November 1958 Radio-Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published 1929 - 1948. All copyrights hereby acknowledged.
Not everyone who visits RF Cafe is a seasoned engineer or technician. Some are just getting into electronics as
part of a career path and/or hobby endeavor and appreciate the availability of entry-level information. As some
oft-quoted sage-type person famously said, "A journey of a thousand miles begins with the first step."
Accordingly, here is a short article explaining the basics of Zener diode operation. Thanks for your indulgence -
or you're welcome - depending on your experience level.
The Backward Diode
By Ed Bukstein
Zener diodes have applications in voltage regulation and wave clipping
Fig. 1 - Characteristic of a Zener diode when a reverse voltage is applied.
Fig. 2 - Voltage drop across Zener diode remains constant even though current may vary over wide limits.
Fig. 3 - Connected in opposite directions, Zener diodes clip extremes of input waveform.
Most technicians have resigned themselves to the fact that the arrow-like symbol used to represent non-thermionic
diodes does not point in the direction of electron flow. Now, to augment the confusion caused by this unfortunate
choice of symbol, comes a new application for the silicon diode. In this application, the diode is intentionally connected
with reverse polarity in a dc circuit.
It is well known that a silicon diode has a low forward resistance and a high back resistance. Not so well known
is that the back resistance suddenly decreases at a certain value of reverse voltage. The voltage required to produce
this breakdown of back resistance is known as the Zener voltage. As shown by the characteristic curve in Fig. 1, the
current is practically zero for all values of reverse voltage up to the Zener value. When the Zener breakdown occurs,
the reverse current suddenly increases. As the characteristic curve shows, in this region the voltage across the diode
is nearly independent of the current flow through it. This ability of the diode to maintain a constant voltage makes
it useful in voltage reference and regulating circuits.
Fig. 2 shows how the Zener diode is connected to maintain a constant voltage across a load. Although current flow
through the diode may vary, due to line voltage or load changes, the voltage across the diode remains constant. The
Zener diode offers many advantages over other voltage regulating elements such as gas-filled tubes. It is smaller,
lighter, mechanically rugged and has a long life. The Zener diode can be manufactured for any value of regulated voltage
from a few volts to several hundred and for operating currents from a few milliamperes to over an ampere.
In general, it is better to use several low-voltage diodes connected in series rather than a single diode of higher
Zener voltage. The advantage of the series arrangement is that the total power dissipation is divided among several
diodes. For this reason, temperature change in each diode is relatively small and stability is improved.
Fig. 3 illustrates the possibility of using two Zener diodes in a squaring or clipping circuit. The two diodes
are connected in opposing directions so that they break down on opposite alternations and prevent the output from
rising above the Zener value. The Zener diode is a relatively recent development and many other uses for it will be
Numerous Zener (voltage-reference) silicon diodes are made by National Semiconductor
Products, Raytheon, Texas Instruments and other manufacturers of semiconductor products and sold through
distributors and mail-order radio and electronic parts supply houses. Among the silicon diodes listed specifically
for voltage-regulator and reference applications are the Raytheon 1N437 and 1N438, and Texas Instruments 650C-653C
and 650C0-653C9. - Editor
Posted August 7, 2014