Diodes are polarised, which means that they must be inserted into the PCB the correct way round.
This is because an electric current will only flow through them in one direction (like air will only flow one way through a tyre valve). Diodes have two connections, an anode and a cathode.
The cathode is always identified by a dot, ring or some other mark.



Diodes Symbols

The pcb is often marked with a + sign for the cathode end. Diodes come in all shapes and sizes.
They are often marked with a type number.
Detailed characteristics of a diode can be found by looking up the type number in a data book.

If you know how to measure resistance with a meter then test some diodes. A good one has low resistance in one direction and high in the other.

There are specialised types of diode available such as the zener and light emitting diode (LED).



Diodes Symbols

Diode Connections Tutorial


Diode Connections

The cathode end of the diode is usually marked in some manner.
Forward Biased Junction Tutorial

Forward Biased Junction Tutorial
Forward Biased Junction Tutorial

Forward Biased Junction



Bear in mind that like charges repel and unlikes attract. When a battery is connected as shown, the negative terminal pushes negative electrons towards the junction.
The positive terminal pushes holes towards the junction.
If the voltage is high enough then the barrier will be overcome and current will flow through the junction.

There is a voltage across the diode. 0.6 for silicon, o.3 for germanium.

The junction is said to be FORWARD BIASED.

The P type is the anode of the diode, the N type the cathode, as shown by the diode symbol.

The resistor limits the current to a safe level.

Transistor Lower Base Bias Resistor Open Circuit Tutorial

There is no potential divider action and the full +9 volts is applied to the base by R1.

This causes a high base current to flow, which in turn causes a high collector current to flow.

There is a large volts drop across R3, causing a low collector volts.

There is a large volts drop across R4, causing the emitter volts to rise.