Power Supply

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300 watt computer power supply

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.

Electrical power supplies

This term covers the 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 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 categorized as a Stabilized power supply.

Power supply types

Power supplies for electronic devices can be broadly divided into linear and switching 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.

Battery power supply

A battery is a type of linear power supply that offers benefits that traditional line-operated power supplies lack: mobility, portability, and reliability. A battery consists of multiple electrochemical cells connected to provide the voltage desired. [1]

The most commonly used dry-cell battery is the carbon-zinc dry cell battery.[2] Dry-cell batteries are made by stacking a carbon plate, a layer of electrolyte paste, and a zinc plate alternately until the desired total voltage is achieved. The most common dry-cell batteries have one of the following voltages: 1.5, 3, 6, 9, 22.5, 45, and 90. During the discharge of a carbon-zinc battery, the zinc metal is converted to a zinc salt in the electrolyte, and magnesium dioxide is reduced at the carbon electrode. These actions establish a voltage of approximately 1.5 V.

The lead-acid storage battery may be used. This battery is rechargeable; it consists of lead and lead/dioxide electrodes which are immersed in sulfuric acid. When fully charged, this type of battery has a 2.06-2.14 V potential. During discharge, the lead is converted to lead sulfate and the sulfuric acid is converted to water. When the battery is charging, the lead sulfate is converted back to lead and lead dioxide.[3]

A nickel-cadmium battery has become more popular in recent years. [4]This battery cell is completely sealed and rechargeable. The electrolyte is not involved in the electrode reaction, making the voltage constant over the span of the batteries long service life. During the charging process, nickel oxide is oxidized to its higher oxidation state and cadmium oxide is reduced. The nickel-cadmium batteries have many benefits. They can be stored both charged and uncharged. They have a long service life, high current availabilities, constant voltage, and the ability to be recharged.

Linear power supply

An 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 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 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 direct current. Linear regulators often provide current limiting, protecting the power supply and attached circuit from overcurrent.

Adjustable linear 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 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 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. A simple, cheap linear power supply would run directly from either AC or DC mains, often without using a transformer. The power supply consisted of a rectifier and a capacitor filter. The rectifier was essentially a conductor, having no sudden effect when operating from DC.

Switched-mode power supply

Main article: Switched-mode power supply

A switched-mode 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 power supply. After the transformer secondary, the AC is again rectified to DC. To keep output voltage constant, the power supply needs a sophisticated feedback controller to monitor current draw by the load.

Modern switched-mode power supplies often include additional safety features such as the crowbar circuit to help protect the device and the user from harm.[5] 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 power supplies have also provided a power good signal to the motherboard which prevents operation when abnormal supply voltages are present.

Switched mode power supplies have an absolute limit on their minimum current output. [6] They are only able to output above a certain power level 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-power 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 power supplies have traditionally been a source of power line harmonics and have a very poor power factor. Many computer 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 sinusoidal AC wave into very small discrete pieces, the portion of the alternating 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 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 actual power used. Filtering capacitor banks may be needed on the building power mains to suppress and absorb these negative power factor effects.

Programmable power supply

Programmable power supplies are those in which the output voltage can be varied remotely. One possible option is digital control by a computer interface. Variable properties include voltage, current, and frequency. This type of supply is composed of a processor, voltage/current programming circuits, current shunt, and voltage/current read-back circuits.

Programmable power supplies can furnish DC, AC, or both types of output. The AC output can be either single-phase or three-phase. Single-phase is generally used for low-voltage, while three-phase is more common for high-voltage power supplies.

When choosing a programmable power supply, several specifications should be considered. For AC supplies, output voltage, voltage accuracy, output frequency, and output current are important attributes. For DC supplies, output voltage, voltage accuracy, current, and power are important characteristics. Many special features are also available, including computer interface, overcurrent protection, overvoltage protection, short circuit protection, and temperature compensation. Programmable power supplies also come in a variety of forms. Some of those are modular, board-mounted, wall-mounted, floor-mounted or bench top.

Programmable power supplies are now used in many applications. Some examples include automated equipment testing, crystal growth monitoring, and differential thermal analysis [7].

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.

High-voltage power supply

High voltage refers to an output on the order of hundreds or thousands of volts. High-voltage power supplies use a linear setup to produce an output voltage in this range.

When choosing a high-voltage power supply, there are several options to consider. Some of these are maximum current, maximum power, maximum voltage, output polarity, user interface, and style. The first four of these characteristics of course depend upon the supply's intended application. There are many available types of user interfaces. For example, the interface may be local in the form of a digital meter, or analog meter. Also, the interface can be remote, as in a computer connection. Numerous styles of high-voltage power supplies are also manufactured. Available models come in printed circuit board mount, open frame (as designed to be incorporated into an instrument), and rack mount. Models with multiple outputs can also be found [8].

Voltage multipliers

Voltage multipliers, as the name implies, are circuits designed to multiply the input voltage. The input voltage may be doubled (voltage doubler), tripled (voltage tripler), quadrupled (voltage quadrupler), etc. Voltage multipliers are also power converters. An AC input is converted to a higher DC output. These circuits allow high voltages to be obtained using a much lower voltage AC source.

Typically, voltage multipliers are composed of half-wave rectifiers, capacitors, and diodes. For example, a voltage tripler consists of three half-wave rectifiers, three capacitors, and three diodes. Full-wave rectifiers may be used in a different configuration to achieve even higher voltages. Also, both parallel and series configurations are available. For parallel multipliers, a higher voltage rating is required at each consecutive multiplication stage, but less capacitance is required. The voltage capability of the capacitor limits the maximum output voltage.

Voltage multipliers have many applications. For example, voltage multipliers can be found in everyday items like televisions and photocopiers. Even more applications can be found in the laboratory, such as cathode ray tubes, oscilloscopes, and photomultiplier tubes.[9][10]

Power supply applications

Computer power supply

Main article: Computer power supply

A modern computer power supply is a switched-mode supply designed to convert 110-240 V AC power from the mains supply, to several output both positive (and historically negative) DC voltages in the range + 12V,-12V,+5V,+5VBs and +3.3V. The first generation of computers power supplies were linear devices, but as cost became a driving factor, and weight became important, switched mode supplies are almost universal.

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 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 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.[11]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.

Welding power supply

Main article: Welding power supply

Arc welding uses electricity to melt the surfaces of the metals in order to join them together through coalescence. The electricity is provided by a welding power supply, and can either be AC or DC. Arc welding typically requires high currents typically between 100 and 350 amps. Some types of welding can use as few as 10 amps, while some applications of spot welding employ currents as high as 60000 amps for an extremely short time. Older welding power supplies consisted of transformers or engines driving generators, while modern ones implement semiconductors and even microprocessors, greatly reducing their size and weight.

AC adapter

A linear or 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 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 power supplies must match the voltage of, and supply at least as much current as, the original 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 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:

  1. 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.
  2. The device itself doesn't need to be tested for compliance with electrical safety regulations. Only the adapter needs to be tested.
  3. Product development becomes faster and cheaper, because the heat produced by the power supply is outside of the product.
  4. The device itself can be smaller and lighter, since it does not contain the power supply.



AC outlet (polarity)

AC-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. Standardized polarity symbols are usually used to label polarity.

Overload Protection

Power supplies should have some type of overload protection. Overload protection is important to protect the electronic equipment hooked up to the power supply and to also prevent overheating, which could potentially lead to an electrical fire. Fuses and circuit breakers are two of the more frequent mechanisms used for overload protection.[12]


A piece of wire is connected between two metal ends. The two metal ends of the fuse are connected by either a tube of glass or plastic which surrounds the wire. If too much current flows, the wire overheats and melts. This interrupts the power supply, and the equipment stops working until the problem that caused the overload is identified and the fuse is replaced.

There are two types of fuses, slow-blow and fast-blow. In a fast-blow fuse, the wire inside the fuse will melt if the current exceeds the rated current, even if it is just for a fraction of second. This concise process is important in electronic equipment where even a small spike in the current could damage the equipment. A slow-blow fuse is designed to only melt when there is a continuous overload. Slow-blow fuses are ideal for motor systems.

Circuit Breakers

One benefit of using a circuit breaker as opposed to a fuse is that it can simply be reset instead of having to constantly replace the blown fuse. A circuit breaker works once an overloaded current causes some element to heat and trigger a spring which shuts the circuit down. Once the element cools, and the problem is identified the breaker can be reset and the power restored.

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 direct current that is used in your car, and most electronics.

Mechanical power supplies


  • SCP:Short circuit protection
  • OPP:Overpower (overload) protection
  • OCP:Overcurrent protection
  • OTP:Overtemperature protection
  • OVP:Overvoltage protection
  • UVP:Undervoltage protection
  • UPS:Uninterruptable Power Supply
  • PSU:Power Supply Unit
  • APSU:Alarm Power Supply Unit


  1. Malmstadt, Enke and Crouch, Electronics and Instrumentation for Scientists, The Benjamin/Cummings Publishing Company, Inc., 1981, ISBN 0-8053-6917-1, Chapter 3.
  2. Malmstadt, Enke and Crouch, Electronics and Instrumentation for Scientists, The Benjamin/Cummings Publishing Company, Inc., 1981, ISBN 0-8053-6917-1, Chapter 3, pg. 66
  3. Xp_Power_Supply_Tech_Guide - TECHNICAL GUIDE 2010/11] THE XPERTS IN POWER (Global Power Solutions 2010.pdf)
  4. Malmstadt, Enke and Crouch, Electronics and Instrumentation for Scientists, The Benjamin/Cummings Publishing Company, Inc., 1981, ISBN 0-8053-6917-1, Chapter 3, pg. 68
  5. Quoting US patent #4937722, High efficiency direct coupled switched mode power supply: The power supply can also include crowbar circuit protecting it against damage by clamping the output to ground if it exceeds a particular voltage.
  6. Quoting US Patent #5402059: A problem can occur when loads on the output of a switching power supply become disconnected from the supply. When this occurs, the output current from the power supply becomes reduced (or eliminated if all loads become disconnected). If the output current becomes small enough, the output voltage of the power supply can reach the peak value of the secondary voltage of the transformer of the power supply. This occurs because with a very small output current, the inductor in the L-C low-pass filter does not drop much voltage (if any at all). The capacitor in the L-C low-pass filter therefore charges up to the peak voltage of the secondary of the transformer. This peak voltage is generally considerably higher than the average voltage of the secondary of the transformer. The higher voltage which occurs across the capacitor, and therefore also at the output of the power supply, can damage components within the power supply. The higher voltage can also damage any remaining electrical loads connected to the power supply.
  10. Miller, Rex. Electronics The Easy Way, 4th ed. Barron's Educational Series, 2002 p. 88-89.
  11. SPCR: Power Supply Fundamentals: POWER SHMOWER or How PSU Power Ratings Mean Almost Nothing
  12. Malmstadt, Enke and Crouch, Electronics and Instrumentation for Scientists, The Benjamin/Cummings Publishing Company, Inc., 1981, ISBN 0-8053-6917-1, Chapter 3.

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