# Power Specifications – CompTIA A+ 220-901 – 1.8

December 4, 2015

Computer power systems have their own language and architecture. In this video, you’ll learn about amps, volts, power calculations, current, voltage, power supply sizing, power supply options, and much more.

When you’re working with power inside of the computer, you’ll probably hear the terms amps, volts, and watts. An amp is an ampere. You’ll also see this abbreviated as the letter A. And this is a measurement of how many electrons will flow past a particular point in a single second.

You’ll also see the term volts. Voltage is also represented as the abbreviation volt or V. And this is referring to effectively the pressure that is pushing those electrons through the connection.

You can think of this in relation to a garden hose. You have water that’s flowing through the hose. If you have a larger hose, you can push more amps, or more water, through that hose. If you were to turn the faucet on a little bit more, you would be creating more pressure in that hose, which is similar to creating more voltage.

To measure the amount of power being used then, we need to calculate the number of watts. The calculation for watts is to simply multiply the number of volts times the number of amps. So if we have a device that is using a 120 volt connection, and it is using half an app, or 0.5 amps, that means it’s using a total amount of 60 watts of power.

Our computers use both alternating current and direct current. Alternating current is often abbreviated AC. And you’ll see it described with this wavy line.

It’s called alternating current because the direction of the current is constantly changing or alternating. It’s a very efficient way to run power over very long distances, which is one of the main reasons we use alternating current on our power lines.

The frequency used on these alternating current connections is very important to us when we’re plugging in our computers. In the United States and Canada, we generally use 110 to 120V of alternating current, or VAC. And it’s running at a frequency of 60 Hertz. In Europe, it’s very different. It’s 220 to 240 volts of AC, running at a frequency 50 Hertz.

The power supplies that are inside of our computers take this alternating current from our wall outlets. And it converts it into direct current, or DC. You’ll sometimes see direct current abbreviated or shown with this long line at the top, and the smaller lines on the bottom. With the DC, your current is always going to move in one direction with a constant voltage.

You may see power supplies described as a single rail power supply or a multi-rail power supply. With a single rail power supply, there is a single 12 volt rail that is providing those 12 volts to the motherboard. It is one single circuit providing one set of outputs for those 12 volts.

A multiple rail power supply is going to have multiple 12 volt rails inside of it. Each one of these rails is going to be on separate circuitry. So they are separated from each other. And each time you connect one of the 12 volt connectors onto the back of that power supply, it’s going to be using a separate rail.

Ideally, you are distributing the load across these multiple rails. And this provides a bit of safety for you. If you were to overload a single rail, you very often get a lot of smoke, and your power supply ceases to function.

But if you can overload a power supply and distribute it across multiple rails, it sees that your power supplies has gotten to a certain point. And it can simply shut down the power supply, instead of burning up any components.

If you read the label that’s on your power supply, you should be able to see whether it’s a single rail or multi-rail, and what the limitations might be on the power. You can see that this power supply allows me to bring in either 100 or 240 volts, depending on where you are in the world. You can bring in either 10 amps or 5 amps, depending on the voltage that you’re using. And it will input a range between 47 Hertz and 63 Hertz from your main power. This power supply will output 3.3 volts at 35 amps and 5 volts at 30 apps, as long as you don’t exceed 200 watts of combined power.

Here’s where the multiple 12 volt rails are. 12 volts, one, two, three, and four are the four multiple rails. Each of those can output 20 amps. And you can see it can go all the way up to 615 watts, but no more than 50 amps across those four rails.

You also have a negative 12 volt at 1.5 amps for 15 watts, and 5 of standby power at 4 amps, for a maximum of 20 watts. And this entire power supply can provide us with 850 watts of power.

Sizing a power supply for your computer is not too difficult, as long as you follow some standard guidelines. You don’t want to overdo things. If you buy too much of a power supply, you’ll simply be spending more money and not using the capabilities of that particular power supply. And just because you get a bigger power supply doesn’t mean your computer is going to go any faster.

The size of the power supply is relatively standard these days. But if you’re running on a very small case or very proprietary system, you may need a proprietary power supply.

You want to calculate the watts that you’re going to need for all of the different components inside of your computer. Find out what your CPU needs, the storage device that you’re using. If you have a video adapter that needs a particular set of power, make sure you calculate and combine all of those together to determine the maximum amount of wattage you’ll need.

If you’re using a separate video card inside of your computer, that’s probably the component that’s going to use the most amount of power. The video card manufacturers already know this. And if you check their documentation, they have a list of what some recommended power supply sizes might be if you’re using a particular model of video card.

Once you calculate everything, a good rule of thumb is to get a power supply that is twice as big as that particular amount. That you have some room to grow. And you’re sure that the power supply itself is not going to be overloaded with anything you add to that computer.

The connectors coming off the power supply may be fixed to the power supply itself, or they might be modular. The fixed connectors are very much like this picture, where all of the connectors are coming out of the inside of the power supply. If you don’t have enough connectors, then you’re going to have to get a different power supply. You can’t easily change these out.

On a modular power supply, you’re adding cables as you need them. So you don’t have a lot of leftover wires once you’ve connected up all of the components. These are little bit more expensive, but your air flow is going to be much better through your computer.

Here’s a power supply that’s half fixed and half modular. So you’ve got all of your fixed cables coming out of the power supply. And you also have room to add things modularly here at the top.

Some power supplies are completely modular. It’s completely empty in the back. And you would simply add cables as you need them.

Power supplies these days come in some very standard dimensions. So whether you have an ATX case or microATX case, the power supply is probably going to fit just fine. You want to check for documentation to make sure it’s going to fit inside of your case.

If you have a power supply from an older computer you’d like to use, you may find that it’s a little larger than some of the newer power supplies. So if you have a microATX case, you may not be able to use one of those older power supplies.

And there’s even some microATX cases that are specially designed to be as small as possible. And in those cases, you might need a very specifically sized power supply.

Here’s a standard ATX case. You can see there’s plenty of room here right at the top for the power supply where we plug in from the wall. And you can see all of the cables coming out and feeding the different components on the motherboard and inside of the case. If it’s a proprietary case, like this small case here, you may need to get a very specific kind and size of power supply to fit into that smaller place.

As we mentioned earlier, the voltages in the United States and Canada are between 100 to 120 volts of AC, running at a 60 hertz frequency. In Europe, the ranges are between 220 to 240 volts of AC, running at a 50 hertz frequency. Those are two very different voltages. So when you’re connecting your computer, you want to be sure your power supply is configured properly.

Some power supplies will have a manual switch on the back that allows you to switch between 110 or 115 volts and the 220 volt connections. If you’re not sure what voltage is in the wall, you might want to get your multimeter and connect it to see what the voltages are coming from that particular connection.

What you don’t want to do is plug a power supply that is designed or configured for 115 volts and connect it to a 230V volt power source. You’ll know very quickly from all of the smoke that you made a very bad decision.

Here’s the back of a power supply that has one of these manual connections between 230 volts and 120 volts. This one is currently set to 230. So if we’re plugging in the United States, we may want to make a change before connecting to the wall outlet.

Some power supplies don’t have a switch on the outside. That means that they will either run at one particular voltage, or they’re auto-switching between two voltages.

This happens to be an auto-switching power supply. We know that because when we look at the label it shows us that the input can range between 100 and 240 volts. This means that we can connect this power supply inside the United States. Or we can take this same power supply to Europe and plug in, and we don’t have to make any configuration changes to the power supply itself.

Category: CompTIA A+ 220-901