Section 01 - What Do All Those Fancy Words Mean?
PSU
Power supply unit. This is the thing that you plug into the electrical outlet and provides your computer with power.
PFC, Active PFC, Passive PFC
PFC is an acronym for
power factor correction Before I can explain what it is, we need to have a little background: your computer is connected to an electrical outlet which provided power via an alternating current (AC). Now before the computer can use this power, the waveforms needs to somehow be synchronized. Simple enough. Now what happens when the power waveforms are out of sync? The power supply has to somehow adjust the signals so that the it can provide (relatively) clean and stable power to the rest of the computer.
- In a power supply with no PFC, the synchronization is done using capacitors. Using capacitors gets the job done however it's not ideal because capacitors introduce noise and voltage drops at peak (and dropping the voltage drops the total amount of power available). The laymans explaination is that there is a reduction in the effiency of the PSU
- In a power supply that has passive PFC, they add some coils that work alongside the capacitors. By having these inductors along with the capacitors, the waveform synchronization is done with less noise being generated. Power supplies with passive PFC require you to "calibrate" them by setting the input voltage (generally there is a secure toggle switch for 115/230v). Even with passive PFC there can still be noise when dealing with 115V power.
- For power supplies with active PFC use "fancy circuitry" to make a dynamic power regulator (which, essentially means you dont need to set the input voltage) and generally has a higher efficiency rating than power supplies with only passive PFC.
From a technical perspective, of the available types of PFC, active PFC results in the most efficient power supply. Of course, whether a PSU with PFC is even needed is up for debate (and will be dealt with later in the VFAQ)
Rails
Power supplies dont just deliver one voltage, they deliver a bunch. Each one of those output voltages is a rail. The most common are +3.3v, +5v and +12v. Of these three rails, the most critical (and often overlooked and forgotten) is the 12v rail.
Twin Rails, Dual +12v, etc
Every power supply has multiple rails however, since we are only seriously concerned with the 12v rail (and thus, colloquially, when we refer to "rail" it almost always means the 12v one) it is nice to have more than one such 12v rail. Why? Because the voltage fluctations on each rail are independent from the next. This means that if you really stress one rail, say with a high end videocard, the power output on another rail will not suffer voltage stability problems.
Many good power supplies will feature dual 12v rails (having more than two is generally an indicator of a very heavy duty power supply) however it should be noted that having two rails does not guarante the power supply to be good nor does the lack of dual rails mark that power supply as being bad. The issue of whether or not dual rails are needed/benificial will be tacked in more detail in the VFAQ
Adjustable Rails/Pots
Although the power supply itself will attempt to deliver power that as close to the 3.3, 5 and 12 spec as possible, high end power supplies will come with adjustable pots (short for potentiometer) which allow users to manually tweak the output voltages so that the power supply in fact, actually output an exact value.
Hold-up Time
In an idea world, PSUs deliver nice clean power, 24x7x365 but in realiy that may not be the case and for very brief moments, there will be a flicker in power. The hold-up time rating of the computer indicates the duration that it can handle without power. Generally speaking 16s is standard with some exceptional power supplies being able to "pretend nothing happened" for up to 20ms and 24ms.
Peak Rating
The peak rating of a PSU is just that, the amount of power that a PSU can deliver for a very brief amount of time (usually less than a minute). Cheaper lower quality power supplies will tend to advertise peak ratings in place of their sustainable ratings.
ATX12, ATX20, ATX24 etc
These terms are generally in reference to the physical cables coming out of the power supply itself. The main cable which provides the bulk of the power to the PSU is most generically rferred to as the
ATX main and sometimes specifically referred to as ATX20, ATX24 or ATX20+4, the extra numbers which indicate the number of pins on the connector.
When Intel released the Pentium4, the processor required additional power and a new power cabled was needed to provide this power (and dubbed the "P4 power connector"), this is a 4-pin connector. Note that although this may commonly be referred to as the P4 power connector, that all modern systems require this connector.
With the advent of high powered, PCI Express based videocards, it was found that even the PCI Express bus was not able to deliver enough power to the system
Tolerance
The tolerance of a power supply refers to how tightly regulated the voltage rails are: in short, it is a measure of how clean the power is. The rating is generally given as a percentage with the lower the value, the better. The ATX specification requires that the voltage be delivered within ±5% of the stated value (and ±10% for the 12V line under load but that's more of a technicality). Most decent power supplies will supply voltages within an even tighter ±3% tolerance and some even go so far as to provide within a ±1% tolerance.