Power related glossary terms


AC (Alternating Current) - It is the current that changes polarity periodically. The electricity in homes, offices etc. is AC.

AC Line – A power line that delivers alternating current only.

AC Line Filter – A filter designed to remove extraneous signals or electrical noise from an AC power line, while causing virtually no reduction of the power line voltage or power.

AC Line Voltage – The voltage commonly delivered by the commercial power line to consumers. In the United States, the two standards are 117V and 234V (~ about 5 percent). The lower voltage is used by most appliances; the higher voltage is intended for appliances and equipment that draws high power, such as electric ovens, cooking ranges, clothes dryers, and amateurs-radio amplifiers. In Europe, 220V is the common standard.

AC Noise – Electromagnetic interference originating in the AC power lines.

Crest Factor - This is the ratio between the peak current and the average current required by the load. Computers normally exhibit a crest factor of 2 to 3, which means the computer draws two to three times the average current for short duration, such as the starting time etc.

Current - It is a quantitative measure of the amount of electricity passing through a circuit. The unit for measuring the current is Amperes.

Current Limiting – The controlling of current so that it does not exceed a desired value.

Current Meter – A usually direct-reading instrument, such as an ammeter, milliammeter, or micrometer, used to measure current strength.

Current Noise – Electrical noise produced by current flowing through a resistor.

DC (Direct Current) - It has either a positive or negative polarity and flows in one direction. A DC charge can be stored more easily and is used for all batteries.

DC Power Supply – A power unit that supplies direct current only. Examples: battery, transformer / rectifier / filter circuit, DC generator, and photovoltaic cell.

Impedance - It is combination of resistance, inductance and capacitance which restricts the current through any device.

Inverter - It is a circuit which converts DC to AC.

Line Frequency
- It is the number of times the AC flows in one direction during one second. Frequency is measured in Hertz (Hz) or cycles per second. The standard power frequency may differ. For example the AC frequency in Malaysia is 50Hz. (i.e. the current changes its direction 50 times per second) while in the U.S.A. it is 60 Hz.

Power Factor - This is the ratio of real power to apparent power (VA/watts). The power factor can be ‘leading’ or ‘lagging’ depending upon the type of load. Inductive loads cause the current to lag and capacitive loads causes the current to lead the voltage.

Power Factor Meter – An instrument that gives direct readings of power factor (lead or lag). One such meter uses a dynamometer-type movement in which the rotating element consists of two coils fastened together at right angles.

Power Rating – The specified power required by an equipment for normal operation.

Power Supply – A device, such as a generator or a transformer-rectifier-filter arrangement, that produces the power needed to operate on electronic equipment.

Power Transistor – A heavy-duty transistor designed for power-amplifier and power-control service.

PWM - Pulse Width Modulation is a technique employed to regulate the output power by changing the pulse width. PWM is employed is SMPS, UPS and many other power control applications.

Rectifier - It is a diode network which changes AC to DC. This is reverse of a inverter which converts DC to AC.

Sine Wave - It is the most simple of all wave forms and is the shape of the AC in homes, offices etc. In a sine wave, the voltage or current changes smoothly from a negative maximum to a positive maximum with changing time.

Square Wave - This waveform is similar to the sine wave except that the transitions from negative maximum to positive maximum are abrupt. Most domestic invertor’s supply a square wave as it is simple to generate.

Transformer - A device used to convert an AC voltage to different AC voltage levels. A transformer is also used to isolate the output AC power from the source.

VA (Volt Ampere) - This is the simple product of voltage and current and is used to express the amount of power. VA gives the apparent power.

Voltage - It is a measure of the potential difference between two points, it is what causes the current to flow from a higher potential. This is measured in volts.

Watt - This is a measure of energy drawn per second by the load. It is calculated by multiplying the VA by the power factor (VA x pf). This gives the true power. The pf varies between 0 for ideal inductive and capacitive loads to 1 for pure resistive loads (incandescent lamps). The pf for a typical computer would be between 0.6 and 0.8 in most cases.

Wave Form - This is the name given to the shape followed by any alternating current or voltage.


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