the PSU board. It is essential that you make the current limit resistors suit your application.
The next section, and also in the parts list of the Builder's Guide, will give details on what
values to use.
I recommend that you use a Yamaha PA-20 or PA-30 power supply. These are neat tidy
external power supplies that keep the dangerous mains voltage away from your modular. The
PA-20 will allow a maximum current of 520mA (0.52A) to be drawn from each rail. While the
PA-30 will allow up to 780mA (0.78A). Remember though you can't use a PA-30 with the 3U
master panel and expect to draw 0.78A out of it without the panel getting too hot. If you are
using the 3U master panel then you should draw no more than 0.52A per rail. Both the PA-20
and PA-30 are centre tapped AC supplies with three wires within their output cable and use a
three pole connector.
Other power supplies are available and they come in lots of different variations. Other than AC
output voltage the two key specifications are output current (please don't call it 'ampage'), and
whether the output is centre tapped or single phase. In almost all cases the outputs of standard
AC output power supplies are single phase. You can tell because they only have a cable with
two wires inside which terminates in a two pole connector.
A single phase AC output supply will allow only you take not much more than a quarter of its
rated current output. For example if you are using a 500mA (or 0.5A) AC wallwart* then the
most current you can take from this power supply module is around 125mA from each rail.
That is, take no more than 125mA from either the +15V supply and 125mA from the -15V
supply.
Various companies make linelumps** with a greater capacity than 500mA. If you can get a
single phase 1A output one than this will be able to drive up to 250mA per rail.
The Oakley PSU can be used with full wave or half wave rectification. The former allows it to
utilise split AC outputs. With full rectification and using a centre tapped power supply the
amount of current taken from each 15V rail can be up to just over half the rated current
output of the power supply. The Yamaha PA-20 supply is rated to give an output voltage of
35Vac (with a centre tap) at a load of 0.94A. Once rectified and smoothed this means that a
maximum current of 0.52A can be drawn from each rail.
Two sets of screw terminal blocks are provided for connecting the low voltage AC power
source to the board and the optional power switch. If you are using a single phase wallwart to
power the PSU module than you need only to use two terminals per terminal block.
The board has four mounting holes for stable placement onto your modular case. Care should
be taken so that the board's various board mounted components do not come into contact with
any part of your modular's enclosure. Use of 10mm hex spacers between the board's bottom
surface and the panel is the preferred option.
The issue 4 PSU board utilises a different way of connecting 0V to the panel than previous
issues. I recommend that the metal panel be securely connected to 0V. Provision on the PSU
board of a chunky screw terminal allows a thick wire connection to be made to a suitable
bonding point on the panel. This is essential if you are using an internal mains transformer but
also has benefits even when you are using an external wallwart or linelump supply.
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