On Sat, 8 Apr 2000, Edward Schernau wrote:
> Mike Bilow wrote:
> >
> > Drives use by far more instantaneous current to spin up then they
> need to
> > run in active idle mode. A big, modern IDE drive can consume
> about 2.0A
>
> [Excellent analysis trimmed]
Thanks!
> Nowadays I think the "danger" comes from two sources. First is
> poor PS quality, and the second is the increased power demands of
> other components.
This is true, but they stick labels on the supplies. If you can't believe
the label, you should not be buying from that vendor. (Then again, the
labels probably also say "No user serviceable parts inside.")
> Cheap PS' will not be able to hold the voltage constant under an
> instantaneous load. Even the best can't, but its a measure of
> how "stiff" the PS is if it can maintain a stable +5V line, under
> sudden load. Dips on this line (and others) can affect the entire
> system.
This is true, but all 3.5-inch and 5.25-inch drives use the 12V line to
spin up the motors. As a result, the loading on the 5V line is minimal
for a drive spin-up, essentially the same or less than for normal
operation. Virtual nothing in the machine except for hard drive motors
and head positioners uses the 12V line, so you can as a general rule
assume that all available 12V rated current is available to you. There
may be exceptions, as you may have something like a tape drive that does
an expensive power-on self-test routine, but ordinarily the hard drives
are the only things eating 12V. (Some CD-ROM drives, especially writers,
also use 12V. I have an Acer CDRW-8432 which even has a cooling fan!)
> Secondly, processors and video cards can suck power. I have 2
> overclocked Celerons running at 522 MHz, EACH with its own Alpha
> cooler + 60x20mm fan.
Do you also drive a Chevy Vega with dual overhead cams and supercharger?
> Where you _can_ run into trouble is with a PS loaded to a good
> percentage of its capacity, THEN hit with an instantaneous load.
> Suddenly your +5V line can bounce around, which can cause memory
> errors, CPU errors, etc.
Another offender here is motherboard manufacturers who leave out most of
the tantalum bypass capacitors indicated on the motherboard engineering
design specification. This makes the motherboard far less able to cope
with instantaneous power supply oddities.
-- Mike
