ECN Forum Forums Technical Discussion Electrical Theory and Applications Light Switches and Meter Readings

If a meter is placed in line with the two wires that make up the switched part of a lighting circuit, without the switch, should there be a reading of 120VAC?

To clarify, the "hot" is what would be switched (hopefully...) There is the conductor going to the top screw of the switch, and a conductor going to the bottom screw. Touch the two conductors together--by flipping the switch so to say--and the light goes on. What if a meter is placed between the two conductors, rather than a switch? Should there be any reading on the meter? Should the light go on?

Thinkgood;

If you place the leads of a Volt Meter across the Terminals of a 1 Pole 1 Throw switch, which has either a Ballast or Incandescent Lamp on the Load side (which is not Barbequed / Failed Open - actually works...), you will find:
<OL TYPE=A>

[*] With Switch Closed - Zero Volts,

[*] With Switch Open - 120 Volts.
</OL>
FYI: above examples assume the system is 120 VAC, and the Switch Contacts are not pitted to death (make very good contact with less than 1 volt loss across the Switch).

So to answer your Q in full, if the Switch is removed and the Test Leads of a Volt Meter applied in its place, there will be a Voltage reading.
The Lamp will not light up, due to the Volt Meter's Impedance limiting the Current in this Series Circuit.
FYI: The Lamp and Meter will be in series, so it is now a series circuit. Highest Voltage will be measured across the highest Resistance / Impedance.

Hope this is what you are looking for!

Scott35

TG, that is a fundamental but important question. With the ususal high-impedance digital multimeter set to and jacked into a 200V-up AC-volt range, expect a reading of 120V with NO lighted lamp. But, if you set to and jack into an 2-amp & up AC-current scale, the meter should read the lamp load current {100 watts = ~0.8 amps} with a bright lamp.

The wiring position on plain-vanilla toggle-switch terminals usually does not matter.




[This message has been edited by Bjarney (edited 03-08-2004).]

That's what I was looking for. Thank you, gentlemen.