This documents the power systems of various computers. Readership is Public.

Psion systems

Organiser II

The organiser II series of machines are designed to be powered by a 9V Alkaline PP3 battery. Power is required continuously to support the internal RAM and to keep the real time clock running. There is no backup battery.

The manual indicates that the battery can be changed while the machine is powered off. If this is done sufficiently fast, the machine will maintain enough internal charge to keep the RAM alive.

The system detects the battery voltage and powers itself off with a low battery warning when this drops below a certain threshold. An alkaline PP3 lasts about 3 months of regular use in a machine that's in good condition. Other types of battery can be used, but if the voltage characteristics are different from those of an Alkaline PP3 the power management software may not make the best use of them.

I now tend to use my machines with rechargeable, low self-discharge NiMH PP3 batteries. These are not ideal but are more efficient and cost-effective than Alkaline PP3s. PP3 batteries are made of a set of 6 cells wired in series. Nominally this gives the NiMH version an operating voltage of 8.4V when they are freshly charged but this is more likely to be about 7.2V for the main part of the discharge cycle. The end condition is sudden as far as the power management firmware is concerned.

Charging these batteries is awkward and is usually done as a timed charge at a very low rate. The performance of each battery will depend on how well matched are the individual cells within them.

External power

Image of Psion external PSU
Organiser II mains power supply
The Organiser II PSU internals
I tend to use a mains power supply for these machines when

Power is provided through the top slot via an adapter. All the top slot peripherals make provision to attach external power.

The power supply itself says it is 10.4V DC at 175mA. In reality, its output floats up to about 13V when unconnected. Internally it is a very simple transformer - bridge rectifier - zener diode (I'm guessing), smoothing capacitor and discharge resistor.

The plug is centre-positive 5.5mm diameter with a 2.1mm inner diameter. The shaft length appears to be 12mm, though I've found a 9mm shaft fits more snugly.

In the absence of a genuine Psion power supply, I've found that a modern universal adapter, with appropriate power connector set to deliver 9V works adequately.


This document is maintained by Pute. Comments should be addressed to PNJ at XQWV dot ORG dot UK.
  1. PNJ, 2016-01-03. Started.

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