Alternating Current (AC)  Electrical current that continuously reverses direction of flow, typically in a periodic fashion. In the United States, standard AC power sources reverse direction sixty times each second (called 60 cycles or 60 Hertz AC). In Europe, the standard is 50 Hertz.

Air-cooled Design that disposes of excess heat without the use of fans or cooling liquids.  Air cooled devices often rely on a heat sink to enhance heat dissipation.

Ampere or Amp  Unit of measure of current flow. At a given point in a circuit, an Amp (1A) equals 1 coulomb of electrons passing in 1 second.

Ammeter A device that measures current flowing through an electrical circuit. An ammeter is typically connected in series.

Attenuation Decrease in signal strength as it passes through a system or medium. Measured in decibels.

American Wire Gauge (AWG)  A standard used to measure the diameter of a wire conductor. The greater the number, the smaller the wire diameter. House wiring typically contains 12-gauge or 14-gauge solid wire. "Doorbell" wire is often 16-gauge stranded wire. Digital circuit boards are sometimes modified using small 22-gauge or 28-gauge wires.

Ball-bearing Fan  A cooling fan with a center hub that glides on precision steel balls. Though more expensive, ball-bearing fans typically last longer than fans made with bushings.

Balun A transformer that presents high impedance to common-mode signals and low impedance to differential-mode signals. It is often used on the input of switching power supplies to suppress common-mode noise.

Blackout An abrupt, unanticipated loss of all electrical power.

Backup Power Supply (BPS)  A second power source that activates if the primary power source fails.

Break An open (disconnected) circuit condition caused by a switch, damage to a conductor, etc.

Breakout Box  A test device that can be attached to a cable, making it easy to connect a wire to any conductor. Some breakout boxes also feature LEDs to monitor signals on each line.

Breakdown Voltage  The maximum AC or DC voltage that may be applied from input to output and/or chassis of a power supply.

Brown-out A planned voltage reduction by a utility company to counter excessive demand on their generation and distribution system.

Burn-in Statistically, if an electrical device is going to fail, it will most often occur during its first hours of operation. Some suppliers first operate new electrical products at their factory for a predetermined period to weed out problems before shipment to customers. This process is known as burn-in. Sometimes, burn-in is conducted with the product under electrical or thermal load, to induce stress and make it easier to identify potential failures.

Bus A common electrical pathway between components or devices.

Canadian Standards Association (CSA)  A Canadian agency that establishes standards and tests products for public safety.

Capacitor An electronic component that can store an electrical charge on conductive plates.

Capacitance Measured in farads, capacitance is an object's ability to store a charge.

CE Products sold in most parts of Europe must be tested and certified to comply with CE standards. These standards include limits on how much electromagnetic interference (EMI) a device may emit, and its susceptibility to EMI induced problems.

Circuit A complete path for electrical flow from one terminal to the other of a power source such as a power supply or battery.

Circuit Board  A sheet of plastic or other material bearing foil or wire conductors that make up an electrical circuit. Often, a photo etching process is used to mass-produce "printed circuit boards" (PCBs) for industry.

Clamp To divert excess voltage to ground.

Clamp Circuit  Designed into surge protectors and suppression devices to clamp (shunt excess voltage to ground) in the event of a power surge.

Clamping Voltage  The desirable, predetermined voltage level maintained by a clamp circuit.

Common-mode Noise  The component of noise that is common to both the DC output and return lines with respect to input neutral.

Compliance Voltage  The output voltage range of of a constant current power supply.

Conductor A material that allows the flow of electricity.

Converter A device used to transform one DCvoltage to another.

Constant Current Power Supply  A power supply that regulates its output current for changes in line, load, ambient temperature and time.

Constant Voltage Power Supply  A power supply that regulates its output voltage for changes in line, load, ambient temperature and time.

Cross-regulation In a multiple output power supply, the percent voltage change at one output caused by a load change on another output.

Crowbar Circuit  An overvoltage protection circuit that rapidly places a low resistance shunt across the power supply output terminals if a predetermined voltage is exceeded.

Current (I)  The flow of electrons through a material. Current is measured in amperes.

Cycles The number of times an alternating current reverses in one second.

Daughterboard A circuit board that forms a subsystem of a larger main circuit board.

Decibel (dB)  A logarithmic measure of signal power determined by comparing an initial reference level to a final measurement.

Derating A specified reduction in output power at elevated temperatures to improve reliability.

Differential Mode Noise  The component of noise measured between DC output and output return.

Dirty Power  An alternating current that includes spikes, surges or other unwanted fluctuations.

Digital Multimeter (DMM)  An electronic device used to measure voltage, resistance, current and other circuit characteristics. Readings are displayed on a digital LCD panel on the DMM.

Direct Current (DC)  Electrical current that flows only in one direction, i.e., does not alternate.

Drift The change in power supply output over a specified period following a warm-up period, when all other characteristics such as line, load and ambient temperature are constant.

Electronic Industries Association (EIA)  A organization whose industry members establish standards for electrical components and consumer electronics products. In 1988, the EIA created a second group called the TIA to administer telecommunications and information technology standards.

Electrostatic Type of force that exists between two electrically charged objects. Objects with a similar charge repel each other, and objects with dissimilar charges attract each other. No current flows in an electrostatic environment.

Electromagnetic Interference (EMI)  An effect of undesirable energy. EMI is typically emitted by switching power supplies, and may be conducted or radiated.

Energy Star  A U.S. government program to reduce unnecessary power consumption and waste.  Energy Star certified computer systems use high efficiency power supplies and advanced power management techniques to help conserve electricity.

Efficiency The ratio of total power output to input power. Efficiency is typically measured at full load and nominal input voltage.

Equivalent Series Resistance (ESR)  The amount of resistance in series with an ideal capacitor that exactly duplicates the performance of a real capacitor.  In high frequency applications, low ESR is very important.

Farad  Unit of capacitance. A capacitor has 1 farad of capacitance when it can store 1 coulomb of charge with a 1 volt potential across its plates.

Faraday Shield  An electrostatic shield between the input and output windings of a transformer, designed to reduce output common mode noise.

Fault-mode Input Current  The input current to a power supply or DC-DC converter, with the output short-circuited.

Federal Communications Commission (FCC)  United States regulatory agency that oversees telecommunications and radio frequency communications.

FCC Class A  United States federal regulations that define permissible levels of electromagnetic emissions from an electronic device operated in a commercial environment.

FCC Class B  United States federal regulations that define permissible levels of electromagnetic emissions from an electronic device operated in a residential environment.

Feed forward  A control technique in which line regulation of a power supply is improved by directly sensing the input voltage.

Ferroresonant Power Supply  An open-loop voltage stabilized power supply in which a portion of the transformer core is driven into saturation by a resonant tank circuit.  The output is derived from the saturated portion of the transformer and is relatively independent of input voltage.

Flyback Converter  Power supply switching circuit that normally uses a single transistor.  During the first half of the switching period, the transistor is on and energy is stored in a transformer primary. During the second half period, this energy is transferred to the transformer secondary and the load.

Foldback Current Limiting  A power supply output protection circuit in which the output current decreases with increasing overload, reaching a maximum at short-circuit.

Forward Converter  A power supply switching circuit in which energy is transferred to the transformer secondary when the switching transistor is on. In this circuit, minimal energy is stored in the transformer.

Full Bridge Converter  A power switching circuit that has four transistors connected in a bridge configuration to drive a transformer primary.

Gain An increase in signal power, typically the result of amplification.

Giga (G)  Metric prefix meaning one billion, or 1,000,000,000.

Green Slang for a device that complies with Energy Star program qualifications.

Ground A voltagereference level in a circuit, typically connected to earth ground at some point.

Ground Fault Circuit Interrupter (GFCI)  A power receptacle that reduces the hazards of ground fault currents that can cause injury or death. Upon detecting a ground fault current in a protected circuit, a GFCI will trip, resulting in the removal of power.

Ground Loop  An unwanted feedback condition caused by two or more circuits sharing a common electrical ground.

Heat Sink  A milled piece of aluminum or other material bearing fins or fingers to increase surface area, and thus dissipate unwanted heat into the air more rapidly than a device with no heat sink attached.

Hertz (Hz)  Repetitions per second.

Hi-pot Short for "High potential" test.  A test to determine if the breakdown voltage of a power supply or transformer exceeds the minimum requirement. A hi-pot test is conducted by applying high voltage between two isolated test points.

Hold-up Time  The period during which a power supply's output voltage remains within specifications following a loss of input power.

Hot-swap To remove and replace a system component while power is on and the system is functional (without first shutting the system down).

Impedance Apparent resistance to the flow of current in an ACcircuit, directly related to the circuit's true resistance to the flow of direct current.

Inductance The ability of a coil to oppose changes in current flowing through it.

Inductor  A coil of wire wrapped a specific number of turns around a core material, used to produce inductance in a circuit.

Input Line Filter  A band-reject or low-pass filter on a power supply's input designed to reduce the level of  line noise being introduced to the supply.

Inrush Current  The peak instantaneous input current drawn by a power supply when it is powered on.

Inrush Current Limiting  A circuit design that limits inrush current to an acceptable level.

Insertion Loss  Decrease in power resulting from the insertion of a component or splice in a circuit.

Institute of Electrical and Electronic Engineers (IEEE)  An organization of scientists, engineers and others that creates standards for the electronics industry.

International Electrotechnical Commission (IEC) A Geneva-based organization that sets international electronics standards.

Inverter A device that converts DC input power into AC output power.

Isolation The electrical separation between input and output of a power supply by means of the power transformer. Isolationresistance (normally in megohms) and isolationcapacitance (normally in picofarads) are generally specified and are a function of materials and spacings employed throughout the power supply.

Isolation Voltage  The maximum AC or DC voltage that may be continuously applied from the input to the output or chassis of a power supply.

Jacket The outermost layer of an electrical cable. Typically made of plastic, rubber or similar non-conductive materials, the jacket encases inner conductors and helps protect against short-circuits, electrical shocks, mechanical damage and environmental hazards.

Kilo (k or K)  Metric prefix meaning 1000.

Layer Winding  A method of winding a transformer whereby the primary and secondary are wound in layers over one another, separated by an insulating layer.

Leakage Current  AC or DC current flowing from the input to the output or chassis of an isolatedpower supply at a specified voltage.

Line Regulation  The change in output voltage in percent as the input voltage is varied over its specified limits, with load and temperature constant.

Linear Regulator  A popular stabilization circuit in which a control device is placed in series or parallel with the load to deliver a constant voltage across the load. The control device is always conducting, and the difference between  input and output power is dissipated by the control device.

Load Regulation  The percent change in output voltage as the load is changed from minimum to maximum, at constant line and constant temperature. The load change may be specified for other than no load to full load, e.g., 30% to full load

Local Sensing  Using the power supply output voltage terminals as the sense points to provide feedback to the voltage regulator.

Magic Packet  A standard for remotely waking a personal computer from sleep mode or a power off condition. Originated by AMD(tm) and Hewlett-Packard(tm), this technique is implemented by sending a special Ethernet or Token Ring message known as a "Magic Packet." The wake-up signal is intercepted by a network interface that remains functional due to power from the PC's standby voltage source.

Mean Time Between Failure (MTBF)  A reliability term that expresses a component's average failure rate in hours.

Mega (M)  Metric prefix meaning one million, or 1,000,000.

Micro (?)  Metric prefix meaning one millionth or 1/1,000,000.

Milli (m)  Metric prefix meaning one thousandth or 1/1000.

Mirrored Replicated by a functional, exact duplicate to provide a real-time backup system.

Motherboard The main circuit board in a computer system or other digital electronic device.

Nano (n)  Metric prefix meaning one billionth or 1/1,000,000,000.

Noise  Signal interference generated by electrical components or external sources that can disrupt or corrupt data transmissions.

Off-line Power Supply  A power supply that operates directly off the AC line, without using a power transformer prior to rectification and filtering.

Ohm Unit of resistance to the flow of electrical current.

Ohm's Law  Physical law that defines electrical voltage in relation to current and resistance. Voltage (E in volts) is equal to Current (I in amps) multiplied by Resistance (R in ohms), or E = IR, or R = E/I, or I = E/R, etc.

Output Current Limiting  An output protection feature that limits the output current to a predetermined value in order to prevent damage to the power supply or the load under overload conditions. The supply is automatically restored to normal operation following the removal of the overload.

Output Impedance  The ratio of change in output voltage to the change in load current.

Output Voltage  The nominal value of the DC voltage at the output terminals of a power supply.

Output Voltage Accuracy  For a fixed output supply, the tolerance in percent of the output voltage with respect to its nominal value under all minimum or maximum conditions.

Overload Protection  An output protection feature that limits the output current of a power supply under overload conditions so it will not be damaged.

Overshoot A transient change in output voltage, in excess of specified output accuracy limits, that can occur when a power supply is turned on or off, or when there is a step change in line or load.

Overvoltage Protection  A power supply feature that shuts down the supply, or crowbars or clamps the output, when its voltage exceeds a preset level.

Parallel Operation  The connection of the outputs of two or more power supplies of the same output voltage to obtain a higher output current than from either supply alone. This technique requires power supplies specifically designed to share a load.

Periodic and Random Deviation (PARD)  A term used to describe the sum of all ripple and noise components measured over a specified bandwidth and stated in either peak-to-peak or RMS values.

Pico (p)  Metric prefix meaning one thousand-billionth (in U.S. and France, called one trillionth) or 1/1,000,000,000,000.

Pi Filter  A filter commonly used on the input of a switching supply or DC-DC converter to reduce the reflected ripplecurrent. The filter usually consists of two parallel capacitors and a series inductance and is generally built into the power supply.

Polarity Electrical condition in which there are two opposing voltage levels or charges, i.e., positive and negative poles.

Post Regulation  A linear regulator used on the output of a switching power supply to improve line and load regulation and reduce output ripple voltage.

Power Fail Detection  An optional power supply feature that causes the supply to send a logical output signal in the event of the loss of line input voltage.

Power Foldback  A power supply feature that causes the input power to be reduced under output overload conditions.

Power Supply  An electrical device that provides a known potential difference in place of a battery. A power supply is typically plugged into a wall outlet, and can be a standalone device, or a fundamental component of a larger electrical product.

Preregulation Regulation at the front end of a power supply, typically by a type of switching regulator. This is followed by output regulation, usually by a linear regulator.

Programmable Power Supply  A power supply that has its output controlled by an external resistor or a digital code.

Pulse-width Modulation  Method of voltage regulation used in switching supplies whereby the output is controlled by varying the width, but not the height, of a train of pulses which drive a power switch.

Push-Pull Converter  A power switching circuit that uses a center-tapped transformer and two switches that are alternately driven on and off.

Rack Mount  Variety of device whose special design enables it to be mounted in a standard cabinet, often in a 19" wide opening.

Rated Output Current  The maximum load current that a supply is designed to provide at a specified ambient temperature.

Redundant Array of Inexpensive Disks (RAID)  A technique for storing data on multiple hard disk drives, to provide higher reliability and faster data access.

Redundant Power Supply  A power supply made by duplicating identical components to provide greater reliability. In the event the primary supply should fail, a secondary supply is activated (or it assumes the entire load instead of merely half the load).

Reference Stable voltage, generally from a Zenar diode, from which the output voltage of a regulated supply is controlled.

Reflected Ripple Current  The AC current generated at the input of a power supply or DC-DC converter by the switching operation of the converter, stated as peak-to-peak or RMS.

Regulated Voltage  Power supply output voltage that is held constant despite fluctuating current drain.

Relay A remote switch activated by an electromagnetic coil.

Remote Sensing Technique of regulating the output voltage of a power supply at the load by means of sensing leads which extend from the load to the regulator. This method compensates for voltage drops in the load leads.

Resistance (R)  Opposition to current flow, measured in ohms.

Resistor (R) An electrical device that inserts a predetermined level of resistance into a circuit.

Resolution In an adjustable power supply, the smallest change in output voltage that can be realized by the adjustment.

Return Name for the common output terminal of a power supply, the "return" carries the return current for the outputs.

Reverse Voltage Protection  Feature that protects a power supply against reverse voltage applied to input or output terminals.

Ripple and Noise  The magnitude of ACvoltage on the output of a power supply, at a specified bandwidth, expressed in RMS or millivolts peak-to-peak. This is the result of feed through of the rectified line frequency, internal switching transients and other random noise.

Series Regulation  The most popular method of linear regulation in which the control device is in series with the raw DC and the load, to achieve constant voltage across the load.

Short-Circuit An abnormal path of little or no resistance across the terminals of a circuit, resulting in an excessive flow of current.

Short-Circuit Protection  A feature that limits the output current of a power supply under short-circuit conditions so the supply will not be damaged.

Shunt Regulation  A method of linear regulation in which the control device is in parallel with the load to achieve constant voltage across the load.

Soft Start  A feature that limits the start up switching currents of a switching power supply, causing the output voltage to rise gradually to its specified level.

Spike A burst of undesirable high voltage on a power line, typically lasting only a fraction of a second.

Split Bobbin Winding  Means of winding a transformer in which the primary and secondary are wound side-by-side on a bobbin, with an insulator between the two windings.

Standby Voltage (SB)  Special output from a power supply in shut-down mode, used to provide enough power to restart a personal computer, or enable a network or modem connection.

Step Change  A instantaneous change in a quality from one value to another.

Strain Relief  A plastic collar or similar contrivance added to a cable at the point it enters a case, designed to help protect the cable from mechanical damage caused by flexing or pulling.

Surge A sudden increase in line voltage of short duration that can sometimes cause damage to electrical devices.

Swell An undesirable condition in which actual voltage lingers above the normal voltage level.

Switching Frequency  The rate at which DCvoltage is switched in a switching power supply or DC-DC converter.

Switching Regulator  A high-efficiency switching circuit that uses a closed-loop system to regulate output voltage, typically be means of a pulse-width modulator.

Temperature Coefficient  The average percent change in output voltage per degree Centigrade change in ambient temperature over a specified temperature range.

Thermal Protection  Internal safeguard circuit in a power supply that shuts the unit down in the event of excess internal temperature.

Tracking A characteristic of dual or multiple output power supplies in which one or more outputs follow another output with changes in line, load or temperature, such that each maintains the same proportional output voltage with respect to common.

Transient Recovery Time  Time required for the output voltage to settle within specified output accuracy limits following a step change in output load current or input voltage.

Transformer A device that uses principals of electromagnetic induction to "step-up" (increase) or "step-down" (decrease) voltage from an AC source.

Technischer Uberwachungs-Verein (TUV)  A German organization similar to U.L. that tests and certifies products for safety.

Undershoot A transient change in output voltage, below output accuracy limits, that can occur when a power supply is turned on or off, or when there is a step change in line or load.

Underwriters Laboratories (UL)  A  independent,  U.S.-based non-profit organization that tests and certifies components and products for safety.

Uninterruptible Power Supply (UPS)  A power supply that continues to provide electricity despite the loss of AC input power. This is made possible by means of a backup battery and a DC-AC inverter or DC-DC converter.

Verband Deutscher Electrotechniker (VDE)  German national standards and test agency.

Volt (V or E)  Electrical unit of voltage or potential difference. One volt is the force necessary to move the sufficient number of electrons through one ohm of resistance to create one ampere of current.

Voltage A measure of electromotive push on an electron.

Voltage Balance  The difference in magnitude, in percentage, between two output voltages of a dual output power supply where the voltages have equal nominal values with opposite polarities.

Voltage Drop  A change in available voltage between two points in a circuit, due to current flowing through resistance. Also known as an IR drop.

Voltmeter An electrical device used to measure the voltage between two points in a circuit.

Wake-on-LAN (WOL)  Another name for Magic Packet technology.

Warm-up Drift  The initial change in output voltage of a power supply from power-on until it reaches thermal equilibrium at nominal line, full load, 25°C ambient temperature.

Warm-up Period  The time required, following initial power-on, for a power supply to meet its performance specifications.

Watt (W)  A unit of power. Equal to 1 joule per second, power in Watts can be calculated by multiplying voltage times current.

Document de Quantum Power Labs, Inc.

We developed this glossary to help you understand power supply terminology. If you spot an error, or have a suggestion to make this list even better, we'd very much like to hear from you.