Re: low voltage in circuit, switch off
- From: Jeff Wisnia <jwisnia@xxxxxxxxxxxxxx>
- Date: Sat, 10 Jun 2006 16:00:43 -0400
Steve wrote:
I'm using a ~20 year old analog multimeter. I measure ~5V on the 30V
scale, but about 0.5V on the 3V scale; so it's just enough to boost the
meter off zero. I was wondering about induction, the circuit in
question is only several meters from the service box, but next to a GFI
outlet.
The other folks keep calling it "induced" voltage, but you only really couple inductively if there's a current in one wire running next to another one, not just voltage on the wire.
From your description it sounds like you have capacitive coupling across the switch. The capacitive reactance behaves like a very high impedance at 60 HZ. For the purpose of understanding what your meter is showing, you could consider it like a very high value resistor across the switch, conducting current from the 120 volt source through your meter's internal load resistance to ground.
Now, your conventional analog meter will have a resistance rating, something like "20,000 ohms per volt". That means, when set to it's 30 volt scale it will present a load of 600,000 ohms, but when set to its 3 volt scale it will present a load of 60,000 ohms.
That's why you see two different voltage readings, the current flow is pretty much constant, as the voltage across the capacitive reactance is nearly 120 volts and varies less than 5 volts between the two measurements, but the meter's load resistance is changing by 10:1, so that relatively constant current creates two significantly different voltages across the meter's input terminals, and that's why your two measurements differ.
Either way, it's nothing to worry about, if your meter is a 20,000 ohm per volt unit the current flow is approximately 5/600,000 amperes, about 8 microamps. You prolly couldn't feel that low a current even if you stuck your tongue on the wire while grabbing metal water pipes with both hands.
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
"Life is like a sewer -- what you get out of it depends on what you put into it."
Steve
Ralph Mowery wrote:
"Steve" <gl_sds@xxxxxxxxxxxxxx> wrote in message
news:1149962292.639614.270200@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I have a circuit in my house with a funny characteristic, and I'd like
to track it down. In that circuit, I measure a slight voltage
difference between power and neutral at a light fixture when the switch
to that fixture is off. Yet, my meter shows no continuity when I try to
measure resistance. Other circuits do not show any voltage when they
are switched off.
I assume this means that the neutral on the peculiar circuit is not at
a common ground with the 'real' ground. It does not strike me as
inherently dangerous, yet it seems like I might as well sort out the
problem. Anybody seen this and have a recommendation on
troubleshooting?
It is probably nothing to worry about. If you are using a digital meter it
is most like picking up some induced voltage from other conductors near it
or the switch could be leaking a very small ammount. How many volts are you
calling 'slight' ?
.
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