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separate 12v rails better than one?

M

mrscintilla

Senior member
I was wondering why 2 separate lower-Amps rails is better than 1 higher-Amp rails?
Say, enermax noisetaker, which has 2 18A +12V rails. Why is it better than a similar enermax with 1 33A +12v rail? I was puzzled because I though the load of cpu and motherboard will be going through a single rail. How can double rails help?

highly technical enough?
 
Two lower ampacity units will dissipate less heat; requiring less active cooling, resulting in less noise.
The noise from two small (or slower speed) fans combined does not sum linearly to equal the noise of one larger fan since decibels are logrithmic. So two small fans is quiter than one larger one; alse applicable is the two lower ampacity supplies will require less cooling because the heat distribution is spread out over a larger area with more power conversion components. This increases cooling efficiency and reduces demand on individual components.

 
no one said there would be 2 fans intsead of 1. I am talking about 2 separate +12v rails of one single PSU here. The analogy to noise is lost on me.. And your reasoning on the logarithm is alarmingly non sequitur.
 
You didn't give much information to go by. Your initial post; what did you mean by better? Quietness, reliability, efficiency, cost??? "Better" is a relative term you know.

Simply put, the two lower ampacity units will have lower heat dissapation requiring less active cooling.

There was no "analogy" to noise. For case or power supply cooling; two small fans is quieter than one large one. I assumed by "better" you meant quieter since the description of one of your items proclaimed "noisetaker". I attempted to explain why it would be quieter. The most common form of active cooling is using fans to push air across a heat transfer medium to gain efficiency. Since lower heat losses mean less active cooling, less air flow is required making the PSU seem quieter. Less air flow is achieved by using a fan that pushes less scfm of air.
 
I think another idea of seperate rails it to take the stress off of one. You dont want any single component stressed for long periods of time so split it up it not only dissipates less heat but effectively takes the stress off of one rail and splits it up.

-Kevin
 
I think it depends on application. I have a dual Xeon system which needs a lot of 12V juice from a single rail to supply all the power needs of the processors along with all the drives a server typically has. Multirail supplies tend to split the juice up leaving you with inadequate power to the processors and VRMs.
 
Originally posted by: mrscintilla
no one said there would be 2 fans intsead of 1. I am talking about 2 separate +12v rails of one single PSU here. The analogy to noise is lost on me.. And your reasoning on the logarithm is alarmingly non sequitur.
Click to expand...

His reasoning on the logarithm makes perfect sense. Since decibels work on a logarthmic scale, their values are not additive. If you have two 50 dB fans, their combined noise will be less than one 60 dB fan.
 
Originally posted by: KMurphy
Two lower ampacity units will dissipate less heat; requiring less active cooling, resulting in less noise.
The noise from two small (or slower speed) fans combined does not sum linearly to equal the noise of one larger fan since decibels are logrithmic. So two small fans is quiter than one larger one; alse applicable is the two lower ampacity supplies will require less cooling because the heat distribution is spread out over a larger area with more power conversion components. This increases cooling efficiency and reduces demand on individual components.
Click to expand...
Two small fans (aerodynamically in parallel, I suppose) with a combined flow rate equal to that of 1 larger fan will sound louder than that louder fan.
 
Hi, I'm interested in this as well & the best reason i've been able to find is the idea that separating the rails provides some isolation from "noise". One rail supplies the motherboard while the other is for the drives, fans etc. As the drives & fans spin up & down the fluctuating demand on the supply does not effect the juice to the M/B leaving it more stable. This may or may not be manufacturer hype but seems reasonable. It may also be a means of compensating for a less stable output to begin with?
 
None of the above, and it's very very doubtful you'll ever find a power supply that sources over 20A on any 12V rail.

There's a UL requirement that any power supply which can source more than 240W (for DC; I think the requirement's really 240VA, but for DC supplies, same thing) has to be in an unopenable cage with the big NO USER-SERVICEABLE PARTS INSIDE label slapped on... and, as I understand it, the unopenable part would apply to the entire computer case, since the power-supply cable leads outside the silver box...

Nobody wants that.
 
It gives two seprate circuits that are not affected by eachother. This is especially useful when you will be using bus powered firewire peripherals and hot-swap caged drives.

When you plug in a bus powered device or a hard drive into the bay, it will spin up causing a voltage sag for a split second in a manner similar to your lights dimming when you start a vacuum cleaner. This can lead to instability or crash. If CPU power is on a different bus(rail), its voltage is not affected by load change on the other bus(for mechanical drives).

 
Originally posted by: KalTorak
None of the above, and it's very very doubtful you'll ever find a power supply that sources over 20A on any 12V rail.

There's a UL requirement that any power supply which can source more than 240W (for DC; I think the requirement's really 240VA, but for DC supplies, same thing) has to be in an unopenable cage with the big NO USER-SERVICEABLE PARTS INSIDE label slapped on... and, as I understand it, the unopenable part would apply to the entire computer case, since the power-supply cable leads outside the silver box...

Nobody wants that.
Click to expand...

?????????????
 
Originally posted by: vanvock
Originally posted by: KalTorak
None of the above, and it's very very doubtful you'll ever find a power supply that sources over 20A on any 12V rail.

There's a UL requirement that any power supply which can source more than 240W (for DC; I think the requirement's really 240VA, but for DC supplies, same thing) has to be in an unopenable cage with the big NO USER-SERVICEABLE PARTS INSIDE label slapped on... and, as I understand it, the unopenable part would apply to the entire computer case, since the power-supply cable leads outside the silver box...

Nobody wants that.
Click to expand...

?????????????
Click to expand...

Hrm. I'm not sure how to answer that. 🙂

Seriously - there's a safety requirement that says any power supply with a rail that can source >240W has to be in an unopenable cage with warning stickers and stuff. And since the power supply cable comes out of its box, the "unopenable cage" would have to be the entire computer case. This would suck.

However, if you split the 12V supply into separate 12V rails which each source <240W, that requirement gets avoided.

Furthermore, it's not about the momentary sag on 12V rails as drives spin up. Modern voltage regulators have really good immunity to both load- and line-side voltage spikes, which is a fancy way of saying they can supply nearly constant voltage even if the input voltage changes, or the output current changes.
 
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