Power Supply missing the -5v source voltage

[MrAl]

Hi there,


Recently i updated the MoBo for my system and the power supply too, and i noticed that
the power supplies these days dont come with a minus 5v supply anymore. That's ok
with me, but interestingly, the hardward monitor is still showing a minus 5v (-5v) supply.
There is a -12v supply still on the power supply, so i was wondering if the newer mobo's
generate their own -5v supply now.
Anyone know anything about this?

[reloadron]

The minus 5 volt requirement was dropped from the ATX PSU Design Guide starting back with around Version 1.2 January 92. Nothing in the system uses it anymore that I know of. I expect to see the -12 volt requirement go away soon too. I am not sure though. On a standard PCI slot B1 is a -12 Volt pin, however on the newer PCI-E slots the -12 Volts is gone.

The only use for the -12 volts I cand think of is maybe custom cards used for data acquisition made by companies like HP and Natl Instruments. Likely even they no longer use it.

Software used to measure the PSU bus often has problems with the non existent -5 volts and also the -12 volts so weird readings are normal.

Ron

[MrAl]

Hi Ron,

Wow that long ago and i didnt notice it until now? Hmmm.
I guess i had older power supplies up until now as they all
had minus 5v rails until this last newest one.

I think i also noticed now that the +12v rails when there is more than
one are independent rails, being regulated separately. That means that
two 20 amp rails can not run a 30 amp device, it would have to have at
least one 30 amp rail to do that.
Any ideas here?

[reloadron]

Hi Ya MrAl

Home computer power supplies are a funny lot. I don't think there are any actual rules that govern their design and build. I frequently reference the ATX12V Power Supply Design Guide but in reality it is just what the name implies, it's a guide with nothing really binding. Anyway as to the 12 Volt rails or 12 Volt power distribution.

Somewhere buried in an IEC Specification (Maybe IEC 60950?) I think there was a statement t the effect the maximum power on any PSU rail could not exceed 240VA (12V X 20A = 240VA) and therefore most power supplies were limited to 20 Amps on any given 12 Volt rail. The idea being that these rails had over current protection to shut down the PSU when a threshold was exceeded. This seemed to fly along well until graphics card manufacturers were building cards requiring a small power sub station to power. Cards that exceeded the overall power requirments of the entire computer.

There was also concerns in the beginning that newer processors were demanding more power and the processor power was being derived from the 12 Volt power. At this point the design guide made a provision for an additional 12 Volt rail to power the processor and we now had a 12V1 and a 12V2 rail. The 12V2 rail being dedicated to the CPU. Then the 12V1 rail could support everything else and neither rail would ever exceed 20 Amps.

You are absolutely correct in that if I have two (or more) regulated 12 Volt rails each capable of let's say 18 Amps I cannot combine them and get 36 Amps. If the PSU is designed correctly. This is true especially if each 12 Volt rail has individual regulation. Not to even mention current limiting. Interesting is when you repaired your PSU and had it open you likely saw two transformers in there. One small and one larger one. The smaller one is always on anytime the PSU has power connected and the rear switch is on. That little guy is for the 5 Volt standby power. The big guy is literally for everything else with multiple secondary taps for all the voltages. Those taps are followed by full wave bridge rectification and then things fan out. Originally there was a 3.3 Volt sense line and all regulation was a function of the 3.3 Volt rail but much of that has changed.

Then things changed I think more driven by marketing but the 240VA thing went away as in poof it was gone! Many 12 Volt ATX Form Factor manufacturers went to hyping a single (monster) 12 Volt rail. Thus began arguments of the pro and con of these creatures.

The following links make for some reading:

The 12 Volt PSU Design Guide

Interesting commentary of single / multiple rails.

A PC Power & Cooling Commentary.

Silverstone's little comments.

There is enough there to make ones head hurt.

Ron

[dyarker]

Question

CPUs run on 5V, or less, why is a 12V supply dedicated to CPU?

[MrAl]

Hi,


The CPU VCore runs on around 1 to 1.30v. The MoBo takes 12v and converts it into
lower voltages for the cores of the CPU. The MoBo has onboard converters just
for the CPU, one for each core.

[dyarker]

Okay, when those guys in linked pages said CPU they meant MoBo because CPU fan and video board would take 12V off MoBo bus "as-is". ??Other 12V bus from power supply would be for hard drive, DVD drive and case fans, right? Seemed to me MoBos used to convert 5V supply to 3.3V or 1.8V for CPU. Guess the power demands of newest CPUs got too big for the amount of current the 5V lines could handle.

Thanks,

[MrAl]

Hi again,


The way i understand it is that they take a lesson from high tension wires. The higher the
voltage the smaller diameter the wire can be.
The CPU requires a certain power, at a low voltage like 1.3 volts, but lets call it 1.2v because
some CPU's can power down to 1v when idle (maybe less i dont know). Ok, so if the CPU
needs 120 watts at 1.2v that means it needs 100 amps at 1.2v. Now what diameter wire
would be required for 100 amps? For a 2 foot run probably 6 gauge wire! Imagine trying
to deal with that We didnt even mention the kind of connector that would be
required for that much current. It would be huge.
Now take that same 120 watts and work it at 12v, and the requirement drops by 90 percent,
to about 10 amps (not including the inefficiency of the soon to be used converter). That's
a wire gauge about number 16 AWG. What a huge difference. Ok, so run two 16 gauge
wires instead of 1 to keep the voltage drop even lower. Now all we have to do is run
two 16 gauge wires from the power supply to the mother board and let the on board
12v to 1.2v converters do their job (1 for each core in the CPU). Nice huh?

Oh yeah, another thing we didnt mention yet is the voltage regulation, and the accuracy
of the voltage regulation that actually gets to the CPU core. If the regulator is on the
mobo, it is closer to the CPU and so the voltage will be more accurate, and also that
gives the mobo manu the ability to control the accuracy of the voltage and also the
absolute level. If the mobo wants to power down to 1.00v instead of 1.2v, it can do
that by telling the on board regulars to do that. If the regulators where in the PS
unit off card, it would be much harder, and a separate set of sense wires and/or
control wires would be necessary too.
Since the mobo manu has control over the voltage now, they can set it to match
the CPU as per whatever is plugged into the socket too. That's a lot more
flexible than having it all done in the PS.

Perhaps Ron can chime in a little here too as he has a lot of experience with the
computer power supplies i think.

[reloadron]

This is going to be mostly speculation on my part.

Looking at motherboard main power connectors (home computer systems) going back to the early 80s the first ones used a pair of 6 pin connectors. There were four pins dedicated to 5 Volts, a single -5 and -12 Volt, a single 12 Volt, four commons and a single power good pin. Both connectors were used side by side to power the motherboard. There was no 3.3 volts at the time. Each connector pin was rated at 5 Amps and the wire was generally AWG 18. Some configurations only used three 5 Volt lines with a key. Anyway there were between three and four 5 Volt lines.

Early processors like the Intel family 8086, 8088, 80286, 80386 (DX), 80486 (SX) and 486 (SX2), all ran with a supply voltage and core voltage of 5 Volts. We can see from the above that there was a focus on 5 Volts since there were three or four 5 Volt pins capable of delivering 5 Amps (a total of 15 to 20 Amps, 75 to 100 Watts).

Eventually as processors evolved and proportionally their demand for power increased they did add a few axillary power connectors to meet demand for power. Many processors required 3.3 Volts and had a 3.3 Volt core voltage. I believe at this point the plan was to continue to directly power the CPU from the system PSU. We were seeing power supplies deliver more and more 3.3 Volt power. I have an old Codegen (generic) PSU here in my bone yard with a single 12 Volt rail rated at 12 Amps but a 3.3 Volt rail rated at 20 Amps and a 5 Volt rail rated at 40 Amps. They were placing tremendous emphasis on the 3.3 and 5 Volt power.

Jumping ahead a few light years we got to processors that could control themselves and adjust their power requirements based on their workload. Somewhere along the way on board (motherboard) CPU Vreg (CPU Voltage Regulation) came along using the 12 Volt rail of the PSU. This became a problem as there was only so much 12 Volt power going to the motherboard. Around 1996 the 20 pin motherboard connector came into play.

In 1996 PC makers started switching to the ATX standard which defined a new 20 pin motherboard power connector. It includes a 3.3 volt rail which is used to power newer chips which require a lower voltage than 5 volts. It also has a standby 5 volt rail which is always on even when the power supply is turned off to provide standby power to the motherboard when the machine is sleeping. The new connector also allows the motherboard to turn the power supply on and off rather than depend on the user to flip a power switch. This connector is polarized so it can only be plugged in pointing in the correct direction.

However, the 20 pin connector only allocated a single (pin 10) 12 Volt line. The emphasis was still on 3.3 and 5 Volt power. However, they finally added a 12 Volt aux connector to provide power directly to the CPU. Initially this was a 4 pin connector with two 12 Volt lines and two commons. The four pin ATX power cable. This later grew to an eight pin cable with four 12 Volt lines and four commons called the eight pin EPS.

Additionally the 20 pin main motherboard connector was expanded to twenty-four pins. There was an addition of a 3.3, 5.0 and 12.0 Volt line plus a common to support the new PCI-E slots.

Suddenly there was a big demand for 12 Volt power to the motherboard. Newer power supplies delivered more and more 12 Volt power. Everything was about 12 Volt demand.

This link is a good example of a now obsolete Vreg used to support P4 Intel processors. It requires both 5 Volts and 12 Volts.

<Edit> I see while I was typing and drinking coffee MrAl was busy posting. </EDIT>

Ron

[MrAl]

Hi Ron,


I was drinking coffee too

[reloadron]

Hi Ya MrAl

A good point I have seen you make numerous times is when considering how to get to a low voltage from a higher voltage, the higher the voltage we can start with the lower the current draw. Considering what the wire and connectors are rated for I think that is why we beat up 12 Volts pretty well.

Ron

[MrAl]

Hi Ron,

Yeah it's interesting how that works when there is true power conversion involved.
Pretty soon we'll be pumping the CPU's 1000 core chip MoBo's with 120vdc ha ha.
The average home computer PS will look like four diodes and a capacitor