Handbook for Sound Engineers

Power Supplies 669

19.1 Power-Supply Terminology

Power Supply. A device that supplies electrical power

to another unit. Power supplies obtain their prime power

from the ac power line or from special power systems

such as motor generators, inverters, and converters.

Rectifier. A device that passes current in only one direc-

tion. The rectifier consists of a positive anode and a neg-

ative cathode. When a positive voltage is applied to the

anode of the rectifier, that voltage minus the voltage

across the rectifier will appear on the cathode and cur-

rent will flow. When a negative voltage is applied to the

anode with respect to the cathode, the rectifier is turned

off and only the rectifier leakage current will flow.

Forward Resistance. The resistance of an individual

cell measured at a specified forward voltage drop or

current.

Forward Voltage Drop. The internal voltage drop of a
rectifier resulting from the current flow through the cell
in the forward direction. The forward voltage drop is
usually between 0.4 Vdc and 1.25 Vdc.

Reverse Resistance. The resistance of the rectifier
measured at a specified reverse voltage or current.
Reverse resistance is in megohms (M:.

Reverse Current. The current flow in the reverse direc-
tion, usually in microamperes (μA).

Maximum Peak Current. The highest instantaneous
anode current a rectifier can safely carry recurrently in
the direction of the normal current flow.
The value of the peak current is determined by the
constants of the filter sections. With a choke filter input,
the peak current is less than the load current. With a
large capacitor filter input, the peak current may be
many times the load current. The current is measured
with a peak-indicating meter or oscilloscope.

Maximum Peak Inverse Voltage. The maximum in-
stantaneous voltage that the rectifier can withstand in
the direction opposite to which it is designed to pass
current. Referring to Fig. 19-1, when anode A of a full-
wave rectifier is positive, current flows from A to C, but
not from B to C because B is negative. At the instant an-
ode A is positive, the cathodes C of A and B are posi-
tive with respect to anode B. The voltage between the
positive cathode and the negative anode B is inversely
related to the voltage causing the current flow. The peak
value of this voltage is limited by the resistance and na-
ture of the path between the anode B and the cathode C.

The maximum value of voltage between these points, at

which there is no danger of breakdown, is termed maxi-

mum peak inverse voltage.

The relationship between peak inverse voltage, rms

value of ac input voltage and dc output voltage depends

largely on the individual characteristics of the rectifier

circuit. Line surges, or any other transient or waveform

distortion, may raise the actual peak voltage to a value

higher than that calculated for a sine-wave voltage. The

actual inverse voltage (and not the calculated value)

should be such as not to exceed the rated maximum

peak inverse voltage for a given rectifier. A peak-

reading meter or oscilloscope is useful in determining

the actual peak inverse voltage.

The peak inverse voltage is approximately 1.4 times

the rms value of the anode voltage for single- phase,

full-wave circuits with a sine-wave input and no capaci-

tance at the input of the filter section. For a single half-

wave circuit, with a capacitor input to the filter section,

the peak inverse voltage may reach 2.8 times the rms

value of the anode voltage.

Ripple Voltage. The alternating component (ac) riding

on the dc output voltage of a rectifier-type power supply.

The frequency of the ripple voltage will depend on the

line frequency and the configuration of the rectifier. The

effectiveness of the filter system is a function of the load

current and the values of the filter components.

The ripple factor is the measure of quality of a power

supply. It is the ratio of the rms value of the ac compo-

nent of the output voltage to the dc component of the

output voltage or

(19-1)

where,

Vrms is the alternating current voltage at the output

terminals,

Vdc is the direct current output voltage at the output

terminals.

Figure 19-1. Peak inverse voltage analysis.

A

B

C

+

ripple factor

Vrms

Vdc

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