output impedance of an Arduino pin is about 26 ohms.
With a 40 ohm external resistor — or a total of 66 ohms —
the current draw was 5V/66 ohms = 72 mA current. The
ripple noise was about 2 mV peak-to-peak, but only a very
small peak at 60 Hz. The voltage amplitude of the 60 Hz
sine wave component is 10^(-89.3/20 )= 34 µV out of 5V.
Many portable applications use batteries as the power
sources. Their properties vary a lot depending on the age
of the battery and the type of battery. The AC ripple noise
is typically at the noise floor of what the scope can
measure. The amount of AC ripple noise from a battery is
not really about the battery, but about how well it is
shielded from external noise sources, and what it is
connected to in its application.
I surveyed a number of 9V batteries and found the
typical output impedance to be about 2. 4 ohms. An
example is shown in Figure 16. However, one battery —
Two AAA batteries in series had an output resistance
of 1.24 ohms. This implies the output impedance of one
AAA battery would be about 0.62 ohms. Its IV curve is
shown in Figure 17.
Before you plug in that power source, try to answer
four important questions:
1. What is the current requirement for your
2. Given the voltage you are going to use, what is the
power dissipation you expect, and can you handle
3. What is the unloaded output voltage and source
resistance of the power source you plan to use, and
what will be the loaded output voltage? Is this
4. How much ripple noise is there on your power
supply and is this okay in your applications?
With the simple measurement and analysis techniques
offered here, you will be able to answer these questions
and eliminate one common source of problems in your
next project. NV
60 July 2015
Eric Bogatin is a physicist on the faculty at CU, Boulder and Front
Range Community College. He is also the Dean of the Signal
Integrity Academy. He leads Arduino workshops at Tinkermill, the
Longmont Hackerspace. His current interests are in simple,
robust, and low cost electronics for high precision physics
experiments. He can be reached at eric@EricBogatin.com.
FIGURE 17. Measured output impedance of two AAA
batteries in series.
FIGURE 16. Measured output impedance of a 9V battery.