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Salvage It!
BY JULIAN EDGAR
A $5 variable voltage power supply
Want to be able to dim lights in, say, a model
rail layout? Or what about varying the speed of
a low-powered motor or regulating the voltage
going to a bike headlight, so that its brightness
stays the same as the battery voltage falls? It’s
all possible for less than $5.00.
T
HIS DESIGN uses a slightly modified car phone charger. The idea
is based on a Circuit Notebook contribution from Timo Mahoney in the
November 2003 issue of SILICON CHIP.
Most car phone chargers use a DCDC switching power supply to reduce
the voltage from the car’s 13.8V to what
ever the phone requires. However, it’s
easy to modify the PC board to give an
adjustable voltage output.
Apart from the car phone charger
(the electronics are normally built into
the cigarette lighter plug), all you need
is a 10kW potentiometer. Car phone
chargers are available in secondhand
shops, at the tip and in the junk rooms
out the back of phone retailers, while a
10kW pot can be scrounged from some
old gear or purchased from your local
electronics parts retailer.
Building it
Fig.1 shows a typical circuit for a
car phone charger. The IC controls the
output voltage, using voltage feedback
provided by resistors R1 and R2. So if
you change the values of R1 and R2,
Inside each of these car phone chargers is a sophisticated DC/DC converter than
can be easily modified to provide a variable output power supply.
88 Silicon Chip
the voltage output will also change.
However, rather than have a fixed
output, we can vary the output at will
by installing an adjustable pot in place
of the two resistors. Then it’s just a case
of measuring the output voltage and
adjusting the pot until the required
voltage is obtained. If you want the
voltage to be set and then forgotten,
use a trimpot. Alternatively, if you
want to be able to vary the voltage at
any time, wire a full-size pot to the
board with flying leads.
The most difficult part is finding the
correct resistors to change. Individual
chargers vary quite a lot, so the com-
Rat It Before You
Chuck It!
Whenever you throw away an old
TV (or VCR or washing machine or
dishwasher or printer) do you always
think that surely there must be some
good salvageable components inside?
Well, this column is for you! (And it’s
also for people without a lot of dough.)
Each month we’ll use bits and pieces
sourced from discards, sometimes in
mini-projects and other times as an
ideas smorgasbord.
And you can contribute as well. If you
have a use for specific parts which can
easily be salvaged from goods commonly being thrown away, we’d love
to hear from you. Perhaps you use the
pressure switch from a washing machine to control a pump. Or maybe you
salvage the high-quality bearings from
VCR heads. Or perhaps you’ve found
how the guts of a cassette player can
be easily turned into a metal detector.
(Well, we made the last one up but you
get the idea . . .)
If you have some practical ideas,
write in and tell us!
siliconchip.com.au
Silicon Chip
Binders
REAL
VALUE
AT
$12.95
PLUS P
&
P
Fig.1 – the circuit of a typical car phone charger. The output voltage is set
by R1 and R2. If these are replaced with a 10kW pot, the output voltage can
be adjusted over a wide range.
H SILICON CHIP logo printed in goldcoloured lettering on spine & cover
H Buy five and get them postage
free!
Price: $A12.95 plus $A7 p&p.
Available only in Australia.
Just fill in the handy order form in
this issue; or fax (02) 9979 6503; or
ring (02) 9979 5644 & quote your
credit card number.
The charger is modified to produce a variable output by substituting a pot
(arrowed) for two resistors. If external adjustment is needed, a full-size pot can
be wired to the board with flying leads. In this case, a multi-turn pot has been
used, which allows for very accurate setting of the output voltage.
ponent designations marked on the
board (R1, R2, etc) will probably not
coincide with Fig.1. Hmmm.
So how do you find the right resistors? The answer is to closely look at
the top and bottom side of the board.
Somewhere, there will be two resistors
that join to a common track at one end
Each of these bike
headlights is run
from a modified
phone charger. The
chargers are used
to drop the voltage
from the available
12V (provided by
sealed lead-acid
batteries that also
power the electricassist bike) to a
regulated 6V.
siliconchip.com.au
but go to different tracks at their other
ends. The track that both resistors join
to will also connect to pin 5 of the IC.
Given that there will only be three or
(at most) four resistors on the board,
it shouldn’t be all that hard to find the
resistor pair in question.
The next step is even easier – re-
move these two resistors and solder
the pot to the board in their place.
The centre terminal of the pot goes
to the track where the two resistors
were originally joined together, while
the outer pot terminals connect to the
remaining two vacant pads.
Note that it, in some cases, it’s easier
to make the connections by soldering
the pot to the track side of the board.
Using it
To test the modified power supply,
you’ll need a source of 12V power and
a multimeter.
First, connect the power supply
to the 12V source (eg, a plugpack),
making sure that you get the polarity
the right way around (the tip of the
cigarette lighter plug is positive). That
done, connect the multimeter to the
output leads of the adaptor and measure the voltage as the pot is adjusted
– the output voltage should vary.
Finally, place a load on the output
(eg, a small 12V bulb) and check that
you can alter the brightness of the lamp
over a wide range. Typically, you can
draw about 0.5A from the unit (depending on the plugpack). If the output
voltage is critical, the pot should be set
SC
with the load attached.
April 2005 89
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