3-2
THE BASIC POWER SUPPLY
Figure 3-1 shows the block diagram of the basic power supply. Most power supplies are made up of
four basic sections: a TRANSFORMER, a RECTIFIER, a FILTER, and a REGULATOR.
Figure 3-1.—Block diagram of a basic power supply.
As you can see, the first section is the TRANSFORMER. The transformer serves two primary
purposes: (1) to step up or step down the input line voltage to the desired level and (2) to couple this
voltage to the rectifier section. The RECTIFIER section converts the ac signal to a pulsating dc voltage.
However, you will see later in this chapter that the pulsating dc voltage is not desirable. For this reason, a
FILTER section is used to convert the pulsating dc voltage to filtered dc voltage. The final section, the
REGULATOR, does just what the name implies. It maintains the output of the power supply at a
constant level in spite of large changes in load current or in input line voltage. Depending upon the design
of the equipment, the output of the regulator will maintain a constant dc voltage within certain limits.
Now that you know what each section does, let's trace a signal through the power supply and see
what changes are made to the input signal. In figure 3-2, the input signal of 120 volts ac is applied to the
primary of the transformer, which has a turns ratio of 1:3. We can calculate the output by multiplying the
input voltage by the ratio of turns in the secondary winding to turns in the primary winding. Therefore,
the output voltage of our example is: 120 volts ac × 3, or 360 volts ac. Depending on the type of rectifier
used (full-wave or half-wave), the output from the rectifier will be a portion of the input. Figure 3-2
shows the ripple waveform associated with a full-wave rectifier. The filter section contains a network of
resistors, capacitors, or inductors that controls the rise and fall time of the varying signal so that the signal
remains at a more constant dc level. You will see this more clearly in the discussion of the actual filter
circuits. You can see that the output of the filter is at a 180-volt dc level with an ac RIPPLE voltage
riding on it. (Ripple voltage is a small ac voltage riding at some dc voltage level. Normally, ripple voltage
is an unwanted ac voltage created by the filter section of a power supply.) This signal now goes to the
regulator where it will be maintained at approximately 180 volts dc to the load.
Figure 3-2.—Block diagram of a power supply.
Q1. What are the four basic sections to a power supply?
Q2. What is the purpose of the regulator?
