The need for a PSU lies in the fact that computers don't work with AC: they need a constant voltage, one that never changes, and it also needs to be of much lower level. Using the same graph scales, it looks something like this:. And because these values are constant, they're called direct current or DC, for short. Time to open the unit and have a look at how it does this! At this stage, we should warn you to not try this if you don't know what you're doing.
Messing about with the insides of a PSU can be potentially very dangerous. There are components inside every unit that store electrical energy, and some store a lot. The layout of this PSU is similar to many others, and although the make and model of the various parts used inside will be different, they fundamentally do the same thing. The mains outlet connection to the PSU is at the top-left hand corner of the picture and the supply essentially runs clockwise around the picture, until reaching the output of the PSU big cluster of colored wires, bottom left-hand corner.
If we flip the circuit board over, we can see that compared to connections on a motherboard , these are broad and deep -- they're designed to have lots of current flowing through them. Something else that's immediately obvious is the big gap running down the middle, like a river cutting a path in a field. This highlights the fact that all PSUs have two clearly defined sections to them: primary and secondary. The former is all about setting up the input voltage so that it can be efficiently changed from the mains supply level; the latter is everything about that change and the processes afterwards.
The very first thing the PSU does to the mains electricity isn't about changing it from AC to DC, or dropping voltage -- instead, it's all about smoothing out the input voltage. Because we have lots of electrical devices in our homes, offices, and business that switch on and off, as well as emit electromagnetic signals, the varying AC is often lumpy and with the occasional spikes the length of the variations isn't constant either.
Not only do these make it harder for the PSU to adjust the mains, it can also damage some of the components inside it. This PSU has two stages of so-called transient filters , the first of which is directly applied to input socket, using 3 components called capacitors to do the job. Think of these as being like a speed bump for sudden changes in the input voltage. The yellow blocks are more capacitors, whereas the green rings wrapped in copper wire are inductors although they're usually called chokes when used this way.
Inductors store electrical energy in a magnetic field but this field also 'pushes back' on the voltage supplying the energy -- so a sudden spike in the voltage results in a sudden kick back from the magnetic field to suppress it. The two little blue discs are yet more capacitors and just below them hidden under a black plastic cover is a metal oxide varistor MOV. These are also used to help counter jumps and spikes in the input voltage; you can read more about different types of transient filter circuits here.
This section of a PSU is often the first sign of where costs have been cut to ensure the model hits a specific budget. Cheaper ones will have less filtering, and the cheapest of all will have none at all which is not what you want! Now that we're all smooth and chilled, let's get on with the day job of a PSU: changing the voltage. Remember that the PSU needs to change an AC voltage that might be averaging volts technically, it's root mean squaring volts, but that doesn't exactly roll off the tongue and hack that down into DC voltages of 12, 5, and 3.
In the picture below, this is the flat black object glued to the chunk of metal which acts as a heatsink. Once again, this is another area where a PSU manufacturer can cut costs, with cheaper components doing a worse job of the AC-to-DC conversion e. Now, if the input voltage peaks at volts which is the case for V mains , then the bridge rectifier will output volts DC.
This gets passed on to the next stage of the PSU and in the one we're looking at, it's called an active power factor correction converter APFC.
This circuit adjusts the current flow in the unit to take into account that it is full of components that store and release energy in a complex way; this can result in the actual power output of the unit being less that what you're supposed to be getting. Other supply units use passive converters, that essentially do the same job.
They're less effective but fine for low power units -- they're also cheaper, so you can guess what kinds of PSUs have these, when they really shouldn't! The APFC can be seen in the image above - those big cylinders on the left are capacitors and they store the adjusted current, before sending them on to the next step in the PSU's chain of processes. Its job is to take the DC voltage and use several field effect transistors to switch the voltage on and off at a very high rate -- it essentially converts the DC voltage back into an AC one.
It does this because the part of the PSU that turns the mains voltage right down into 12 volts is a transformer. These devices use electromagnetic induction and a set of two coils of wire one having more loops in the coil than the other to step down the voltage; however, transformers only work with an alternating voltage.
There's also a fan opening at the back of the power supply that sends air out the back of the computer case. The side of the PSU facing outside the case has a male, three-pronged port that a power cable, connected to a power source, plugs into. There's also often a power switch and a power supply voltage switch. Large bundles of colored wires extend from the opposite side of the power supply unit into the computer.
Connectors at the opposite ends of the wires connect to various components inside the computer to supply them with power. Some are specifically designed to plug into the motherboard while others have connectors that fit into fans, floppy drives , hard drives , optical drives , and even some high-powered video cards. Power supply units are rated by wattage to show how much power they can provide to the computer.
Since each computer part requires a certain amount of power to function properly, it's important to have a PSU that can provide the right amount. The very handy Cooler Master Supply Calculator tool can help you determine how much you need.
For most people, the noticeable differences just speak to the physical connection plug on the motherboard. Choosing one over the other depends on the type of motherboard that's being used. The newest standard, ATX12V v2.
Motherboards using ATX12V 2. ATX motherboards use a pin connector. One situation where the pin count comes into play is when deciding if a particular power supply works with your system. ATX12V-compliant power supplies, although they have 24 pins, can actually be used on an ATX motherboard that has a pin connector.
The remaining, unused four pins will just sit off of the connector. If your computer case has the room, this is a completely doable setup. However, this doesn't work the other way around.
If you have an ATX power supply that therefore has a pin connector, it won't work with a newer motherboard that requires all 24 pins to be connected. The extra four pins were added with this specification to supply extra power through 12V rails, so a pin PSU can't provide enough power to run this kind of motherboard.
Everything contained in the computer chassis is powered by the power supply. For example, the motherboard, RAM , CPU , hard drive , disc drives , and most video cards if the computer has one are all drawing power from the power supply.
Any other external devices and peripherals , such as the computer monitor and printer , have a power source or draw power over the data cable like some USB devices. If the computer is a laptop or an All-in-One computer , the display is powered by the computer power supply.
While the computer is on, the fan s inside a power supply should always be running. If the fan is not running spinning , either the computer is not working or the fan has failed and the power supply should be replaced. Some power supplies have variable controls that may increase or decrease the speed of the fan depending on its temperature. However, it should always be spinning.
Home Dictionary P - Definitions. The voltage thus obtained is then switched on and off at very high speeds using electronic switching circuitry, effectively producing a high-frequency square wave voltage effectively, a series of dc pulses. A light and relatively inexpensive high-frequency transformer can then be used to produce the required dc output. The dc output voltage and current are regulated kept constant using a feedback controller that increases or decreases power output in accordance with variations in load current.
It does this by increasing or decreasing the duty cycle essentially, this means increasing or decreasing the number of voltage pulses produced by the switching circuitry in a given time frame. Note that most PSUs can shut themselves down if load current exceeds a certain threshold, reducing the possibility of damage to the computer system or its user in the event of an electrical fault such as a short circuit.
The same principle applies to the absence of a load current or a very low load current , since the PSU cannot operate correctly below a certain power output level and will shut down if insufficient load current is detected.
When first turned on, it can take half a second or so for the power supply to stabilise and start generating the correct dc voltages required by the computer. The power supply therefore sends a signal to the motherboard called the Power Good signal, once it has carried out its internal tests and is satisfied that the power outputs are all as they should be. The motherboard must wait for this signal before powering up the system.
A power surge or momentary power failure will sometimes cause a short interruption in the Power Good signal, which will cause the system to reboot when it is resumed. Note also that for practical reasons the different voltages produced by a power supply unit are actually produced by several different switched-mode supplies that are linked together within the PSU, each of which varies its output according to component power requirements. One recent trend in PSU design has been the concept of a modular power supply , in which cables can be attached to the PSU via connectors at the power supply end , allowing the user to install only the cables they actually need.
The idea is that the omission of cables that are not required will reduce clutter inside the case and improve airflow. It also provides more choice in the type of power cable the user can install e. Serial ATA or Molex for hard drives. Critics of this development have pointed out that electrical resistance will be increased due to the greater number of electrical connections. Proponents point out that the increase in resistance is very small.
In practical terms however, problems are only likely to occur if the connectors are old and worn in which case the connection may be a loose one or the connection has not been made correctly during installation. The obvious answer is to replace old cables and check all connections prior to first use.
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