Measuring Voltage with Step-Down Adapters: Questions & Answers

[whiterabbot1]

I am currently building an energy monitor according the the specifications of this site,http://openenergymonitor.org/emon/node/59. I understand how the current is measured but I am confused by the voltage measurement circuit.
I do not understand how the step-down adapter can correctly measure voltage usage from one outlet. I understand that the lights and outlets are in series but I was reading that typically dryers and air conditioners are typically on a separate circuit. So how can the voltage used from these appliances be measured from the one outlet?

Thanks.

 

[berkeman]

I am currently building an energy monitor according the the specifications of this site,http://openenergymonitor.org/emon/node/59. I understand how the current is measured but I am confused by the voltage measurement circuit.
I do not understand how the step-down adapter can correctly measure voltage usage from one outlet. I understand that the lights and outlets are in series but I was reading that typically dryers and air conditioners are typically on a separate circuit. So how can the voltage used from these appliances be measured from the one outlet?

Thanks.

The source imedance for your AC Mains power is very low, so unless there is a fault somewhere, you can measure you whole-house AC Mains voltage from any outlet in your house. This assumes that your two phase loads are reasonably well balanced (USA comment).

 

[whiterabbot1]

I know that you can but I do not understand why you can. If you could point me to some good resources that would be great.

Thanks.

 

[dlgoff]

When berkeman says,

The source impedance for your AC Mains power is very low...

(bold p added by me), implies that the transformer that is providing this power is rated so the voltage won't sag when you take power from it. Also, the power company is able to deliver such large amounts of power that flipping on a light switch has really no effect on lowering what can be supplied.

Here's a couple of links that you might find interesting.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld2.html#c1"
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html#c1"

Regards