Questions about ATX power supply outputs....what happens when you combine them? - Page 1
Forum section: electronics / beginners

author: slikvik
author_rank: Contributor
timestamp: August 16, 2024, 10:23:25 am
content: I want to use my spare PSU to power some projects but I need to work out the max amps for the lines? I've seen people on YouTube combining the 12v lines, and the 5v etc.My PSU says +12V1 8A and +12V2 14A. If I join those together does that give me 22 Amps?Side question....what happens if you wire the +12V and the -12V together?Or +12V and +15v?

author: wasedadoc
author_rank: Super Contributor
timestamp: August 16, 2024, 10:33:48 am
content: No, you cannot join them to get 22A.

author: magic
author_rank: Super Contributor
timestamp: August 16, 2024, 11:06:11 am
content: On most PSUs there is one internal 12V rail which powers all of those outputs, but usually there is per-output current sense resistor and overload protection to avoid dumping all the available 12V power into one cable in case of a fault.In such a PSU, paralleling multiple outputs should give more current than a single output. But if a PSU has truly independent rails, they may end up not sharing load equally and one will fault and likely shut down the whole PSU.Observe maximum combined load limit on all 12V outputs.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 16, 2024, 11:25:38 am
content: i would say depends,yes some old psu had one 12vdc linenow you have multiples separated 12vdc lines and their current monitoringBUTI think you have a new psu standard who re-combine them or not, to provide a single main 12 vdc line and the motherboard must convert it to many voltages internallyyes load SHARING IS IMPORTANT ... the weakest will affect the strongestand dont play stupid by combining +12 and -12v. or mix different voltages to get some 17v 24v or else get a dedicated power supply for this

author: Siwastaja
author_rank: Super Contributor
timestamp: August 16, 2024, 11:33:44 am
content: Depending on how individual lines are regulated or sensed (if at all, itcouldbe just all internally paralleled), answer is anything between 8A and 22A. Very likely closer to 22A than 8A, and hopefully more than 14A.

author: slikvik
author_rank: Contributor
timestamp: August 17, 2024, 02:51:53 pm
content: Sorry, I'm a bit confused by these answers (I guess that's why I posted it in the beginner forum!)All the videos I've watched that convert an ATX PSU to a more generic PSU (Drone Workshop being a good one) take all the 12v wires and combine them, all the 5V and combine them etc etc.Also this open source ATX to binding post board shows the outputs being combined:https://www.electronics-lab.com/project/breakout-board-for-atx-power-supply-benchtop-power-supply/Oh WAIT!!?? Am I mistaking the two 12V pins on the 24 pin ATX connector as being both 12v1 and 12v2???Are both of the ATX connector pins eitheroneof 12v1OR12v2?Is the other 12v rail on one of the other connectors (GPU or second MB)??If that is the case, how do I know which connector is the 12v1 and which is one 12v2, given they have different amp ratings?(I've attached the label from my PSU for reference)To put it another way....If I bought one of those 24Pin ATX to Binding post adapters from eBay (similar to the open source board above), what amps will I be getting, 8 or 14?

author: magic
author_rank: Super Contributor
timestamp: August 17, 2024, 03:43:46 pm
content: You can combine all yellow cables on the 20/24-pin connector because they come from a single 12V rail. Same for red and orange. Motherboards also short them.Typically, 12V1 is the rail for the 20/24-pin connector and disk connectors, while 12V2 is a more or less separate rail for the 4/8-pin CPU connector. Larger PSUs may have more 12V rails for 6/8-pin PCIe GPU connectors.Your PSU may enter protection when you pull more than 8A from the ATX connector.

author: SuzyC
author_rank: Frequent Contributor
timestamp: August 17, 2024, 05:11:35 pm
content: An ATX P/S has a large 24 pin connector to a Motherboard and at least one 4-pin connector with both supplying the same 12V power from a same single connection PCB area on the ATX P/S PCB.The ATX 4-pin connector is rated at 8-Amp because it only has two yellow wires to carry up to 8-Amp current, while the 24-pin connector has much more 12V yellow wires and therefore can supply a larger total current because of the many yellow wires in parallel available to supply 12V power.You can try paralleling two ATX power supplies, but it would be necessary to prevent surges by connecting both 12-V outputs together before turning on the ATX supply. In this situation, it would be necessary to switch on both ATX's at the same time if a 12V load exceeds the single ATX 12V current output rating.Chances are it will work fine this way and supply about twice the current.On some ATX power supplies, not loading the 5V and 3.3V outputs will cause these output voltages to trip overV protection before the 12V max current output is reached.

author: magic
author_rank: Super Contributor
timestamp: August 17, 2024, 07:03:38 pm
content: Not really.Usually, 12V1 is the motherboard and disks and 12V2 is the CPU.These Molex connectors are rated for some 6A or 8A per pin IIRC and many PSUs allow even more through them.Paralleling two separate PSUs may easily result in the one with higher output voltage providing all the current until it reaches its limit and shuts down.

author: slikvik
author_rank: Contributor
timestamp: August 19, 2024, 09:00:51 am
content: Fully understood! Thank you all!

author: amyk
author_rank: Super Contributor
timestamp: August 20, 2024, 01:30:57 am
content: I recommend to check that the outputs you want to parallel are actually shorted together inside the PSU, using a multimeter. I've read that many newer designs use a set of buck regulator modules to generate the output voltages from an internal higher-voltage bus, and if two 12V outputs from different modules with their own references and feedback loops get joined together, bad things can happen.

author: MathWizard
author_rank: Super Contributor
timestamp: August 20, 2024, 04:57:37 am
content: OP what sort of things do you want to power ? What model PSU is it?If you just wanted to use it for basic 12V, 5V and 3.3V hobby breadboard stuff, most people would never run out of power, or have to worry about the load on each rail. The -12V rail is not that powerful, I think they might only be rated for 0.5Amps.But they are pretty overpowered for a lot of stuff, and will happily provides 10's of amps to thing's that will get destroyed before you notice the smoke. And even a near short, with 10's of amps, can be 12V, and the PSU is still happy.If I was going to use a computer PSU for breadboard stuff, I'd use it as a pre-supply, and make some circuits with little less output voltage, but with some overcurrent protections. So even using 7809/7909's, they might have 1-2A shutdown current, and overtemp protections. Or LM317/337, for quick easy low power rails. And have switches, and even actual fuses too, right I forgot them.