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Old 11-10-2012, 10:31 PM   #1
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voltage across a switch


should i have votage between the 2 conections on a switch when off? wth a dig volt meter i m reading 50 volts when the switch is off. is this normal? thanks

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Old 11-10-2012, 10:32 PM   #2
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voltage across a switch


Yes, you are reading voltage thru the light bulb.

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Old 11-11-2012, 10:24 AM   #3
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voltage across a switch


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Yes, you are reading voltage thru the light bulb.
I disagree. When the switch is off, the circuit is open. There is no current through the light, the light is OFF, the full line voltage (presumably 115-120V) should appear across the switch terminals.

When the switch is on, the circuit is closed. There will be no voltage drop across the switch, the full line voltage is across the light, and the light in ON.
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Old 11-11-2012, 10:50 AM   #4
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voltage across a switch


Both of you are right.....depends on how the bulb is wired.

If power is ran to the bulb and the switch is a switch leg, then yes, you will see the full voltage across the switch with the switch off.

But if power is ran to the switch....assuming the hot is being switched....one side of the bulb is tied to neut....the other side to the switch....with the switch off....you will read some voltage...though I find 50Vac to be a bit high unless the bulb is a CFL. The only way you could get that high of a reading is with no bulb, a CFL or LED or a really low wattage (high resistance) bulb.
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Old 11-11-2012, 03:46 PM   #5
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voltage across a switch


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Originally Posted by ddawg16 View Post
Both of you are right.....depends on how the bulb is wired.

If power is ran to the bulb and the switch is a switch leg, then yes, you will see the full voltage across the switch with the switch off.

But if power is ran to the switch....assuming the hot is being switched....one side of the bulb is tied to neut....the other side to the switch....with the switch off....you will read some voltage...though I find 50Vac to be a bit high unless the bulb is a CFL. The only way you could get that high of a reading is with no bulb, a CFL or LED or a really low wattage (high resistance) bulb.
I still have to disagree. Whether the switch is inline or in a switch loop is irrelevant... electrically the two are the same.
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Old 11-11-2012, 08:57 PM   #6
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voltage across a switch


With the switch closed (light on) you should read nothing at the switch, however with the switch open you have a difference in potential and should read line voltage
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Old 11-11-2012, 09:08 PM   #7
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voltage across a switch


You will read voltage either way.
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Old 11-11-2012, 09:11 PM   #8
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voltage across a switch


a voltmeter reads the potential difference so if the switch is closed (light on) you should read zero volts since your ur just touching the 2 leads together, when its open (light off) you should be getting the 120 volts.
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Old 11-11-2012, 09:19 PM   #9
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voltage across a switch


For almost all household electrical situations, measuring voltage across the two switch terminals does not give "useful" information. Voltage should be measured from neutral to a desired test location, or from ground to a desired test location, or from either of the hot leads of a 240 volt circuit to a desired test location which could be the other hot lead.

When a voltmeter is used, a minute current flows through the meter to register a non-zero reading.

When you see a non-zero voltage reading across a switch that is flipped off, the voltmeter has completed a circuit and the minute current flows.

If the light bulb is removed, or for some kinds of fixtures where the bulb lights up by having an arc struck inside, you will measure zero volts across the switch when flipped off also. The resistance of the voltmeter is such that current will not jump across the electrodes in the bulb (strike the arc) so, although the voltmeter completes the portion of the circuit at the switch, the circuit is not complete through the bulb and no current whasoever flows.

A tiny neon bulb is connected directly into a circuit (it also has a resistor in series). This bulb has two electrodes across which an arc is struck, and no filament, so measuring across a switch that controls this bulb and is switched off will show zero voltage.

A neon sign has a transformer whose primary is connected to the supply line and whose secondary is connected to the neon tube, the latter with the two electrodes at the respective ends of the tube with an arc to be struck the entire length of the tube. The transformer primary will conduct electricity at all times when energized, even if the secondary circuit is not complete. (The primary conducts more current when the tube is lit). When measuring across a switch that controls the primary current and is flipped off, the voltmeter completes the primary circuit but not passing enough current to cause enough secondary voltage to light up the neon tube, and the meter will show a nonzero reading, probably at least a third of the supply voltage.

For an incandescent bulb the filament completes the circuit when the switch is flipped on or the voltmeter probes are placed across the switch that is flipped off. The ratio of the voltage across the meter to the voltage across the bulb is equal to the ratio of the resistance of the meter to the resistance of the bulb. Almost always the result is that nearly the full line voltage is measured across the switch.
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Last edited by AllanJ; 11-11-2012 at 09:35 PM.
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Old 11-12-2012, 11:58 AM   #10
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voltage across a switch


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I still have to disagree. Whether the switch is inline or in a switch loop is irrelevant... electrically the two are the same.
Electrically...yes....measurement wise...not......and you showed yours wired different than what I was describing.

In my house...all of my lights have the hot wired to the fixture...the switch leg is on the neutral side.......

I should have also stressed the importance of the type of bulb. With an incandescent....the resitance is so low that in an open ckt condition, there will be no voltage drop across the bulb. But, if using a CFL, I suspect that it will pretty much look like an open ckt....might be worth testing.

I wish the OP would respond back with the type of bulb he had.

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