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November 1958 Radio-Electronics
 [Table
of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
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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 an 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 basic physics and application of the of
backward diode, which
is akin to a Zener diode
and tunnel diode in
that it is meant to operate in the reverse bias region. National
Semiconductor, Texas Instruments (TI), and
Raytheon were the manufacturers
in 1958 when this article appeared in Radio-Electronics magazine.
National Semiconductor was swallowed up by Texas instruments in 2011. 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 forthcoming.
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 July 16, 2020
(updated from original post on 8/7/2014)
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