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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Redundant power supplies
Date: Mon, 28 Oct 2024 16:11:40 -0700
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On 10/28/2024 3:22 PM, Lasse Langwadt wrote:
> On 10/27/24 07:54, Don Y wrote:
>> Most of my boxes have dual power supplies.  Most
>> don't give me an option as to how they are used/configured;
>> one shits the bed, the other is there to cover the load.
>>
>> I have always *assumed* they were configured to SHARE the load.
>>
>> I picked up another box that gives me the option of NOT operating
>> them redundantly (what the hell does the "extra" one do, just
>> sit around??).  And, when in the redundant configuration,
>> allows me to choose which is the "primary".
>>
>> This suggests one is carrying the load and the other is switched
>> in (even if passively) when that one fails.
>>
>> Is there any advantage to this over a "sharing" configuration?
> 
> I would guess if you wanted to power one of them from a "preferred" powersource 
> and the other from a another source that you don't want
> to load unless you have to

Then why not say "Connect PS#1 to your preferred source of power,
if you have one"?  I.e., why add a setting AND hardware that
allows the system to "pick" one over the other?

The downside would be that access to the PDU might not be convenient,
especially for the guy running the farm (its no fun walking down the
hot aisle!)

>> And, why would I ever want to *disable* PFC?
> 
> when you have an UPS that doesn't have sinewave output, they don't always play 
> nicely with PFC

Possibly.  The reason I have been given has to do with energy efficiency
(even at low loads -- like "sleep" power).