ZL2PD Fixed Switchmode Power Supply
This page describes how to modify a surplus
switchmode phone charger to create a lightweight fixed voltage output power
supply for other uses.
old cellular phone charger was originally made by Salcom Oy in Finland
for an Ericsson GSM cellualr phone. I used it to build a lightweight AC
power supply to power a microprocessor emulator. Other types of
chargers and power supplies can easily be changed to suit a variety of
applications by following the details described below.
I've mentioned elsewhere on this website, I travel quite a bit in
connection with my work. A day or so before one of my trips, I suddenly
decided I just had to take my Philips 8051 microprocessor emulator with
me. I had been working on another microprocessor based project in my
spare time, and it felt close to completion. A long gap away from the
project would have really set me back. With some free weekends in some
fairly remote places coming up, I reckoned that if I took the emulator
with me, I could finish the project and make better use of my time.
These sudden whims of mine can have unexpected results. Fortunately,
this time, the outcome was positive. It resulted in the construction of
a lightweight space saving power supply for my emulator, and, as a
result, the completion of that software. This project is also a good
example of how I modify small switchmode power supplies to suit
emulator is quite compact. But as I was packing it for my trip, I
suddenly realised that the emulator's power supply was going to be a
bit of a problem. It's an old transformer powerpack, rated for
continuous operation at 9V and 500mA. I originally built it perhaps ten
years ago, adding a switchmode single IC regulator to a simple
transformer-rectifier power pack.
This power pack was heavy. Really heavy. Normally it's no problem. I
just use it around my office, with the emulator plugged into my laptop
or PC. (See the photo to the left) But for a long trip, the power
supply's weight could make it impractical to take the emulator. It
weighted in at almost 0.8 kg! (Just under 2 lbs for those disadvantaged
folk still living in the non-metric world) No way was I going to carry
that around in my suitcase.
(Those that know my travel habits will smile - Here I was worrying
about an extra kilo when my bag often weighs in at 20++kg at the
check-in counter. But my motto is - A kilo saved from one item is an
excuse to carry another kilo of something else….)
around in my cardboard box of salvaged power packs, I found an old
Ericsson celphone charger with a rating of 7.6V/600mA which looked like
it would be ideal for the task. But the output voltage of 7.6VDC was
too low for the emulator. My emulator needs at least 8.5V into the on-board regulators to give
Having determined that it was a switchmode power supply, confirmed by
the charger's label, the next step was to open the charger to see if
the circuit was suitable for the planned modification. I don't bother
if the power supply lacks an optocoupler or if it is poorly constructed
with components which appear to have been dredged from the bottom of a
river. This is something I frequently see in chargers on cordless
phones with odd-sounding brand-names. In this case, the charger was a
high quality design with a clean layout.
Opening the charger case was arguably the most difficult part of the
whole project. It really took quite a bit of force to open it. Unlike
an earlier Ericsson charger which I modified several years before, one
with an identical part number which was simply screwed together, the
two halves in this unit had been glued together in the factory, and
this took quite a bit of effort with a pair of screwdrivers to
gradually prise the two halves of the case apart along the plastic
The photo at the top of the page shows the power supply just after I'd
opened it up.
(Sorry - I just have to add this note here to
keep those salivating lawyers at bay)
Do not attempt the
modifications described here unless you fully understand the risks
associated with mains voltage switch mode power supplies and you are
confident in your ability to complete any evaluation, measurements and
Circuit voltages can
easily rise above 300VDC, and, under some circumstances, to
considerably higher voltages, especially if modifications are not
carried out correctly. Voltages and currents present in these circuits,
both before and after modification, can therefore pose potentially
lethal risks to the unwary.
Do not attempt to copy
these designs or to modify your own switchmode AC power supplies unless
you have adequate expertise and experience with such circuitry.
Furthermore, you should only attempt these modifications if you can,
firstly, accurately analyse the specific power supply you are planning
In short, don't come
complaining to me if you kill yourself while attempting to modify one
of these power supplies!!
charger's internal layout is very tidy and key components can be
readily identified. (Right click on the photo at the left here to see
the internals of this power supply) This allowed the appropriate
modifications to be
determined easily and the required changes to be made rapidly.
Once opened, the design was found to be completely different from a
previous Ericsson charger I had modified for another use. I quickly
determined this latest one used a TL431 as the voltage detector, with
the output voltage set by two resistors. (See the diagram and photo below) Feedback to the primary side was via an optocoupler.
And, within a little over ten minutes, I had completed the
modification. All that was required was a extra 100k resistor in
parallel with R2 to give an output voltage of exactly 9V. The photo
below shows the resistor mounted on the underside of the PCB. I used a
wooden toothpick as a pointer in the photo.
I also removed the original DC output connector and replaced it with a
short DC cable with an emulator-compatible DC connector. I clamped the
new cable in place to the PCB with a nylon cable tie using the old DC
connector's mounting holes.
Typical output voltage detection system using a TL431 shunt regulator
uses a two resistor voltage divider (R1 and R2)
A simple single resistor modification allowed this power supply to be
converted to the required output voltage of 9V. The added resistor -
the toothpick is pointing to the resistor - is at the upper left
hand corner of the PCB
Experimenting with resistor values on an open power supply with mains
AC voltage applied is key to determining the appropriate values to use.
But it is potentially a very dangerous procedure. I used a variable
preset resistor and a couple of fixed resistors to find the correct
component value to use in the final modification, with the components
roughly soldered to the back of the PCB.
To make it safer, I wedged the
board in a short length of plastic square section tube during these
tests, and covered as much of the board as I could to avoid any
accidental contact with the dangerous and potentially lethal voltages
present in the power supply. The preset resistor was adjusted with a
I must stress the need to take great care with
this part of the process if you choose to try this for yourself. Read
the other warnings on this page too before you try these modifications.
Before beginning any modifications, I also took a really careful look
at the circuit board layout. I noticed that the unmodified power supply
output was protected against overvoltage by a PCB fuse and a 12VDC
zener. Since the planned modification would also give an output voltage below
12V, this arrangement could be left unchanged. The unit also
featured an extra SMD LM334 IC constant current regulator circuit fed
from the output voltage rail which then fed a 'Power On' green LED
indicator. A quick check with the LM334 datasheet before making any
modifications determined that this circuit would work fine with the
increased output voltage.
After testing, the power supply case was reassembled with rapid setting
glue ('Superglue'). I removed the old Ericsson label and replaced it
with a new label detailing the revised power supply performance. This
paper label was glued in place and covered with heavy duty clear
self-adhesive plastic film.
output of the power supply with this modification delivered exactly
9.0VDC. Under full load (500mA), the output voltage fell by only 30mV,
a better performance than the original transformer power supply. Also,
the power supply remained very cool, indicating excellent efficiency.
The new power supply was put on the scales just to see how successful I
had been in reducing the power supply weight. The new switchmode power
supply weighed in at 100 grams! (3.5 oz) That's about 1/8th of the
weight of the original unit. Size-wise, it occupies a similar volume,
but its shape made it much easier to pack in the same box with the
emulator. So I count it as a successful space-saving modification too,
The power supply has worked very well during my lengthy travels. I even
finished the microprocessor project!
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