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Old 03-31-2008
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Stefan555 Stefan555 is offline
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If we take Be-quiet Straightpower 550W as an example. It has the following spec:

+3.3V +5V +12V1 +12V2 +12V3 +12V4 -12V +5vsb
32A 30A 18A 18A 18A 18A 0,5A 3A

Total comibined output on +12V: 41A (492W) = The PSU's maximum capabiliy on +12V
On each rail on +12V we are allowed to draw 18A according to the spec. A theoretical maximum output of 4*18A = 72A (864W). But 864W is more than the PSU's total output of the psu (550W) and even more than the psu's capability on +12V, 41A (492W).

4*18A = 72A only tells us we are allowed to draw maximum of 18A on each of the four rails on +12V, but not on all of the rails at the same time. The maximum we can draw from +12V at the same time is 41A (492W).

The gap between the theoretical output on +12V of 72A (864W) and the capability of 41A (492W), is the headroom the spec gives us to use "unused amps" on one rail on +12V, on another rail on +12V.

In reality, the gap is bigger, all psu's with a specified maximum output of 18A on the rails, have an OCP which will shut the psu off if the draw from one of the rails on +12V exceeds 20, 22 or even 25A.

The OCP's are always programmed to shut the psu off at a higher current than specified on the label, no matter what is printed on the label.
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