Tutorial 4 - Diodes

Diodes are one-way electrical valves that allow current to flow in one direction only.

Knowledge of diode action is not essential, but it will help you to understand the action of diodes and transistors.  This is covered in the extension material

Consider these two circuits:

In circuit A the diode is forward-biased so the current flows and the motor will turn.

• In circuit B the diode is reverse-biased, the current will not flow and the motor will not turn.

• Diodes have a low resistance when they are forward biased, and a very high resistance when they are reverse-biased.

• This means that current can flow one way only.

The diagram below shows the four main kinds of diode:

Diodes are made from semi-conductor materials like silicon and germanium.

Voltage Characteristic of the Diode

The pictures show an experiment to plot the voltage-current characteristic for a semi-conductor diode.

The circuit diagram is shown:

The voltage and current can be recorded for the forward biased diode. We can turn the diode round for the reverse biased diode.

A forward biased diode starts to conduct at the junction voltage (for silicon, about 0.6 V). A small increase in the forward voltage leads to a large increase in forward current.

Here is the characteristic graph from an actual experiment:

If we show the reverse biased  characteristic as well, we see:

If we reverse the voltage, we get a tiny leakage current of no more than a few micro-amps. At a certain voltage, anything from 10 V to 2000 V, depending on the doping, the insulation of the barrier layer breaks down suddenly and a sudden increase in current occurs. The breakdown voltage is the voltage at which this happens. In many diodes, this would result in burn-out.

Germanium diodes switch on at 2.2 V and the breakdown voltage is about 100 V.

Light Emitting Diode

The light emitting diode (LED) has replaced indicator lamps in many devices.  In recent years LEDs have been used in torches and domestic lighting.  They last a lot longer than filament lamps and use a lot less electrical energy. 

Additionally the light from LED domestic lamps is more pleasant than the light from fluorescent low-energy bulbs.

The forward voltage is about 2.2 V. The characteristic is shown below.

Notice that the graph is a little wobbly, and a smoothed out curve is added. A better curve would have been obtained by repeating the readings and taking an average.

The voltage current characteristic of the LED has a similar shape to the graph of the ordinary diode. The LED can only tolerate a small reverse voltage. A reverse voltage of 20 V will destroy an LED.

It is very easy to ruin LEDs with currents in excess of 30 mA. To prevent this we put a current limiting resistor in series with the LED.  Failure to do so can make the LED explode.

Zener Diode

The circuit is used to investigate the Zener diode.

The graph shows the characteristic for a Zener diode from an actual experiment.

Notice that the zero point is in the middle of the two sets of batteries. The top set provides a positive potential difference, while the bottom set provides a negative potential difference. You could use this arrangement for any diode.

The Zener diode is designed to be used in a reverse biased configuration. Its behaviour is very like an ordinary diode in forward-bias, but a typical reverse-biased breakdown voltage is –5.6 V, and there is a very rapid rise in current.

In its reverse biased configuration, the zener diode will hold the output voltage at a constant 5.6 V. It can be described as a voltage clamp. If the voltage is above 5.6 V, a current will flow through the diode, which results in the voltage being held at 5.6 V. Zener diodes are used as simple voltage regulators.

The graph shows how diode limits the voltage, which remains steady at 5.6 volts while a small current is taken. The Zener diode can only take a limited current, and requires a current limiting resistor.

Calculating the value of the current limiting resistor is done in the same way as the LED.

Data Sheets

We often refer to data sheets that are found in retailers’ catalogues. As well as price, we might get data on:

• Current that the component can carry

• Temperature that the component can tolerate.

• Case style.

• Working voltages.

Here are some data for some high power diodes from a catalogue:

Diodes are used a lot in rectification.  We will look at that more closely in Power Supplies.