You probably also know that different hardware components need different voltage values to function. So for example USB ports work at 5V but DDR4 RAM needs between 1.2 and 1.35V for proper operation. The goal of the power supply is to be able to provide the energy that each component needs, but if the source enters between 220 and 240 volts via the plug (between 100 and 125 in America), what then?
The source’s 12V, 5V and 3.3V rails, explained
To start with, you should know that although the 220V enters the power supply through the outlet, it is alternating current, but a PC runs on direct current, so one of the first components to go into operation is the AC / DC converter, which converts alternating current into direct current. This converter changes the current to 12V, the main voltage with which the source operates.
In the diagram above, you can see some of the main components of a power supply; In addition to the AC / DC converter, you can notice that we also have a + 5V converter and a + 3.3V converter, thus generating the three rails with which a power supply works: 12V, 5V and 3.3V. In summary, when the current enters the source, it converts it to direct current at + 12V, and from these 12V, it then generates two additional rails, one of 5V and the other of 3.3V, each one. with his own independent circuits.
Now comes a little complicated concept, because if the source works with these three rails and RAM, for example, it needs 1.35V to run, doesn’t the source provide too much? Indeed it is, but for this motherboards have their own regulators and converters, so that they can change the voltage supplied by the power supply according to the needs of each hardware component that needs to be powered.
As we said, each component needs a certain voltage to work, and the purpose of the source is to provide you with the voltage that is closest to what you need so that the motherboard has to run as little as possible. , no more no less. So if 1.35V is needed for RAM, the source 3.3V rail will be used to supply it because it is closest. However, if a fan is running at 12V, it will of course use the 12V rail.
Why is only one voltage not used?
Explained that now comes the next question which curves the loop even more: why then does the source not provide the voltage 12V and that it is the motherboard which modifies it accordingly? The answer is as simple as it is simple: because it has so much more Efficiency When it comes to converting the voltage of a power supply that the motherboard.
So, one of the reasons this is done is that when converting the voltage the power supply is better prepared and changes it at the source in a much more efficient way than the motherboard would, this which He would need to integrate circuits much more advanced than today. The motherboard’s job is much finer to say the least when it comes to voltage regulation, while the power supply does it more efficiently but coarser.
The current design of power supplies, with rails with independent circuits for each of them, has proven to be the most efficient because in the old days, in the first AT and ATX power supplies before the ATX12V specification, the conversion voltage was done later, but they found that doing it at the source greatly increased efficiency and therefore also reduced the heat generated.
Imagine that the source only supplies 12V to the motherboard, and that the latter is responsible for regulating this voltage according to the component it must supply. However, a lot of heat would be generated in addition to additional work on the motherboard that would involve integrating much more complicated and, above all, large circuits. Otherwise, why can the source do it directly and efficiently?