inputs every 10 msec. For about 50 measured points
during the ramp-up, this is an on-time of about 0.5 sec. As
long as the current is on for just one second every so
often, I am not worried about the power consumption in
For an on-time of 0.5 sec, I needed an RC time
constant of about 0.1 seconds. With a 1K ohm series
resistor, this means an integrating capacitor of about 0.1
sec/1000 ohms = 100 µF.
The voltage across the capacitor drives the base
voltage, which then drives the current through the
transistor. The voltage drop across the 10 ohm load
resistor is a direct measure of the current through it.
Figure 10 shows an example of the input voltage from
digital pin 13 and the resulting voltage across the 10 ohm
This final system implemented with an Arduino
Redboard from SparkFun ($19.95) and a small protoboard
is shown in Figure 11.
Analyzing the I-V Curve in Excel
I wrote a simple sketch for the Arduino which sent a
0.5 sec digital pulse on pin 13 to drive current through the
transistor from the power supply. I used two analog input
pins to measure the voltage of the power supply and the
current through it. Chan1 was a direct measure of the
current from the supply, while chan2 was a scaled value of
the supply output voltage.
Since many power sources are 15V or more, I used a
voltage divider circuit to offer voltage drops from 1x, 0.5x,
and 0.25x. This scaled the supply voltage to a range the
58 July 2015
FIGURE 12. Measured I-V curve for the wall wart,
how the output voltage of any power supply decreases
with added current draw. The internal resistance is a
good metric of how much voltage drop to expect.
FIGURE 11. The final automated I-V system based on an
Arduino with an integrating circuit and current load to a power
source. The wires coming off from the bottom connect to the
power supply to characterize.
Choosing the Right Input Mode
to a Scope
Most scopes have three input modes for a channel: 50
ohms, 1 megohm input, and AC coupled. The 50 ohm input is
most useful when measuring signals in the 100 MHz and
above bandwidth. It will terminate the coax cable connecting
the scope to the device-under-test and prevent the artifact of
reflections in the cable.
The 1 megohm input is the general-purpose setting to
look at signals from DC to about 100 MHz, depending on the
nature of the probe. The AC coupled input puts a 100 nF
capacitor in series with the 1 megohm. This means DC
signals are blocked. The time constant is about 0.1 sec (100
nF x 1 megohm), so signals above about 5 Hz are passed
through to the receiver pretty much unchanged.
FIGURE 10. Voltage on pin 13 of the Arduino and the
voltage across the 10 ohm load resistor — a measure of the
current from the power source. The square signal is the
voltage on pin 13 and the ramping signal is the voltage
across the 10 ohm resistor.