ATX Motherboard 24-PIN inner-workings - Page 1
Forum section: electronics / projects

author: IamSynthetiC
author_rank: Regular Contributor
timestamp: October 02, 2020, 12:36:10 pm
content: Greetings,I have been repairing enough PC motherboards lately that i got tired of having a huge PS power supply on my bench, filling my already limited space with unused cables, whenever i want to test the power rails.I got the idea of making a power supply PCB only for the motherboard, that would save me space and also (provided i design it correctly) perform some measurements on the power output. While working on it a came accross some issues that i could find an answer to, therefore i came to the expertsThere are several pins from each rail going to the motherboard. Should the pins of each rail be connected together? I performed a continuity test on a (broken) motherboard, and the meter "beeped" for a moment but then stoped as the resistanse rose to ~1+kΩ. Capacitor ? Why ?Also i couldnt find any info on what to do with the PG and PS_ON pins. I know the PS_ON turns on the PSU but does the motherboard itself turn it on or do I need to do something?And the PG gives a "power good" pulse, but what for? what are its characteristics and how do i interpet them?And finaly, how much current does a motherboard need to start up, could i use some 18650 batteries for that?Thanks for your time*Bellow, i pinned pictures of an ATX-24 pinout, and a 3D model of my (prototype) design, to give an idea

author: george.b
author_rank: Frequent Contributor
timestamp: October 02, 2020, 01:43:04 pm
content: ATX Specification, version 2.2:https://cdn.instructables.com/ORIG/FS8/5ILB/GU59Z1AT/FS85ILBGU59Z1AT.pdfPower Supply Design Guide for Desktop Platform Form Factors:https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide.pdfAs for your questions:Quote from: IamSynthetiC on October 02, 2020, 12:36:10 pmYes, the pins belong to the same rail, except for 12V1 and 12V2 (the latter is found on the auxiliary power connector). Why they weren't tied together on your motherboard? I don't know.QuoteOne usually has to push a button in order to power the machine on, yes?When you push it, the motherboard - the part of it that is powered by the 5VSB - drives PS_ON# low, and it remains in that state until it's time to shut the PSU down. Things like WOL, that run off the 5VSB, may cause this signal to be asserted "on its own".PWR_OK comes up (asserted by the PSU) to signal that the rails have come up to spec and it's okay to power the machine on. This is to prevent attempting to power up in an undervoltage condition, before the rails have come up.QuoteThat depends entirely on what kind of system you're running. Do the relevant math. The 5VSB should supply around half an amp, but once PS_ON# is asserted, you'll power everything up, including the processor. Unless you're powering the motherboard up without processor...?

author: janoc
author_rank: Super Contributor
timestamp: October 02, 2020, 03:11:28 pm
content: Quote from: IamSynthetiC on October 02, 2020, 12:36:10 pmThe Power Good signal was traditionally used to release the motherboard/computer from reset once the power supply has stabilized/passed self-tests. The same signal is also specified to go back low at least 1ms before the power rails go out of spec when the AC power is lost.The idea is to put he machine into "safe" state (typically reset) so that data or some hardware isn't damaged due to the processor or something else being in an undefined state when the power rails are out of spec.This is all documented in the ATX spec.

author: magic
author_rank: Super Contributor
timestamp: October 02, 2020, 03:48:52 pm
content: Quote from: IamSynthetiC on October 02, 2020, 12:36:10 pmTake a small 200~300W PSU (there are some with only 10~12cm length or so), take it apart, remove all cables you don't need.Or try Pico PSU and a beefy 12V power brick feeding it. Your only power hog to worry about will be CPU, rest of a motherboard gets by with just a few amps per rail (in any remotely modern machine).

author: rdl
author_rank: Super Contributor
timestamp: October 03, 2020, 01:00:10 pm
content: Take a look at some Flex form factor power supplies. They're not much bigger than a couple of sticks of butter. I have a Seasonic SFF250SUB, 250 watts and runs fanless up to about 75 watts.