I’ve been looking for a more convenient way to power some 1990s era computers/palmtops using USB: specifically the Psion Series 3a (hardware dates from 1995) and the Psion MC400 (“mobile computer” i.e. laptop from 1989).
- the Series 3a needs 9V DC with a +ve outer / -ve inner
- the MC400 needs 12V DC again with a +ve outer / -ve inner
Back in the olden days, i.e. before USB became a viable option for power, everything needed its own proprietary connector/polarity/voltage. Given USB’s status as a de facto power supply/source I looked for some USB to 9V and USB to 12V converters to bring these old devices into the modern world. I’ve had success powering these old Psion devices from USB DC-DC converters, models similar to this Adafruit version
I made some current consumption measurements on the old devices to make sure the new USB-DC converters were up to the job. Multimeter readings on the MC400’s 12V external supply show current draw is approx. 50mA when idle up to about 120mA when busy.
I used 2 separate approaches – for the first approach I kept the purchased lead in one piece and used a converter/adapter on the end of the cable, for the second approach I removed the end of the purchased lead and used a connector block to connect a standard Psion connector from an old dead Psion mains PSU.
These are really basic DIY solutions, they could be smartened up with soldered connections/connectors. A much more sophisticated design could use a USB-C PD module configured to directly supply 9V/12V but this would necessitate the use of a PD-compatible USB-C supply/source as an whereas and the ubiquitous USB-A-port-based charger is still much more abundant.
My Frankenstein creations: I made the one below to power the Psion Series 3a. The 2.1/5.5mm plug connector on the output provides 9V but the polarity is -ve outer +ve inner. The 2.1/5.5mm socket with screw terminals is connected to a standard psion connector and handles the polarity conversion.
For the one below (to power the MC400) I removed the switch and 2.1/5.5mm connector and used a small connector block to connect a standard Psion connector from an old mains PSU.
(current display reads “0.00A” as the MC draws less than 50mA when idle)
Room 7609 
For some reasons I didn’t notice this particular post previously. While I have several original power adapters it seems to be useful to use USB chargers instead (you know, the original power brick is so big heavy). Thanks!