other types of sources and look at what to expect. You
will never look at a power source the same again.
Not All Wall Warts are the Same
Almost every low cost consumer product today uses
an external wall wart as its primary power source. The
output is usually a DC voltage from 5V to 30V. If you get
rid of the original product, don’t get rid of the wall wart.
Re-use it for your next project.
I have a box of extra wall warts I’ve been collecting,
and use them over and over again. Figure 2 shows some
of that collection. Look on the outside of any wall wart
and you will see a rating for the DC output voltage and
the current rating. You can see some of these values in the
figure. Don’t take these numbers too seriously. They are
more of a guideline. Before you use the wall wart — or any
power source — you’ll want to characterize it using the
tests described here.
I grabbed one of these wall warts that had 12V
output/500 mA stamped on its side. We’ll also look at
what this really means and why this doesn’t tell the whole
To test the properties of a power source, we need to
be able to measure the voltage at its output not only
when it is unloaded, but also when a resistive load is
attached which would draw comparable current as with
the intended application.
What we want to look for is the DC voltage levels and
the AC noise — usually in the form of 60 Hz or 120 Hz
ripple. The best way of doing this is using two channels of
a scope. If you only have one channel available, use it to
measure the AC ripple and use a digital multimeter
(DMM) to measure the DC voltage.
The AC coupled mode of the input to a scope is the
perfect setting to use to look at 60 Hz or 120 Hz noise on
the power supply voltage.
My personal favorite scope for general lab
applications is the Digilent Analog Discovery scope. It has
two channels — each with 14-bit vertical resolution — 100
Msamples per second, and an excellent user interface. I
can add a math function which can calculate the average
voltage — even the peak-to-peak voltage. On top of that,
you can do an FFT (Fast Fourier Transform) of the
measured voltage signal to look at the spectrum of the
The system I used to perform the voltage
measurements in this article is shown in Figure 3. The
scope is set up to measure the unloaded voltage from a
wall wart, using a simple power plug to connect to the
wall wart. Channel 1 was set for DC, 1 megohm input,
and channel 2 was set for AC coupled input.
July 2015 53
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FIGURE 2. A selection of some of the wall warts I’ve collected
and reused for my projects.
FIGURE 3. Analog Discovery scope with two channels
connected to measure the voltage on the power rail.
Use a Light Bulb as a Load Resistor
For future reference, if you need a 13 ohm high power resistor, a
75 watt incandescent light bulb will do. A 100 watt incandescent light
bulb has a resistance of about 10 ohms. These are the resistances
when cold. They increase as they get warmer. Of course, due to the
internal electronics of a compact fluorescent light bulb (CFL), you can’t
use one as a resistive load.