Basic information

[Islam Ghunym]

In an ATX 24 PSU oupot there is +12V, +5V, +3.3, +5VSB, -12V, -5, PS-ON and a couple of other sensor wires.

 

The +12V goes to SATA/PATA drives, CPU, GPU and?

+5V goes to?

3.3V?

+5VSB used for standby features and it is not important to exist?

-12V?

-5V?

[Dutch_Master]

The negative voltages are remnants of ancient PC's, which used TTL logic, amongst other things. Today, they're still used for RS232 ports (if your mainboard has a COM header on it, that's the one).

 

As for the 5VSB: try booting a PC without it. Good luck

 

The 3V3 line is a common voltage for modern logic circuits, so very much in demand. Likewise the 5V line.

 

PS: PATA does not exist, that interface is named IDE, the PATA name is a marketing trick invented during the IDE/SATA transition period.

 

PPS: you could have found pretty much all answers with a simple search online:

https://en.wikipedia.org/wiki/Power_supply_unit_(computer)

[Islam Ghunym]

26 minutes ago, Dutch_Master said:

The negative voltages are remnants of ancient PC's, which used TTL logic, amongst other things. Today, they're still used for RS232 ports (if your mainboard has a COM header on it, that's the one).

 

As for the 5VSB: try booting a PC without it. Good luck

 

The 3V3 line is a common voltage for modern logic circuits, so very much in demand. Likewise the 5V line.

 

PS: PATA does not exist, that interface is named IDE, the PATA name is a marketing trick invented during the IDE/SATA transition period.

 

PPS: you could have found pretty much all answers with a simple search online:

https://en.wikipedia.org/wiki/Power_supply_unit_(computer)

Thx for help, although this hovers around my questions to things I didn't ask about and didn't quite answer any of my questions with a full answer with some wrong info, but still appreciated

[mariushm]

The power supply standard is very old, I think from the times of 486 computers if I'm not mistaken.

Back then, the processors were made with older processes, which required them to use higher voltages...  some processors worked at 5v and later processors started to use 3.6v, then 3.3v then 2.8v and lower.  By the times of Super Socket 7 (amd k6-2) we were down to around 2.1v if my memory is correct.

 

This is the reason why there's so many 3.3v and 5v wires in the connector, the processor was powered directly from the 5v wires, maybe a small voltage regulator converted 5v to 3.6v or 3.3v or whatever the processor needed.

 

The atx connector only has 2 wires with 12v because 12v was only supposed to be used for mecanical hard drives and floppy drives (for the motors spinning the discs) and maybe for extension cards, for example for audio amplifiers on a sound card, or for a dial up modem.

If you look at the ISA slot pinout  or the PCI slot pinout you'll see that there's basically only one pin for 12v, while there's multiple pins for 5v and 3.3v

 

By the times of Pentium 4, processors started to consume quite a lot of power, let's say more than 50 watts, and that made it more difficult for processors to be powered from 5v because of the high current - the higher the current, the more losses you're gonna have in the wires between the power supply and the motherboard. At the same time, we had more smarter and better chips to convert 12v to the voltage processor needs with very good efficiency, and the price for these also went down, so the choice was made to add a 4 pin CPU connector to bring 12v to the circuit which powers the processor.

 

-5v was required for ISA slots and for some types of chips - a specific method of manufacturing chips made it necessary for chips to be supplied with positive and negative voltages so ram chips, cache chips, some "glue"  chips, even some processors required both negative and positive voltages.

By the time PCI slots were introduced, the current manufacturing techniques for chips (CMOS) become the standard and this process no longer requires negative voltages, so -5v was no longer needed. It's not present in the PCI slot pinout and in a later version of the ATX power supply specification, the -5v was marked as obsolete and then deprecated (no longer required, can be missing). That's why in some power supplies, there may be a wire missing from the 24pin connector, that's where the -5v wire used to be.

 

3.3v was used with PCI cards ... some were 3.3v, some were 5v, a few were universal (accepting either voltages and using voltage converters on the card)

 

3.3v also gained a bit more popularity with AGP video cards for a brief moment, until they upgraded the AGP to using 1.2v (but power hungry video cards added a molex connector to get 12v from power supply)

They also added 3.3v in the SATA connectors, so the SATA connectors had 3.3v , 5v and 12v.

BUT, it didn't catch on, because a lot of computers users didn't want to upgrade their power supplies and simply used molex to sata adapter cables which only had 5v and 12v, so hard drive manufacturers and optical drive manufacturers couldn't rely on the SATA connectors actually having 3.3v - so they simply added a voltage converter to convert 5v to whatever voltages they need on the circuit board.

 

Nowadays, 3.3v will be used a bit for onboard devices (audio chip, network chip, maybe extra usb controllers). Some motherboards may use 3.3v to power the ram sticks, converting it to 1.2v ..1.35v DDR4 needs, or 1.5...1.65v for DDR3 chips. BUT, I think most motherboard makers use 5v instead or 3.3v

 

M.2 connectors have only 3.3v but I don't think a lot of motherboard manufacturers connect these m.2 connectors directly to the power supply - the voltage is so low that there would be some voltage drop on the wires and in the traces on the motherboard from the 24pin connector to where the M.2 connectors are, so the M.2 devices wouldn't get 3.3v.  I think most motherboard manufacturers add a dc-dc converter circuit to convert 5v or 12v to 3.3v for M.2 devices.

 

5v stand-by is stand-by power . The chipset is powered from that voltage. The chipset monitors the power button, if you press it, it checks the power supply to see if it's ready to supply the other voltages and if so, it tells the power supply to fully turn on.

 

The chipset also let's you enable some things like wake on lan, or wake on keyboard press, or wake on other signals. For example, when fax modems were still common, you could configure an option in bios to have a pci slot powered while the pc is shut down, so if the fax modem in that slot received a fax call, the modem could tell the bios to turn on the PC so that software on the PC could receive incoming fax and save it to disk.

 

Or think of laptops for example... when you open the screen, a magnet in the lid releases a switch which tells the chipset you want the pc to start, so the bios wakes the laptop from sleep/standby

 

-12v is still used with serial ports, which were common in network switches, and still common in industrial machines. While your motherboard may no longer have the connector on the IO shield, it may still have the header where you could plug a ribbon cable with the serial port connector and have a functional serial port.

 

Your motherboard will most likely function perfectly fine without a -12v wire present, but the serial ports would not work.