4 ms·

> I wonder if it is possible to run more than one board off the same PSU. Absolutely. There's something I can't remember about signal lines that need to be set

> I wonder if it is possible to run more than one board off the same PSU. Absolutely. There's something I can't remember about signal lines that need to be set up, but yet, people have been running multiple computers off as single supply for a long long long time now. Also worth warning people ahead of time, it's hard to predict what kind of specific power consumption a motherboard will have. There's so many power rails going in. 3.3V, 5V, 12V, -3.3V... Making sure that each rail is adequately rated for additional motherboards can be a challenge. Ideally one would test each supply line, one by one, to try & figure things out, & compare against rated specs of the power supply. Intel's been pushing a new spec called ATX12VO, short for 12-volts-only, which does away with all the other rails & has the motherboard expect 12v. This would make it much easier to understand what power consumption is. I do seem to recall some of the new ATX specs (3.0?) also kind of expect power-consumers to declare how much power they expect to use, as a safety/planning feature, which could complicate this one easy-to-do hack (unless there's some way for the additional motherboard/motherboards to also declare their power draw). This might only be ATX3.0's new 12VHPWR connector, which works alongside PCIe 5.0-- not sure if it also impacts motherboards or not.

robocat 4y ago
> Making sure that each rail is adequately rated for additional motherboards can be a challenge Buy a cheap inline tester and leave it always in? Something like: https://www.google.co.nz/search?q=Power+Supply+Tester+Digital+LCD+20%2F24+Pin+PSU+ATX+BA4 https://www.google.co.nz/search?q=Power+Supply+Tester+Digita...
paulmd 4y ago
These days the 3.3v rail and -3.3v rail are practically vestigial (iirc the major consumer on those rails was floppy disk drives...) and 5V is mostly used to drive SSDs and USB devices. Almost everything runs off 12V these days - which is the whole point behind the 12VO standard, just make everything use 12V and step it down at the point-of-consumption for the handful of things that still need something else. So basically I wouldn't worry about motherboard compatibility due to 5v, 3.3v, and -3.3v rails - there just isn't much of anything still being done on those rails, and the things that do use it pretty much are the same across motherboards (because they're things that are plugged into the motherboard, not the motherboard itself).
- mschuster91 4y ago
  > and 5V is mostly used to drive SSDs and USB devices. The most important 5V rail is 5VSB - it powers wake-on-LAN and on some systems wake-via-bluetooth. The interesting question is, is it more efficient to always let 12V->5V regulators run for 5VSB or to use an extremely small separate regulator?
  - glowingly 4y ago
    To be fair, something is stepping it down from AC, and many modern PSUs step everything down from a common DC rail using DC-DC conversion (i.e, 12V, 5V, 3.3V, etc are all generated from a source DC voltage). Then things like the CPU and GPU have long moved off of the 3.3V rails to harness the 12V rail for DC-DC conversion in various VRM stages, down to <<2V. My main appreciation for ATX12VO (12 volts only) is the MB can choose to right size the 5V and 3.3V rails appropriately. M.2 slots are driven from 3.3V only (even if M.2 devices step it up or down for their specific uses). A board with 4 M.2 slots will have a different potential 3.3V power demand than a board with only 2 slots. Typically, a decent board will have 1 M.2 2230 A or E slot for WiFi, and 1-2 M.2 2280/22110 M (and rarely B) slots for M.2 storage. PCIe x16 lanes will have 9W of 3.3V, as part of their 75W total power (the remaining 66W is 12V). As noted earlier, 5V is heavily I/O driven. A board with 15 USB ports can expect to need more 5V than a stingy board with only 8 (both are accounting for front I/O headers). I'd rather the MB have to right-size the 3.3V and 5V rails, than forcing every PSU to include copious amounts of 3.3 and 5 just to cover every edge case. Sure, it's a tiny bit of potential efficiency. It's also passing a bit of cost from the PSU to the board itself. Sadly, it does seem ATX12VO's main impact will be lost. OEMs are going to continue with their custom PSUs with unique connectors and the DIY vendor + consumer market has largely turned their backs to this.
  - paulmd 4y ago
    > The interesting question is, is it more efficient to always let 12V->5V regulators run for 5VSB or to use an extremely small separate regulator? To complicate this further, full system power-off is not the only time you want to do wake-on-lan... if you want to wake from system sleep then you have to be running the 12V rail anyway, because that 12V rail drives the VRMs which drive Vcore and Vmem. No 12V rail means you lose your memory and CPU state... As such I don't think you can really frame this as "having a separate 5V regulator with the 12v rail turned off", or at least that use-case must exist in addition to the deep-sleep use-case, it cannot supplant it entirely. Anyway though, power-supply 5V rails are usually much much larger than they need to be, and that has higher loss inside a transformer. Even if you make that a solid-state thing inside the transformer, moving power at 5V is less efficient than moving it at 12V and stepping it down at the point-of-consumption. So actually the answer is probably that it's more efficient to always run a step-down regulator.

robocat 4y ago

> Making sure that each rail is adequately rated for additional motherboards can be a challenge Buy a cheap inline tester and leave it always in? Something like: https://www.google.co.nz/search?q=Power+Supply+Tester+Digital+LCD+20%2F24+Pin+PSU+ATX+BA4 https://www.google.co.nz/search?q=Power+Supply+Tester+Digita...

paulmd 4y ago

These days the 3.3v rail and -3.3v rail are practically vestigial (iirc the major consumer on those rails was floppy disk drives...) and 5V is mostly used to drive SSDs and USB devices. Almost everything runs off 12V these days - which is the whole point behind the 12VO standard, just make everything use 12V and step it down at the point-of-consumption for the handful of things that still need something else. So basically I wouldn't worry about motherboard compatibility due to 5v, 3.3v, and -3.3v rails - there just isn't much of anything still being done on those rails, and the things that do use it pretty much are the same across motherboards (because they're things that are plugged into the motherboard, not the motherboard itself).

mschuster91 4y ago
> and 5V is mostly used to drive SSDs and USB devices. The most important 5V rail is 5VSB - it powers wake-on-LAN and on some systems wake-via-bluetooth. The interesting question is, is it more efficient to always let 12V->5V regulators run for 5VSB or to use an extremely small separate regulator?
- glowingly 4y ago
  To be fair, something is stepping it down from AC, and many modern PSUs step everything down from a common DC rail using DC-DC conversion (i.e, 12V, 5V, 3.3V, etc are all generated from a source DC voltage). Then things like the CPU and GPU have long moved off of the 3.3V rails to harness the 12V rail for DC-DC conversion in various VRM stages, down to <<2V. My main appreciation for ATX12VO (12 volts only) is the MB can choose to right size the 5V and 3.3V rails appropriately. M.2 slots are driven from 3.3V only (even if M.2 devices step it up or down for their specific uses). A board with 4 M.2 slots will have a different potential 3.3V power demand than a board with only 2 slots. Typically, a decent board will have 1 M.2 2230 A or E slot for WiFi, and 1-2 M.2 2280/22110 M (and rarely B) slots for M.2 storage. PCIe x16 lanes will have 9W of 3.3V, as part of their 75W total power (the remaining 66W is 12V). As noted earlier, 5V is heavily I/O driven. A board with 15 USB ports can expect to need more 5V than a stingy board with only 8 (both are accounting for front I/O headers). I'd rather the MB have to right-size the 3.3V and 5V rails, than forcing every PSU to include copious amounts of 3.3 and 5 just to cover every edge case. Sure, it's a tiny bit of potential efficiency. It's also passing a bit of cost from the PSU to the board itself. Sadly, it does seem ATX12VO's main impact will be lost. OEMs are going to continue with their custom PSUs with unique connectors and the DIY vendor + consumer market has largely turned their backs to this.
- paulmd 4y ago
  > The interesting question is, is it more efficient to always let 12V->5V regulators run for 5VSB or to use an extremely small separate regulator? To complicate this further, full system power-off is not the only time you want to do wake-on-lan... if you want to wake from system sleep then you have to be running the 12V rail anyway, because that 12V rail drives the VRMs which drive Vcore and Vmem. No 12V rail means you lose your memory and CPU state... As such I don't think you can really frame this as "having a separate 5V regulator with the 12v rail turned off", or at least that use-case must exist in addition to the deep-sleep use-case, it cannot supplant it entirely. Anyway though, power-supply 5V rails are usually much much larger than they need to be, and that has higher loss inside a transformer. Even if you make that a solid-state thing inside the transformer, moving power at 5V is less efficient than moving it at 12V and stepping it down at the point-of-consumption. So actually the answer is probably that it's more efficient to always run a step-down regulator.