Semi Conductors - Diodes
Diodes fall broadly into two types - rectifier and signal. Rectifier diodes are used in power supplies to convert a.c. to d.c. They may have to handle high currents and voltages but normally work at low frequencies (typically 50 Hz). Signal diodes normally handle much lower power but may be required to operate at much higher frequencies (many MegaHertz).
Diodes allow current to pass through them in one direction only. In the forward direction, the resistance is low (typically 5 ohms - measured on an analogue ohmmeter). In the reverse direction, the resistance is very high. (Greater than 20 Meg-ohms).
Uses include rectifiers in power supplies, signal demodulators and back EMF limiters in inductors.
Semi Conductors - Operational Amplifiers
This chip is designed to accept two inputs on pins 2 and 3. The difference between the inputs is amplified to give the output. If the voltage on pin 2 is greater than pin 3 the output is negative. If pin 3 is greater than pin 2 the output is positive. The Op-Amp gives a voltage gain as opposed to a transistor which gives a current gain.
Uses: Switching, amplifiers, oscillators and analogue computing.
Semi Conductors - Transistors
Transistors are used as electronic switches and current amplifiers. They have three "legs" - base , collector and emitter. A small current to the base switches on the transistor allowing a much larger current to flow between collector and emitter. The ratio of the collector current to the base current is known as the "amplification" or "gain" of the transistor (hfe = Ic/Ib).
They come in various different shapes and sizes and it is important to decide which type is required at the outset.Factors to be considered are voltage and power handling capacities, cost and availability. Transistors require a base voltage of at least 0.6V with respect to the emitter to "turn on". (Darlington Pair transistors require 1.2 V). A limiting resistor - typically 1K is required in the base circuit of the transistor stage to prevent overrunning and hence overheating and destruction of the transistor. Transistors come in NPN and PNP types. It is important to use the correct type when designing and building circuits.
Semi Conductors - Timer
This chip is designed to function as a monostable (one stable state) multivibrator or as an astable (free running) multivibrator. It contains the equivalent of approximately 20 transistors,resistors, diodes etc. It is used for timing or switching circuits which give pulsed or digital outputs.