There’s no need to
select a current-limiting
resistor. Just connect a
voltmeter across the
diode and measure its
breakdown voltage at a
consistent and safe
power level. The circuit
will also safely test LEDs
— including white LEDs
that multimeters can’t
test and other low
voltage diodes in their
forward direction, as
well as MOVs (
metal-oxide varistors) and
other protective devices
with higher voltage
external power supply
to the tester allows devices that break down well above
50V to be tested easily, also. With all my projects and the
numerous uncharacterized devices lying around my shop,
this zener tester has proven to be the most regularly used
equipment on my bench!
If you’re not familiar with zener diodes, then an
introduction is appropriate. Zeners are so useful they
should be found in every experimenter’s component
collection. The zener diode is a specialized silicon diode
that looks like a normal silicon diode in the forward
direction. However, in the reverse voltage direction, it
exhibits a low leakage current like a normal diode until a
voltage — the so-called “zener voltage” — is reached. At
that point, it sharply exhibits a controlled constant
breakdown voltage despite increasing current.
All diodes start to heavily conduct at some voltage in
the reverse direction, but are operated well below this
breakdown voltage to avoid damage. Zeners, on the other
hand, are designed to be operated in their breakdown
region, and that breakdown is carefully engineered for
specific voltages. For example, the 1N4728 to 1N4764
family of zener diodes includes breakdown voltages from
3. 3 volts to 100 volts in 37 steps — a much broader range
than common fixed-voltage three-terminal IC regulators.
These diodes can also be put in series to achieve almost
any voltage desired, and different diode families come in
various wattage ratings from 200 m W to over 10W.
Zeners are commonly used as voltage regulators,
reference-voltage standards for op-amp designs, and as
protection devices to safeguard components from overvoltage conditions. Applications like relay or solenoid
drivers and switching power supplies — like the one
described in this article — commonly use zeners for
protection (like D2 in Figure 1).
They also find use (instead of a simple series resistor)
in dropping one voltage down to a lower voltage, and in
clipping AC voltage waveforms. They are even used as
noise generators in RF impedance bridges. Best of all —
particularly for experimenters on a budget — typical low
power zener diodes cost just pennies each.
The Zener Diode Tester
The diode to be tested is connected between the red
and yellow terminals, and the voltage is read across these
terminals when the pushbutton is depressed to test.
Connecting a current meter between the yellow and black
terminals allows current measurement while testing if
desired, or replacing the current meter with an external
power supply allows extending the tester’s range to well
above 52 volts.
A flashing red LED indicates when voltage is present
across the test terminals. A yellow LED indicates that the
output is at the limit of 55V. When no diode is present or
the zener diode has a breakdown voltage above 55V, this
LED is illuminated. The LED also indicates correct
functioning of the internal circuit for initial testing and
serves as a safety reminder that a potentially dangerous
voltage is present across the output terminals. When the
test switch is released, the output is quickly discharged to
zero volts for safety.
The internal circuitry of the tester is easily modified
and simulated to change the voltage range or for use in
other applications. These modifications and simulation
■ FIGURE 1.
November 2014 25