Power Supply F.A.Q. - September 2007

[Pwnzilla]

In response to the number of questions I have seen regarding PSU recommendations, I have compiled an F.A.Q to help others make an informed decision regarding a PSU for their computer. This may not answer all questions, but it should provide enough information to allow one to select an appropriate PSU for their given application.

Q. What is the function of the computer power supply unit (PSU)?
A. The PSU converts AC electricity into regulated DC voltages which power the various components within a computer system. Almost all computer PSUs are switching mode power supplies. This means the PSU switches off/on upwards of 100,000/sec. This makes for a more efficient PSU than a linear (non-switching) power supply. The conversion of AC to DC always requires some signal filtration and some loss of energy as heat. The lower the loss, the higher the efficiency. Efficiency is defined as the ratio between DC output and the AC input necessary to achieve that output which is expressed as a percentage. The higher the percentage the more efficient the PSU converts the power. A good rule of thumb for a quality PSU is it is at least >/= 80% efficiency.

Q. What are the different voltages listed for the PSU?
A. Several different voltages are necessary for a computer to function. Each voltage supplies the requirements of specific functions within the computer. The primary voltages necessary for computer operation are +12v, +5v, +3.3v, & 5v standby (SB).

Q. What computer functions do each voltage type supply?
A. The following functions/voltages are utilized:
1. The +12v is the rail that supplies power to most main components. The motherboard uses DC to DC conversion of the 12v rail to supply the <1.5vDC necessary to power the processor (CPU). The +12v rail also powers video cards through 6-pin & 8-pin PCIe connectors. The +12v rail also provides the power for drive and fan motors to operate via the 4-pin molex connectors.
2. The +5v rail is utilized by the motherboard and many of its component parts.
3. The +3.3v rail supplies system memory, video cards, and other circuits.
4. The -12v circuit provides for backward compatibility of some serial port circuits.
5. The +5v standby (SB) stays on so long as the computer is plugged into an AC outlet and the main switch on the PSU is ON. It powers the circuitry in the motherboard that tells the PSU to power-down or power-up. It is also the rail that provides power to USB devices that do not utilize an external power source.

Q. What are the connectors on a PSU and what is their function?
A. Most modern PSUs will have the following connectors:
1. A 24-pin or 20+4-pin main power connector for the motherboard.
2. A 4-pin or 8-pin connector for the motherboard (the 8-pin version is mainly for high-power CPU or dual CPU motherboards).
3. One to four 6-pin or 8-pin PCIe connectors for high-power video cards.
4. Various number of 4-pin molex connectors for PATA hard drive, optical drive, fans, lights, etc…
5. Various number of 15-pin SATA power connectors for SATA hard drive or optical drive.
6. 4-pin floppy drive connector

Q. Which PSU do I need to run my computer?
A. It depends to a degree, on what CPU/motherboard you have in your system. The following changes have been made to the ATX specification over the years:
1. ATX uses a 20-pin main power connector (Pentium III and early AthlonXP).
2. EPS12V uses a 24-pin main connector, 8-pin secondary connector, & optional 4-pin tertiary connector (Xeon/Opteron servers)
3. ATX12V uses a 20-pin main connector, 4-pin secondary connector, & 8-pin tertiary connector (Pentium 4, late AthlonXP & Athlon64)
4. ATX12V 2.0 uses a 24-pin main connector & 4-pin secondary connector (Pentium 4, Core 2 Duo, & Athlon64 w/PCIe bus)
5. ATX12V 2.2 uses a 20/24-pin connector, one ATX12V 4-pin connector (Pentium 4, Core2 Duo, and Athlon64 with PCI Express). Many manufacturers add a 4 + 4-pin, or 8 to 4-pin secondary connector, which can be used as the secondary EPS12V connector for Xeon/Opteron server specification.
Most current generation systems will utilize either ATX12V 2.0 or 2.2.
Note: Some system builders (Dell notably) utilize proprietary PSU/motherboard designs that are not compatible with ATX standards. There are some PSU manufacturers that produce Dell compatible PSUs (PC Power & Cooling notably).

Q. This PSU says it has multiple +12v rails. What does that mean?
A. Each voltage output rail has a maximum current capability expressed in amperes (A). 240VA is the limit on some consumer electronics safety standards. The 18A current limit on the 12V is intended to keep the VA in the PSU output cables below that 240VA limit. This is achieved by placing a simple limiter to keep the current below 18A on any 12V line. Almost all PSUs limit the output on any single cable to <20A & virtually none have more than one 12V rail. All the 12V wires connect to the same 12V transformer. Marketing has taken this configuration and advertised it as having “multiple independent rails”.

Q. This PSU is advertised as a 600 watt PSU. What does that mean?
A. This means total maximum output of all the cables from all the voltage rails should add up to 600 watts output. As a rule of thumb, 80% of this output should be on the 12V rail (combined +12v & -12v). Is this adequate to meet you needs? The answer is, maybe. First you have to determine your systems requirements. You can “Google” PSU calculator and find several sites that will give you approximate power requirements for your system. This will give you an approximation of what capacity PSU you will need to purchase. However, generic, low-cost, and/or unscrupulous manufacturers will commonly falsely label their units either blatantly, or utilize test methods that deviate from standards to achieve that rating. In addition, the ATX specification calls for a +/- 5% tolerance on the +12v, +5v, & +3.3v rails. It also specifies +/- 10% on the other rails. The voltage monitoring circuitry/software in motherboards is not precise enough to provide truly accurate measurement. The only accurate means of checking this is through the use of a multi-meter across the outputs of the various rails. Some erroneously believe that higher voltage is always better. This is not true. Excessively high voltage can cause damage. It is far better that the PSU maintain the rail voltages within the specified limits. What video card(s) you plan to install will also factor into your decision. You will need enough PCIe connectors to power your choice of video card. Some require none, some one 6-pin, some two 6-pin, and some one 6-pin & one 8-pin. A dual card set-up (SLI/Crossfire) would double that requirement.

Q. This PSU advertises that it has active pfc (power factor correction). What is that?
A. Active pfc uses switching regulator technology with active elements such as IC, FET, and diodes, to create a PFC circuit. This circuit has a theoretical power factor of over 95%, it reduces total harmonics noticeably, and automatically adjusts for AC input voltage. However, it requires a complex EMI filter and an input source circuit, which is more costly to build. This is as opposed to passive pfc which uses passive elements such as a ferrite core inductor on the input source to create a countering reactance. While easily applied to the existing power circuitry without much modification, the power factor is low (60 - 80%), the AC input must be chosen (115VAC / 230VAC), and the harmonics produced from the difference between the capacitance and the inductance are hard to control. Significant electromagnetic noise can result. Basically, active is preferred over passive but it does add cost to the unit.

Q. Why is there such a price difference between PSUs with the same power rating?
A. For the reasons listed above. A quality PSU in the 600w range will run around $90-120. You may find one a bit cheaper if on sale or through a rebate.

Q. Why should I spend so much for just a PSU?
A. The PSU is what supplies the power to every component in your system. It does not make very good sense to trust a $400+ video card, $100+ motherboard, $200+ CPU, $100+ RAM, etc…to a $50 PSU by my logic. If you buy a quality PSU with sufficient requirements from a reputable manufacturer you should receive years of trouble free service from it. You buy a cheap PSU, with mislabeled/over-specced ratings, from a generic supplier (who probably isn’t the manufacturer) you can almost guarantee you will regret the decision. If not immediately, then as time goes on.

In conclusion, when you are deciding upon a PSU you should consider the following items:
1. Is it marketed by a reputable established PSU distributor?
2. Does it have sufficient output on all rails to meet the system requirements (should have a bit of headroom to allow for potential future upgrades)?
3. Does it have the requisite connectors necessary to hook up all the components?
4. Does it have active pfc (not required, but recommended)?
5. “Google” the PSU make/model for independent hardware site reviews. Do they tend to rate it well?
6. Does the price seem in line with the market for this rated PSU?
7. Does it state an efficiency rating of >/= 80%?

If all these answers are acceptable, then you have probably found the “right” PSU for your system. Hope this helps answer some questions.

[FluffyBunni]

The colors scare me =X

[Pedro]

sounds good, you kinda left out explaining amperage and how it varies by video card

[Pwnzilla]

Originally Posted by Pedro

sounds good, you kinda left out explaining amperage and how it varies by video card

I hit the 10,000 character limit. Had to delete some stuff.

[Pedro]

details

still, you could have replied to yourself and continue on
but very good overall

[Vulgra]

Nice post, should be stickied.

I cant stress the first "question" enough however, one thing I would NEVER buy no name is a PSU.