Understanding Multiple 12V Rails - Help?

[nheather]

I am frequently reminded how important it is to have a power supply that can deliver a lot of stable clean 12V.

I'm also aware that it isn't as simple as adding the current available on the 12V rails.

For example my PSU has three 12V rails rated at 18A, 18A and 16A (52A in theory). But there is a caveat that you can only draw a total of 34A at 12V.

My question is if your PSU has dual 12V rails and has a bundle of wires rather than being modular how can you tell whether you are connecting to 12V1 or 12V2?

Specifically I'm having trouble with a HD4890 on my son's computer. It keeps crashing even at settings at which the previous 1800XT could manage - I suspect a power problem.

It is a TRUST PSU with rails as follows

3.3 - 32A
5V - 30A
12V1 - 12A
12V2 - 16A

Spec limits 3.3 and 5 combined to 170W but no limit on 12V (so assume we can get 28A).

My concern is that it only has one PCIe, so I bought a hard disk power (old style big) to PCIe adaptor.

It's quite likely that the PSU just isn't upto it but one thought was

how do I know if I am using both 12V rails or just trying to take everything off one rail?

Any ideas before I buy a new PSU.

Cheers,

Nigel

 

[EsaT]

It is a TRUST PSU with rails as follows
3.3 - 32A
5V - 30A
12V1 - 12A
12V2 - 16A

Spec limits 3.3 and 5 combined to 170W but no limit on 12V (so assume we can get 28A).

And I'll call it as Never trust-PSU with probably 20A 12V at most considering whole thing stinks like some ancient design p.o.s. (+commonly made from substandard components)

 

[nheather]

I tend to agree with you. It's done fine upto now but the load has been well below it's stated spec.

Can anyone tell me what dual 12V rails actually means.

Does it mean that there are two (or three in some cases) seperate 12V supplies and the connectors are shared between them.

Or does it mean there are 2 (or 3) sepearte 12V supplies which are parallelled into a single suplly and all the connectors are conneced to that single supply.

If the former, how do you know that you are not connecting all your stuff onto just one of the rails?

Cheers,

Nigel

 

[EsaT]

Can anyone tell me what dual 12V rails actually means.

It means this:
http://tiny.cc/d2P8b

They are only artificial current limiters and rails are as separate as wall sockets behind different fuses are "different power lines".
Biggest difference is just that in case of your house exceeding one limit doesn't shutdown your whole house like it does in PSU.

 

[nheather]

Okay sort of following you but still a little puzzled.

In my PC there is a decent 550W which claims to have 3 12V rails

12V1 - 18A
12V2 - 18A
12V3 - 16A

But does caveat that with a 34A maximum.

So using your analogy, if they are sat on the same ring circuit like the wall sockets where do the three different limits come in?

Is it just that there are three load sharing 12V supplies all feeding the same 12V line.

Cheers,

Nigel

 

[EsaT]

So using your analogy, if they are sat on the same ring circuit like the wall sockets where do the three different limits come in?

Count how many fuses there are in fuse box and you'll get number of "power lines coming to your house" according to logic of BS (aka marketing) departments.

 

[Sasahara]

ATX Spec (though not all PSUs totally follow ATX Spec these days) requires the 12V 4 pin connector to be a seperate line from the rest so that's probably the 12A line, I'm pretty sure that's only powering the CPU.

So that would leave 16A for everything else, which certainly isn't enough.