On Tue, 24 Oct 2006 17:34:36 GMT, (Doug Miller)
wrote:

In article , Mark Lloyd wrote:
On Tue, 24 Oct 2006 13:35:09 GMT, (Doug Miller)
wrote:

In article m, David
Nebenzahl wrote:
Doug Miller spake thus:

In article m, David
Nebenzahl wrote:

dpb spake thus:

Al Bundy wrote:

wrote in news:1161608374.490859.297690
:

Can anyone tell me why I would get a 27 volt reading on a 120v circuit.
I have a old home but some of the home has been re-wired. I am
remodeling my bath room and removed old florescent lights. I tested
the power at the connections and got a reading of 27v. I climbed in
the attic and it looks like the wire is coming from a junction box with
several other wires. It looks like the work was done by a pro. The
upstairs has 2-20amp (connected together) circuits.

You are using a low impedance (resistance) meter ...

Precisely backwards...a _high_ impedance meter can load a circuit and
read "phantom" voltages. For such tests of household wiring circuits
an inexpensive analog meter is probably more reliable than the digital.

Nope, *you've* got it backwards: high impedance = high sensitivity. For
a meter with an impedance in the megohm range, it's very easy to pick up
stray, "phantom" voltage readings.

Ummmm.... that's exactly what he said. What's "backwards"?

The part where he said "a high impedance meter can load a circuit". I
shoulda been more clear; a high-impedance meter reads phantom voltages
because it loads the circuit *less*, not more.

Since when does a high impedance place less load on a circuit than a low
impedance??

Since "impedance" took on its current meaning.

Please explain.

Would you be saying you don't know what "impedance" means?

A high impedance source (such as that 27VAC probably is), is
essentially a voltage source in series with a resistor. The effect of
this is that any attempt to draw current from this source will lower
the voltage, potentially to near zero. Phone lines are like this.

A high impedance load (such as a digital meter or VTVOM/FETVOM) will
draw very little current from a source (not lowering the voltage very
much). Note that such a meter is necessary for some sensitive
electronic circuits (that would be disturbed by a low impedance load).

An analog meter on a high impedance source will give inconsistent
readings on different ranges, since it has a different impedance on
each range, loading the circuit differently. Adding a (120V 60W) light
bulb in parallel will make the impedance MUCH lower.
--
62 days until the winter solstice celebration

Mark Lloyd
http://notstupid.laughingsquid.com

"How could you ask be to believe in God when there's
absolutely no evidence that I can see?" -- Jodie Foster