Elenco Variable Voltage Power Supply - Manual (Page 3)

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AC to DC CONVERTER
The AC to DC converter consists of
diodes D1 and D3 and capacitor C1.
Transformer T1 has two secondary
windings which are 180 degrees out
of phase. The AC output of each
winding is shown in Figure 2A and 2B.
Diodes are semiconductor devices
that allow current to flow in one
direction. The arrow in Figure 3 points
to the direction that the current will
flow. Only when the transformer
voltage is positive will current flow
through the diodes. Figure 3 shows
the simplest possible rectifier circuit.
This circuit is known as a half wave
rectifier. Here, the diode conducts only
half the time when the AC wave is
positive as shown in Figure 2C. Use of this
circuit is simple but inefficient. The big gap
between cycles requires much more filtering to
obtain a smooth DC voltage.
By the addition of a second diode and
transformer winding, we can fill in the gap
between cycles as shown in Figure 4. This
circuit is called full wave rectification. Each
diode con-ducts when the voltage is positive.
By adding the two outputs, the voltage
presented to capacitor C1 is more complete,
thus, easier to filter, as shown in Figure 2F.
When used in 60 cycles AC input power, the
output of a full wave rectifier will be 120 cycles.
Capacitor C1 is used to store the current
charges, thus smoothing the DC voltage. The
larger the capacitor, the more current is stored.
In this design, a 2,200
m F capacitor is used,
which allows about 2 volts of AC ripple when
one half amp is drawn.
Figure 2
Voltage Waveform for Supply
A) Transformer
Winding AB
B) Transformer
Winding BC
C) Output of
diode D1.
D) Output of
diode D3.
E) Total of diodes
D1 & D3.
20V
F) Output of capacitor C1
Ripple depends on load
current (expanded).
Figure 4
Full Wave Rectifier
D1
D3
C1
Figure 3
Half Wave Rectifier
C1
D1