[geeks] power supply question...

[Scott Howard]

On Sat, Feb 22, 2003 at 11:33:35AM -0800, Lionel Peterson wrote:
> I was wondering, if I have a box with, say, a redundant power supply
> "complex" of two 450 watt power supplies, and the system only draws,
> say, 300 watts in operation - how much total power is drawn by the
> system:

f) The total will be around 350-400 watts across both, with the extra
above what the system is drawing being for losses in the power supplies.

> e) depends on the design...

it will depend on the design as to how much is drawn by each supply. Some
setups will draw the above 400 watts as 200/supply, others will draw it
as 350 watts from one (the "active") and 50 watts from the other (the
"standby").

But when it comes to provisioning both of the circuits (they are on
different circuits, aren't they?) keep in mind that if you loose one
circuit the other will jump to full. I saw one recently where a customer
lost one of the two power boards to the rack. All the redundant supplies
then started drawing full current from the other side (rather than the
normal half) and it overloaded and tripped that one too! ie, you need
to provision based on both power supplies drawing 450 watts.

  Scott