# measuring ac voltage across a load

Discussion in 'General Electronics Chat' started by the brain, Jul 15, 2009.

1. ### the brain Thread Starter New Member

Jun 16, 2009
3
0
I know when you measure voltage across a switch, in an ac circuit, if the voltmeter reads 0 volts the switch is closed, if the meter reads 120 volts the switch is open. If you measure voltage across a load such as a fan or resistor does the same rule apply. Or because of the load resistance will the meter read different. Let's say the fan is working and you measure voltage across the fan from line to neutral should the meter read 120 volts or 0 volts. And vice a versa if the fan is not working should you read 120 volts if there is line voltage across the fan. Maybe the fan has an open winding what should you read on the meter.

2. ### sissow2 Member

Jul 14, 2009
16
0
Its going to measure the $V_{\tiny{rms}}$ drop if you measure it across a resistor (or anything for that matter). If its the only resistor in the circuit, then the drop is going to be 120. If you measure one of two equally valued resistors in series, the drop will be 60.

Across a fan things get a little funky because the fan has an inductive property to it, so the voltage drop you measure will depend on the frequency of the supply.

Last edited: Jul 15, 2009
3. ### DC_Kid Distinguished Member

Feb 25, 2008
756
16
you'll measure line voltage
you'll measure line voltage

4. ### russ_hensel Distinguished Member

Jan 11, 2009
823
50
The answer to this depends on the source impedance of the supply. A power line is usually treated as 0, so open circuit and closed circuit voltage are the same. Of course it is not exactly 0 so.....

5. ### mik3 Senior Member

Feb 4, 2008
4,846
67
To read 0V across a load it means that it is shorted or that the power supply has failed.

To read a non zero value it means that the load is good or open circuit.