15.4.4 – Diode

Diode is a semiconductor which only allows current flow in one direction.

It is a non-ohmic component hence does not obey Ohm’s law.

circuit symbol for a diode

A common type of diode is a light-emitting diode (LED). These are used as a light source in more and more places as they have very long lifespans and are very energy efficient, turning most of the electrical energy into light (very little heat produced).

circuit symbol for LED

Forward Bias

In the above circuit the diode is forward biased and current will flow through it.

We assume it has negligible resistance.

The bulb will light up!
Reverse Bias

In the above circuit the diode is reverse biased and current will not flow through it.

We assume it has infinite resistance.

The bulb will not light up!

 

 

The I-V Characteristic Graph for a Diode

For negative p.d.s (i.e. reverse bias) the current is insignificantly small showing a very high resistance – we can approximate this to infinite resistance.

For positive p.d.s (i.e. forward bias) (at least after ∼0.5 V) the current becomes very large showing a very low resistance – we can approximate this to zero resistance.

 

Example
At which of the following points does the diode have the lowest resistance?

Answer: D

The ratio V/I has the smallest value at point D compered to the other points.

 

Alternating Current & Direct Current

Direct Current (d.c)

A battery provides a constant voltage, which will drive a constant current through a fixed resistor.

The voltage of the emf source remains at a constant value and does not change with time.

The current-time graph for the current flowing through the 3 Ω resistor would look like this:

i.e. it is also constant and does not vary with time. This type of current is referred to as direct current or d.c..
Alternating Current (a.c.)

Let us now replace the battery with an alternating source:

The voltage of this source is actually constantly charging like this:

Note that the magnitude is constantly changing – it is not just flipping direction.

This alternating emf will cause an alternating current to flow through the circuit:

 

Diodes and Alternating Currents

Alternating Current and Diodes
Consider an LED connected to an alternating current source.

Will the LED be lit?

For half the time, the left side of the source will be positive and the current will try to flow anti clockwise around the circuit.

This is forward biased for the LED, so it allows current to flow and the LED will light up.

for the other half of the time, the right side of the source will be positive and the current will try to flow clockwise around the circuit.

This is reverse biased for the LED and so current cannot pass and the LED does not light up.

Thus, connected to an a.c. source, an LED will flash on and off.

The current flowing through the LED will be similar to this:

The resulting wave only has current flowing in one direction. But the current only flows for 50% of the time – for the other 50% the current is zero. This is said to have undergone half-wave rectification.

 

 

 

Rectifying A.C.

Diodes can be used to rectify an alternating current (convert it to d.c.).

Four Diodes placed in the following arrangement will result in a current that only flows one way through the light.

Half the time the current will be flowing like this:
 Then the direction of the emf changes and the current will flow like this:Of course, for a lamp like shown above, the direction of current flow is not important (it will glow whichever way the current is flowing through it).

However, if the lamp had been replaced with an LED, the direction of current flow would be of significance.

The current flowing through the centre LED would be like this:

<< Back D.C. Circuits

 

2025 Physics Lessons