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Power Supply Unit (PSU) FAQ


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Ok... this is a place that we can kinda orginize many of the commonly asked questions about Power Supplies.

 

Feel free to submit to this thread in a Q&A format... a staff member will edit this message from time to time with the new FAQ. :)

 

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Q: How do you measure voltages?

 

A: With a digital multimeter. Preferably at the main ATX connector, but molex connectors/AUX connector are also fine as well. The yellow wires are the 12v line, red is 5v, orange is 3.3v, and black is ground. Software monitoring programs like MBM5/Speedfan/BIOS readings are all dependant on the onboard motherboard sensors, which vary too widely in quality to be considered accurate.

 

To measure the voltages with a digital multimeter it is recommended to read the manual of the digital multimeter first before you attempt to measure the voltages. Set the digital multimeter to vDC (volts: direct current) and plug in the red and black measuring probes in according to your manual. The black probe goes to any ground (I use a black wire from a spare molex connector) while the red probe goes to what you want to measure; in this case the wires coming from the PSU. For measuring voltages directly from the main ATX connector, squeeze the red measuring probe in between the plastic and the wire just enough where the probe can touch the metal to get a reading.

 

The main 3 voltage rails (3.3v/5v/12v) should have a voltage tolerance of +/- 5% from baseline. In other words the 3.3v rail should have a reading from 3.135 to 3.465v, the 5v line should be within 4.75 to 5.25v, while the 12v line should have a reading from 11.4 to 12.6v. The closer the voltage reading from the baseline the better. It is also important to take different measurements of the PSU while the PC is idle and under full load (e.g. running a combination of prime 95, ATITool, 3D Mark, etc.) to see how the PSU can handle the additional electrical load.

Edited by lilkev715

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  • 4 weeks later...

Q: My case came with a (high wattage) power supply. Is it good enough?

 

A: Most likely, no. A cheap power supply's rating is the maximum output, while a good one (Antec, Enermax, Thermaltake, Fortron/Sparkle/FSP, TTGI, CWT, etc) is the sustained output. Also, when a cheap power supply fries, it usually takes other components with it, while a better power supply only fries itself.

 

Q: How do I tell whether I have a good power supply?

 

A: First, the brand is an important factor, and another good indication of quality is the weight of the power supply. Better PSUs weigh more.

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  • 3 weeks later...

Q: How much power does a power supply use (both AC & DC)?

 

A: If a system consumes 200watts DC, then the power supply will only consume 200watts DC. It does not matter how many watts a PSU is rated for. -->For example if the system consumes 200watts DC but the PSU is rated at 500watts DC, the system will still consume 200watts DC(not 500watts DC).

 

Maximum DC power output can be affected by the PSU's internal temperature. Most manufacturers rate the maximum DC output at a internal temperature of 25C. This temperature is not a real-world scenario as most PSU internal temperatures are in the range of 35-40C+. As the temperature inside of a PSU increases, the maximum DC output decreases.

 

Because of the nature of switching PSU's, the amount of DC consumed does not equal the amount of AC consumed. Componnents inside a computer cannot utilize AC for power, so the AC needs to be converted to DC, thus "switching". In the switching process, some of the energy is lost being switched which also correlates to the efficiency of a PSU. Most PSU's have a efficiency rating of around the 70% mark on average. So, if a PSU (with 70% AC to DC efficiency) consumes 200watts DC, then it will consume 285 watts AC.

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  • 1 month later...
  • 3 months later...

Q: My motherboard has a ATX 24-pin connector, do I have to purchase a 24-pin ATX PSU? Will my regular 20-pin PSU work? How about a 20-pin to 24-pin ATX adapter?

 

A: The Intel 915/925/925XE and the nVidia Nforce4 motherboards have a 24-pin ATX connector that is backwards compatible with a 20-pin PSU. It is recommended that a 24-pin PSU is used, but a 20-pin ATX PSU will work absolutely fine as well. The pins on the ATX connector are keyed to prevent incorrect installation.

 

If a 20-pin ATX PSU is used in a 24-pin slot on a motherboard, the extra 4 pins on the 24-pin ATX connector will not be used at all. There is a misconception that the ATX 12v "P4" 4-pin connector would have to be plugged into those extra 4 pins. The 4-pin P4 connector does not plug into the extra 4 pins on a ATX 24-pin connector if a 20-pin ATX PSU is used. The extra 4 pins on a 24-pin ATX connector are 3.3v/5v/12v/ground wires. The ATX 12v P4 4-pin connector is 2 X 12v and 2 X ground wires.

 

Here is my own personal test of a 20-pin ATX connector plugged into a 24-pin ATX slot. For higher-power setups like a SLI setup it is recommended to have a native 24-pin ATX connector.

 

The 20-pin to 24-pin ATX adapters are pretty much worthless in reality. The reason for the move from a 20-pin ATX connector to a 24-pin ATX connector is to provide more overall power. Each individual wire can only supply so many amps. These "adapters" simply split some of the wires from the 20-pin ATX connector in order to end up with the 24-pin connector. These splitted wires cannot provide the full amount of power compared to a native 24-pin ATX connector. Also any adapter that is not native to the copper wire itself from the PSU increases the electrical resistance of the electricitry coming from the PSU, through the adapter, and to the component(s) it is attached to. This increased electrical resistance can result in lower voltages compared to a direct connection from the PSU without a adapter.

Edited by lilkev715

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  • 3 months later...

Q: What is PFC?

 

A: PFC is Power Factor Correction. So what is the Power Factor of a system, and why does it need to be corrected? Well, when a system uses AC power, it uses Complex Power, which has 2 components: Real Power, and Reactive Power. Real Power is power that is useable by the system, and Reactive Power is power that is generated but not used by the system, and often is dissipated as heat. Think of these quantities as a right triangle, with Real Power being the base, Reactive Power being the height, and Complex Power being the hypotenuse. Power Factor is Real Power divided by Apparent Power (which is the magnitude of Complex Power), so the closer you are to Unity Power Factor (PF of 1), the less Reactive Power is being used. Power Factor also represents the phase difference between the voltage and current phasors in the complex plane.

 

PFC can be presented in 2 ways, passive and aggresive. Passive PFC is done through a fixed component in parallel with the source, the component either being a capacitor or an inductor depending on the reactance of the source. This extra component acts as a balance, bringing the Power Factor closer to unity by adding to or subtracting from the source reactance. However, it is not easy to control the inductance or capacitance needed, and usually only brings the PF to about 0.6 or 0.8. Active PFC is done with more modern technologies, using an integrated circuit to correct the Power Factor to about 0.95.

 

So what are the benefits to having a high Power Factor from Active PFC units? It is said that Active PFC reduces the electric bill and that these PSUs are more efficient. Both statements are untrue. The only way for Active PFC to save you money is if you have a high volume of machines using it, then maybe it will save you money. The efficiency statement is also false. Because the Active PFC unit has a seperate circuit that uses power, these power supplies are often less efficient than Passive PFC units.

 

The real benefit to having an Active PFC unit is that it draws less current than a Passive PFC unit. Obviously this helps reduce stress on the wires in your home. Also, less current allows a user to use a lower rated UPS if one is needed (thanks for reminding me lilkev715). Finally, as I said in the beginning, using less Reactive Power means less heat.

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  • 1 year later...

Firstly, the image Firestorm is refering to in his post re: ATX connection to facilitate non-loaded PSU activation ;)

 

post-3835-1160109376.jpg <- Any black wire connection will do for this jumper configuration

 

 

Q: How do you determine a good/quality build PSU?

 

A: Unfortunately, name, place of manufacture or weight are not sufficient to indicate the quality of any given PSU, as some PSU's on the market are merely 're-badged' units. Capacitors used in the construction of PSU's are as important if not more so than the heatsinks or any other given components, the reason is essentially capacitors filter the current (highs & lows) to a mere (steady) ripple, a capacitor in poor condition essentially venting contents or bloating (see image below) thus becomes unable to perform this regulation of current, with the end user experiencing ever increasing fluctuation in their PSU's output & system instability.

 

post-3835-1160110765_thumb.jpg <- Note the domed appearance at the top of the cap'

 

One such example of a poorly 'fitted out' PSU is the Antec TruePower 380, equiped with Fuhjyyu capacitors (a known failure ridden component) it's lifespan is severely reduced... sadly not all faulty capacitors are as obvious with no physical signs of deterioration :(

 

 

EDIT: It has come to my attention that Antec has begun using OST & Teapo capacitors in their newer released PSU's, still not the best capacitor selection but definitely a better choice over Fuhjyuu :)

Edited by tazwegion

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  • 1 year later...

well i was wondering if this psu is a good one. i had posted earlier a topic about it , but didnt get much replies as its not sold world wide.

i could not get any reviews also.

The details on the psu are as follows:-

 

 

 

MODEL: vip-400EX(pfc).

AC INPUT -220V-5A. 50-60HZ.

MAX OUTPUT -400W.

DC OUTPUT : +3.3V - 30.0A(ORG),+5V-28.0A(RED),+5sb-2.0A(PUPL). +12V1DC-14.0A(YEL) + 12V2DC-15.0A(YEL/BLK), -12V-0.5A(BLUE).

P.G.SIGNAL(GRAY),GROUND(BLACK).

(+3.3V & +5V = 150W MAX)

 

My question is , how much is the max output if this psu, and its got one 120mm-fan , which sucks air from cabinet and blows out cooling it self. it is quite heavy also, and its got a sporty look ( Purple), 24-pin also.

 

Vip is a branch of POWER-SAFE.

 

looking forward to read reply eagerly! The configuration below in my signature is the one i am using presently with this psu. And it does not get too hot, just warm! 363 WATT ON 100% LOAD

thank you

Edited by get_saif

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  • 6 months later...

get_saif

 

A late reply to this question but it may be of interest to others.

 

well i was wondering if this psu is a good one. i had posted earlier a topic about it , but didnt get much replies as its not sold world wide.

i could not get any reviews also.

 

I am generalising here but if the power supply is not sold world wide and you can't find any reviews on the internet this would indicate to me that it is probably a poor quality, low budget unit.

 

MODEL: vip-400EX(pfc).

AC INPUT -220V-5A. 50-60HZ.

MAX OUTPUT -400W.

DC OUTPUT : +3.3V - 30.0A(ORG),+5V-28.0A(RED),+5sb-2.0A(PUPL). +12V1DC-14.0A(YEL) + 12V2DC-15.0A(YEL/BLK), -12V-0.5A(BLUE).

P.G.SIGNAL(GRAY),GROUND(BLACK).

(+3.3V & +5V = 150W MAX)

 

My question is , how much is the max output if this psu, and its got one 120mm-fan , which sucks air from cabinet and blows out cooling it self. it is quite heavy also, and its got a sporty look ( Purple), 24-pin also.

 

The maximum output is quoted at 400W but if you add up all the available individual outputs which comes to 603W there are obviously other limiting factors to take into account. The combined +3V3 and +5V limit is 150W which for example limits you to say 15A on the +3V3 line (thats 49.5W[volts x amps]) and 20A on the +5V line (thats 100W[volts x amps]) for a total of 149.5W. This then leaves you approximately 234W for the 12V1 and 12V2 lines (the +5VSB and -12V lines at their maximum are only 16W). Thats a total of only 19.5A available (watts / volts) on the combined 12V lines.

 

This illustrates why you have to be very careful when selecting a power supply and don't just look at the total power. Have a good look at the quoted combined maximums and decide whether it meets the requirements of your system.

 

I feel that weight these days is not, as it once used to be, an indication of a good power supply. Increased power supply efficiency, improved technology and electronic devices are all leading to lighter power supplies. The heaviest component inside the power supply is the main switching transformer and heatsink size, being aluminium, makes very little difference.

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