Power supply
From Wikipedia, the free encyclopedia

Power supply is a reference to a source of electrical power. A device or system that supplies electrical or other types of energy to an output load or group of loads is called a power supply unit or PSU. The term is most commonly applied to electrical energy supplies, less often to mechanical ones, and rarely to others.

Contents [hide]
1 Electrical power supplies
1.1 Computer power supply
1.2 Domestic mains adapter
1.2.1 Linear power supply
1.2.2 Switched-Mode power supply
1.2.3 Polarity
1.3 Uninterruptible power supply
1.4 Alarm Power Supply Unit
1.5 Power conversion

Electrical power supplies
This term covers the mains power distribution system together with any other primary or secondary sources of energy such as:

Conversion of one form of electrical power to another desired form and voltage. This typically involves converting 120 or 240 volt AC supplied by a utility company (see electricity generation) to a well-regulated lower voltage DC for electronic devices. For examples, see switched-mode power supply, linear regulator, rectifier and inverter (electrical).
Batteries
Chemical fuel cells and other forms of energy storage systems
Solar power
Generators or alternators (particularly useful in vehicles of all shapes and sizes, where the engine has rotational power to spare, or in semi-portable units containing an internal combustion engine and a generator) (For large-scale power supplies, see electricity generation.) Low voltage, low power DC power supply units are commonly integrated with the devices they supply, such as computers and household electronics.
Constraints that commonly affect power supplies are the amount of power they can supply, how long they can supply it for without needing some kind of refueling or recharging, how stable their output voltage or current is under varying load conditions, and whether they provide continuous power or pulses.

The regulation of power supplies is done by incorporating circuitry to tightly control the output voltage and/or current of the power supply to a specific value. The specific value is closely maintained despite variations in the load presented to the power supply's output, or any reasonable voltage variation at the power supply's input. This kind of regulation is commonly categorised as a Stabilized power supply.

Computer power supply

Computer power supplyMain article: Computer power supply
A computer power supply typically is designed to convert 110-240 V AC power from the mains, to several low-voltage DC power outputs for the internal components of the computer. The most common computer power supplies are built to conform to the ATX form factor. The power rating of a PC power supply is not officially certified and is self-claimed by each manufacturer. The more reputable makers advertise "True Wattage Rated" to give consumers the idea that they can trust the wattage advertised.

Domestic mains adapter
A linear or (rarely) switched-mode power supply (or in some cases just a transformer) that is built into the top of a plug is known as a "wall wart", "power brick", "plug-in adapter", "adapter block", "AC adapter" or just "power adapter". They are even more diverse than their names; often with either the same kind of DC plug offering different voltage or polarity, or a different plug offering the same voltage. "Universal" adapters attempt to replace missing or damaged ones, using multiple plugs and selectors for different voltages and polarities.

Because they consume standby power, they are sometimes known as "electricity vampires" and may be plugged into a power strip to allow turning them off. Expensive switched-mode power supplies can cut off leaky electrolyte-capacitors, use powerless MOSFETs, and reduce their working frequency to get a gulp of energy once in a while to power for example a clock, which would otherwise need a battery.

This type of power supply is popular among manufacturers of low cost electrical items because

Devices sold in the global marketplace don't need to be individually configured for 120 volt or 230 volt operation, just sold with the appropriate AC adapter.
The device itself doesn't need to be tested for compliance with electrical safety regulations. Only the adapter needs to be tested.

Linear power supply
A simple AC powered linear power supply usually uses a transformer to convert the voltage from the wall outlet (mains) to a different, usually a lower voltage. If it is used to produce DC a rectifier circuit is employed either as a single chip, an array of diodes sometimes called a diode bridge or Bridge Rectifier, both for fullwave rectification or a single diode yielding a half wave (pulsating) output. More elaborate configurations rectify the AC voltage at first to pulsating DC. Then a capacitor smooths out part of the pulses giving a type of DC voltage. The smaller pulses remaining are known as ripple. Because of a fullwave rectification they occur at twice the mains frequency (in USA it's 60 Hz doubled to 120 Hz - or the UK, it's 50 Hz, doubled to 100 Hz). Finally, depending on the requirements of the load, a linear regulator may be used to reduce the ripple sometimes also allowing for adjustment of the output to the desired but lower voltage. More elaborate versions used by circuit designers are adjustable up to 30 volts and up to 5 amperes output. These often employ current limiting. Some can be driven by an external signal, for example, for applications requiring a pulsed output.

In the simplest case a single diode is connected directly to the mains and uses a resistor in series with a more or less fixed load to recharge a battery. This circuit is common in rechargeable flashlights.

Switched-Mode power supply
A switched-mode power supply (SMPS) works on a different principle. AC mains input is directly rectified, obtaining DC voltage. Then this voltage is changed back to AC by using electronic switches, but with a much higher frequency (typically 10 kHz ¡X 1 MHz). Higher frequencies require smaller transformers. Then on the transformer secondary the AC is again rectified to DC. To keep output voltage constant, the power supply needs a sophisticated feedback controller - typically a single IC chip.

Polarity

Diagram explaining standard symbols for polarity.AC-to-DC adaptors have polarity (positive or negative). It is necessary to use an adaptor with the correct polarity to avoid damage.

Uninterruptible power supply
Main article: Uninterruptible power supply
An Uninterruptible Power Supply (UPS) takes its power from two or more sources simultaneously. It is usually powered directly from the AC mains, while simultaneously charging a storage battery. Should there be a dropout or failure of the mains, the battery instantly takes over so that the load never experiences an interruption. Such a scheme can supply power as long as the battery charge suffices, e.g., in a computer installation, giving the operator sufficient time to effect an orderly system shutdown without loss of data. Other UPS schemes may use an internal combustion engine or turbine to continuously supply power to a system in parallel with power coming from the AC mains. The engine-driven generators would normally be idling, but could come to full power in a matter of a few seconds in order to keep vital equipment running without interruption. Such a scheme might be found in hospitals or telephone central offices.

Alarm Power Supply Unit
Main article: Alarm Power Supply Unit
The Alarm Power Supply Unit (APSU) takes its power from the AC mains and converts this to a DC supply while simultaneously charging a 12-24 volt storage battery. During a mains power dropout or failure, the battery instantly takes over so that the load never experiences an interruption. The APSU's main function is to power Network Access Devices (Modems, Switches, Routers) and includes an IP stack that supports either SNMP, UDP, TCP/IP Alarm monitoring protocols or SMTP, POP email clients. The APSU can report a mains failure, battery low etc and most importantly has the ability to reboot Network Access Devices to ensure the communication gateway is always available. Designed to provide increase reliability of broadband circiuts where details of such failures are able to be easily reported to any conventional Alarm monitoring service provider and/or the APSU owner.

Power conversion
The term "power supply" is sometimes restricted to those devices that convert some other form of energy into electricity (such as solar power and fuel cells and generators). A more accurate term for devices that convert one form of electric power into another form (such as transformers and linear regulators) is power converter. The most common conversion is AC-DC. This is a conversion from the household current AC, to the DC current that is used in your car, and most electronics.