These sudden whims of mine can have unexpected results. Fortunately, this time, the outcome was positive. In this case, it was the construction of a lightweight space saving power supply for my emulator. This project is also a good example of how I modify small switchmode power supplies to suit specific applications.
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….)
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 seams.
The photo at the top of the page shows the power supply just after I'd opened it up.
(Sorry - Just have to add this note 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 changes safely.
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 to modify.
In short, don't come complaining to me if you kill yourself while attempting to modify one of these power supplies!!
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.
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 featured output voltages 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.
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, I think.
The power supply has worked very well during my lengthy travels. Of course, I still haven't finished that microprocessor project…