# Thread: Power Supply Units Explained.

1. ## Power Supply Units Explained.

Power Supply Units, Explained
Xenocrates, contributor

Based on discussions in a previous thread, I realised that a number of us still don't fully understand a computer's power supply unit, which is by far the single most component in your rig. Concordantly, consider this a quick crash course in electronics, after which I'll move on to specifics about computer power supplies. If you are a do-it-yourself builder, build machines for a living, or otherwise tinker with your own hardware, this thread is for you. UTECH Comp. Sci. students should already know much of this stuff. Never-the-less, a refresher course never killed anyone.

Ok, let's learn about some basic priciples of electronics that you need to understand before you can make sense of anything else in this article:
• Voltage - Is the measure of the potential to produce an electrical current between the highest and lowest differential in an electrical circuit. The wider the differential, the greater the voltage and thus the greater the amount of burst electricity that is produced.

Electricity is measured in volts. A cathode and an Anode are both a simple example of a differential circuit. An electrical circuit creates this differential using capacitors, resistors, diodes and other components of which it is comprised. The spark produced between differentials in a circuit is a certain number of volts (denoted ##v, e.g. 12v or 12 volts), depending on the differential in the electrons on either node inside the circuit. Your power supply is rigged to produce a fixed differential on various circuits that exist inside it. There are usually about 5 differential circuits in a power supply unit which cater to specific components on your motherboard. But before you can understand that, you need to appreciate something else:

• Amperage - Also known as "Current" Is the rate of flow of electrons between two differentials. Note the subtle difference between amperage and voltage. Current is measured in amperes (or Amps for short). It is denoted on your power supply as ##a, for example: 18a = 18 amps. Every circuit in your powersupply produces a specific voltage at a certain number of amps. Think of amps as the speed of electricity and voltage as the volume. When a certain volume of electricity (volts) travels at a certain speed (amps), you get:

• Wattage - Which is also known as "Power". Wattage = Volts x Amps. So when a power supply is rated as 500 Watts, it is the voltage of each circuit inside the power supply multiplied by their individual amp ratings. Each circuit inside a power supply produces what is known as a:

• Rail - Every power supply carries anywhere from 4 to as many as 9 rails. Each rail is rated for a specific voltage, and will always carry that voltage no matter how many devices are connected to it. However, each rail has an AMPERE RATING. The more devices you connect to each rail, the lower the available amperage to the rest of your computer system on that specific rail. I'll explain the significance of this in just a bit. But first, you need to understand how volts and amps work.

[Cont'd...]

2. ## Power Supply Units, Explained (Cont'd)

Demystifying Power

The acceptable market rating on power supplies is the Watt. However, this is nothing more than deceptive marketing. You can actually have a 600 Watt power supply that costs \$100 that fails within 3 months with a high end rig, only to have a 450 Watt power supply that lasts for 3 years on the same rig. Why does this happen?

This is because no matter how many watts a power supply is rated for, it means NOTHING if the amps are split up too thinly across each rail to support the devices in your box.

In order to understand this principle, there're two key things that you need to know about electricity:

Lemma #1: Voltage can be split across channels and remain the same in an electrical circuit.

So let's say you can have 12 volts on a molex (4 pin) power connector that you would use to power one IDE drive. Let's say that you put in an additional hard drive and a DVD-RW burner. Now each of these two devices needs 12 volts to run. But you only have one available molex coming from your power supply. What do you do? You get a molex splitter. It's nothing more than a female molex with two male molex connectors extended out of it. You connect the female to the one molex coming from the power supply, and then the two males to your hard drive and DVD-RW respectively. Voila, both devices power right up with no problems.

Why does this work?

Because Voltage can be split across two or more electrical pathways and remain the same. Why is this possible? Because voltage is a rating of the potential for electrical discharge. So long as your hard drive and dvd-rw can create a wide enough potential opposite the cathode connection from your power supply for their rated differential, 12 volts will always exist in both devices and they will always work.

The same is NOT true for Amps. Hence:

Lemma #2: Amperes, or current WILL split across an electrical pathway and become reduced.

Remember that Amps is a measure of the rate of electrical flow. Think of a raging river rapid. You'll note that the water runs at relatively the same speed throughout the main body of the river. However, when the river splits up into various off channels, the water speed dies down on each pathway. This is because the total kinetic energy of the running water is split up along each path. If all those paths were to run together again, they would reproduce the same water speed as existed before the river split up.

Electricity behaves in exactly the same way.

Every power supply has an Ampere rating. You can find out the total amps of a power supply by summing all of the amps on each power rail on the power supply.

[Cont'd...]

3. ## Power Supply Units, Explained (Cont'd)

Reading a Power Supply Specification Chart

Observe the image below:

When checking on the specifics of a power supply, ALWAYS pay attention to the line that says "DC OUTPUT". That's where all the pertinent information is. Now let me explain what all the mumbo jumbo means:

1. +3.3v - This is the rail that powers your south bridge processor (e.g. the ICH9 class chips on the new P35 Intel boards) and some of the newer on board chips that come with modern motherboards. Each chip on your motherboard (except the main CPU) gets +3.3 volts.

2. +5v - This rail powers your USB ports and devices connected to them (including devices which are powered by a usb connection). This includes everything from peripheral devices (mouse, keyboard, web cam) to storage devices (flash/thumb drives, external hard drives, digital cameras, etc.). The more amps available on this rail, the more devices you can connect to it. So if you plug in a USB keyboard to your machine and it doesn't work, chances are, this rail doesn't carry enough amperage to run it. This rail also powers your RAM chips, as each stick needs +5v to run.

3. +12v - This is the main rail on your PSU. It powers everything else in your computer. This includes your main processor, it's fan, your case fans, hard drives, optical drives (i.e. DVD/CD±RW), add-on cards, your PCI-Express lanes, the GPU, your sound card.... everything else. Your PSU may have one or more +12v rails. All molex (4 pin) connectors run from the 12v rail. We'll talk about the significance of this in just a bit.

4. -12v & -5v - These rails exist to support legacy devices, like floppy disk drives and ISA cards. Therefore, you'll notice that there's minimum amperage delivered to these rails. You can safely ignore these rails if you're running a fairly recent rig.

[Cont'd...]

4. ## Power Supply Units, Explained (Cont'd)

12 Volt Rails and the Wattage Deception

Review the image above once more. You will note that this PSU provides:

• 30 Amps on the 3.3 volt rail. - This means that your southbridge processor and every other chip on the mother board have to divide up 30 amps among themselves.

• 50 amps on the 5 volt rail. - This means that your memory chips, your usb devices, your peripheral input devices, thumb drives, etc. all have 50 amps to split up among themselves. 50 amps is pretty good for a 5 volt rail. This means that whichever rig runs on this power supply can connect a truck load of devices to the computer via USB ports. You can even add a USB hub and go nuts. 50 amps is plenty - especially since most of these devices rarely use more than 5 amps each.

• 35 amps on the 12 volt rail. - That means this power supply can only support a baseline configuration. If it uses a Core 2 Duo processor, I doubt that the motherboard will be able to support more than two or three IDE/SATA drives at most. A high end GPU is absolutely OUT of the question for this power supply, as those often need 30 amps to themselves to run any respectable game. So if you put in any GPU more powerful an a ATI 1300 class, it would leave only 5 amps to be split up among the processor, your hard drives and other peripheral devices.

Pay attention to your 12 volt rail!

If you notice that one of your hard drives suddenly goes kaput, 99% of the time, it's because the 12 volt rail is running too low on amperage to power it properly. The same goes for CD/DVD drives that suddenly start mass producing coasters. Your 12 volt rail is being taxed so heavily (usually by your new GPU or some similar power hungry device), that your other 12 volt devices are suffering because there aren't enough amps to go around after your massive 8800GTX sucked up all the amps from the 12 v rail! Hard drives are especially vulnerable to death because of amp inadequacy on the 12 volt rail. If they don't get the right amount of amps to write data, then they will most likely cause the HD head to smear the platters and cause irreversible, physical damage to the disk.

Let's look at another power supply unit. Examine the image below:

Right off the bat, what do you notice about this unit? If you guessed that it has multiple 12 volt rails, then you guessed right! But do you notice that each 12v rail only carries 18 amps? There is a problem here. What if your GPU requires 22 or even 30 amps to run? Do you think it will work? It probably will - until you run a game that really taxes your GPU.

If you think that it will absorb 18 amps from one rail and then siphon 12 amps from another, then unfortunately, it doesn't work that way. This is the deception that Power Supply Makers use to sell high wattage PSUs. They will tell you that a PSU is rated at 500 or even 600 watts (like the one shown above) and they may even tack on "SLI Ready" to the box so that it flies off store shelves. However, DO NOT BE DECEIVED. Know this:

NO DEVICE IN YOUR COMPUTER WILL BE ALLOWED TO 'SUM' CURRENT FROM MULTIPLE RAILS.

Remember that amps split across electrical circuits. This means that even if your 8800GTX sucked up 18 amps from one rail, your PSU will not take amps from subsequent rails to satiate your GPU. Multi-rail PSUs are designed to stabilize the current on each rail. Siphoning current from a second or third rail will only put the rest of your other devices at risk. That's the whole point of multi-rail PSUs.

[Cont'd...]

5. It is not a deception... it is called one word.... starts with an e....

EFFICIENCY.... that must be added to your formula... look in specs on newegg they are always ther.... yesterday the day before last year and the year before...

VOLT x AMPS x EFFICIENCY

Notice the coolmax seem to not add up... go and get the efficiency and see if it doesn't add up... i betcha gonna be surprised....

6. ## Power Supply Units, Explained (Cont'd)

Why some PSUs have Multiple 12 Volt Rails

What is the real significance of having two (or more) 12 volt rails?

Intel, in their 2.01 specifications for acceptable power supply for the new standard of processors (circa 2004), stipulated that each power supply that conforms to this standard should allow for the increasing power demands of newer devices. Thus, each rail should:
• Not exceed 240 VoltAmperes (roughly 18 amps)

• Cater to secondary or ternary rails as needed to support the other devices if necessary.

This they assert, will provide for more stability in the PSU, such that if a rail should collapse, then it won't take down the entire PSU. It would just mean that only one or more devices would fail, but the main rail available for the Processor and other critical motherboard components would still run, thus allowing for a graceful shut down of the machine if possible.

There is only one problem with this specification:

THEY MADE THIS STANDARD BEFORE HIGH END GPUs EXISTED!

What does this mean? First, we need to understand some simple concepts:

• A high end GPU (Graphics Processing Unit) is any video acceleration device that requires more than 18 amps to run.

• Virtually every GPU produced since 2005 requires at least 22 amps to run.

• GPUs which use 2 slots on your mother board and have their own on board cooling unit usually need 30 amps to run effectively. Many 8800GTX/S cards need 30 amps to run - standard.

STOP RIGHT NOW.

Go check the box on your new 8800GTX/S and see how many amps it needs to run. Then pull open your computer case and look at the amp rating on the 12v rail(s) of your PSU. After you regain your composure, read this:

Lemma #3: If the Amps required on your GPU exceeds the amp rating on any 12v rail on your power supply, then EXPECT YOUR PSU TO FAIL WITHIN A YEAR.

Even if your PSU can fire up your 8800GTS/X, when it starts to rev really hard on a next gen game like Crysis, your PSU will eventually give out suddenly, and your computer will either suddenly reboot, crash to a blue screen or just power off. You might smell burning copper wire as well. Pray that it doesn't take your 300 gb hard drive with it when it dies. More on this later.

But wait, it gets better:

Intel, in their 2.2 specification, later said that a PSU isn't required to have multiple rails to support a power hungry system. This, they observed, after the fact, when the nVidia 8800 GTS/X cards came out and started demanding insane power from PSUs, which they realised still wasn't being adequately provided for on multi-rail PSUs. So even though most new PSUs are coming out with multi-rail configurations, most of them will not be enough to satiate your monster GPU.

Look twice before you buy another PSU. I'll tell you what to look for later.

[Con'td...]

7. ## Power Supply Units, Explained (Cont'd)

Power Factor Conversion

Because your computer is a digital device (and not analog like your TV or your refridgerator), it needs Direct Current to run (aka 'DC'). However, Alternating Current (aka 'AC') comes from your wall socket (hereafter referred to as JPS). Your Power Supply Unit changes Alternating Current to Direct Current so that your computer can use it to produce ones and zeroes. However, in the conversion process, harmonics are produced.

Harmonics are undesirable in a PSU, because it reduces efficiency in the conversion process. The inefficiency stems from the fact that electrical units which thrive on AC power, require what is known as apparent power. Here're some notes:

• Apparent Power = Real Power + Reactive Power. Apparent power is what you normally see rated on your UPS. Apparent Power is rated in Volt-Amperes or VA.

• Real Power - The actual wattage used by your computer.

• Reactive Power - The increase (or decrease) of power demanded by your rig at any given time.

• Power Factor = Real Power / Apparent Power. It must always be 1 (also known in electrical jargon as "unity") or close enough to it for a power supply to be considered "efficient".

If your rig features a high end GPU, then chances are, that when it kicks into high gear (like when you turn on stuff like anisotrophic filtering and 4x Anti-Aliasing at some insane 1600x1280 resolution), it's GOING to demand more power from your PSU. This is an example of what what constitutes reactive power - when your rig demands more out of the PSU than that which it provided at rest, while AC power is coming to your PSU from JPS. The presence of reactive power causes Real Power to be < Apparent Power, thus creating a power factor of less than 1. Reactive power is produced by devices that exhibit capacitance. There are thousands of such devices in a computer rig.

When reactive power is introduced to your PSU, it's going to draw more power from JPS, thus causing an increase in the loss of energy from the power grid as heat. This creates a quandry for JPS, in that they start loosing money to your slice of the grid, because there is an increase in consumption of Apparent Power from your home. This is bad for JPS, beacause you're not being charged by voltamperes (Apparent Power), but rather by kilowattage per hour (Real Power). So ultimately JPS is loosing money on a daily basis!

Remember that watts = volts x amps. That will remain steady going into your PSU from JPS. This means that although the AC current going into your PSU increases, the wattage being produced from your PSU remains the same. What they (JPS) will tend to do in this scenario is to compensate for the loss of power by charging you a little extra on your next bill. The actual kilowatt usage on your bill will be "padded" to make up for the loss. Usually the amount is negligable - but not non existent. Ultimately, it is JPS that looses this battle, and that's when they apply to the OUR for an increase in electricity rates.

A power supply unit with PFC will compensate for this creation of harmonics, and thus create a Power Factor of 0.9 or higher. If your rig is going to be running high end SLI configurations, multiple case fans, a zalman heatsink/cooler, multiple optical and hard drives, you're going to want to ensure that your PSU of choice features Active PFC. There's Passive PFC, but don't get those types. The efficiency rating is still fairly low, and it will only make JPS approach the OUR more frequently.

[Cont'd...]

8. Just checked.... ther efficiency is between 65 & 70 percent... yeah they suck... i know... thats why a new standard was define called 80+... google it... it is an official certification for power supplys who are over 80% efficient... Xeno all the mumbo jumbo boils down to what mi saying.... and the power supply makers tell up front... I am 1000 watts but 50% efficient.... what that means i am really a 500watt power supply.... str8 marketing
Last edited by MaxFactor1; Nov 18, 2007 at 10:35 PM.

9. ## Power Supply Units, Explained (Cont'd)

Common Power Supply Myths

Myth #1: Massive wattage will cover what devices can run in any given rig

This is not true. It is the Amperage on your 12v rail that does. Wattage is just Amps x Volts. If there isn't enough amperage on a given 12v rail to power specific devices, then expect your PSU to die after continued aggressive use. Most computer systems don't need more than 450 watts to run. If you're going to be running a SLI configuration, you want to be looking at 500 watt systems with individual rails carrying in excess of 30 amps per rail. It is cheaper to buy a PSU that carries only a single rail that has all the amperage you need, instead of a PSU with multiple rails, with insufficient amperage on each rail.

Myth #2: A cheap PSU with high wattage is more cost effective than an expensive PSU at the same wattage.

Again, TOTALLY FALSE! Three main reasons why:
1. No PFC - A PSU without Power Factor Conversion is very cheap, because PFC circuits (especially active circuits) are expensive to build. Cheaper PSU's also don't feature high efficiency in their AC to DC conversion, which you will see directly reflected on your JPS bill! Ideally, you want to buy a PSU which has a power efficiency of at least 80%. Anything lower than that, while cheaper, will cause JPS to ship your bill in a box. Guaranteed.
2. No Fail-Over Circuit - Furthermore, cheaper PSU's don't carry a fail-over circuit (hence why they're cheap) that allows the PSU to sacrifice itself in the event of failure. These are the PSUs that when they fail, they take your expensive \$400 8800GTS card and your 300 Gb hard drive with it. Buyer Beware! Look for PSU's in the \$50 ~ \$100 range. The higher the cost, the higher the quality of the device, the higher the Mean Time Before Failure (MTBF) and the higher the likelihood of the existence of a failover circuit.
3. Lacking Features - Finally, cheaper PSU's tend to not carry PCI-E, enough SATA, or Molex connectors for your monster rig. Even though they're more expensive, go for PSU's with sheathed or modular connectors. It makes your setup a LOT tidier and increases air flow in, and thus the lifespan of your rig.

Myth #3: Only cheap PSUs from specific brands suffer from DOA.

FALSE.

Every brand of PSU has shipped a unit that is delivered DOA (dead on arrival) at some point. Unfortunately, that's the reality of doing business over the internet. Merchants are busy serving thousands of customers all over the continent simultaneously. They don't have the time to check every single unit before it ships. Thus, every once in a while, a unit will ship DOA. However, the likelihood of an established, costly name brand unit shipping DOA is far lower than the cheaper models at similar ratings. The reason for the more expensive units (aside from the additional features) is that they employ a production line of quality testers to make sure their units don't ship as dead weights.

Beware of branded units that you can only find at one merchant. Those units are usually cheap production devices which the manufacturer pays the merchant to over stock because they won't sell in a standard brick and mortar shop where people can test the units before somebody buys and walks out the door. Those units (e.g. Rosewill @ NewEgg) make their money from the shipping that you are not refunded when you have to RMA the dead unit. Again, buyer beware.

[Cont'd...]

10. ## Power Supply Units, Explained (Fin)

Myth #4: It doesn't matter how many amps are on one rail. Just sum all the amps in the unit on all the 12v rails to know what GPU you can run.

This is a vicious lie. Amperes don't cross rails. No PSU will do that. The PSU would nuke one or more devices on your motherboard to make that possible. This is exactly why multi-rail PSUs exist. Make sure that at least one rail in your PSU carries enough amperes for your video card. If you're building a new PC, check the amp requirement on the box before you select a power supply.

If you're going after a monster rig, you're better off buying a PSU with only one rail with enough amperage to satiate your power hungry GPU. Most multi-rail PSU manufacturers assume that nobody is rich enough to put two 8800GTS/X cards in one rig. They assume that you're more likely own ONE 8800GTS/X card and one lower end GPU. This is why most multi-rail PSU's are inadequate. Furthermore, multi-rail 12v units are more conducive to ATI crossfire configurations.

Myth #5: I need more than one power supply unit.

If you're building a time machine, then sure. If you're building a server, again, sure. If you're building a system that is running two high end GPUs, RAID, an Intel Quad Core Extreme processor, water cooling, one of those monster Zalman CPU coolers (like the 9700CPS LED), 4 gigs of DDR3 Memory with their own heat sink + fan, and you have the intention of overclocking EVERYTHING - then sure. Go for it.

However, if you're not living with MaxFactor or are otherwise not planning on building a rig that would cause the lights in Jamaica to go dim for a second when you power it on, then don't be stupid. Save your money. Enthusiast configurations rarely see a noticeable performance gains that justify going overboard. Most people don't need resolutions higher than 1280x1024. Most games don't use it. So don't tax your system (or your pocket) unnecessarily.

Hopefully this would've helped you understand a lot of the requirements for this key element in building your new rig. If you have any additions or corrections you'd like to make, please feel free to add as you see fit.

Happy hunting.

[fin]

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