The Zener Diode
June 1961 Popular Electronics

June 1961 Popular Electronics Table of Contents Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

According to online sources, the Zener diode was named after American physicist Clarence Zener, who first described the Zener effect in 1934 in his primarily theoretical studies of the breakdown of electrical insulator properties. His work led to the Bell Labs implementation of the effect in the form of the now-familiar electronic device. Commercial availability of the Zener was fairly recent in 1961 when this article appeared in Popular Electronics magazine. It was just beginning to replace vacuum tube regulators like the OA2 and OB2. If fact, it seems strange to see both Zener diodes and vacuum tubes in the same schematic. Before small, inexpensive integrated circuit voltage regulators hit the scene, Zeners were the go-to quick and dirty voltage references. They only have two leads to deal with, and are good enough for non-critical applications, so you will still find them widely used.

The Zener Diode

By R.J. Shaughnessy

Now widely used as regulating elements in control and similar circuitry, zener diodes have yet to become part of the average hobbyist's or experimenter's stock in trade. However, by keeping a few simple facts in mind and becoming familiar with some of the zener's applications, you'll be designing your own special circuits in no time. In fact, you'll find that zener diodes can actually improve the performance of your older designs.

What Are Zener Diodes?

Zener diodes, or silicon regulators-as they are also called, are simply semiconductor versions of the familiar vacuum-tube voltage regulators - the OA2, OB2, etc. However, while vacuum-tube regulators can provide regulation only at certain specific voltages and over limited current ranges, silicon regulators can be made to work at almost any desired voltage and over a wide current range.

From the outside, these devices look exactly like ordinary semiconductor diodes. Inside, too, their construction is much the same. In fact, you can even use them as rectifiers, provided the a.c. peak value does not exceed the characteristically low peak inverse voltage of the zener. The curve of Fig. 1 shows a typical 6-volt zener diode characteristic and will help make this point clear.

The forward side of the curve (anode positive) looks just like that of an ordinary diode; useful current flows as soon as the forward voltage reaches about 0.7 volt. In the reverse direction (anode negative), you can see that the zener would never make the grade as a really practical rectifier because of its low breakdown voltage. In this sense, it behaves as though it were a normal rectifier that had developed high leakage at this low voltage.

If this really were the case, however, the unit would very likely destroy itself through excessive current flow in a very short time. Not so the zener diode. The ability of this device to withstand high reverse currents and go on working while in a saturated state is actually its most important characteristic and makes it very useful in voltage-regulating applications, as we'll see shortly.

The curve also shows that breakdown occurs at about 6 volts, and that this voltage remains pretty constant even though the current varies from a few ma. to more than 100 ma.

Breakdown Voltages

Ever wonder how the word "zener" came to be associated with these devices ? The answer is simple: in the early days of semiconductor research, much of the information on breakdown mechanisms was derived from still earlier research on dielectric materials by Dr. Carl Zener. Later, when semiconductor researchers began to utilize breakdown effects in certain silicon devices, they called these devices "zener diodes" in recognition of Dr. Zener's pioneering efforts.

The point at which breakdown occurs can't be predicted exactly, although it can be controlled within certain limits. It is largely determined by the type of silicon used in the manufacturing process.

Breakdown of the crystal junction occurs in a natural distribution about the desired value. For example, a factory run of 6-volt units might have breakdowns in the range of 5.6 to 6.5 volts - a distribution span of about 15%. If closer tolerances are needed. the manufacturer must select special units from within this distribution. Naturally, because of the additional labor involved, these cost much more than regular units - especially if the desired tolerance is less than about 5%.

In practice zener diodes are readily available with voltage accuracies of 20%, 10%, and 5%. Very close tolerances can be obtained by connecting lower voltage units in series in special packaged assemblies called reference elements.

Temperature Effects

As with all semiconductor devices, temperature plays an important role in the operation of zener diodes. A temperature coefficient is directly related to the saturation voltage, as shown in the curve of Fig. 2. The coefficient approaches 0.1% per degree centigrade at higher voltages, passes through zero in the region around 5 volts, and becomes negative at lower voltages. The importance of this effect can be appreciated when we consider that a 10-volt unit at normal room temperatures (20°C) will measure 10.5 volts in an equipment cabinet where the temperature is 75°C. his voltage change could be disastrous in certain applications. However, the danger can be avoided by selecting two lower voltage units with smaller coefficients. For example, if we had used two 5-volt units in series - each with a temperature coefficient of 0.002% - we would still have the desired 10 volts. But the change with temperature would be only a small fraction of a millivolt within the same range.

Dynamic and Static Resistance

The basic factor in the regulating ability of a zener diode is its dynamic resistance. This is an expression for the change in saturation voltage with a small change in current. It can be measured by observing the a.c. voltage developed across the diode when a small a.c. current is superimposed on the operating d.c. current. Because a.c. is used in these measurements, the term "a.c. resistance" might have more meaning than dynamic resistance, although both terms mean the same thing in this connection.

Values of dynamic or a.c. resistance vary from less than one ohm in high-current, low-voltage units to several hundred ohms in low-current, high-voltage units.

The d.c. or static resistance of a zener diode depends on its operating point and is simply the operating voltage divided by the operating current. In Fig. 1, this point has been set arbitrarily at 6 volts and 20 ma.; the static resistance is therefore 6/.02 or 300 ohms.

We can use the same curve again to obtain an approximate estimate of dynamic resistance. Select two values of saturation current that are spaced equal distances above and below the d.c. operating point; then draw vertical lines upwards from these points until the voltage axis is intersected. The dynamic resistance can now be found by dividing the peak-to-peak voltage by the peak-to-peak current. For the particular diode shown, this figures out to be about 10 ohms. In practical measuring equipment, the peak-to-peak current change is restricted to about 10% of the d.c. operating current.

Zener Diodes As Circuit Elements

In essence, a zener diode is a device which, when saturated in the reverse direction, will maintain an almost constant voltage across its terminals. The most popular application for a device of this type is, of course, voltage regulation. Figure 3 shows a zener diode connected as a shunt-regulating element across a load represented by resistor To analyze the circuit operation, let's assume that the input voltage, E_IN increases and see how this affects the output voltage. Note that the positive terminal of the input supply is connected to the cathode of the zener, and that the current flowing in the series resistor R_S is the sum of the diode and load currents. Let us assume also that the diode's characteristic is that shown in Fig. 1.

You'll remember we said that dynamic resistance was associated with alternating or changing currents and voltages, and that it was the basic factor in the regulating ability of a zener diode. Now let's put these facts to work for us.

We have assumed that the diode in the circuit is the unit whose characteristic is shown in Fig. 1; its dynamic resistance therefore is about 10 ohms. Since we've set the operating point arbitrarily at 6 volts and 20 ma., the static resistance is 300 ohms.

Initially, conditions around the circuit are not changing and can be represented as shown in Fig. 4( A) . We see the input voltage of 10 volts across two resistors - 4 volts across R_S and 6 volts across R_D, which represents the parallel combination of the zener and the load resistor R_L. Now let's increase E_IN by 2 volts. Since the increase is a changing quantity and shifts the diode's operating point, we must analyze its effect on the circuit by means of the dynamic rather than the static resistance. Figure 4(B) shows how the 2-volt change will distribute itself around the circuit.

As soon as conditions settle down and become static again, the operating point will have shifted to a new position and the circuit voltages will now be the sum of the original voltages and those resulting from the 2-volt change. The new distribution will therefore be as shown in Fig. 4(C). Referring back to Fig. 4(B), it is obvious that lowering the dynamic resistance of the zener or increasing the value of R_S will improve the regulating action of the circuit. Increasing R_S, however, will mean a corresponding increase in input voltage to maintain the proper current levels.

Using Zener Diodes

The fact that a zener diode will maintain an essentially constant voltage drop independent of current suggests its use as a source of grid-bias voltage. The circuit conditions of Fig. 5 are such that the tube requires a bias of 6 volts at a cathode current of 20 ma. Figure 5(A) shows how this is obtained by conventional means with a cathode bias resistor of 300 ohms and a bypass capacitor of 150 μf. Figure 5(B) shows how a zener diode can be substituted for both of these components.

The bias voltage is the zener voltage; because of its low dynamic resistance, the diode will hold the voltage constant even during wide swings of plate current. The bypass capacitor is not needed either, because the dynamic resistance is roughly equal to the capacitor's impedance even at low signal frequencies.

Should you have need of a number of different voltages, you can build the voltage divider shown in Fig. 6 by connecting several zener diodes in series across a suitable d.c. supply. This divider would be useful around the workbench as an accurate voltage source for calibrating meters or as a stable d.c. bridge supply. In fact, it's applicable whenever a dependable, long-lived voltage source is required.

In certain critical, vacuum-tube circuits, every possible precaution must be taken to stabilize key voltage points against drift. The effects of plate voltage drift are particularly troublesome in high-gain d.c. amplifiers, and call for regulation not only of the B+ supply but of the tube's a.c. heater supply also.

Figure 7 shows how zener diodes can be used as effective a.c. voltage regulators. Two diodes must be connected back-to-back in this application to prevent forward conduction in both at the same time as the a.c. waveform switches back and forth from plus to minus. With this arrangement, one of the diodes will always be biased in the reverse direction for either polarity of a.c. The diodes effectively clip the peaks of the waveform; as a result, the voltage at the heater is a flat-topped wave independent of line voltage changes.

As you might guess, special, double-anode units have been developed for this type of application. These units contain two zener diodes connected cathode-to-cathode inside a single package. This design simplifies mounting and can save considerable space in crowded chassis areas.

The application possibilities of zener's are far too numerous to be described individually here. But whatever the project being worked on, chances are that one or two of these devices, properly placed in the circuit, will improve its performance. Once you have used zener diodes and come to depend on their excellent performance and reliability, you'll wonder how you ever got along without them in the past.