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What's the Practical Difference Between Single and Multi-Rail modes?

jidenshaotoko

7 months ago

So I got an HX850 to replace my RM650X and I read there's a switch to change rail modes. I haven't seen the same switch on my current RM650X and I was wondering what exactly this does.

According to what I read on Corsair's website, this is more of a safety measure, but I'd really like to know is whether running on multiple would have any negative impact on any given component under load. For example, does running it on multiple limit the power of an RTX 2080Ti to the point it'll throttle under load so it doesn't go beyond the imposed limits of this mode?

Comments

  • 7 months ago
  • 4 points

Multiple rails is like the breaker box in your house. In case of a short circuit, a breaker will cut the power before something burns.

If you have a single rail power supply, then if your graphics card slow short-circuits, it starts demanding infinite power from the PSU. In single-rail PSU there's nothing to stop it, so it will pull increasingly more power, up to your PSU's total capacity. The cable and connector will melt and burn far before it reaches the over-power protection of the PSU.

If you have a multi rail PSU and your graphics card short circuits, then it will also demand increasingly more power. But then it will hit one of the OCP trip points assigned to the cable feeding it, which will trip the protection and **** everything down, far before it reaches so high power draw that anything melts or burns.

  • 7 months ago
  • 1 point

Ah got it. So if that's the case, it's probably recommended to have it on multiple, right?

  • 7 months ago
  • 2 points

Yes, it is. Corsair HX850 has multiple rails configured in such way that each PSU-side connector gets its own rail. Each rail has OCP set at 40A, so in case of a short circuit it will **** off when it reaches 480W power draw from that connector, but also the limit is high enough that the GPU doesn't reach it under normal operation. 2080 Ti will never draw 480W unless it short circuits.

  • 7 months ago
  • 1 point

I see. Good to know. Thanks!

  • 7 months ago
  • 1 point

If you're running a typical single-GPU desktop build, it's unlikely to make a difference.

A single 12V rail design, in principle, permits you to distribute your load ("at" or below the total 12V capability of the PSU) on the 12V rail as you please. (There are still particular limitations on connectors and cords that you should respect, though.) Splitting across multiple 12V rails has you distribute your load more carefully, or else you could trigger a PSU safety. (Obviously, with a low power system, distributing load in an acceptable manner is relatively trivial.)

If things are working well for you, there's no need to rock the boat. Besides, to analyze how your components are distributing load on the rails, we'd need to know how you're connecting them to the PSU.

  • 7 months ago
  • 1 point

If things are working well for you, there's no need to rock the boat. Besides, to analyze how your components are distributing load on the rails, we'd need to know how you're connecting them to the PSU.

Interesting. Is there more than one way to connect them to the PSU?

  • 7 months ago
  • 1 point

Safety is one aspect as others have mentioned, but voltage regulation is another.

Whenever a power supply is subjected to a quickly changing load demand, the voltage at the supply will dip. You can often see this in house wiring whenever an appliance in the house starts running, you might notice the (incandescent) lights get a little bit dimmer. That's because the sudden power demand elsewhere caused the supply to require delivery of a high transient load which caused the voltage to dip some.

Take that idea and put it into the terms of the billions of transistors in the GPU turning on and off and you can get a similar effect, albeit on a much smaller scale. Even older power supplies on single-rail systems are built to anticipate and correct these issues, but theoretically a multi-rail system allows the voltage dips to be much lower overall, or distributed across different rails so there's no realistic voltage swing.

Overall it's far from a big deal, and won't make any difference to most people.

tl;dr: Variable power requirements can change voltage levels on the supply, but multi-rail can help alleviate the issues. Not a big deal either way with how well-built most modern supplies are.

  • 7 months ago
  • 1 point

Multiple rails don't have any effect on output voltages or voltage regulation. There's only one +12V tap coming off the main transformer and only one +12V pi filter; multiple-rail is simply extra voltage shunts monitoring current on groups of wires.

What you say would have been true if multiple-rail power supplies had multiple 12V generation circuits inside (like the very old 2009 Corsair HX1000), but that's not the case. They only have one 12V output with extra current monitoring components.

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