Definitions

Power (P) is the rate at which energy is used. The most common unit is the watt. One watt is the use of one joule of energy per second. One joule is the amount of energy required to lift one kilogram by one meter.

Power is the product of voltage and current.

Volt (V or E) It is the unit of potential difference or electromotive force in the MKS (meters-kilograms-second) system equal to the potential difference between two points for which 1 coulomb of electricity will do 1 joule of work in going from one point to another. It is the potential difference required to make a current of one ampere flow through a resistance of one ohm.
Ampere (A) it is the unit of electric current in the MKS system defined in terms of the force of attraction between two parallel current carrying conductors. It is equal to one coulomb of charge flowing across a surface in one second. One ampere is the current flowing through one ohm of resistance at one volt potential.
Coulomb is a standard unit of charge equal to 6.24 x 1018 electrons and is equal to one ampere second.

 

Alternating Current (AC) is one that periodically reverses direction.

Direct Current (DC) is one that travels in only one direction.

Ohm is a measure of electrical resistance.

Ohm’s Law

In the 18th century, scientist thought that electrical current was a sort of fluid where both positive and negative charges moved. Consequently, circuit theory initially defined current as the movement of the positive charge. When it was discovered that electrical current was the movement of negative charge, it was too late to rewrite the technical literature. As a result, circuit theory continues to use conventional (positive) current flow.

Basic Power Supply

A power supply consists of four principle parts:

  1. Transformer
  2. Rectifier
  3. Filter
  4. Regulator

A simple regulated DC power supply converts the AC voltage from the power utility, to a constant DC voltage. This process requires four unique stages:

  1. Voltage transformation (step-up or step-down)
  2. AC rectification (half-wave of full-wave)
  3. Filtering (capacitors and perhaps inductors)
  4. Regulation (discrete circuit or IC)


Some switched-mode power supplies dispense with the transformer entirely.

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