zack
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