Diodes and Rectifiers
Diodes

When the voltage on the anode is positive in respect to the cathode, the diode conducts and the lamp lights. The diode is said to be forward biased.


When the voltage applied to the anode is negative in respect to the cathode, the diode does not conduct and the lamp will not light. In this condition the diode is said to be reverse biased.

A forward biased diode

A reverse biased diode

Rectifiers

What is a rectifier?

A rectifier is a circuit that converts an AC current into a DC current. They are used where a DC supply is required but only an AC supply is available. Electronic systems such as fire alarms and security systems require a DC supply to operate and are therefore likely to include a rectifier circuit.


Half-wave Rectifier

A half-wave rectifier consists of a single diode in series with a load, which in the example below is a resistor. The AC supply produces a voltage which alternates between a positive and negative in one complete cycle. This means that the current flows in one direction and then the other repetitively.


During the positive half-cycle of the supply voltage, the anode voltage is positive in respect to the cathode, therefore it is forward biased and current flows through the resistor and a voltage is developed across it. When the supply voltage enters the negative half-cycle, the anode is negative in respect to the cathode, this means that the diode is reverse biased and no current flows through it, and as a result, no voltage is developed across the resistor.


The half-wave rectifier produces a pulsating voltage, although the voltage across the resistor does not look like DC because its voltage pulsing, it is in fact DC because the current never reverses direction. A half-wave rectifier produces pulsating DC.

Vs

R

The supply voltage (Vs)

The voltage developed across the load (VR)

Full-wave Rectifier / Bridge Rectifier

A full-wave, or bridge, rectifier uses four diodes arranged in such a fashion as to direct the current through the circuit in a way that ensures that the current is directed to the load in one direction only. As with the half-wave rectifier the bridge rectifier produces pulsating DC, but the power supplied by the negative half-cycle is not lost.

Positive half-cycle

During the positive half-cycle of the supply voltage, diodes 2 and 4 are forward biased because their anode voltages are positive in respect to their cathodes. The voltages on the anode and cathode of diodes 1 and 3 are all positive, therefore no current can flow through them so it appears that they are disconnected.


The current is therefore directed through the load resistor with point a being positive and point b being negative.

Negative half-cycle

During the negative half cycle the direction of current flow reverses as shown in the diagram. In this case diodes 1 and 3 are forward biased because their anode voltages are positive in respect to their cathodes. The voltages on the anode and cathode of diodes 2 and 4 are at the same voltage, therefore no current can flow through them.


The current is directed in such a way that, despite the supply current direction changing, the current through the load resistor continues to flow in the same direction as during the positive half-cycle.

D1

D2

D3

D4

The supply voltage (Vs)

The voltage developed across the load (VR)

D1

D2

D3

D4

The diagrams below explain how the bridge rectifier works...

Smoothing

A capacitor can be added to the output of the rectifier to reduce the pulsing effect. The capacitor acts like a reservoir of electricity, charging up as the output voltage of the rectifier increases and then discharging into the resistor during the periods where the output voltage of the rectifier drops. The diagram below shows how the smoothing capacitor improves the quality of DC delivered to the load resistor. Some ripple will still remain on the load resistor but will be greatly reduced, increasing the value of capacitance reduces the ripple.

The supply voltage (Vs)

The rectified and smoothed DC voltage across the load resistor

a

b

a

b

+

+

-

-

A diode is a semiconducting device which allows current to flow in one direction only. It has many applications in electrical circuits such as:

Rectification                       -    Converting AC into DC

Back EMF protection         -    Protecting sensitive components from reverse voltages developed by inductive loads

Polarity protection              -    Preventing circuits from being damaged through incorrect polarity

A silicon rectifier diode

A capsule diode

(High current)

Stud diodes

The operation of diodes

Circuit Symbol

Anode

Cathode

An AC voltage - Alternating in direction

A DC voltage - One direction only