Introduction, Theory of operation, Voltage step-down function – Elenco 015V Power Supply Kit User Manual

Page 3: Ac to dc conversion - rectification, Figure 1, Figure 2 figure 3a figure 3b, Figure 4, Figure 5, Figure 7a figure 7b, Figure 6

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The electrical power that is supplied by your power
company is what is known as Alternating Current or
AC. This current is constantly changing amplitude and
polarity (see Figure 1). The electrons (current) move
back and forth at a rate of 60 times per second. This
type of current is not suitable for most electronic
circuits. It must be converted into a Direct Current or
DC which moves in one direction only. Your Multi-
Purpose Power Supply does exactly that. It converts

AC into DC. It also
allows you to vary the
amplitude

of

the

voltage from 0-15V
(0.25A - 12Vmax).
Also, your power
supply has an output
of

8.5VAC,

0.5A

without regulation.

-2-

INTRODUCTION

A block diagram of the system is shown in Figure 2. It
consists of five basic circuits needed to convert 120
volts of AC to a usable 0-15 volts of DC. We will
analyze each circuit for a better understanding of
power supplies.

VOLTAGE STEP-DOWN FUNCTION

In this circuit, 120 volts of AC is
reduced to two 13VAC out of
phase voltages. Thirteen volts
is the rms (effective) value. The
peak voltage is really 18 volts.
This step-down is achieved by
the use of a transformer. The
winding of the transformer is
shown in Figure 3A and the
voltage across each winding is
shown in Figure 3B. In a
transformer, the magnetic field
produced by the 120 volts of
current is induced into the
secondary winding. The voltage
across the secondary winding is
a ratio of the number of turns between the primary and
secondary winding. If the ratio was 1:1 (equal turns),
then the secondary voltage would be the same as the
120 volt input. In our transformer, the ratio is 120:13.
Therefore, the secondary voltage will be 13 volts. If
we were to put an oscilloscope between the common
lead of the two secondary windings and looked at the
other leads, we would see that the two voltages are
180 degrees out of phase. This is because the
windings are going in opposite directions from the
common point. This is an important point in converting
the AC to DC as we will see in the next section.

AC to DC CONVERSION - RECTIFICATION

This circuit that converts AC to
DC consists of two diodes D1
and D2. The purpose of a
diode is to pass current in only
one direction (see Figure 4). If
we were to take a battery and
connect it to a diode and lamp,
as shown in Figure 5, the lamp would only light when
the diode is in the conduction direction. If we replace
the battery with a transformer secondary winding, the
diode will only conduct on the positive cycle of the
voltage as shown in Figure 6. Note that the output of
the diode is a half-wave rectification with a hole in the
center. This voltage would be hard to filter out. It is
desirable to fill in this area. This is done by something
known as full wave rectification, which is using a
second winding out of phase with the first.

Figure 7A shows the circuit for a full wave rectification.
Note that diode D1 conducts the previously described
and diode D2, one half cycle later. The diodes
conduct only when the voltage goes positive and no
current flows on the negative half. The resulting
output voltage waveforms are shown in Figure 7B.
Note that there is no longer a gap between cycles.
This will make it easier to filter the output voltage.

THEORY OF OPERATION

Figure 1

336V

+168V

–168V

or

Time

Peak to

Peak (PP)

or 120V

rms

Voltage

Stepdown

Rectification

AC to DC

Voltage and

Current Control

DC

Filtering

Reference

Voltage

0-15V

Output

13VAC

Figure 2

Figure 3A

Figure 3B

120VAC

18V

AC Peak

AC Peak

–18V

18V

–18V

36Vpp

36Vpp

Figure 4

Direction of
Current Flow

Figure 5

Diode

Battery

Not Lit

Lamp

Output D1

Output D2

D1 & D2

D1

D2

Figure 7A

Figure 7B

Diode

Battery

Lamp

Figure 6

Resistor

Diode

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