Elenco Variable Voltage Power Supply User Manual

POWER SUPPLY

MODEL XP-15

Instruction Manual

Copyright © 2013 by Elenco®Electronics, Inc. REV-A 753020
All rights reserved.

No part of this book shall be reproduced by any means; electronic, photocopying, or otherwise without written permission
from the publisher.

ELENCO

®

Figure 1

Simplified diagram of positive power supply

120VAC

Input

18VAC

20VDC 0 - 15V

Regulated

Output

Transformer
120V to 18V

AC to DC
Converter

Voltage

Regulator

CIRCUIT DESCRIPTION

INTRODUCTION

The XP-15 Power Supply features an output voltage variable from
0 to 15V at 0.3 ampere maximum current. The voltage is regulated
to within 0.1V when going from no load to full load. Varying the input
AC voltage from 110 to 130V will have practically no effect on the
output voltage. This is because of the specially designed IC circuit
used in the XP-15. Severe overloading or even short circuiting the
output will not damage the supply. Special turn-off circuits in the IC
sense the overload and turn off the output.

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

-1-

TRANSFORMER

The transformer T1 serves two purposes. First, it reduces the
120VAC input to 18VAC to allow the proper voltage to enter the
rectifier stage. Second, it isolates the power supply output from the
120VAC line. This prevents the user from dangerous voltage shock
should they be standing in a grounded area.

SPECIFICATIONS FOR XP-15 POWER SUPPLY

Output Voltage 0 - 15VDC

Output Current 0.3A @ 12V, 0.2A @ 15V

Load Regulation 0.1V

Line Regulation 0.1V

Ripple Max. 0.01V rms

Short Protection IC THERMO

Output Impedance 0.3W

-2-

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 2,200mF 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