Home project using current transformers [duplicate]

Question

Possible Duplicate:
How to measure power consumed by mains device

I have the following question:

In my home I have certain devices such as my fridge etc. of which I would like to measure their powerusage. For practical reasons I would like to use current convertors (CT's) to do the measuring for me. At the moment I have some current convertors with a rated secondary output of 1A. Yet I do not seem to find a device that allows me to accomodate this output and convert it to kwh. As a plus I would like that the measuring devices has a pulse output for usage in an other project I am working on.

Can anyone give me advices on this matter?

Sincerely

Answer 1

Measuring current isn't enough to find effective power, even if you know the voltage.

\$ P = V \cdot I \cdot cos(\phi) \$

where \$\phi\$ is the phase difference between current and voltage. Only for purely resistive loads voltage and current are in phase, and then

\$ P = V \cdot I \$

So you need to measure both current and voltage, not only to find the phase difference, but also because you need to know the voltage at each moment, because that will fluctuate as well.

There are special data acquisition ICs for this, which have two ADCs (Analog-to-Digital Converters) which measure current and voltage simultaneously and multiply that to get instantaneous power. Integration over time gives you energy (in kWh).

You'll find ICs with a serial interface for a microcontroller which allows you to get real-time data, or others which give you a pulse for an energy unit, for instance 10 pulses per kWh.

Further reading
Energy measurement products at Analog Devices