FIGURE 1. Small Signal Diodes. The horizontal scale is simply the setting of Channel A: 200 mV/div.
The vertical scale is the setting of Channel B divided by the series resistance, 300Ω, that is,
5V/300Ω = 16. 6 mA/div; (a) silicon diode 1N4148; (b) point contact diode.
and plugging the transformer directly
into the AC line. An adjustable power
resistor could then control the peak
current in the DUT. However, this
arrangement applies full voltage to the
device under test and there might be
fireworks if the test voltage exceeds
the device rating. The autotransformer
allows the voltage and current to be
increased gradually while watching for
FIGURE 2. Diode Curve Tracer.
signs of stress in the device, so it’s a
For a lower-cost version, one
could use a transformer with multiple
secondary taps to replace both the
autotransformer and the step-down
transformer, and use a switch to select
the desired voltage. This is the
approach used in the unit designed by
Craig Taylor (see Resources).
Small Signal Diodes
Figure 1 shows screenshots of the
Syscomp DSO-101 PC oscilloscope
functioning as the display in Figure 2,
to compare two small signal diodes.
Figure 1a is a 1N4148 silicon diode
with a threshold voltage of approxi-
FIGURE 3. Junction Transistor Measurement.