shown in Figure 29. Note: Refer to
"Breakdown
Voltage Measurement"
if the oscilloscope being used
has less than 1 Mn of horizontal input impedance.
TESTING ZENER DIODES
The procedure for testing Zener diodes is almost
the same as for testing signal and rectifier diodes.
In fact, the forward characteristics of the diodes are
essentially identical and the test procedures would
be the same, except that forward voltage measure
ments are seldom used for Zener diodes. Zener
diodes are designed to be used in the reverse volt
age breakdown mode. In this region, a large change
in reverse current occurs while the Zener voltage
remains nearly constant. Because of this character
istic, they are most often used as voltage regulators.
The zener voltage value (reverse voltage break
down value) may be measured with the curve tracer
and oscilloscope. This is the parameter that is most
often checked for a Zener diode. Set up the curve
tracer and oscilloscope as described for reverse
voltage measurement of signal and rectifier diodes.
r-T-···r·-·r·-..
r ......... ::1:· ......... 1 ......... , ... -..... ·-·-·····I······-··-:
0
'---·! .. --.. i.-.. ) .... ,l.. ... i
6
.3 VOLT .... j ........ ,_._)
2
I ! I I !
-
ZENER
I i
�1
-
'
1
:
filgl,
'
i
I
i
l
..
j..
!
i
I
I
I
H±
i
_.i�t=lJ __ tH·
10
a
6 VR 4
2
o
SHARP ZENER KNEE
To obtain the most accurate voltage reading pos
sible, calibrate the full scale oscilloscope horizontal
sensitivity to a convenient value slightly above the
Zener voltage. For example, for
a
6-volt diode, cali
brate full scale at 10 votls; for a IS-volt diode, cali
brate full scale at 20 volts; etc.
Be sure the POLARITY switch is set to display the
reverse voltage condition. Increase the SWEEP
VOLTAGE control setting to display the Zener re
gion as shown in Figure 30. No reverse current
should flow until the reverse breakdown voltage
value is reached. At that point, there should be a
very sharp "elbow" and a very vertical current
trace. If the "elbow" is rounded or the vertical cur
rent trace has a measurable voltage slope, the Zener
diode is probably defective. Read the Zener voltage
value from the display as accurately as possible.
There may be variation of Zener voltage from one
diode to another of the same type number. If the
Zener voltage value falls outside the limits that can
be used for your application, sele
c
t another one that
will meet the requirements.
10
8
6
4
VR(vol!s)
2
SLOPPY ZENER KNEE
0
Figure 30. Typical Zener Diode Curves
25