2.1 Characteristics of diode, and diode as semiconductor device.

2.1.1 Schematic diagram and physical structure of diode.

2.1.2 Label the anode and cathode.
2.1.3 Schematic diagram of diode that is forward biased and
reverse biased.
2.2 I-V Characteristic curve for silicon diode :
a. Knee voltage
b. Forward current (in miliampere scale)
c. Reverse current (in microampere scale)
d. Breakdown voltage
e. Burning level
2.3 Apply diode in rectifier circuits
2.3.1 Draw following rectifier circuits :
a. Half-wave rectifier
b. Full-wave rectifier (centre-tapped)
c. Bridge rectifier
2.3.2 Operation of rectifiers in (2.2.1).
2.3.3 Using equation for following values :
a. Average current
b. Average voltage
c. Root mean square voltage
2.3.4 Sketch the items in 2.3.1.
2.3.5 Multimeter to measure AC in rectifier circuit
 1.0 Introduction

• A diode is an electrical device allowing current to move through it in one

direction.

• Diode is a semiconductor device.

• Combination of N-type and P-type materials.

• 2 terminals : Anode and Cathode.

Figure 1 : Diode
 2.0 Schematic and physical structure

Anode

P Conventional Current Flow

Cathode

a) Physical Structure b) Schematic Symbol

Figure 2 : Physical Structure & Schematic Symbol Of Diode

 3.0 Diode’s
3.0 Bias

There are 2 types of bias; Forward Bias and Reverse Bias

3.1 Forward Bias

- Definition : Anode get more positive voltage than
cathode OR cathode get more negative voltage than anode.

- Schematic Circuit Of Forward Bias :

  Figure 3 : Diode in forward

biased
 3.0 Diode’s Bias

3.2 Reverse Bias

- Definition: Anode get more negative voltage
than cathode OR Cathode get more positive voltage
than anode.

- Schematic Circuit Of Reverse Bias :

Figure 4 : Diode in reverse

biased
 4.0 I-V Curve Of Diode

If : Forward Current
Vf : Forward Voltage
Ir : Reverse Current
Vf : Reverse Voltage

Figure 5 : I-V Characteristics

 Definition Of Terms In I-V Curve
i). Forward Current (If)
• Currents flow through diode during forward bias.
• Usually measured in miliampere (mA) unit.

ii). Reverse Current (Ir)

• Small current (leakage current) which flow during reverse bias.
• Usually measure in microampere (A) unit.

iii). Knee Voltage

• The voltage level where the forward currents increased drastically (Si = 0.7
V, Ge = 0.3 V).

iv). Breakdown Voltage

• The voltage level where the reverse currents increased drastically.
• If the currents is bigger than the breakdown voltage’s level, the P-N
joints will burn and damage. the diode will get damaged. At this point, the
diode behave more like a shorted wire and hence current flows through it
easily. 
 Diode In Rectifier Circuit

• Rectifier was circuit network that can convert a.c voltage to d.c voltage
(changing from 2 poles to 1 poles only).

RECTIFIER

Block Diagram Of Power Supply

 Diode In Rectifier Circuit

 Conversion of alternating current (or voltage) into direct current (or voltage)
is called rectification.

 A diode is well-suited for this job because it only conducts when forward-
biased.

 Types Of Rectifiers :
a). Half-wave rectifier
b). Center-Tapped Full-wave rectifier
c). Bridge rectifier
 Half-wave Rectifier

 An a.c voltage is applied to a single diode connected in series

with a load resistor, RL.

 Schematic diagram :
 Half-wave Rectifier

Circuit Operation :

During the positive half-cycle of input a.c voltage :

When M is positive, the diode D is forward biased (ON) and conducts.

While conducting, diode act as a short-circuit so that circuit current flows.

Positive half-cycle of the input a.c voltage is dropped across RL.

The output waveform is similar to the input voltage waveform.

Half-wave rectifier during the positive half-cycle

 Half-wave Rectifier

During the negative half-cycle of input a.c voltage :

When point M becomes negative, the diode is reverse-biased
(OFF),so it does not conduct current.
Diode act as open-circuit, so there is no current flow.
There is no voltage drop across RL.
The negative input half-cycle is suppressed.

Half-wave rectifier during the negative half-cycle

 Half-wave Rectifier

Waveforms for both cycles :

Output voltage waveforms : Only appear during positive half-wave.

 Full-wave Rectifier

 The full-wave rectifier circuit using two diodes and a centre-tapped

transformer, which usually taken as the ground or zero voltage
reference point.
 The purpose of centre-tapped transformer is to get two a.c output with
the same value but in inverse phase.

 Schematic Diagram :

M
 Full-wave Rectifier

Circuit Operation :

When input a.c supply is switched on, the ends M and N of the
transformer secondary become positive and negative alternately.

When Terminal M is positive, G is at zero potential and N is at negative

potential :

Diode D ia in forward biased and D is in reverse biased.

1 2
Current flows along M, D , C, RL dan G.
1
As a result, positive half-cycle of the voltage appears across R .
L
 
When Terminal N is positive, G is at zero potential and M is at negative
potential :

Diode D ia in forward biased and D is in reverse biased.

2 1
Current flows along N, D , C, RL dan G.
2
As a result, positive half-cycle of the voltage appears across R
L.
 Full-wave Rectifier

Output voltages for the both half-cycles :

Current keeps on flowing through RL in the same direction in both half-cycles of the a.c
input. It means the output voltage waveforms of the both half-cycle have a same shape.

Frequency of the rectified output voltage is twice the supply frequency.

* VNG : D2 operated
* VMG : D1 operated
 Full-wave Rectifier

VMG

VNG
 Bridge Rectifier

 Bridge rectifier requires four diodes, but the transformer use did not center-
tapped and has a maximum voltage across its secondary.
 Schematic Circuit :
 Bridge Rectifier

Circuit Operation :
When Terminal M of the secondary is positive and N is negative :
Diode D1 dan D2 become forward biased, when diode D3 dan D4
become reverse biased.
Current flows along M, E, D1,A, RL, B, C,D2, F and N. Produce a
positive half-wave waveform across RL.

When Terminal N of the secondary is positive and M is negative :

Diode D3 dan D4 become forward biased, when diode D1 dan D2
become reverse biased.
Current flows along N, F, D4,A, RL, B, C,D3, E and M.

Produce a positive half-wave waveform across RL.

 Bridge Rectifier

Output voltage for both half-cycle :

Current keeps flowing through load
resistance RL in the same direction
during both half-cycles of the a.c input
supply. It means the output voltage
waveforms of the both half-cycle have
a same shape.

Its frequency is twice that of the

supply frequency.
Formulas
 Vp sec Vpp sec

Vp sec
Vpp sec

Vpsec – 0.7V
2
 EXAMPLES

Example 1: Solution :

i. Output voltage, Vo
A half wave rectifier build in 60 Known that, Vo = Vp sec – 0.7V
Vrms in the secondary of the
transformer. Calculate:- From formula, Vp sec = √2 x Vrms
= √2 x 60
= 84.85 V
 i. The output voltage.
ii.Average voltage of the circuit Therefore, Vo = Vp sec – 0.7V
= 84.85 – 0.7
= 84.16V

ii. Average voltage, Vavg

Known that, Vavg = Vo
π
= 84.16
π
= 26.7 V
 EXAMPLES
Example 2:
In a centre-tap full wave rectifier,
load resistance is 2KΩ. The a.c.
supply across the primary winding
is 220V. Taking transformer turn
ratio N1/N2 = ½ and neglecting
diode resistance. Determine:-
i. Output voltage
ii. Average voltage
iii. Average current

Vpsec

220Vpp G
Vpsec 2k
 EXAMPLES

(110)
70 v

70 v
35mA
 EXAMPLES

Example 3:
A full wave bridge rectifier with a 120 Vrms sinusodial input at secondary
side had a load resistor of 1KΩ. If silicone diodes are employed,
determine:-
i.Average voltage available at the load.
ii. Average current

120Vrms

1:2
EXAMPLES
5.3 Application Of Rectifiers in AC meters.

• a.c. meter could not properly function if directly connected to a.c.

because meter’s needle direction movement will be fluctuating at
every half cycles positive and negative.

• If frequency which involved is high frequency so the needle will only

vibrate in zero value because the needle cannot lagging reading.

• rectifier will cause wave which in positive cycle only can be

measured. So, the meter read average value only (Vavg).