screwdriver tester

It's magic dust!

Even with no ground connection to complete a resistive circuit, your body has capacitance to the rest of the universe, so charges and discharges each half cycle of the AC with a small capacitive current. This small capacitive current (sub milliampere) is well below your threshold of feeling, but is above the threshold to light a small neon bulb.

If the AC voltage were much higher (say, hundreds of thousands of volts, like what is carried on the high voltage transmission lines that cross the continent) the capacitive currents they would drive into your body capacitance would reach the threshold of feeling, even if you were suspended from a helicopter.

That, plus the very low (< .1 ma) current required by a small neon lamp, plus the unipolar capacitance of your body. The tester has a large resistance in series with the lamp, so you won't get stung even if you are grounded.

You're acting as a large value resistor. Enough current passes to light the neon bulb without giving you a shock.

Graham

neon

really, I thought it had something to do with capacitance with AC combined. Oh well, back to school. snort snort..

neon

Are you saying that the body (or wood) is incapable of making a resistive connection ?

Graham

the neon

connection ?

Your body is one plate of a capacitor, and the earth is the other plate. It has nothing to do with resistance, in this application. it is like running your hand along the glass of a fluorescent lap that doesn't come on when you apply power.

Not at all, merely that in this case the conductance is negligible compared to the capacitive reactance.

Hope This Helps! Rich

If you placed one 60 hz 120v wire upstairs in a wooden house with no other wiring or metal objects and the neon lamp still lit I would guess that trickle energy and the wire voltage that made the neon bulb light converted into trickle radiation into the air space surrounding our candidate holding the other end of the wire.

I think radiation resistance through our candidate is as good an answer as capacitance on our candidates skin to some earth grounding surface ten feet away.

At higher frequencies possible, but 50/60 hz, I would need to ask Radium.

Usenet was a wild frontier 10 years ago. I remember ;-) Then Google invited itself to our campfire.

• * * Christopher

Temecula CA.USA

neon

The wood is an insulator. The current flows through you because of capacitance.

Depending upon the moisture content, wood may be a poor insulator.

So the screwdriver comes with a waterlogged wooden stool?

neon

A beginner question here: Since it is due to capacitance, does it mean that the neon lamp will be turned ON for only a few second when we're testing it with DC voltage? Because capacitance has the characteristics to block DC when it's fully charged right?

Yes. One flash, till you discharge the DC voltage on your body and the plastic surfaces of the screw driver handle. again.

But don't take my word for it. Series connect a stack of 12 each 9 volt transistor radio batteries, with one terminal grounded. Touch the other terminal that with the screw driver, and it should produce a single faint blink. Have someone else reverse the end grounded and tough the other end and you should get another blink.

That would be true if you were testing a DC circuit. Usually, those things are used on AC mains, and it's that AC that flows through the capacitive reactance of your body capacitance to ground. It's not much current (probably way less than one mA), but neon bulbs don't need much.

Hope This Helps! Rich

zhafran, if you do this, use EXTREME CAUTION. 108 volts is enough to drive lethal current through your skin. If you insist on doing this test, then keep one hand in your pocket.

Good Luck! Rich

More likely a few {micro,milli}seconds; the capacitance involved is a few picofarads.

Right.

Does it also applicable to high AC voltage?