Handbook for Sound Engineers

674 Chapter 19

(19-3)

where,
Irect is the current per rectifier,
0.5 is a constant from Table 19-1,
Idc is the rectified ac current, which is the dc current.

This is the current each rectifier must carry. Next, the
ac voltage required from the transformer is determined
by the equation

(19-4)

where,
Vac is the transformer voltage,
1.11 is a constant from Table 19-1.

This is the voltage as measured from each side of the
transformer center tap; the total voltage across the
secondary is 55.50 Vrms.
The peak inverse voltage is

(19-5)

where,
Vac is the secondary ac voltage per leg,
2.82 is found in Table 19-1.

If a rectifier with the required piv rating is not avail-
able, two or more may be connected in series to obtain
the desired piv rating. Unequal values of piv ratings may
be used, provided the lowest rating is greater than half
of the total piv rating needed.
Parallel operation of rectifiers can be used obtain
higher current ratings. However, because of a possible
imbalance between the units due to the forward voltage
drop and effective series resistance, one unit may carry
more current than the other and could conceivably fail.
To prevent this, small equal value resistors must be
connected in series with each individual rectifier to
balance the load currents, as shown in Fig. 19-4.

19.2.4 Three-Phase Power Supplies

Three-phase power supplies are common in the industry
but are seldom used to power audio circuits directly.

The are used as the input power to power an entire sys-

tem—for instance, a portable high-power outdoor rock

system. To see the characteristics of three-phase sup-

plies, see Table 19-1.

19.3 Filters

A power-supply filter is a series of resistors, capacitors,

and/or inductors connected either passively or actively

to reduce the ac or ripple component of the dc power

supply.

19.3.1 Capacitor Filters

A capacitor filter employs a capacitor at its input, as

shown in Fig. 19-5A. Power supplies with an input

capacitor filter have a higher output voltage than one

without a capacitor because the peak value of the recti-

fier output voltage appears across the input filter. As the

rectified ac pulses from the rectifier are applied across

capacitor C, the voltage across the capacitor rises nearly

as fast as the pulse. As the rectifier output drops, the

voltage across the capacitor does not fall to zero but

gradually diminishes until another pulse from the recti-

fier is applied to it. It again charges to the peak voltage.

The capacitor may be considered a storage tank, storing

up energy to the load between pulses. In a half-wave

circuit, this action occurs 60 times per second, and in a

full-wave circuit, it occurs 120 times per second.

For a single-phase circuit with a sine-wave input and

without a filter, the peak inverse voltage at the rectifier

is 1.414 times the rms value of the voltage applied to

the rectifier. With a capacitor input to the filter, the peak

inverse voltage may reach 2.8 times the rms value of the

applied voltage. This data may be obtained by referring

to Table 19-1.

Irect=0.5uIdc

=0.5u 1

=0.5A

Vac=1.11uVdc

=1.11u 25

=27.75Vrms

piv=2.82uVac

=2.82u27.75

=78.4Vrms

Figure 19-4. Small resistors connected in series with each

rectifier to balance the current through each unit of parallel-

connected rectifiers.

R 1

R 2

R 3

R 4

D 1

D 2

D 3

D 4

+

+

+

+

+

Load