analog input pin could handle. I use
enough voltage drop to keep the
maximum voltage on chan2 less than
the 3.3V reference.
The sketch turns on pin 13,
ramping the current from the power
supply, and measures the supply
current and the loaded voltage. These
values are printed to the serial
I used PLX-DAQ to read the serial
port and bring the data into a
spreadsheet. The I-V data is plotted
automatically as it comes in. I take the
first five values to fit a linear trend
line, and display the DC voltage and
the slope which is the internal
resistance. A steeper slope means a
higher internal resistance.
When I click on the connect
button in the PLX-DAQ control box,
the Arduino is triggered to start the
loop, the data is taken, printed to the
serial port, read into Excel, and plotted
— all automatically. Figure 12 is an
example of the measured I-V curve for
this wall wart.
We can see that the model for a
constant internal resistance is a pretty
good one. The 7. 22 ohm internal
resistance measured with this
automated system is very close to the
7.1 ohm resistance I measured
With this automated system, we
can now explore the features of
different power sources.
A Survey of
Not all wall warts are created
equal. I routinely use a 9V wall wart
from SparkFun all the time. It is rated
for 9V at 650 mA. Figure 13 shows
the I-V curve and the AC ripple noise
when loaded by 17 ohms, or drawing
530 mA. The peak-to-peak noise is
remarkably low. The peak-to-peak
voltage is 4. 5 mV out of 9V, or less
than 0.05% even loaded down. Its output impedance is
The USB port is often used to supply current. The
measured I-V curve and AC ripple with a 42 ohm load
drawing 120 mA is shown in Figure 14. It is a remarkably
accurate 5V regulated supply with an output impedance
of only 0.4 ohms. The AC ripple of the USB source I used
was 4 mV peak-to-peak, with no discernable peak at 60
Hz or 120 Hz.
Sometimes one of the digital output pins of the
Arduino is used to power a component. Figure 15 shows
its I-V curve and AC ripple noise when set to HIGH. The
July 2015 59
FIGURE 13. Characteristics of a 9V wall wart from SparkFun.
FIGURE 14. USB power performance.
FIGURE 15. Arduino digital output pin as a power supply.