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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

11-10-2012, 10:32 PM   #2
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Yes, you are reading voltage thru the light bulb.

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11-11-2012, 10:24 AM   #3
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Quote:
 Originally Posted by Regination 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.

 11-11-2012, 10:50 AM #4 JOATMON     Join Date: Aug 2011 Location: S. California Posts: 10,978 Rewards Points: 1,116 Blog Entries: 2 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. __________________ Even if you are on the right track, you will still get run over if you just sit there. My 2-Story Addition Build in Progress Link ... My Garage Build Link and My Jeep Build Link
11-11-2012, 03:46 PM   #5
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Quote:
 Originally Posted by ddawg16 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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 11-11-2012, 08:57 PM #6 Member     Join Date: Aug 2012 Location: Winnipeg, MB, Canada Posts: 97 Rewards Points: 75 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
 11-11-2012, 09:08 PM #7 Member   Join Date: Dec 2006 Posts: 17,158 Rewards Points: 6,652 You will read voltage either way.
 11-11-2012, 09:11 PM #8 Newbie   Join Date: Nov 2012 Posts: 12 Rewards Points: 10 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.
 11-11-2012, 09:19 PM #9 Member   Join Date: Nov 2007 Location: Nashua, NH, USA Posts: 7,917 Rewards Points: 1,440 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. __________________ The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit. Last edited by AllanJ; 11-11-2012 at 09:35 PM.
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11-12-2012, 11:58 AM   #10
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Quote:
 Originally Posted by Dave632 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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