Okedokey
Well-Known Member
Hello Marcuz and ceewi1
Firstly, I would like to apologise to oscaryu1, the administrators of this forum for my conduct, not my most proud posts to date. I still standby the intent of what I said, however I will say it in PM, or at least more constructively next time.
Anyway, ceewi41 thank you for your intelligent approach. You are correct in many ways; however here is my concerns/thinking.
When a PSU has multiple 12v rails you combine them to get a rough idea of the total available amps on the 12v rail, but it's not always a real accurate indicator of the 12v amp rating nor its suitability for the given task (e.g. HD3870X2). Most multi rail PSUs are actually fed from a single 12V source and split into smaller rated rails. Using the Alienware 750w as an example: It has four 18A +12v rails and a 16A rail that are fed by a single rail. I know what you're thinking... (3x18A)+ (1x16A) =70A. Well it doesn't quite work that way.
To determine the actual amp rating of the 12v rail you have to divide the total wattage available for the combined 12v rails (which is why I asked this question) by 12V. (Watts / Volts=Amps). But that’s not all that concerns me. The following is key imho.
Now the problem with multiple-rail PSU's (thus my suggestions to get a powerful single rail - and why most manufactures are moving away from them) is this. A large, single 12V rail (without a 240VA limit) can transfer 100% of the 12V output from the PSU to the computer, while a multi-rail 12V design has distribution losses of up to 30% of the power supply's rating. Those losses occur because power literally gets "trapped" on under-utilised rails. For example, if the 12V rail that powers the CPU is rated for 18A and the CPU only uses 7A, the remaining 11A is unusable, since it is isolated from the rest of the system.
Since the maximum current from any one 12V rail of a multiple-rail PSU is limited to 20A (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. With power requirements for multiple processors and graphics cards continuing to grow, the multiple-rail design, with its 240VA limit per rail, is basically obsolete. I guess my point here is that although this is not about over-current shut-downs, that PSU is bad quality imho, has low powered 12V rails and (at least anecdotally) the 12V rail seems to droop (I agree with your view about software monitoring of 12V not being accurate). This can (as you know) stress components causing excessive heat production.
Marcuz as I said in post 31, I think this is what you will need to do, test the PSU under load with a multimeter (cheap to buy), or borrow another quality PSU. I do feel that your issues will be resolved by getting a new quality (preferably single rail) PSU. I cannot be sure, because we need better information (i.e. measured voltages during load e.g. 3DMark06). I would suggest borrowing one from a friend if possible.
Firstly, I would like to apologise to oscaryu1, the administrators of this forum for my conduct, not my most proud posts to date. I still standby the intent of what I said, however I will say it in PM, or at least more constructively next time.
Anyway, ceewi41 thank you for your intelligent approach. You are correct in many ways; however here is my concerns/thinking.
When a PSU has multiple 12v rails you combine them to get a rough idea of the total available amps on the 12v rail, but it's not always a real accurate indicator of the 12v amp rating nor its suitability for the given task (e.g. HD3870X2). Most multi rail PSUs are actually fed from a single 12V source and split into smaller rated rails. Using the Alienware 750w as an example: It has four 18A +12v rails and a 16A rail that are fed by a single rail. I know what you're thinking... (3x18A)+ (1x16A) =70A. Well it doesn't quite work that way.
To determine the actual amp rating of the 12v rail you have to divide the total wattage available for the combined 12v rails (which is why I asked this question) by 12V. (Watts / Volts=Amps). But that’s not all that concerns me. The following is key imho.
Now the problem with multiple-rail PSU's (thus my suggestions to get a powerful single rail - and why most manufactures are moving away from them) is this. A large, single 12V rail (without a 240VA limit) can transfer 100% of the 12V output from the PSU to the computer, while a multi-rail 12V design has distribution losses of up to 30% of the power supply's rating. Those losses occur because power literally gets "trapped" on under-utilised rails. For example, if the 12V rail that powers the CPU is rated for 18A and the CPU only uses 7A, the remaining 11A is unusable, since it is isolated from the rest of the system.
Since the maximum current from any one 12V rail of a multiple-rail PSU is limited to 20A (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. With power requirements for multiple processors and graphics cards continuing to grow, the multiple-rail design, with its 240VA limit per rail, is basically obsolete. I guess my point here is that although this is not about over-current shut-downs, that PSU is bad quality imho, has low powered 12V rails and (at least anecdotally) the 12V rail seems to droop (I agree with your view about software monitoring of 12V not being accurate). This can (as you know) stress components causing excessive heat production.
Marcuz as I said in post 31, I think this is what you will need to do, test the PSU under load with a multimeter (cheap to buy), or borrow another quality PSU. I do feel that your issues will be resolved by getting a new quality (preferably single rail) PSU. I cannot be sure, because we need better information (i.e. measured voltages during load e.g. 3DMark06). I would suggest borrowing one from a friend if possible.
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