# screwdriver tester

Discussion in 'Electronic Basics' started by [email protected], Oct 8, 2007.

1. ### Guest

when i stand on a wooden stool and touch a live wire i dont get a
shock because circuit is not complete with ground as wood is a
insulator but when i touch a screwdriver tester to that live wire the
bulb lights up even while i am standing on the wooden stool how can a
bulb light without getting earthing?

2. ### JamieGuest

It's magic dust!

3. ### John PopelishGuest

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.

4. ### Stephen J. RushGuest

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.

5. ### EeyoreGuest

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

Graham

6. ### JamieGuest

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

7. ### EeyoreGuest

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

Graham

9. ### Michael A. TerrellGuest

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.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

10. ### Rich GriseGuest

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

Hope This Helps!
Rich

11. ### Guest

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

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

* * *
Christopher

Temecula CA.USA
http://www.oldtemecula.com

12. ### sparkyGuest

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

13. ### craigmGuest

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

14. ### Jon SlaughterGuest

So the screwdriver comes with a waterlogged wooden stool?

15. ### zhafranGuest

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?

16. ### John PopelishGuest

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.

17. ### Rich GriseGuest

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

18. ### Rich GriseGuest

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

19. ### NobodyGuest

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