BUILD A WALL WART POWER MONITOR By David Goodsell
As an experimenter, I use wall warts all the time to power circuit boards,
microcontroller boards, and even finished projects. However, during the
checkout phase of a new circuit, wall warts present a problem. How do you
measure their output when they’re plugged into a board or project box?
Figure 1 illustrates the problem and Figure 2 presents the solution.
The Need
Why is there a need to measure the output of a
wall wart, you might ask? There are two reasons:
current and voltage. For example, is your circuit
drawing the predicted amount of current? Is the current
more than the wall wart is designed to handle? Is the
voltage sufficient to drive any downstream regulators
under full load conditions? Or, is the voltage too high,
putting a large heat burden on the regulators?
That’s where the unit described in this article
comes in. I call it the Wall Wart Power Monitor. It’s a
self-contained, in-line, battery powered unit that
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February 2011
measures both DC current and voltage. It accepts four
of the most common types of wall wart plugs and, in
turn, has four identical output plugs, ready to hook to
your project.
Requirements
Coming from a background in the aerospace
industry, when I start a new project I immediately think
of generating a 200 page System Requirements
document. Since I’m retired now, I have to keep myself
in check. A list of basic requirements is still useful, so,
Requirements:
Other Desirable Features:
1. Uses power from the wall wart to drive the
circuitry.
2. Has an LED to indicate when unit is on.
■ FIGURE 1. The old way to measure voltage and current
from a wall wart.
