help: monitor power supply lines

[Tim]

Tim:
>> The power supply is internally regulated.  If it's set to provide 5
>> Volts from its output terminals, that's what it does.  However, if
>> there's a drop across the wiring and there's less than 5 Volts at the
>> motherboard, the power supply will not know about it.  It doesn't sense
>> at that point of the circuit.
>> 
>> I don't know if it's still the case, but some computer supplies only
>> really regulated some of the voltages.  e.g. It'd keep the 5 Volts at 5
>> Volts, but the 12 Volt supplies were less controlled.  Since some
>> supplies were derived from a common source, it's presumed that adjusting
>> one keeps the rest in step, but that doesn't always happen in practice.
>> 
>> Some motherboards may have voltage regulators on the board, but that
>> could only help against too much voltage, they can't increase supply of
>> what's not there.
 

Robin Laing:
> Not fully true.

Which bit, that's a little vague?

> The power supply is supposed to regulate the power.

Yes, though how well some do that is a bit of a problem...

> That is why monitoring is important with a calibrated system.

Monitoring is important, either way, but you can't tell what you're
monitoring without calibration.  Calibration is important if you're
going to monitor.  You've got the cart before the horse.

> If there is a significant drop on the wiring between the power supply 
> and the motherboard, there is a major current draw to cause the 
> resistance in the wire to drop that much voltage. V=I*R  And R is very
> small.

Yes.  Nothing unexpected there.

But the PSU is going to regulate the supply, and compensate for changes,
up to a point.  The big problem is that it senses the voltage at its
side of the wiring, before the voltage drop.  It'll keep its end at 5
Volts, for instance.  The other end will remain proportionally lower,
but unknown to the power supply.

Unless, of course, you have a power supply that has an external sensor
point.  However, that's still only sensing power at the plug.  It's not
sensing whether there's 5 Volts across the whole motherboard.

> In my case the power supply monitor on the mother board stated that
> the voltage was at 5.1 volts, even in the BIOS.  When I checked with a
> volt meter, the voltage was closer to 4.5 volts.  This was on one of
> the unused leads from the power supply.  Using your analogy, the 
> motherboard/ps wiring actually increased the voltage.

No.  The monitoring is merely incorrect.  I don't trust BIOS monitoring
to be accurate.

> The cost of shipping the PS back under warranty was more then the PS
> was worth. 

Unfortunately that's often the case.  The hassles of dealing with
warrantees means that many will never make a claim (it's too expensive,
privacy and confidentiality issues for storage devices, you're left
without something you need for a long time, etc).

-- 
(This computer runs FC7, my others run FC4, FC5 & FC6, in case that's
 important to the thread.)

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