Re: OT?: Major PC hardware failure

Puddin' Man wrote:


Please: For future reference, what is the SB in +5VSB?

Thanks,
P

It stands for "+5 volts stand by".

When the computer sleeps, +5VSB continues to run. It may be
implemented, as two separate power supplies, within the power supply
box. The +5V standby circuit, isn't particularly efficient.

You can see why that is, in this diagram. The lower left corner
shows "second supply" and a 78L05 based linear regulator. A linear
regulator isn't nearly as efficient as a switching regulator circuit.
On some power supplies, while the computer sleeps, you may even feel
a tiny cloud of warm air, near the power supply. (Or perhaps, just
as the fan starts to spin, when you press the button on the front.)

http://www.pavouk.org/hw/en_atxps.html

The primary side of the supply, charges up the main capacitors
to a high voltage (like 300 volts DC, or 115 * 1.414 * 2 for voltage
doubler). Switching circuits, are used to make an A.C. signal from
that voltage, through the primary side of the various transformers.
The "second supply", has a switching circuit on its primary side.
Transformers only pass AC, and that's why it has to be converted
to AC. A high frequency is used, and the transformers needed are
relatively small physically. That's how they manage to all fit
in the box (use of frequencies much higher than 60Hz line frequency).

But when the voltage comes out the other side, the voltage isn't
regulated. There is no backward path, from the output side to the
input, saying "I need more voltage" or "I need less voltage". The
switching circuit is functioning open loop. The output voltage
is higher than 5V. It then passes through the 78L05 which
trims it to 5V exactly. And that is what provides standby power,
power for the sticks of RAM, when the computer is sleeping.
The 78L05 gets warm in the process, and would be one source of heat.

The main part of the supply, has a feedback path, between secondary
and primary. And that allows the main outputs to be used, without
adding additional devices for regulation. The main part of the
supply, then has a path that can say "I need more voltage". And that
means the power supply automatically regulates as a function of the
load. If the output voltage drops, the feedback says to pump up
the output, to compensate. The +5VSB "secondary supply", doesn't have
that feature. The switching stage in that one, may vary a bit,
and the 78L05 cleans up the resulting voltage and helps it
meet the ATX spec.

Paul
.

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