# Computer Power – CompTIA A+ 220-1001 – 3.7

A computer’s power supply is one of the most critical components of the system. In this video, you’ll learn how the power supply provides DC power voltages for use inside of your computer.

In this video, we’re going to talk about computer power. But before we do that, I want to give you some very important warning messages.

The first one is you should always disconnect from any power source when you’re working on any kind of device. A computer, a laptop, a mobile device, or anything else. You must also remember that some devices will store charge in capacitors. This charge could provide an electric shock, so you want to be sure that you are discharging any capacitors before you begin working on that device.

You should also be sure that you never connect any part of your body to any part of an electrical system. I have heard of cases of people connecting themselves to an electrical ground. This is something you do not want to do. Electricity does not respect you. It will shock you. It could electrocute you. You want to be sure to respect electricity when you’re working with these computing devices.

Your computer or your laptop uses DC power, direct current, to be able to operate. But the power that’s coming out of your wall outlet is probably alternating current, or AC. We use these power supplies to convert 115 volts AC or 220 volts of AC, depending on where you live in the world, into something your computer can use. Most often, that’s 3.3 volts of DC power, 5.5 volts of DC power, and 12 volts of DC power.

This is one of the most important components in your computer. If your power supply isn’t working, some or all of the components in your computer will fail to work, as well.

In very broad terms, we can describe electricity flowing through a wire as water might be flowing through a hose. An ampere, or an amp, you might also see this abbreviated as a capital A, is the rate of electron flow past a point in one second. So you can think of this as the diameter of the hose the water is flowing through.

The voltage, which you might also see abbreviated as volt or a capital V, is the pressure of the electrons that are being pushed through that particular wire. If we compare that again to water flowing through a hose, the voltage would be how open that faucet is allowing the water to flow through that hose.

Another electrical measurement you might see is the number of watts. You could also see this abbreviated as a capital W. This is considered to be a measurement of real power use. If you wanted to calculate this, you could multiply volts times amps and that would equal the number of watts. For example, if you have a 120 volt electrical line and you’re using 0.5 amps, then your measurement of real power use is 60 watts.

If we go back to our concept of water, the total amount of water that would be flowing through that hose would be a combination of the pressure of the water, or the volts, multiplied by the diameter of the hose, or the amps.

We’ve already made reference to AC and DC power, so let’s look at these two types of currents. Alternating current is often abbreviated as AC and is often referenced as a line with these curves. As the name implies, alternating current is constantly reversing direction as it flows through the electrical lines. This allows us to efficiently distribute AC power over very long distances.

If you look at the characteristics of AC power, you’ll see that these frequencies of change are a little bit different depending on where you live in the world. For example, in the United States and Canada, it’s common to see AC power running at 110 to 120V, and the frequency of change of that AC power is 60 Hertz or 60 cycles per second.

In other places in the world, such as Europe, it’s common to see 220 to 240 volts of AC power and their frequency of AC voltage runs at 50 Hertz.

The other type of current that we’ll reference is direct current or DC. This is often shown as a single line on the top with smaller lines underneath. The primary characteristic of direct current is that it’s moving in a single direction with a constant voltage.

So you can already see there’s differences in how we use alternating current in different parts of the world, with the United States and Canada using one type of voltage and frequency, and Europe using a completely different set of voltages and frequencies.

One way that we’re able to choose which one of these we’d like to use is by having a switch on a power supply that designates what type of power system we’ll plug into. There’s usually a switch on the back of the power supply that allows you to set the input to match the type of power you’re connecting to. There’s usually two options on the switch, one for 110 or 115 volts, and the other option is usually for 220 or 230 volts.

The type of power you would set them will depend on what you’re connecting to. If you’re not sure what type of power you’ll be connecting to, then you may want to get a multimeter connected to those wall outlets and see exactly what the voltage might be.

Many power supplies are auto switching power supplies, which means they will automatically determine what the input voltage is and will adjust itself accordingly without you having to set any manual switches on the power supply.

If you’re connecting a power supply and the power input is not providing enough voltage, there’s usually not any permanent damage. But if you connect a 150 volt power supply to a 230 volt power source, you can seriously damage the power supply or more of the components inside of that computer.

Here’s a good look at that manual switch that you might find on the back of a power supply. You want to make the change to that switch, and then connect it to the power source. You might want to check with the specifications of your power supply to see if there’s a manual switch involved or if it’s an auto switching power supply.

Inside of your computer, that power supply will be providing a number of different voltages for the components on a motherboard. The primary motherboard connector will be this 24 pin connector. That’s very easy to find on a motherboard, it’s one of the largest components on the motherboard itself. It’s commonly providing plus 3.3 volts, plus and minus 5 volts, and plus and minus 12 volts for the motherboard and the motherboard components.

On older motherboards, you may find this connector is 20 pins instead of 24 pins. That was the original standard for ATX motherboards. There were an additional 4 pins added for a total of 24 on the newer motherboards that are using the PCI Express bus.

If your power supply is providing you with a 24 pin connector but your motherboard has a 20 pin connector, you can still plug-in to that same connection. There will, of course, be four pins that don’t fit into that exact connection. Some power supply connectors even allow you to disconnect those four pins to move them out of the way of the connector.

Here’s a 24 pin connector on a motherboard. You’ll notice the individual pins of the connector are keyed. Some of the pins are square and some are a bit rounded off at the top. This means that there is only one way that you could possibly plug-in, this 24 pin connector.

Here’s the view from the connector side and again, we see these key connections that are perfectly square and others that are a little bit rounded along one side of the connector. Whenever you’re installing any type of connector onto the motherboard, you want to use as little force as necessary. So you’ll make sure that you line up this connector and gently push it down into the connection itself.

Most of these power connectors will also have a lock on them. So to remove this power connection, you have to push down to release that connection from the bottom. This means that this connector is not going to accidentally pull out of the power connection, but it also means you need to be very careful when removing it, making sure that you release the lock before pulling it out of this connection.

We’ve already seen that the power supply in your computer provides different voltages. These different voltages are designed to power different components on your motherboard. You’ll also find that these voltages are referenced as a positive voltage or a negative voltage. This is referring to the difference in electrical potential if you’re using an electrical ground as your common measuring point.

For example, if you’re standing at the front door of your house, the second floor would be plus 10 feet from where you are and the basement would be negative 10 feet from where you are. A power supply is commonly providing positive 12 volts to larger components that are on your motherboard, things like PCI express adapters or drive motors.

You might also see that your power supply can provide positive 5 volts to motherboard components, although these days, many of those motherboard components have shifted to use positive 3.3 volts. And those plus 3.3 volts components would be things like your memory slots, M.2 slots, and other motherboard components.

It’s more uncommon to find negative 12 and negative 5 volts used on a modern motherboard, but some older motherboards may be using negative 12 volts for an integrated network connection or older serial ports. And if your motherboard supports PCI connections, there are some PCI cards that need negative 12 volt.

You might find some of the older power supplies that can provide negative 5 volts, but your modern power supplies probably won’t provide this that’s because these were most commonly used for the older ISA bus adapter cards, and even some of the older isa BUS adapter cards didn’t even use negative 5 volts. Of course, your modern motherboards are not going to have any ISA slots on them, and therefore they don’t require negative 5 volts.

Here’s a good example of some of these power characteristics on a power supply. This power supply supports 100 to 240 volts of AC input, so it will work no matter what country you happen to be in. It requires 10 amps or five amps of input, depending on the voltage that’s being used, and it supports 47 Hertz through 63 Hertz of frequency on those power inputs.

The amount of DC power provided by this power supply is 3.3 volts, 5 volts, you have multiple 12 volt rails, a negative 12 volt, another negative 12 volt, and a positive 5 volt that you use for standby power.

Power supplies are usually rated by the number of watts they can support. There’s usually a large number for the overall wattage. This particular power supply can support 850 watts of power, but you can also see that there are separate wattages depending on the different voltages for this power supply.

You want to size the power supply to match the type of computer it’s going in. You don’t want to purchase a power supply that’s too large because you’ll be paying too much, and having a larger power supply doesn’t provide you with any additional performance improvement.

Most computer cases are very standardized. You’ll find that most power supplies will fit into most computer cases. But if it’s a proprietary computer, you may find a specialized power supply shape that you can only get from that manufacturer.

To size the power supply, we need to determine how many watts we’ll be using. So you want to add up all of the different devices inside of your computer– your CPA, your storage devices, your video adapter– to determine how many total watts you’ll be using. If you’re not quite sure, there are a number of online calculators that can help you estimate the right numbers for your computer.

If you’re not using the onboard video of your CPA and you instead have a separate video card, that’s probably the device that’s going to be using the most amount of power inside of your computer, and usually the video card manufacturer will provide you with a list of recommended power for that particular card. A good rule of thumb is to pick a power supply that hits about 50% of your requirement. That will keep your costs down, but still allow you room to grow in the future if you add more components to your computer.