Household Wiring Question

[JohnCU]

When you take a voltmeter, and test the two wires connected to a regular on/off switch (just one wire going in, one going out), shouldn't the voltmeter read 120 volts, since the switch is just an open circuit when it is in the off position?

 

[flamingelephant]

uhhhh if the circuit was DC what would be true, but its AC, so its not.... right?
i believe AC current switches polarity 60 times a second.... so if you switch your probe ends 60 times a second then yes, it would

 

[JohnCU]

Originally posted by: flamingelephant
uhhhh if the circuit was DC what would be true, but its AC, so its not.... right?
i believe AC current switches polarity 60 times a second.... so if you switch your probe ends 60 times a second then yes, it would

Voltmeter has an AC mode to read the RMS value of the voltage.

 

[JohnCU]

bump for that one dude who's name I can't think of but knows a lot about electrical wiring.

 

[wedi42]

yes

 

[MrPhelps]

It would be the same as putting the probes in a duplex. You should get 120volts.

 

[JohnCU]

Originally posted by: MrPhelps
It would be the same as putting the probes in a duplex. You should get 120volts.

I thought so, I'm not getting anything... maybe that's why my ceiling fan wasn't running at full speed...

 

[drnickriviera]

Uh maybe i'm missing something, but the 2 wires coming out of a light switch are both hot (well the same wire, just one is switched). There is no potential difference so there will be no voltage reading. Now you can test it against the ground wire and should get a reading then.

 

[JohnCU]

Originally posted by: drnickriviera
Uh maybe i'm missing something, but the 2 wires coming out of a light switch are both hot (well the same wire, just one is switched). There is no potential difference so there will be no voltage reading. Now you can test it against the ground wire and should get a reading then.

Well, when the switch is open (off position), there is no current getting to the second wire, so it's just a break in the circuit, so it's an open circuit and should be registering 120 volts (RMS) I would think.

 

[ktehmok]

The switch leg going to the device (a light bulb in this case), is only connected to the neutral by a thin filament in the light bulb & won't show up on a volt meter. You won't see it register unless there is a load on the wire, like when it's turned on.

The whole point of the filament is to cause resistance to ground, therefore heating up until it glows, & voila! A light bulb!

 

[JohnCU]

Originally posted by: ktehmok
The switch leg going to the device (a light bulb in this case), is only connected to the neutral by a thin filament in the light bulb & won't show up on a volt meter. You won't see it register unless there is a load on the wire, like when it's turned on.

The whole point of the filament is to cause resistance to ground, therefore heating up until it glows, & voila! A light bulb!

Must be me confusing ideal circuit models and real world stuff...?

 

[Wizkid]

It's kinda like you are trying to measure the voltage in series... and that doesn't work.

You are measuring A vs. A if the switch is on (zero) or A vs. nothing if the switch is off (also zero).

 

[HomeBrewerDude]

why are you measuring the voltage anyways?

 

[Scarpozzi]

It takes 2 wires for A/C to work. A light switch breaks only one of these wires. You'd have to either test against ground to get 120 or find the other wire in the circuit to get a reading.