From Wikipedia, the free encyclopedia
A power supply (sometimes called a power supply unit or PSU) is a device or system that supplies electrical or other types of energy to an output load or group of loads. The term is most commonly applied to electrical energy supplies, less often to mechanical ones, and rarely
to others.
[edit] Electrical power supplies
This term covers the mains power distribution system together with any other primary or secondary sources of energy such
as:
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.
[edit] Computer power supply
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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.
[edit] 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", "adaptor block", "AC adaptor" 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" adaptors 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.
[edit] 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.
[edit] 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 — 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.
[edit] 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.
[edit] Uninterruptible power supply
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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.
[edit] 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.
[edit] Mechanical power supplies
[edit] Terminology
- SCP
- Short circuit protection
- OPP
- Overpower (overload) protection
- OCP
- Overcurrent protection
- OTP
- Overtemperature protection
- OVP
- Overvoltage protection
- UVP
- Undervoltage protection
- UPS
- Uninterruptable Power Supply
[edit] References
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This article has been tagged since March 2007. |
