July
1998, Issue 96
A
PIC-Based AC Power Meter
by
Rick May
Questioning
your power bill? Rick shows you how to build a tool
to make sure your power bill stays right on target—a
portable AC power meter that displays the power delivered
to and consumed by your house.
When
I watch the old mechanical wattmeter on the house spin,
I marvel at just how low tech this device is. No fancy
displays, no RF transponders sending telemetry to roving
meter-reader trucks (at least not in my neighborhood).
I
always wondered why I couldn’t go out and buy a
small hand-held instrument that I could use to figure
out just what was causing that old-fashioned meter outside
to spin so fast.
A
few years ago, I came across a novel circuit design
by Stephen Woodward [1]. He used a quad optoisolator
(conventionally a nonlinear device) to generate an analog
voltage proportional to the power consumed by a load.
It used optos for safety, and, well, it was just neat.
So,
off I set to marry some type of micro to that analog
front end. I wanted to build a hand-held, portable AC
power meter that could display the power delivered to
a load. The result: see Photo 1.
Photo 1—Here’s my dream come true—a
hand-held portable AC power meter.
|
By
tossing in a little math and some numerical integration,
I could also display the energy consumed by a load.
For a little added challenge, I used a PIC microcontroller
that has no multiply or divide instructions.
