Handbook for Sound Engineers

Tubes, Discrete Solid State Devices, and Integrated Circuits 321

In a series arrangement, the most important consideration
is that the applied voltage be equally distributed between
the several units. The voltage drops across each indi-
vidual unit must be very nearly identical. If the instanta-
neous voltage is not equally divided, one of the units may
be subjected to a voltage exceeding its rated value,
causing it to fail. This causes the other rectifiers to absorb
the piv, often creating destruction of all the rectifiers.
Uniform voltage distribution can be obtained by the
connection of capacitors or resistors in parallel with the
individual rectifier unit, Fig. 12-12. Shunt resistors are
used for steady-state applications, and shunt capacitors
are used in applications where transient voltages are
expected. If the circuit is exposed to both dc and ac,
both shunt capacitors and resistors should be employed.

When the maximum current of a single diode is
exceeded, two or more units may be connected in
parallel. To avoid differences in voltage drop across the
individual units, a resistor or small inductor is
connected in series with each diode, Fig. 12-13. Of the
two methods, the inductance is favored because of the
lower voltage drop and consumption of power.

Zener and Avalanche Diodes. When the reverse
voltage is increased beyond the breakdown knee of the
diode characteristics as shown in Fig. 12-11, the diode
impedance suddenly drops sharply to a very low value.
If the current is limited by an external circuit resistance,
operating in the “zener region” is normal for certain
diodes specifically designed for the purpose. In zener
diodes, sometimes simply called zeners, the breakdown
characteristic is deliberately made as vertical as possible
in the zener region so that the voltage across the diode is
essentially constant over a wide reverse-current range,

acting as a voltage regulator. Since its zener region

voltage can be made highly repeatable and very stable

with respect to time and temperature, the zener diode

can also function as a voltage reference. Zener diodes

come in a wide variety of voltages, currents, and

powers, ranging from 3.2 V to hundreds of volts, from a

few milliamperes to 10 A or more, and from about

250 mW to over 50 W.

Avalanche diodes are diodes in which the shape of

the breakdown knee has been controlled, and the

leakage current before breakdown has been reduced so

that the diode is especially well suited to two applica-

tions: high-voltage stacking and clamping. In other

words, they prevent a circuit from exceeding a certain

value of voltage by causing breakdown of the diode at

or just below that voltage.

Small-Signal Diodes. Small-signal diodes or general-

purpose diodes are low-level devices with the same

general characteristics as power diodes. They are

smaller, dissipate much less power, and are not designed

for high-voltage, high-power operation. Typical rating

ranges are:

Switching Diodes. Switching diodes are small-signal

diodes used primarily in digital-logic and control appli-

cations in which the voltages may change very rapidly

so that speed, particularly reverse-recovery time, is of

Figure 12-12. Rectifiers connected in series.

D 1

D 2

D 3

D 4

D 1

D 2

D 3

D 4

R 1

R 2

R 3

R 4

C 1

C 2

C 3

C 4

V+ V+

V– V–

Figure 12-13. Rectifiers connected in parallel.

IF (forward current) 1–500 mA

VF (forward voltage drop at IF) 0.2–1.1 V

piv or prv 6–1000 V

IR (leakage current at 80% prv) 0.1–1.0 μA

D 1 D 2 D 3 D 4

R 1 R 2 R 3 R 4

V+

V–

V–

D 1 D 2 D 3 D 4

V+

L 1 L 2 L 3 L 4