# Circuit description – Elenco Triple Supply w/ 12.6VAC CT @ 1A User Manual

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CIRCUIT DESCRIPTION

Introduction

Congratulations on your purchase of a very versatile power supply. Our engineers have carefully designed the
XP-720 to give you years of trouble-free performance.

The Model XP-720 Power Supply features three solid-state DC power supplies and a 12.6VAC center-tapped
output. The first DC supply gives a variable output of positive 1.25 to 15 volts at up to 1 ampere. The second
gives a negative 1.25 to 15 volts at 1 ampere. The third has a fixed 5V at 3 amperes. All DC supplies are fully
regulated. A special IC circuit keeps the output voltage within 0.2V when going from no load to 1 ampere. The
output is fully protected from short circuits. This supply is ideal for use in school labs, service shops or anywhere
a precise DC voltage is required.

Power Supply

Figure 1 shows a simplified circuit diagram of the positive supply. It consists of a power transformer, a DC
rectifier stage and the regulator stage.

Transformer

The transformer T1 serves two purposes. First, it
reduces the 120VAC input to 17VAC and 8VAC to allow
the proper voltage to enter the rectifier stages. Second,
it isolates the power supply output from the 120VAC
line. This prevents the user from dangerous voltage
shock, should the user be standing in a grounded area.

AC to DC Converter

The AC to DC converter consists of diodes D1 and D3
and capacitor C5. Transformer T1 has two secondary
windings which are 180

O

out of phase. The AC output

at 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 of 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 require much more filtering to
obtain a smooth DC voltage.

By 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 conducts when the voltage is positive. By
adding the two outputs, the voltage presented to capacitor C5 is more complete, thus easier to filter, as shown
in Figure 2E. When used in 60 cycles AC input power, the output of a full wave rectifier will be 120 cycles.

Capacitor C5 is used to store the current charges, thus smoothing the DC voltage. The larger the capacitor, the
more current is stored. In this design, 2200µF capacitors are used, which allows about 3 volts AC ripple when
one amp is drawn.

-2-

Figure 1

Simplified diagram of positive power supply

120VAC

Input

17VAC

20VDC

1.25 - 15V

Regulated

Output

Transformer

120V to 17V

AC to DC

Converter

Voltage

Regulator

Figure 2

Figure 3

Figure 4

Voltage Waveform for Supply

A) Transformer

Winding AB

B) Transformer

Winding BC

C) Output of

diode D1.

D) Output of

diode D2.

E) Total of diodes

D1 & D2.

20V

F) Output of capacitor C1