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Old 03-10-2011, 11:40 AM   #1
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Voltage measurement not as expected on 3-way


Newbie learning about electricity.
I got a digital multimeter, and have been measuring various devices.

I have a pair of 3-way switches controlling the basement lights.
I measured voltages on one of the 3-way switches.

When the lights are off, I measure 120 volts between the 2 traveller terminals, which is what I expect to see.

With the lights on, I measure 80 volts between the travellers. I get 120 volts between the hot traveller and ground, and I get 13 volts between the other traveller and ground. I get 120 between the common and ground, and 120 between the common and non-hot traveller.

Why do I see only 80 between the travellers when the circuit is on?

Thanks.

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Old 03-10-2011, 01:03 PM   #2
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Voltage measurement not as expected on 3-way


Quote:
Originally Posted by bw777 View Post
Newbie learning about electricity.
I got a digital multimeter, and have been measuring various devices.

I have a pair of 3-way switches controlling the basement lights.
I measured voltages on one of the 3-way switches.

When the lights are off, I measure 120 volts between the 2 traveller terminals, which is what I expect to see.
Why would you expect this? In this case the meter is in series with the lamp filament and the to leads - hot and neutral - but since the meter has a 10 meg ohm input impedance there is no (read very little) current flow.
Quote:
With the lights on, I measure 80 volts between the travellers. I get 120 volts between the hot traveller and ground, and I get 13 volts between the other traveller and ground. I get 120 between the common and ground, and 120 between the common and non-hot traveller.

Why do I see only 80 between the travellers when the circuit is on?

Thanks.
With the lights on, one traveler is floating so you will have false readings.


Last edited by a7ecorsair; 03-10-2011 at 01:04 PM. Reason: phrasing
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