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Old 08-19-2009, 07:43 PM   #1
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Odd behavior of voltage tester across switch terminals?


I tested a wall switch today, and found with my voltage tester probes to each of the brass terminals on the switch, the tester would light when the switch was off, and would not light when the switch was on.

Why is this?

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Old 08-19-2009, 08:39 PM   #2
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Odd behavior of voltage tester across switch terminals?


When you place the tester leads across the switch, you are placing them in series with the load with the switch off. If you unscrew all the bulbs in the fixture, the tester should go dark.

Turning on the switch shunts the leads, making the light work.

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Old 08-19-2009, 08:40 PM   #3
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Odd behavior of voltage tester across switch terminals?


Voltage testers need to be in parallel with the circuit to read voltage. When you had the switch closed (on) you were trying to read in series. With the switch open(off), you were reading parallel to the circuit because the wire going to your light was reading through the bulb to neutral.

Had the neutral been disconnected from your light you would still read 0 volts.
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Old 08-19-2009, 11:11 PM   #4
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Odd behavior of voltage tester across switch terminals?


BTW. this (putting a high wattage (300w.) incandescent bulb in series with the circuit) is another way of checking (when working inside a junction box) whether you have a short circuit or a high load. If the bulb lights up dim, you're dealing with a high load, and you could connect both leads. If it lights up BRIGHT, then you have a SHORT. A small wattage light bulb or VOM won't distinguish between the two conditions! (Now more than ever)Don't Drink and Drive!!!
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Old 08-19-2009, 11:16 PM   #5
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Odd behavior of voltage tester across switch terminals?


It requires a very small amount of current (say, a couple hundredths of an ampere to light your test probe. It requires somewhat more, say half an ampere, to light a typical household incandescent lamp. The current will be the same anywhere in a simple circuit, here from panel to switch through tester to other switch terminal to light to neutral back to the panel. With the switch turned off and the only path going through the tester, the current in the whole circuit will attain and also be limited to that the tester takes so the tester lights up and the regular light does not.

Voltage wise, the voltage across any component in that simple single loop circuit is proportional to the resistance of that component. The actual rule (Ohm's Law) is voltage equals current times resistance and as we said for a simple loop circuit, the current is the same in each component. The voltages across each of the components respectively all add up to the input voltage. Let's say a voltmeter has a resistance of a thousand ohms (it's really much more). Let's say the regular lamp, when cold (not lit) has a resistance of a hundred ohms (it's really less). The wires have some resistance but negligible for this math problem. So if you measured the voltage across the switch terminals with the switch off and with the regular lamp still screwed in, using this example you will measure voltage of 1000 (resistance of the meter) divided by (1000 plus 100), or resistance of everything including the meter, or about nine tenths of the 120 volts there.

In a real live situation only a few microamperes flow through the meter and the meter's resistance represents more than 99% of the total circuit resistance and therefore the voltage across the test leads and therefore reported by the meter is essentially the entire circuit voltage, if no other circuit component is present between the test leads. If there is another component between the leads, for example you put the test leads across the terminals of a lit lamp, then the current flowing is much greater, the voltage across that other component (and the meter test leads) equals the current multiplied by the resistance of said other component. The few microamperes going through the meter changes this voltage (the measured voltage) by a negligible amount.

Normally it is not meaningful to measure voltage across switch terminals. More useful is from each switch terminal to neutral (or you can generally get away with measuring from a switch terminal to ground).
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Last edited by AllanJ; 08-20-2009 at 07:15 AM.
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Old 08-19-2009, 11:45 PM   #6
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Odd behavior of voltage tester across switch terminals?


Quote:
Originally Posted by darsunt View Post
I tested a wall switch today, and found with my voltage tester probes to each of the brass terminals on the switch, the tester would light when the switch was off, and would not light when the switch was on.

Why is this?

Your tester is looking for a difference in potential, which it will find when there is a break in the circuit (ie: switch turned off) and when there is no break (switch turned on), both terminals on the switch are said to be at the same potential, and the light wll not light on your tester.
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Old 08-19-2009, 11:46 PM   #7
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Odd behavior of voltage tester across switch terminals?


With the switch off, there is a voltage present across the open circuit, so the tester lights up. With the switch on, the effect is like touching the two tester leads together, so it doesn't light.
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Old 08-20-2009, 12:08 PM   #8
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Odd behavior of voltage tester across switch terminals?


Thats how I test cartridge fuses without having to remove them from their holders. If I read the correct voltage across the fuse, I know the fuse is bad.
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Old 08-20-2009, 02:52 PM   #9
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Odd behavior of voltage tester across switch terminals?


Hey Darsunt!

Did you understand any of this? wow! It's all good stuff but, IMHO it's not exacly laymans terms.

When the switch is ON, the electricity will only flow to the light 'cus it has less resistance than your little tester. When the switch is OFF the electricity will flow through your tester and the light but not enough to glow the lightbulb.


oh...when working with switches, put one lead of your tester to ground and use the other to check for voltage one one screw at a time.... if you do this you will see clearly how the switch works!

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Old 08-20-2009, 05:05 PM   #10
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Odd behavior of voltage tester across switch terminals?


Hey pete

he said the light is on when the switch is off; your explanation is backwards!
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Old 08-20-2009, 08:32 PM   #11
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Odd behavior of voltage tester across switch terminals?


I think it is percolating in. The testor measures the voltage DIFFERENCE, such as when you stick the testor leads into a socket. One side has 120V coming from the breaker box, while the other has 0 V going to the breaker box.

When the switch is on there is negligable voltage difference between the terminals.
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Old 08-20-2009, 09:28 PM   #12
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Odd behavior of voltage tester across switch terminals?


^^^exactly!!
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Old 08-20-2009, 09:28 PM   #13
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Odd behavior of voltage tester across switch terminals?


Your the man!

....you answered your own question in the best possible way!

_pete

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