AC vs DC and RMS values
Mains electricity is AC: it reverses direction many times per second (50 Hz in Nigeria), unlike the steady DC from a battery. Because AC constantly changes, its effective value for delivering power is quoted as an RMS (root-mean-square) value: .
Example 1: An AC supply has a peak voltage of 340 V. Find its RMS voltage.
-- close to Nigeria's standard 230 V mains supply.
Transformers
A transformer changes AC voltage using two coils wound on a shared iron core: , where is the number of turns on each coil. A step-up transformer has more turns on the secondary; a step-down transformer has fewer.
Example 2: A transformer has 100 turns on its primary coil and 500 turns on its secondary. If the primary voltage is 12 V, find the secondary voltage.
-- this is a step-up transformer.
Diodes and rectification
A diode conducts current in only one direction, blocking it in the other. This makes diodes useful for rectification -- converting the back-and-forth current of AC into the one-directional current (DC) that most electronic devices need. A single diode gives half-wave rectification (only one half of each AC cycle passes through); four diodes arranged in a bridge give full-wave rectification, using both halves of the cycle.
Transistors
A transistor is a semiconductor device that can act as an electronic switch or amplifier -- a small current or voltage at one terminal controls a much larger current flowing between the other two. This switching ability is the basis of all modern digital electronics.