Computer Power Supplies and the amount of wires.

[Bskitter]

Hello Everybody.

I would like to know why copmuter power supplies go so heavy on the 12v, 5v and Ground wires.

For instance. the 24 pin mother board connector.



8 of those wires are just for ground.
5 of them are just for 5 volts.
4 of them are for 3.3 volts
2 of them are for 12 volts

Why are there no power supplies designed where its just 1 thick black wire all the way to the motherboard from the power supply and then at the 24 pin connector it branches out to the different pins.
And the same thing goes for the 12,5 and 3.3 volt rails.

[Monkeh]

Because that's hugely inflexible and exceedingly hard to terminate?

[mdszy]

Have you ever built a computer? There's a lot of cable routing/management you have to worry about. One huge thick cable would be impossible to route. Multiple smaller cables is much easier to make a connector for and can carry high currents without sacrificing flexibility.

[fourtytwo42]

So many reasons, also helps reduce localized high current densities in the terminating pcbs

[Bskitter]

So many reasons, also helps reduce localized high current densities in the terminating pcbs

I see, could this be explained further ?



Above is basically what I want to do instead I will use much thicker wire (10 AWG)

Byron

[debininja]

I see, could this be explained further ?

You don't want point contacts where high current wires converge. You know stick welding? Those electrodes have over a hundred amps (multiple hundreds?) flowing through them into a teensy point that, when shorted to the metal you want to weld, heats up, melts, and joins them together.

You want to spread the current out over a wide surface area, that way, there are no "hotspots" where the resistance will heat up and destroy the PCB. Even though PSUs don't do 100+Amps, the 12V rail for a typical 600Watt PSU can supply ~63Amps continuous, and, coupled with the bulk capacitors, if you suddenly hook it up to a load with discharged capacitors, you can very easily weld the connector to the input point.

[Nusa]

Every connection in a wire harness is additional cost and a potential point of failure for a manufacturer, so cost and QC are better when they minimize the number of connections. Using only one wire size in a harness also makes things easier.

But there's nothing stopping you from doing it for yourself. I think you'll find it's not worth the effort on a motherboard connector. It's not a very long wire run in the first place.

It makes a little more sense on those (graphics card?) connectors you picture. And the risks and QC are all on you.

[RGB255_0_0]

So many reasons, also helps reduce localized high current densities in the terminating pcbs

I see, could this be explained further ?



Above is basically what I want to do instead I will use much thicker wire (10 AWG)

Byron

What happens when you need to connect to 8-pin cards?

[mariush]

Hello Everybody.

I would like to know why copmuter power supplies go so heavy on the 12v, 5v and Ground wires.

For instance. the 24 pin mother board connector.



8 of those wires are just for ground.
5 of them are just for 5 volts.
4 of them are for 3.3 volts
2 of them are for 12 volts

Why are there no power supplies designed where its just 1 thick black wire all the way to the motherboard from the power supply and then at the 24 pin connector it branches out to the different pins.
And the same thing goes for the 12,5 and 3.3 volt rails.

The connector you see there is the "improved" version, the tiny 4 pin extension was added later in such a way as to preserve backwards compatibility.

When the first ATX power supplies were made, the processors and the memory was powered from 5v and 12v was used only for computer fans and motors in mechanical drives and optical drives. That's why you see only one 12v wire in the 20pin block.
The NC pin was in previous versions used for  -5v , which was present in the ISA slots and often used in sound cards for the audio amplifier on the boards (for example power opamps with -5v .. +12v ). Since a few versions of the ATX standard, the -5v is deprecated and can be missing. So one of the ground wires was for this -5v wire.
-12v is also on its way out and on most modern motherboards it wouldn't be a big deal if it's gone. It's pretty much used only for serial ports these days, and those are pretty much only as headers on most motherboards and people don't use them.
5v was also used a lot by PCI slots
3.3v I guess was used mostly by onboard stuff like onboard audio chips, extra sata controllers , firewire, onboard network card...   and some PCI slots are 3.3v and 5v (universal)

Some versions of AGP had 3.3v but I think by that time most motherboards used dc-dc converters to produce the agp voltages from 12v .. unless i'm very wrong, by the time of agp 8x they were already down to 1.25v or something like that.

The ATX connector itself has a rating of 9A per circuit and the contacts have a guaranteed resistance of 10mOhm .. however, the ATX standards drop the maximum current per circuit to something like 7-8A per circuit (ex 8A per circuit in the case of 4 pin cpu power connector and 7A per circuit for the EPS 8 pin cpu power connector)

So assuming 7A per circuit, the 20 pin block would have only 4 circuits x 7A = 28A on 5v and  3 circuits x 7A = 21A on 3.3v ... which when you think about was not much when mb+cpu+memory was powered from that connector..

Also keep in mind that while we see AWG16 or AWG18 wires used these days which can carry up to 16-20 amps easily per circuit with low resistance per meter (i think it's 21mOhm per meter for AWG18) , back then it wasn't uncommon to use AWG20 wires which have about 33 mOhm resistance per meter and are rated for around 10A per circuit - so it made sense to use multiple circuits to reduce the current per circuit and have less voltage drop on the individual wires.

The extra 4 pins in the 24 connector is a latter addition, which helps mostly with 12v .. as often the pci express slots are powered from those two 12v wires along with fan headers.

Note that recently some power supplies use one of each 5v and 3.3v pins as Vsense wires instead of carrying power to the motherboard (or in addition to carrying power, they hook an extra vsense wire to one of those pins for extra voltage stability)
 

[Bskitter]

Thank you for all the feedback gentlemen.

The reason I ask is because I am modifying an old Apple G5 to ATX and this case has a 1KW power supply, but it is obviously not a standard ATX PSU.

So I will be converting it to ATX.

Ill create a post once all the parts arrive.

Byron

[Nusa]

What do you plan to do about the 25V rail that the G5 supplies provided? You won't find that on an ATX.

Or did you mean you wanted to use the G5 case as a home for a modern ATX motherboard? That's been done many times over the years.

[ed_reardon]

I think it's quite an elegant thing really.

It's a widely adopted standard, and it allows people with zilch electrical knowledge to plug and play.  Sometimes they're the sort of people who bleat about how many "Watts they have" and when you push them ever-so-slightly further as to what they mean by "Watt" or what it does they do this -> 

Either way,  I don't think it's a particularly inelegant solution,  it makes building and maintaining PC's a lot easier.

[mariush]

I personally wish some companies would just team up and decide on a new standard. Intel has stated that they don't wish to change the standard and that if anyone wants to, they can come up with something on their own.

I wrote about this on another forum as well... My main beef is with the amount of space such connector takes up on ITX boards, especially when a 20-40w cpu on such motherboard is powered from a separate 4 pin cpu power connector.

Here's what I'd change:

* get rid of -5v and -12v  (no need for them, or if really needed motherboard can produce them locally)
* get rid of 3.3v completely (it would be more efficient to use a dc-dc converter on the motherboard close to the devices that need 3.3v instead of wasting power through cables). Only SATA has 3.3v wires external of the motherboard but no sata device uses 3.3v on PCs because people may use molex to sata adapters and then there's no 3.3v lines.

* get rid of POWER_ON and POWER_OK and replace them with TX and RX  ( some i2c or SMBUS or light USB / USB 1.1 - basically some 1-10mbps communication between power supply and motherboard)

The chipset or whatever can query the power supply and figure out the wattage, get some stats about number of connectors, maybe get live fan rpm speed of the fan , maybe get voltages and current usage on various connectors through this bi-directional link.  Also, users could override fan profiles on power supply (but not below some hardcoded preset for safety reasons, i mean user shouldn't be able to kill the psu by disabling the fan)
The link would also be good for software on computer to talk to the power supply for example to tell it to change rgb leds or crap like that ... just has to be some kind of protocol (thinking something like FTP or HTTP commands and return codes)

There would be either a 4 pin connector  [ 5v SB | TX | RX | GND ]  or maybe a 8 pin connector  where the bottom would be  [ 5v | 5v | GND/KEY | GND]
The bottom 5v line could be optional (the power supply could tell the motherboard if it can provide that or not)  - for example if the motherboard is powered from a 19v DC In connector and a chip on board produces the 5v SB from 19-20v in (see below). Some motherboards may be designed to rely only on that high voltage for everything and have their own dc-dc converters to produce all the other voltages.

If the extra 5v is present, Basically once the power supply is turned on, it can use a relay or something to turn off the 5vSB and connect the 5vSB  wire to 5v dc-dc converter inside the power supply.  3 pairs of 5v should be enough for around 15-30A of 5v (considering the amount of USB 3 ports on motherboards and backwards compatibility i don't think getting rid of 5v now would make sense)

* add 20v +/- 1v
** would make it possible to power small ITX motherboards directly from DC In jack (19v laptop adapter) OR by plugging a 4 pin cpu power cable near the DC In jack. The VRM would be optimized for 19-20v -> < ~1v conversion instead of having 12v ..19v input range.  If powered from a DC In jack, some tiny chip could be powered from DC In jack to "talk" to the chipset and pretend it's a regular power supply and provide 5v SB for the chipset and whatever else is needed.

Make 4 pin connector and 8 pin connector for cpu use 20v ... a single 4 pin connector would now be able to carry enough power even for a 95w tdp cpu and would be enough for itx and matx boards. If you need more just have 2 x 4 pin connectors close together or as one 8 pin connector.
20v would also be useful for  usb power delivery which can do up to 20v and 5a if i remember correctly .  There's also quickcharge 3  charge scheme from mobiles ( where charger starts with 5v and can negociate with phone to rise voltage up to 20v in 0.2v steps, up to 18w )
Motherboard can use 20v in - 3.3v out dc-dc converter to produce 3.3v as needed with very high efficiency

Could even switch from using 12v fans to using 24v fans and then they'll work just fine from 19-20v directly (dc in jack or power supply)


* keep 12v for backwards compatibility at least for a few years.  But instead of the big atx connector just have an 6-8 pin pci-e connector going to the motherboard (for fans and the pci-e slots)

12v would also be needed for molex and sata connectors ... but really i'm thinking it would make more sense to invent some new connector and simply send 20v to various devices ... nowadays some hard drives use 5v for the motors so the 12v wire is wasted , ssd drives can also peak at around 10w from 3.3v-5v during writes and 3.3v wire in sata is not used... just have each hard drive use dc-dc converters on it to produce the voltages they want from 20v.  a dc-dc converter at the volume of hard drives and ssds manufactured would be less than 1$ for each device.

For backwards compatibility they could make  new connector to 12v/5v dc-dc converters

Video cards could be redesigned to have the VRM on them capable of tolerating 20v , later designs could have separate connectors (one for the 20v and one for 12v) and eventually just have them use 20v only.

So basically optional 4-8 pin connector for 5v + 5vSB + communication with psu  (can be unused on motherboards with 19v DC In Jack) ,  4-8 pin for 20v CPU , and 6-8 pin pci-e for 12v (fans, pci-e slots etc)

backwards compatibility would be fairly easy .. just make a tiny adapter board in which you plug the 4-8 pin 5v +data , a pci-e 6-8pin and a basic micro on the adapter board can be hardcoded to talk to the psu and "create" the power_ok and ps_on wires for the atx connector. if needed a voltage inverter can also create the -12v voltage directly on the adapter .. and a dc-dc converter could be used to create 3.3v from 12v

[tooki]

Given that USB Power Delivery is moving us slowly towards higher voltages, it would really make sense to move toward a higher base voltage and rely more on local regulation via DC-DC converters. (Unless I'm mistaken, we're already doing local regulation damned near everywhere in a PC anyway.)

That said, unlike with mains wiring and charger cables, I don't think the length, weight, or cost of power cables in a PC is a particularly large factor.

[Bskitter]

What do you plan to do about the 25V rail that the G5 supplies provided? You won't find that on an ATX.

Or did you mean you wanted to use the G5 case as a home for a modern ATX motherboard? That's been done many times over the years.

Yes the case will be converted to ATX shortly but the original PSU fits perfectly in the case and is powerful and just needs to be converted to ATX Molex connectors.



Here are the outputs of the power supply. You see the thick Black and Grey wires on the left ? Those are 10 AWG wires that fed the CPU's directly.



Here are the exact ratings of the power supply I will be using.

My Plan is

12.0V MPU1 will go to the graphics card (GTX 1060)

12.0V MPU2 will go to the CPU header on the mother board.

12.0Va, 5.1V and 3.3V will go to the motherboard connector

12.0VB and 5.1V will go to DVD Room and Hard Drives.

POWER_ON, STANDY BY and 5V STAND BY will also be connected.

The only thing that will be missing is -12V.

[tooki]

Those cases and PSUs are lovely beasts. (As a student tech, I once set up two entire labs of then-brand-new first-gen G5's for the art department... What a sight! I used the FireWire Target Disk Mode to clone the installs from one to another, and since OS X isn't running in that mode, the OS-controlled fan management isn't enabled, so the 9 fans in those things all ran at full blast, instead of the roughly 10% they ran at in normal use. That literally had the drapes in the room swinging back and forth!)

Pity a G5 is useless as a computer these days. Glad to see the lovely cases and PSUs being upcycled!