The Oakley RPSU module
This power supply board will allow the conversion of a suitable low voltage alternating
current (AC) to be rectified, smoothed and regulated. The module is designed to be fitted to a
suitably large metal panel which functions as a heatsink for the two regulator devices attached
to the RPSU circuit board. The metal panel should have adequate airflow around it and it is
expected that it will be part of the metal enclosure your rack project is built into.
It is possible, if the RPSU is only powering a small amount of electronics, that the unit can be
used with either no heatsinks or just small clip-on types. However, it is up to the builder to
determine whether or not this is possible. If in operation the two power devices get too warm
to touch for more than ten seconds you will need additional heatsinking. If the RPSU is
powering just one HVM it will need heatsinking.
The voltage output of the power supply module is a split rail 15V supply. This means it
generates both +15V and -15V. That is, two power rails, one of a positive voltage, the other a
negative one. These voltages are measured with respect to a common ground which is
normally connected, via your house's wiring, to the earth that you stand on. The voltage
across both rails is 30V, with the common ground sitting exactly in the middle of this at 0V.
The output current capability is the maximum current you can draw out of the power supply.
The current taken from the supply is determined by the amount and type of modules you are
connecting to the power supply. One HVM will draw around 80mA from the +15V supply
and 80mA from the -15V supply.
The Oakley RPSU features current limiting but only what is built into the two power devices.
Although this will probably stop your project from bursting into flames it is not to be relied on
if the fault continues for a long period of time. The RPSU is fitted with fuses and the correct
choice of fuse will probably save your project in the event of a major breakdown.
If you are using a single phase 500mA (or 0.5A) AC wallwart* then the most current you can
take from this power supply module is around 125mA from each rail. This is enough to drive
one HVM. You may think that a 500mA power pack should be able to drive up to 250mA for
each rail. But simple arithmetic does not work when you are dealing with conversion from AC
to DC. The various parts of the power supply require power to work and this reduces the
useful power we can actually draw from the supply. The rule of thumb is that the AC current
drawn is roughly 1.8 times that of the DC current drawn. This means we can take 278mA
from both supplies, that is 139mA for each rail. To allow for some safety margin we drop this
down to 125mA per rail which allows for a 10% overhead. This is plenty enough for one
HVM but not much else.
Various companies make wallwarts* and linelumps** with a greater capacity than 500mA. If
you can get a 1A output one than this will be able to drive up to 250mA per rail. With any
single phase supply you will be using your RPSU in half wave rectification mode. Single phase
mains adapters are the most common and have only two wires coming from them. They are
normally connected to the rack with a barrel type plug.
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