Posted on: July 1, 2014

AMP: Quantitative unit of measurement of electrical current.

It is the shortened term of the ampere, which is the SI unit used to express the amount of current or electrons flowing in an electrical circuit.

Named after a French physicist, Andre Marie Ampere, an ampere is the is the amount of current that flows through a resistance of one ohm if a potential difference of one volt is applied.
In an electrical circuit, the current flows when electrons move from a point of positive potential to a point of low or negative potential. The number of electrons in a unit charge is expressed in coulombs, where 6.2×1018 electrons make up one coulomb.

One ampere is equal to a flow of 6.2×1018 electrons per second or one coulomb of electric charge per second, through the cross-section of a conductor. In general, amperes are seen as measurements of the flow rate of charged particles per unit time.

The symbol of an ampere is A.

A = C/s where C is the coulomb and s the time in seconds.

All electrical equipment are required to displayed their electrical power rating in terms of the required voltage, current and power ratings. The voltage is expressed in Volts, current in amperes and power in watts. The current drawn by an electrical equipment or distribution systems is dictated by the power it consumes and the operating voltage.

The current can be measured using a various methods and an ammeter is the common tool used for measuring low values.

The ohms law and power equation are important when working with the equipment or circuit’s electrical ratings.

  • Ohms law I = V/R Power P = VI
  • Where I = current in amperes,
  • V = voltage in Volts
  • and P = power in Watts

Different equipment have different voltage and current requirements and it is important to understand these when choosing the accompanying components such as power supplies, sockets, power blocks and other electrical accessories and protection gear such as fuses, circuit breakers and switches.

A typical laptop adapter power rating reads

  • Input AC 100-240V 50/60HZ,
  • 130-170VA, Output D.C 20V 3.5A,
  • Maximum input current 1.5A,
  • Maximum output power 70W

Other examples are

  • A hearing aid rated at 1mW and 1.4V will have a current rating of 0.7mA
  • 60 W Car headlights powered by 12V car battery will consume 5A
  • A 1-2 KW starter motor at 12V draws 80-160A

When choosing power supplies and other associated circuit components, it is important to match the input of the equipment to the output of the supply. This ensures that the circuit and components in the power supply circuit operate within the safe limits; otherwise, if the load draws more current than the supply’s capacity, an overload may occur and lead to damage of the power unit.

Power Supply Glossary