5 volt power supply>

5 volt regulated power supplies


When fitting extra drives to a PCW care must be taken not to overload the power supply of the machine. This applies particularly when fitting 3.5" drives to the PCW8256,8512 and 9512, machines which were originally supplied with 3" drives. The Amstrad 3" drives use 5 volts for the electronics and 12 volts for the motors, and the power supplies of these machines have been designed with this in mind. It is very tempting when fitting a 3.5" drive internally to simply use the existing connector, which fits OK, but most modern standard 3.5" drives use 5 volts for all functions and the 12 volt pin is simply not connected. Therefore the drain on the 5 volt supply is heavier than intended, which can lead to alarming distortion of the screen whenever the drive runs, and eventual failure. Tweaking the power supply to give slightly over 5 volts so that it pulls down to 5 volts when under this load is not an option as far as I am concerned, although I've known it to be done.

A neat solution is to use a small electronic circuit which, interposed between the power supply connector and the drive connector, converts the 12 volt line to a regulated 5 volts and adds it to the existing 5 volt line.

The heart of the voltage conversion circuit is a small integrated circuit type 7805 which has three legs. The 78xx series voltage regulators are easily obtained and are quite cheap. The xx part signifies the voltage e,g 7812, 7815. In our case we need the 7805 which gives a regulated voltage of 5 volts DC.from an unregulated supply which must be over 5 volts. There are several versions. we need the 1 amp version.

The diagram below shows the 7805 5 volt voltage regulator approximately twice full size.

A typical circuit is shown below. It can be built on a scrap of veroboard, incorporating suitable connectors. The 7805 chip gets quite hot and requires a substantial heatsink. Indeed the chip could be bolted to the drive casing and connected to the circuit board by soldered wires.

Capacitors "C" of 100nf to 300nf provide smoothing and a resistor "R" may be needed to lower the input voltage if a 12 volt input is used and depending on how much current is taken. This can be determined in practice.

An even better solution, particularly if fitting more than one drive externally is to have a separate 5 volt power supply. This could use the same type of 5 volt regulator circuit circuit as described above, but get it's input from an inexpensive power supply unit of the type that plugs into a standard wall socket. This unit need not be regulated but should be of at least 800Ma and supply 6 to 12 volts DC.
On the other hand you may want to make a custom-built 5 volt power supply to suit your own requirements and in this case the booklet "Power Supply Projects" by R A Penfold and published by Bernard Babani will give you a lot of help.The copy I have cost 2.50.

The whole range of PCW is not over-generous with power supply capacity and in my view needs some help in one of the ways outlined above.

Ron King

Email address ... ron@king27.freeserve.co.uk