Computer Power – CompTIA A+ 220-1101 – 3.5

A power supply is a critical component on every computer. In this video, you’ll learn about power specifications, recognizing power connectors, and how to size a power supply.

In this video, we’re going to talk a lot about computer power. But before we do that, I want to make sure you understand that if you are working inside of your computer, you should always disconnect from the power source when you are working on that device. This is a non-negotiable best practice. And if you hear anyone else tell you that it’s OK to keep everything plugged in, you should completely disregard that advice and always disconnect from every power source.

Another challenge you might have is that some components will have capacitors that will store power. So even when you’re unplugged from the power source, those devices could still shock you. You should always follow the manufacturer’s instructions to ensure that all of the power in that device has been dissipated.

You should also never connect any part of your body to any part of the electrical system. And that includes the electrical ground. Do not connect yourself to any part of the electrical ground system. Electricity follows some very specific rules. And it doesn’t care whether you’re playing by the rules or not. If you happen to be in the way, there’s a possibility you could get shocked. You should always respect electricity and make sure that you are always disconnecting yourself from any power source.

Your computer motherboard and components commonly use Direct Current to provide power, or DC power. But your power sources are receiving Alternating Current, or AC power, from the power outlet. That means that we’ll need to do some type of conversion between alternating current and direct current. One way you would do that is through the power supply of your computer, which converts that input of AC power to provide an output of DC power.

And it’s very common for these power supplies to provide 3.3V volts, 5 volts, and 12 volts of DC power. We often say that this is the most important component inside of your system because if your power supply isn’t working, nothing is going to work.

One measurement that we use with power is an ampere or an amp. So sometimes see this abbreviated as a capital A. This describes the rate of electrons that are passing by a particular point in one second. You can think of this as electricity flowing through a wire, is very similar to water that’s flowing through a hose. And if you have a larger diameter hose, you’re able to support a larger number of amps.

Another important measurement is voltage. This is sometimes abbreviated as volt or a capital V. This is referring to how much force is pushing that water through the hose. If you open or close a faucet to change the amount of water that’s being pushed through a hose, you can think of this as changing how much voltage is going across a wire.

If you start looking at power supplies that are used on computers, they’re often rated by the number of watts. A watt is a measurement of the real power use for that power system. And it’s often abbreviated with a capital W. If you wanted to calculate the number of watts that would be used, you take the volts that are in use, multiply it by the number of amps, to have the total number of watts. So if you’re using 120-volt power with a device that uses 0.5 amps, then you have a total measurement of 60 watts of real power use.

Alternating current is abbreviated as AC. And as I mentioned, that is the type of power that we are receiving from the outlet that’s in the wall. We’re commonly distributing power using alternating current because AC power can be sent over very long distances. The reason we call it alternating current is that the direction of the current is constantly alternating between one direction and another.

We often refer to the number of times this current alternates as a total number of cycles or hertz. It’s very common in the US and Canada to have 110 to 120V of alternating current. And it’s alternating at a rate of 60 times a second or 60 hertz. In Europe, you’ll commonly find 220 to 240 volts of alternating current coming from your wall outlet. And it commonly uses a 50-hertz frequency.

Well, if alternating current is going into our power supply, then direct current, or DC, is what is coming out of our power supply and onto the motherboard. With DC, we don’t have this current going back and forth. And just like the name implies, a direct current is moving in one direction only.

With these differences in power systems, if we take a computer from one country and move it to another country, then we’ll want to be sure that the power supply is able to support these changes in voltage. In the US, of course, it would be 120 volts of AC running at a 60-hertz frequency. And in Europe, 230 volts of AC running at a 50-hertz frequency. Your power supply may have a switch on it that allows you to change between 120 volts and 230 volts.

If you’re not sure exactly what voltage is coming out of your wall outlet, you can use your multimeter and determine exactly what the voltage might be. With some power supplies, you may not have a switch on the back of the power supply. Instead, the power supply itself can automatically switch between 120 volt and 230 volts.

If you’re not using an auto-switching power supply, then you want to be very careful of what power source you’re connecting to. You would never want to plug in a manually switched 120 volt power supply into a 230 volt power source, or you’re going to see sparks and a lot of smoke come from that power supply.

Here’s a better view of this switch that’s on the back of the power supply. We would disconnect from our power source. We would set this appropriately to 230 volts or 120 volts, and then reconnect the power to that power supply.

This power that’s going into the power supply is now going to convert it to direct current. And it’s not going to convert it to one single voltage of direct current. Instead, there will be multiple voltages that will be used by different components on your motherboard.

Another characteristic of this direct voltage is that we specify it as either a positive value or a negative value. The voltage is the difference in electrical potential as compared to the electrical ground. And where you measure from becomes very important.

If you think about standing in the front door of your house, the second story of your house is 10 feet away from you. But since that is above you, we often refer to that as being plus 10 feet. The basement of your house is also 10 feet away from you. But it’s underneath your feet, so it would be negative 10 feet away. Both of these are 10 feet away but you can see we are measuring it in different ways and referring to it as a positive or a negative.

Higher voltages are commonly associated with components inside of your system that need additional power. So things like PCI Express adapters, hard drive motors, or fans might require plus 12 volts to be able to operate. Some power supplies can also output a plus 5 volt of DC power. And these are usually reserved for our legacy motherboards. Most modern motherboards do not use plus 5 volts and instead are using plus 3.3V volts.

And on those more modern motherboards, you have M.2 slots, some RAM slots, and other components on the motherboard that will take advantage of those 3.3 volts. Some power supplies also have a separate line for a plus 5 volts SB. The SB stands for a Standby voltage. Many power supplies also provide plus 5 volts of SB, or Standby voltage. That standby is used when your system is in a hibernate mode. And the standby is constantly checking to see if you’re going to push the power button to bring that sleeping computer back to life.

Some older components on your system may use negative 12 volts for integrated local area network connections or some of the older serial ports that are in your system. And some motherboards might provide negative 5 volts of power. That minus 5 volts is really for legacy systems. Most modern motherboards won’t need minus 5 volts from the power supply. And you’ll even find some models of power supply that don’t even provide minus 5 volts as an option.

Here’s an example of a power supply with its specifications written right on the side of the unit itself. This power supply supports an input that ranges from 100 volts all the way up to 240 volts. And that certainly covers the 120 volts we need in the US and the 230 volts you might find elsewhere in the world. This also supports either 10 amps or 5 amps of input, depending on whether you’re using 120 volts or 230 volts.

And, of course, there are differences in the frequencies used on those power systems. And this power supply supports 47 hertz through 63 hertz.

This power supply also supports a wide range of output. You can see the different voltages that are supported from the output of this individual power supply. For each one of these outputs, you can also see the max current that’s supported for this particular power supply model. And if you add up all of the DC voltage that’s supported by this power supply, it can support a total of 850 watts.

We get power from the power supply to the motherboard through this very large 24-pin connector. This is our main motherboard power. And it provides the 3.3 volts, plus or minus 5 volts, and plus or minus 12 of DC power.

This was originally a 20-pin connector. And you might still see that on some older motherboards. Most modern systems will use this full 24 pins of motherboard power. If you do run across one of those older motherboards, you can still plug in this 24-pin connector. It’s just that the four pins at the end will not be connected to anything on that motherboard.

Here’s a good view of that connector. You can see that these individual pins are keyed so there’s only one way you can plug in that power block into this connector. And you can see where the keys are on this connector. There’s a rounded key on this pin. And this pin is clearly squared off on the end.

With everything unplugged from your system, you would connect the power connector from the power supply directly to the motherboard. This connector also includes a clip. So when you push it down into the connection, it not only pushes in place but it locks itself down. And the only way to remove this connector is to push down on the clip and remove the entire connector from the motherboard.

Our computers at home probably have a single power supply inside of that system. But if you’re working in a data center or with other servers, you may find devices that have multiple power supplies on the back. These are three separate servers. And all three of these servers have two separate power supplies on the back. You can see that only one side of the power supply is currently connected.

Each one of these power supplies can technically handle 100% of the load. If you have both of these plugged in at the same time, they will evenly distribute the load across both of them. And if one of these power supplies fails, the other power supply takes the entire electrical load of the system.

If you then need to replace one of these power supplies, they’re very commonly hot-swappable. So there’s a clip right on the top of this when you push down. And you can slide out the entire power supply and simply replace it with a new one without powering down the server.

If you look at the power supply in your system, there may be a connection for power on one side. And then on the side going to the motherboard, there may be a fixed set of connectors that not only include your motherboard power but also power for all of the other components on your system as well. There are obviously some limitations with this because if you run out of a particular type of connector, you’ll have to replace the entire power supply.

Some power supplies are built to be modular. Instead of having a cable that is permanently attached to the power supply, you can decide exactly what you’ll plug in and use as few or as many connections as you might need. This means you’ll have fewer wires inside of your system. And you’ll have better airflow through the system, as well.

Here’s a hybrid version of a fixed and modular power supply. You can see the fixed wires coming out of the power supply. And then you can choose which one of these modular connectors you might need. The power supplies I like to use in my systems are completely modular. So you would have a system with no wires coming out the back, and you would decide exactly which ones are necessary for your particular implementation.

We’ve already seen that there are a lot of nuances to the power supplies you might use. But you generally would size a power supply based on how many watts it can support. This particular power supply is an 850 watt model. And you can see the documentation for that max power down here at the bottom.

You would obviously need to balance the maximum capacity of the power supply with how much power you’re going to need for that system. You can certainly purchase a power supply that supports a very high number of watts. But that means it’s going to be also very expensive compared to smaller power supplies. And having additional capacity in your power supply doesn’t change the overall speed or efficiency of your computer.

Fortunately the physical size of these power supplies is very standardized. So you should be able to purchase a power supply from any manufacturer, and that should fit just fine in your computer case. The only times when this might be a problem are if you’re using a very old system or one that’s proprietary that does not conform to these modern standards.

To determine the proper size for a power supply for your system, you’ll need to perform some calculations. Fortunately, there are a lot of websites that provide calculators that can help you with this. But you can also calculate this manually by determining the wattage you would need for all of the components inside of your system, including the CPU, storage devices, video cards, and anything else inside of your computer case.

If you’re installing a separate or discrete video card, then you’ve probably already seen that those cards use a lot of watts. So the documentation of that video card will often tell you what a good starting place might be for the power supply that you would use.

Once you perform these calculations and you have a pretty accurate number of how many watts your system might need, you’ll need to purchase the power supply. A good rule of thumb would be to take the wattage that you’ve calculated and double that number. This would provide you with a power supply that can not only run efficiently for the amount of work that you’ll be doing, but it also provides you with some room for upgrades in the future.