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 laptop power supplies


This term covers the 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 laptop 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 torque to spare, or in semi-portable units containing an internal combustion engine and a generator) (For large-scale laptop power supplies, see electricity generation.) Low voltage, low power DC laptop power supply units are commonly integrated with the devices they supply, such as computers and household electronics.
Constraints that commonly affect laptop power supplies are the amount of power they can supply, how long they can supply it 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 laptop power supplies is done by incorporating circuitry to tightly control the output voltage and/or current of the laptop power supply to a specific value. The specific value is closely maintained despite variations in the load presented to the laptop power supply's output, or any reasonable voltage variation at the laptop power supply's input. This kind of regulation is commonly categorised as a Stabilized laptop power supply.


laptop power supply types
laptop power supplies for electronic devices can be broadly divided into linear and switching laptop power supplies. The linear supply is a relatively simple design that becomes increasingly bulky and heavy for high current devices; voltage regulation in a linear supply can result in low efficiency. A switched-mode supply of the same rating as a linear supply will be smaller, is usually more efficient, but will be more complex.


Linear laptop power supply

A home-made linear laptop power supply (used here to power amateur radio equipment)An AC powered linear laptop 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 is used. A capacitor is used to smooth the pulsating current from the rectifier. Some small periodic deviations from smooth direct current will remain, which is known as ripple. These pulsations occur at a frequency related to the AC power frequency (for example, a multiple of 50 or 60 Hz).

The voltage produced by an unregulated laptop power supply will vary depending on the load and on variations in the AC supply voltage. For critical electronics applications a linear regulator will be used to stabilize and adjust the voltage. This regulator will also greatly reduce the ripple and noise in the output DC current. Linear regulators often provide current limiting, protecting the laptop power supply and attached circuit from overcurrent.

Adjustable linear laptop power supplies are common laboratory and service shop test equipment, allowing the output voltage to be set over a wide range. For example, a bench laptop power supply used by circuit designers may be adjustable up to 30 volts and up to 5 amperes output. Some can be driven by an external signal, for example, for applications requiring a pulsed output.

The simplest DC laptop power supply circuit consists of a single diode and resistor in series with the AC supply. This circuit is common in rechargeable flashlights.


Ac/dc supply
Main article: AC/DC (electricity)
In the past mains electricity was supplied as dc in some regions, ac in others. Simple, cheap, linear laptop power supplies running directly from either ac or dc mains, often without a transformer used. They used a rectifier and capacitor filter; the rectifier was essentially a conductor, having no effect when operating from dc.


Switched-mode laptop power supply
Main article: Switched-mode laptop power supply

A computers switched mode laptop power supply unit.A switched-mode laptop power supply (SMPS) works on a different principle. AC mains input is directly rectified without the use of a transformer, to obtain a DC voltage. This voltage is then sliced into small pieces by a high-speed electronic switch. The size of these slices grows larger as power output requirements increase.

The input power slicing occurs at a very high speed (typically 10 kHz 1 MHz). High frequency and high voltages in this first stage permit much smaller step down transformers than are in a linear laptop power supply. After the transformer secondary, the AC is again rectified to DC. To keep output voltage constant, the laptop power supply needs a sophisticated feedback controller to monitor current draw by the load.

Modern switched-mode laptop power supplies often include additional safety features such as the crowbar circuit to help protect the device and the user from harm.[1] In the event that an abnormal high current power draw is detected, the switched-mode supply can assume this is a direct short and will shut itself down before damage is done. For decades PC computer laptop power supplies have also provided a power good signal to the motherboard which prevents operation when abnormal supply voltages are present.

Switched mode laptop power supplies have an absolute limit on their minimum current output. [2] They are only able to output above a certain wattage and cannot function below that point. In a no-load condition the frequency of the power slicing circuit increases to great speed, causing the isolation transformer to act as a tesla coil, causing damage due to the resulting very high voltage power spikes. Switched-mode supplies with protection circuits may briefly turn on but then shut down when no load has been detected. A very small low-wattage dummy load such as a ceramic power resistor or 10 watt light bulb can be attached to the supply to allow it to run with no primary load attached.

Power factor has become a recent issue of concern for computer manufacturers. Switched mode laptop power supplies have traditionally been a source of power line harmonics and have a very poor power factor. Many computer laptop power supplies built in the last few years now include power factor correction built right into the switched-mode supply, and may advertise the fact that they offer 1.0 power factor.

By slicing up the sinousoidal AC wave into very small discrete pieces, the portion of the AC current not used stays in the power line as very small spikes of power that cannot be utilized by AC motors and results in waste heating of power line transformers. Hundreds of switched mode laptop power supplies in a building can result in poor power quality for other customers surrounding that building, and high electric bills for the company if they are billed according to their power factor in addition to the kilowatts used. Filtering capacitor banks may be needed on the building power mains to suppress and absorb these negative power factor effects.


Uninterruptible laptop power supply
Main article: Uninterruptible laptop power supply
An Uninterruptible laptop 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.


laptop power supply applications

Computer laptop power supply
Main article: Computer laptop power supply
A modern computer laptop power supply is a special type of switched-mode supply designed to convert 110-240 V AC power from the mains, to several low-voltage DC power outputs known as +12v, -12v, +5v, +3.3V and +5v StandBy. The first computer laptop power supplies were linear devices, but as power requirements increased, the transformer size became too large and heavy and a change to a more efficient but more complex switch-mode supply was needed.

The diverse collection of output voltages also have widely varying current draw requirements, which are difficult to all be supplied from the same switched-mode source. Consequently most modern computer laptop power supplies actually consist of several different switched mode supplies, each producing just one voltage component and each able to vary its output based on component power requirements, and all are linked together to shut down as a group in the event of a fault condition.

The most common modern computer laptop power supplies are built to conform to the ATX form factor. The power rating of a PC laptop power supply is not officially certified and is self-claimed by each manufacturer.[3]A common way to reach the power figure for PC PSUs is by adding the power available on each rail, which will not give a true power figure. The more reputable makers advertise "True Wattage Rated" to give consumers the idea that they can trust the power advertised.


AC adapter

Switched mode mobile phone chargerA linear or switched-mode laptop 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 pack", "plug-in adapter", "adapter block", "domestic mains 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. Replacement laptop power supplies must match the voltage of, and supply at least as much current as, the original laptop power supply.

The least expensive AC units consist solely of a small transformer, while DC adapters include a few additional diodes. Whether or not a load is connected to the power adapter, the transformer has a magnetic field continuously present and normally cannot be completely turned off unless unplugged.

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 laptop 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 laptop 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.
Other advantages of using separate power adapters are: removing the transformer as a source of heat, separating mains voltage, and making the devices smaller.


Polarity

Diagram showing negative tip polarity on the right and positive tip polarity on the leftAC-to-DC adapters have polarity; even if the plug fits into a device, the positive and negative connections may be oriented the wrong way. It is necessary to use an adapter with the correct polarity to avoid damage.


Power conversion
The term " laptop 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.

 

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