Elenco Triple Supply w- 12.6VAC CT User Manual

POWER SUPPLY

MODEL XP-720

Instruction Manual

ELENCO

®

Copyright © 2013 , 1997 by Elenco®Electronics, Inc. All rights reserved. Revised 2013 REV-E 753270

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

Input Voltage 110-130VAC
Current Protection 1A

Output Voltage 1) 1.25-15VDC @ 1A
(at 120V input) 2) –1.25 - –15VDC @ 1A

3) 5VDC @ 3A

4) 6.3, 12.6CTAC @ 1A

Output Regulation 200mV each supply

Line Regulation 100mV each supply

Ripple Max 5mV rms

Current Protection 1A limit 2-15VDC

3A limit 5VDC

Short Protection 1A limit 2-15VDC

3A limit 5VDC

Output Impedance 0.2Ω ±15VDC

0.06Ω 5VDC

SPECIFICATIONS ON XP-720 POWER SUPPLY

-1-

OPERATING INSTRUCTIONS

1. Check the voltage rating of the equipment to be powered. Care must be taken not to exceed this rating.

2. Plug the line cord into a 120V, 60Hz AC outlet.

3. Adjust the voltage control to desired voltage. Load variation will have practically no effect on the voltage
setting due to the special regulation circuit.

4. Connect the positive lead of your equipment to the red output terminal and the negative lead to the black
terminal when using the positive supplies. For the negative voltage use the yellow terminal.

5. Care must be taken not to exceed the current reading, as the supplies will turn themselves off if overheated.

6. The Model XP-720 has an added feature of having the positive and negative supplies being able to be
stacked to give up to 30V at 1 ampere. Simply use the negative 1.25 to 15V output (yellow jack) as your minus
supply connection. For the positive connection, use the positive 1.25 to 15V red terminal. This will allow you
to obtain a variable 2.5V to 30V at up to 1 ampere out.

CAUTION: The maximum output current for the AC section is 1 amp. Overloading the output or shorting the

6.3VAC to ground will damage the transformer.

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 180Oout 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 half­wave 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) To t al o f d io d es

D1 & D2.

20V

F) Output of capacitor C1

Ripple depends on load
current (expanded).

Half Wave Rectifier

Full Wave Rectifier