SCREWDRIVER / VOLTAGE TESTER like this
work using following principle:
There is a high resistance resistor (limits current
through the device to safe sub-milliamprere level maximum)
and a neon bulb (lights at very low current).
The circuit is the same as used on neon lamp indicator
lights found on switches etc.. only difference being
that the current limiting resistor has higher resistance
(lower current) and safer design (will not arc over or fail on
any normal use or overvoltages).
When the tip is connected to live wire (or is near it so that there is
enough current through capacitance) there is a small current
that flows to your body to ground and makes the neon bulb to
put out some light. If you are touching ground with
other hand, the maximum current flows through your body
(limited to safe value with the current limiting resistor).
If you are not toucvhing ground the current flows to ground
through the capacitance between your body and ground
(still enough current
This kind voltage tester can fail in case you are very well insulated
away from ground sources. In that case there is not enough
"leakage" through you that the would make the neon bulb to flow.
Information on neon bulbs:
Tomi Engdahl (http://www.iki.fi/then /)
Take a look at my electronics web links and documents at
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