| 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. 
Barrier |
At the junction, electrons fill holes so that there are no free holes or electrons there. The actual junction becomes an insulating layer. This barrier must be overcome before current can flow through the P-N junction.
Diode Characteristic Curves |
An electronic gate opens to let part of a signal through, and then shuts to reject the remainder. It's like separating sheep from goats, using a real gate. In the circuit, the cathodes of the diodes are more positive than the anodes. They are reverse biased and non conducting. The output of the circuit is isolated from the input. When the negative gating pulse comes along, the cathodes become more negative than the anodes. The diodes are forward biased and conduct. The output is connected to the input. During the duration of the gating pulse, the input signal appears at the output. as shown by the lowest waveform. The cathode end of the diode is usually marked in some manner.
Diode Voltages |
| To forward bias a diode, the anode must be more positive than the cathode or LESS NEGATIVE. To reverse bias a diode, the anode must be less positive than the cathode or MORE NEGATIVE. A conducting diode has about 0.6 volts across if silicon, 0.3 volts if germanium.
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. |
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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). |
