Circuit Description
The line lump outputs a two phase AC signal each one with a peak of nearly 25V with respect
to its centre tap output. The difference between the two phases is that one is completely out of
phase with the other. It's a bit like an audio balanced output in that respect. Both phases
should be present for the power supply to work correctly.
Each phase is separately fused. Note that the fuses should be anti-surge, or ‘slo-blo’. The
standard convention is for a ‘T’ in front of the value, where T is for time lag. For example,
T2A would mean a two ampere anti-surge fuse. Anti-surge fuses have a higher thermal mass
than fast blow fuses to prevent them blowing when they see a short burst of current. The
inrush current when the supply is first switched can be very large and we don’t want our fuses
blowing at that point.
D8, C12 and R12 provide a low current DC feed to a standby AC in LED. This lights up
whenever the line lump is connected and powered up. It is wired in before the modular's
power switch to remind you that the power on switch is actually a standby switch and does
not switch the line lump off.
It is possible to run the Oakley PSU module on a single phase. This means that the unit could
be connected to a single output AC adapter, ie. one with just two leads to pins 1 and 2.
However, the regulators and the AC adapter will have to work harder in this application. It is
therefore not recommended to run the PSU in single phase unless the output current is going
to be less than 250mA or so.
The raw AC is fed to a bridge rectifier based around D9, D10, D11 and D12. This is the
classic bridge rectifier circuit. Although it is drawn somewhat differently in the schematic than
the usual ‘bridge’ style. If you think about a diode as passing current through it just one way,
you should be able to work out why the voltage across C13 and C14 ends up as only positive,
and across C7 and C8 as only negative.
I have used 1N5401 diodes in this place. These are 3A devices and are plenty large enough to
cope with any abuse the power supply is given.
The outputs of the rectifiers supply current to the two smoothing capacitors. These act as
reservoirs of charge when the AC voltage dips below its peak output. You can think about the
rectifiers as merely topping up the reservoirs 100 times a second (120 times in North
America), whilst the capacitors actually provide the energy to keep the modular powered.
The smoothing capacitors are generously rated both in terms of voltage and capacitance. It is
essential that you use good quality components here and that they have sufficiently high ripple
current rating. Since this PSU could possibly be supplying up to 1A per supply rail, a single
smoothing capacitor could have nearly 2A running through it. The issue 4 Oakley PSU uses
two smoothing capacitors in parallel per supply rail. The current through each capacitor is,
therefore, roughly half that what it would be with just one larger smoothing capacitor per rail.
This reduces the internal heating of the capacitor thus increasing longevity. Since the
capacitors are in parallel the individual capacitance can be half of what is required for a single
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