Single Rail vs Multi-Rail PSUs?

Cherlyn Tefera: It's pretty much better protection because of limited amps on each rail. I think this should explain it"To make a long story short, "rails" are group of wires with an independent over current protection (OCP) circuit.For example, on a power supply with a single rail, all +12 V wires are connected to the same OCP circuit. If the manufacturer says that OCP on this particular model is configured at 30 A, for example, the PSU will shut down if you pull more than that from the +12 V outputs.This configuration, however, is agaisnt UL's (Underwriter Laboratories) guidelines, which state that no home appliance should deliver more than 240VA per rail. Meaning 20 A per +12 V rail (12 x 20 = 240 W).So the manufacturers started to use multiple rails as a way to make their PSU's to comply with UL guidelines.On a power supply with two rails, there are two independent OCP circuits. Some +12 V wires will be connected on the first circuit (first rail) and some wires will b! e connected on the second circuit (second rail).Continuing our example, supose that this PSU has each of their OCP circuits set at 18 A. So the maximum current you can pull from +12 V is 36 A, however if you pull more than 18 A from any rail the power supply will shut down. This can be a problem on some configurations, where the PSU will shut down even if you are pulling less than its maximum capacity.In summary, the single rail design has as advantage the power supply not shutting down if you are below the PSU maximum current capacity, however in a case of overload of a component due to a malfunction the PSU won't probably shut down as it will "think" that it is running inside the specs (it will shut down only if the component pulls more than the PSU total capacity -- i.e. the single rail capacity).With multi-rail the advantage is that the PSU offers a better protection, as the PSU will shut down if a component pulls more than the value under the OCP is configured -- which! is a smaller value than compared to single-rail designs (e.g.! 18 A instead of 36 A with the same PSU built as a single-rail instead of a dual-rail).But the disadvantage is that the OCP may shut down the PSU even if the components are working just fine, if you are using power-hungry components.Which one is the best? This is an endless discussion that I don't have an opinion."...Show more

Theresia Fashaw: Agreed. :-)