The Rectifier Diode

The diode will pass electric current in one direction only.

It is used in power supplies as a rectifier to convert AC voltages to DC voltages.

The diode is the simplest form of semiconductor device used in electronics and is formed using a single P-N junction.

conventional current flow

Cathode -Anode +

NP

electron flow

Simple Rectifier Circuit

The simplest rectifier circuit is called the ‘Half-Wave’ rectifier as it only converts one half cycle.

Since current passes in one direction only, the negative portion of the waveform is blocked.

0 t

+

-

Vo

Vi

I D

-

ac mains supply

+

ViVo

dc (unsmoothed)

voltage

VF

Rectifier Diode Characteristics

The rectifier diode has a low resistance in the forward direction and a very high resistance in the reverse direction. It therefore passes current in one direction

only. A diode which is reverse-biased will not pass a significant current, however if

the reverse voltage rating is exceeded the diode will break down. This voltage is called the maximum reverse voltage (VRRM ).

VF

IF

(mA)

-IR

(µA)

Forwardbias

VR0.4 0.6

Reversebias

breakdown Voltage(VRRM )

silicongermanium

The Rectifier Diode – CharacteristicsActivity

Silicon Diode (1N4001)Select your components and instruments place them on the breadboard.

Connect the components to form the circuit shown using link wire.Set power supply voltage to 15 volts

Adjust current from 0 to 20mA in 2mA steps and record your results

Germanium diode (OA91)Repeat the above for the germanium diode.

R1470Ω

DC POWER SUPPLY

15V

VR11kΩ

VF

Multi-meter set to volts(2V)

+

+

IF

Objective: To measure and compare the characteristics of a silicon and germanium rectifier diode.

R1470Ω+

-TEST

DIODE

Multi-meter set to current(100mA)

The Rectifier Diode – Characteristics ActivityBreadboard layout and Connections

Select your components and instruments place them on the breadboardas shown and connect the test instruments.

AmmeterIF

-+VoltmeterVF

+ -

DC Power Supply

+ 15v

R1470Ω

VR11kΩ

- TEST DIODE

The Zener Diode

The Zener diode is a silicon junction diode designed to operate in the reverse conduction mode and provide a stable voltage across its terminals.

It is used to provide a reference voltage in comparator circuits, power supply circuits and constant current sources.

It can also be used to provide a regulated supply to a load but has limitations where the load can change.

conventional current flow

Cathode +Anode -

Zener Diode ApplicationsThe zener diode provides a stable voltage (VZ) from a varying source voltage (VI).

Circuit analysis

ILOAD

IZ

R+

-

+

-

VZ

IT

VI

VR

IT = IZ + IL amps ohmsR = VI - VZ

IT

VR = VI - VZ volts

R+

-

+

-

Regulated (Fixed) output voltage,

(VO )

Unregulated input voltage

(VI )

Typical application

Zener Diode Characteristics

The zener diode will pass electric current in both directions, however it is usually operated in the reverse bias mode.

Its resistance is high in the ‘reverse’ direction until the zener breakdown voltage (VZ ) is reached. At this point the diode conducts as its resistance becomes very

low, maintaining a constant voltage across its terminals.

VF

IF

(mA)

-IR

(mA)

Forwardbias

VR 0.6VReverse

bias

Zener breakdown Voltage

(VZ )

Breakdown current

(IZ )

Diode Types and Markings

Diode types range from small signal glass and general purpose ceramic to high current stud mounted.

Applications include power supply rectifiers, protection and detector circuits for radio tuning.

Diodes are identified using an alphanumeric code and the cathode is identified by a bar.

Package styles range from miniature glass bead to large metal stud. Stud mounted styles print the diode symbol orientated to identify the polarity.

1 watt rectifier diode

30 watt rectifier diode

Germanium signal diode

The PN Junction

A p-n junction is a piece of semiconductor material in which part of the material is p-type and part n-type.

At the junction donated electrons called majority carriers diffuse into the p-type material and the acceptor holes diffuse into the n-type material

The area in the region of the junction becomes depleted of holes and electrons due to re-combination and is called the depletion layer.

Impurity atoms(fixed)

p - type material n - type material

Holes (mobile carriers)

Electrons (mobile carriers)

Electrons (negative charge carriers)

Holes (positive charge carriers)

Depletion layer