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Don't spend big
bucks on batteries
for your Sony
Walkman ®. This
simple voltage
regulator will let
you power it from
a plugpack supply
or from your car's
cigarette lighter
socket.
The
If you like listening to your personal
player in the car or while relaxing in
your favourite chair, then this project
is a must. It's a very simple voltage
regulator circuit that's designed to
conserve the player's batteries for those
times when you want to listen "on the
move".
We've christened the project The
Eliminator because that's just what it
does - when it's plugged in, it effectively eliminates (or bypasses) the player's internal battery. In use, The Eliminator is simply connected between a
plugpack supply (or car battery) and
the player's DC socket. Its job is to
accept the +12V (nominal) from the
36
SILICON CHIP
By MARQU E CROZMAN
plugpack supply and regulate it to the
DC voltage required to run the player.
Because it only uses low-cost parts,
the savings in batteries will very
quickly pay for this project. It's also a
snack to build and get going. In fact,
we reckon that even a rank beginner
could build this design.
The circuit can also be easily customised to meet your requirements.
Some personal players require two
AA cells (3V), while others require
three (4.5V) or even four (6V). For this
reason, we've designed the project so
that it can deliver either 3V, 4.5V, 6V
or 9V. All you have to do is select one
resistor value from a table.
An important safety aspect of this
project, particularly for beginners, is
that no mains wiring is required. Instead, it's designed to run from a car
battery or from a DC plugpack supply.
You can use any DC plugpack supply
with a rated output voltage of 9-12V.
However, the project is not just confined to powering personal players
from a DC supply. Because it was originally intended for use in a mainspowered project, the PC board also
has provision for a four-diode bridge
rectifier. By adding this bridge rectifier, the circuit can be powered from a
15VAC (or less) mains transformer.
In this application, however, the
02
1N4004
~.....__ _ _ _ _ _ _ _,-..,-•3V
D3
1N4004
10
+
16VW
-
Fig.1: the circuit is
based on an LM317
3-terminal adjustable regulator. The
output voltage can
be varied between
3V & 9V by choosing
the value ofR2, as
shown in Table 1.
LED1
OVO---,.__
___.,_ _ _ _ _ _ _ _ _ _...,.._OV
THE ELIMINATOR
TABLE 1
R2
Output
180 ohms
3V
330 ohms
4.5V
470 ohms
6V
750 ohms
9V
four diodes in the original bridge rectifier have been replaced by a single
diode (Dl) and a wire link. We don't
need the bridge rectifier, since we are
using a DC supply only.
Circuit details
Refer now to Fig.1 which shows the
circuit details.
As you can see, The Eliminator is
based on an LM317 3-terminal positive voltage regulator. This device is
capable of delivering up to 1.5A (with
adequate heatsinking), has an output
voltage range from 1.2-37V, and features internal thermal overload and
short circuit protection, In short, it is
virtually "blow-up" proof.
Let's take a closer look at how it
works. The+ 12Vrail from the plugpack
(or car battery) is applied to the input
of the LM317 regulator via reverse
polarity protection diode Dl. In operation, this regulator is designed to
provide 1.25V between its OUT and
ADJ (adjust) terminals but note that
the current flowing through the ADJ
terminal is negligible.
This makes the device extremely
easy to use, since we require just two
resistors to set the output voltage. Since
the voltage between the OUT and ADJ
terminals is fixed at 1.25V, it follows
that the current through Rl and R2 is
also fixed. This gives a simple formula
for the output voltage, as follows:
V001 = 1.25(1 + R2/Rl)
In our circuit, Rl is fixed at 120Q
while R2 is selected from Table 1 to
give the required output voltage. In
our case, we used a value of 180Q to
obtain a 3V output. If you want a 4.5V,
6V or 9V output, then just select the
appropriate resistor value from Table
1.
Alternatively, you could replace R2
with a lkQ trimpot to make the output
continuously variable over the 1.259V range.
A lO0µF electrolytic capacitor is
used to filter the incoming supply
PARTS LIST
1 PC board, code SC04104921,
48 x 61mm
1 aluminium case 100 x 58 x
45mm (Jaycar Cat. HB-5100)
1 Dynamark front panel label, 55
x 190mm
4 68A x 10mm machine screws,
nuts & lock washers
6 PC stakes
1 200mm length of medium~duty
hook-up wire
Semiconductors
3 1 N4004 diodes (D1 -D3)
1 LM317 voltage regulator
1 5mm LED (LED 1)
Capacitors
1 100µF 25VW PC electrolytic
2 10µF 25VW PC electrolytic
Resistors (0.5W, 1%)
1120Q
1 R2*
* The value of R2 must be selected ·
from Table 1 or a 1kQ trimpot can
be used instead
voltage, while the output of the regulator is filtered by a l0µF capacitor to
ensure stability. Notice also that the
ADJ terminal is bypassed using a second lOµF capacitor, to improve the
ripple rejection.
D2 protects the LM317 from any
reverse voltages which may generated
across the output by inductive or capacitive loads, while D3 discharges
the lOµF capacitor on the ADJ termi-
li:i
"'...
C>
~
u
c,i
ov--1
+T2v-
D1
~
9,oouF
+
Fig.2: the circuit is shown
here wired for a 3V output
but this can be varied by
changing the value of the
180Q resistor (see Table 1).
Mount the LM317 regulator
at full lead length & with its
metal tab adjacent to the
edge of the PC board.
Fig.3: here is the full-size etching
pattern for the PC board.
MAY1992
37
TABLE 2
Brand
Plug Size
Sanyo
1mm
Sony & Aiwa
1.3mm
Casio
1.6mm
JVC
3.1mm
Others: generally 2.1 mm or 2.5mm
Note: there is no fixed convention as
to which terminal is positive & which is
negative. Check the socket of your
player - there is usually a small
diagram next to it to indicate the wiring
polarity
The PC board is mounted on the base of the metal case & secured using machine
screws, nuts & spacers. Take care with the polarity of the external leads.
nal if the output is short circuited.
Finally, LED 1 and its associated
3300 current limiting resistor provide
power on indication.
Construction
All the parts are mounted on a small
PC board coded SC04104921 - see
Fig.3.
Begin the assembly by installing PC
stakes at the six external wiring points,
then install the wire link. You now
have to select RZ from Table 1 to obtain
the required output voltage. We wanted
an output voltage of 3V, so we used a
value of 1800 as shown in Fig.3. If you
want an output voltage of 6V, for example, just substitute a 4800 resistor
instead.
The remaining resistors can now be
installed, followed by the diodes and
the capacitors. Make sure that the diodes and capacitors are oriented ex-
actly as shown in the diagram.
Finally, the LM317 regulator can be
installed. Mount it with its metal tab
towards the edge of the board and
leave its leads at full length.
The completed board assembly is
housed in a small metal case which
also acts as a heatsink for the LM317
regulator. Mark out and drill the three
mounting holes in the base to accept
the PC board, then mount the board
using 5mm spacers, machine screws
and nuts.
When the board is in place, mark
the mounting hole for the LM317 on
the side of the case, then remove the
PC board and drill the hole to 3mm.
Carefully deburr this hole so that you
get a perfectly smooth surface, then reinstall the PC board in the case.
Fig.5 shows the mounting details
for the LM317. As shown, its metal tab
must be isolated from the case using a
mica washer and insulating bush.
Smear both sides of the mica washer
with heatsink compound before bolting the assembly together. When this
has been done, use your multimeter
(set to a high resistance range) to confirm that there is no connection between the tab and the case.
If the meter indicates a short, remove the assembly immediately and
locate the source of the problem (eg, a
piece of metal swarfmay have punched
through the mica washer).
Next, attach the adhesive label to
the lid and drill a hole to accept the
LED bezel. This done, mount the LED
in position and connect it to the PC
board using short lengths of hook-up
wire. Take care with the LED polarity
INSULATING
MICA
WASHER
·"·~)I
SCREW
r
Dllilill(3
----CASE
t
T0220
DEVICE
The
Eliminator
Fig.4: this is the full-size artwork for the front panel.
38
SILICON CHIP
Fig.5: the LM317 3-terminal
regulator must be isolated from
the case using a mica washer &
insulating bush. Smear all
mating surfaces with heatsink
compound before bolting the
assembly together, then use your
multimeter to confirm that the
metal tab has been correctly
isolated from the case.
correct - the polarity indication will
usually be moulded into the plastic
adjacent to the socket.
For in-car use, the input lead is
simply a length of figure-8 cable attached to a cigarette lighter plug. Again,
make sure that you get the polarity
correct. If a plugpack supply is used,
tbe input lead will come directly from
the plugpack itself. You can either
connect the plugpack supply to The
Eliminator via a DC socket mounted
on the side of the case or you can
solder the wires directly to the PC
board.
The LM317 regulator uses the case as
a heatsink. Make sure that it is
electrically isolated from the case as
shown in Fig.5 .
MARK ONE
COMMUNICATIONS
Specialising in:
Mitsubishi , Motorola &
NEC mobile phones
INSTALLATION &
SERVICE
7DAYS
.
Testing
Double check all wiring and component orientation before you assemble the case. All OK? If so, screw the
two halves of the case together and
connect the unit to your 12V power
supply. If everything is working OK,
the LED will come on. Check the output voltage and polarity with your
multimeter- it should be very close to
the selected value.
Alternatively, if you substituted a
lkQ trimpot for R2 , you simply adjust
the pot until you get the desired output voltage.
SC
- the cathode lead is the shorter of the
two (see Fig.1).
The assembly can now be completed
by installing the input and output
leads. These are fed through holes in
the ends of the case and secured by
cordgrip grommets. You will have to
fit the output lead with a DC plug to
suit your particular player (see Table
2) . Make sure that you get the polarity
obile Phone
18) 29 5888
18) 21 4555
275 Ryedale Rd,
Eastwood,
NSW2122.
Motorola Micro-T•A•C
Cellular Telephone
Phone (02) 489 7777
Fax (02) 489 7774
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MAY 1992
39
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