increased heat dissipation but a good heatsink will keep the temperature down to a safe level.
If you can't comfortably touch a power device for more than ten seconds it is too hot.
The 3U wide 5U high panel design has only been tested to work with a maximum of 0.52A
per rail when driven from a PA-20 or PA-30. A 4U or 5U high 19” panel is capable of
considerably more – a 4U 19” panel has been tested successfully using a mains transformer
with a 1A load and 25V across the smoothing capacitors. However, I would advise you to do
your own experiments carefully otherwise you run the serious risk of overheating.
Finally, I think modular systems work better with a number of smaller power supplies rather
than one big one. This is because as power is distributed around a large modular it gets
'corrupted' by the connected modules and the cabling. The cables have resistance and as
current flows through the cables the voltage at the end of the cable gets reduced. The more
current that flows and the longer the cable the worse the problem.
Having multiple power supplies does create the not insignificant question of what to do with
the multiple grounds you will now have in your system. These can all be connected together
and later in this document I will present one way of doing this.
Now let us look in more detail at the negative part of the power supply. The negative supply
works in a similar way to the positive regulator but uses an op-amp, U3, and discrete
components. The reference for the negative rails comes not from a precision reference but
from the output of the +15V regulator. This means that the negative output voltage will track
the positive one, but not vice versa I should add.
The op-amp is wired as a simple inverting amplifier although its difficult to see this at first
glance. The input is via R9 which is connected to the +15V output. The feedback is provided
via R8 which is connected to the negative supply output. The output of the op-amp drives the
base of the pass transistor, Q2, via R11, which in turn controls the level of the negative
output. Q2, a TIP145, is a PNP Darlington transistor. This is actually two transistors in one
enclosure – but it can treated as one device with a larger than normal current gain (hfe) and
twice than normal base-emitter voltage of 1.2V. Having a high gain requires only a small
amount of drive current from the op-amp via R11 even when the load on the power supply is
relatively high.
U3 will act so that its output will force all the current flowing through R9 to be passed onto
R8. This is one of the op-amp golden rules; that is, no current shall flow into the input pins so
long as feedback is maintained. To do this the op-amp must force the pass transistor to pass
enough current though it to establish exactly -15V at the output.
Q3 and R3 form part of the current limit circuit. If the voltage across R3 exceeds 640mV then
Q3 will turn on dragging current away from Q2’s base and thus lowering the output voltage of
the negative rail accordingly.
The op-amp U3 derives its positive power from its own 5V supply based around a simple
7805 regulator chip. This allows the negative regulator to function correctly when the +15V
rail is under stress and allows the negative rail to fall gracefully when the mains power is
switched off.
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