Electronic – What’s all this “rails” nonsense on a computer power supply

atxpower supply

This is in danger of becoming a computer question, but I've always wondered why do computer power supplies have so many 12V "rails". I have a 800W dead supply with four 12V 16A "rails", but internally, they are all connected to the same transformer, use the same diodes and the same capacitors – in fact, they are all electrically connected to each other. So why call them separate rails (given they are in fact the same rail), is it just marketing or is there some proper reason?

Best Answer

Allow me to quote Wikipedia: Power supply unit (computer):

In computer power supplies that have more than one +12V power rail...

  • Multiple 12V power supply rails are separately current limited as a safety feature; they are not generated separately.
  • ... the IEC 60950 standard, which requires that no more than 240 volt-amps be present between any two accessible points. Thus, each {12 V} wire must be current-limited to no more than 20 A; ... Unlike a fuse or circuit breaker, these limits reset as soon as the overload is removed.
  • Because of the above standards, almost all high-power supplies claim to implement separate rails, however this claim is often false; many omit the necessary current-limit circuitry,{5} both for cost reasons and because it is an irritation to customers.{1} (The lack is sometimes advertised as a feature under names like "rail fusion" or "current sharing".)


So it's apparently for exactly the same reason that electricians claim that different outlets in your house are on "different circuits", even when they are all electrically connected to each other. Each circuit is allegedly current-limited with a fuse or circuit breaker. And yet it is an irritation to customers when a circuit breaker blows because I have too much stuff plugged into it, when that same stuff works just fine if they are plugged into outlets in different rooms, or if someone replaces that fuse with a penny :-/.