output voltage. Due to their susceptibility to damage from voltage surges,
silicon diodes shall also be protected by selenium surge cells or varistors
and by current limiting fuses against over-current surges. A high speed
rectifier fuse should be installed in one leg of the ac secondary and one in
the dc negative output leg.
Meters. In order to conveniently measure the output current and
potential, the rectifier should be furnished with meters for reading these
values. The meter should not be continuously operating but should be switched
into the circuit as required. This not only protects the meter from
electrical damage from surges but, when the meter is read, it moves from zero
to the indicated reading. Frozen meter movements are easily detected in this
manner. Often, one meter and a two position switch are used to measure both
potential and current. Current is usually measured using an external current
shunt. Output voltage and current can also be conveniently measured by the
use of portable meters used across the rectifier output and the current shunt.
Standard Rectifier Types
Single-Phase Bridge. The circuit for this type of rectifier is
shown in Figure 32. This type of rectifier is the most commonly used type of
rectifier up to an output power of about 1,000 W. Above 1,000 W, the extra
cost of three-phase types is often justified by the increased electrical
efficiency of the three-phase units. The rectifying unit consists of four
elements. If any one of the rectifying elements fails or changes resistance,
the other elements usually also fail. Current passes through pairs of the
rectifying elements through the external load (structure and anode circuit).
The active pair of elements alternates as the polarity of the alternating
current reverses while the other pair blocks the flow of current. The result
is full-wave rectified current as shown in Figure 33.
Single-Phase Center Tap. The circuit of a single-phase center tap
rectifier is shown in Figure 34. This type of rectifier has only two
rectifying elements but produces full-wave rectified output. However, since
only one-half of the transformer output is applied to the load, the
transformer required is considerably heavier and more costly than in
single-phase bridge type units. This type of unit is also less sensitive to
adjustment than the single-phase bridge type; however, it is electrically more
Three-Phase Bridge. The circuit for a three-phase bridge rectifier
is shown in Figure 35. The circuit operates like three combined single-phase
bridge circuits that share a pair of diodes with one of the other three
bridges. There are three secondary windings in the transformer that produce
out-of-phase alternating current supplied to each pair of rectifying elements.
This out-of-phase relationship produces a direct current output with less
alternating current "ripple" than the single-phase type, only 4.5 percent.
Due to the reduction in alternating current ripple, three-phase bridge
rectifiers are more electrically efficient than the single-phase types, and
the extra initial cost of the unit is often justified by savings in supplied
power, particularly for units of over 1,000 W capacity.