What does is a "dual 12volt" power supply?

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What does is a "dual 12volt" power supply?

Postby Johnny Mnemonic » Mon Apr 10, 2006 8:38 pm

What does it mean when I see a power supply advertised as Dual +12V: Yes?
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Postby profofpcs » Mon Apr 10, 2006 8:58 pm

in most cases there is 2 separate 12v rails. each getting power from a separate circuit. some cases its just a fake separation, as the liberty i use is. but its still touted as dual 12v rails.

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Postby jonnyGURU » Tue Apr 11, 2006 7:21 am


In all fairness, I wouldn't say "most" dual 12V rail power supplies truly have seperate regulation between the two 12V rails and I wouldn't say that those that have both 12V rails on the same plane are "fake."

For any power supply to have truly independent rails, the AC must be rectified to DC by literally twice as many components than you would find in your "typical" power supply. If you look inside your "typical" power supply and a "high end" power supply, you'll find there really isn't twice as many components... just larger components.

This is where people typically say the voltages are on the same "plane."

What typically happens is the 3.3V and 5V are rectified onto one plane (which is why you almost always have 3.3V + 5V maximum combined wattage ratings and why they're never additive) and the 12V rail or rails will be rectified onto another plane.

On each of these planes, the voltage is regulated to their specific "rails" by transistors.

By definition, when someone says the dual 12V rails are "fake" I naturally think that there is no independent regulation of the 12V rails. This isn't really true. The rails are on the same plane, but they are independently regulated.

If you look at almost any single, dual, tri or quad rail power supply, you'll find that there's a maximum combined 12V rail. Typically, this number is NOT equal to the sum of the two 12V rails. This is because the rectification of the DC output is limited to less than what the two transistors can actually regulate. You might have two FET's that can easily handle 240W each, but if the maximum wattage the components that are rectifying the DC output to those FET's is only 400W, then you certainly can not load both rails up and put a 480W load on your combined 12V rails.

And for the record, hardly any power supply that doesn't happen to list total combined wattage has independent planes. They just choose not to share the information and are well within their right since they are providing per rail specifications and total maximum output. This is all that is outlined in the ATX12V specification.

When you see a power supply with truly independent regulation, it's rarely independent regulation for each of the 12V rails. It's usually that the 12V rails are independent from the 3.3V and 5V rails, which means the power supply can be crossloaded with lopsided loads and your voltage won't go hay wire.

So even a PC Power and Cooling 1Kw PSU has "fake" 12V rails. The 12VDC is rectified with a limit of 66A (70A peak,) yet each 12V rail has regulation that can independently handle 16A, 16A and 36A by themselves.
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Postby Karlsweldt » Tue Apr 11, 2006 8:45 am

Regardless of the capacity of the power source, any device that is added to it or removed, such as motors starting/stopping, will cause a surge or "ripples" in the voltage level. This can be problematic to sensitive circuits. The separation of the +12 volt sources is intended that one rail provide power to those devices that are not sensitive to surges, while the other is more stable.
This is not a new idea.. many older PSUs had up to 3 separate +12 volt soruces, from the same core.. each regulated in its own field. Same too with some +5 volt sources. Some older large PSUs had separate core transformers that supplied separate +5 and +12 sources. Typically, many server designs emply one PSU for the mainboard, and another for the drives. The one for the drives will normally have only the +5 and +12 sources.. up to 38 amps on the +12 volt source! I found one with 50 amps on the +5 volt rail.. nice 'welder'?
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