# In-05-02 Diode Application

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```					Abu Dhabi National Oil Co. ADNOC Technical Institute

INSTRUMENTATION
INDUSTRIAL ELECTRONICS II

UNIT 2 DIODE APPLICATIONS

DIODE APPLICATION DATE OF ISSUE 8-DEC-09

PAGE 1 OF 16 IN-05-02

UNITS IN THIS COURSE

UNIT 1

BASIC SEMICONDUCTOR THEORY

UNIT 2

DIODE APPLICATION

UNIT 3

THE CONTROLLED DIODE

UNIT 4

TRANSISTORS

UNIT 5

DIODE APPLICATION DATE OF ISSUE 8-DEC-09

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Paragraph 2.0 UNIT OBJECTIVES 2.1 INTRODUCTION 2.2 THE DIODE AS A RECTIFIER 2.2.1 Half-wave Rectification 2.2.2 Full-wave Rectification 2.2.3 The Bridge Rectifier 2.2.4 Smoothing Circuits 2.2.5 The Integrated Circuit Regulator 2.2.6 The Zener Diode 2.2.7 Light Emitting Diodes and Photo-Diodes 2.2.8 The Opto-Coupler 2.3 OTHER DIODE APPLICATIONS 2.3.1 The Voltage Clamp 2.3.2 Switch Protection 2.3.3 Reverse Current Protection Page 4 5 5 5 6 7 8 10 11 12 14 15 15 16 16

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ADNOC TECHNICAL INSTITUTE 2.0 UNIT OBJECTIVES The student will be able to : Draw a simple circuit diagram and explain the action of a half-wave rectifier circuit.  Draw a simple circuit diagram and explain the action of a full-wave rectifier circuit.  Draw a simple circuit diagram and explain the action of a bridge rectifier circuit.      With the aid of a diagram, explain rectifier smoothing circuits. Explain the uses of the integrated circuit regulator. With the aid of a diagram, explain the use of the Zener diode. Explain the LED and photo-diode and their uses in the opto-coupler. With the aid of simple sketches, show how the diode is used as a clamp and for reverse current protection.

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ADNOC TECHNICAL INSTITUTE 2.1 INTRODUCTION The aim of this unit is to show the practical use of a PN Junction semiconductor diode in electronic circuits.

2.2 THE DIODE AS A RECTIFIER TVs, computers, instrument loops, etc. all work on D.C. diode is to change an A.C. supply to a D.C. The A.C

workshop supply must be changed to D.C. The most common use of a supply, called RECTIFICATION. There are various methods of doing this, as follows:

2.2.1 Half-wave Rectification
D D

+
A.C. SUPPLY R + + + A.C. SUPPLY


R

+







PULSED DC OUTPUT





+

PULSED DC OUTPUT

Figure 2-1 Half-Wave Rectifier
Figure 2-1 shows a half-wave rectifier circuit. The diode D only conducts on the positive half cycles and the current flowing through the load resistor R will produce only positive half cycles. If the diode is reversed it will only conduct on negative half cycles and the pulsed D.C. output will be negative.

DIODE APPLICATION DATE OF ISSUE 8-DEC-09

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ADNOC TECHNICAL INSTITUTE 2.2.2 Full-wave Rectification
A

+
D1

CENTRE TAP +

+ -

B

R

+

D2

+ + +
PULSED DC OUTPUT

Figure 2-2 Full-Wave Rectifier
The half-wave rectifier is not efficient as only half the input produces a useful output. The full-wave rectifier (see Figure 2-2) is more efficient because both halves of the A.C. input are converted into DC output. Operation  The A.C. input supply is applied to a transformer which has the secondary coil split into two halves by a centre tap.  When end A is positive with respect to end B, the diode D1 conducts the current which flows through the load resistor (R) as shown ( ……….. ………….)  When end B is positive with respect to end A, the diode D2 conducts the current which direction as before (  flows through the load resistor in the same )

The output is now continuous positive going pulses and a full wave is produced in the load resistor.



If the diodes are reversed, the output pulses will all be negative going.

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ADNOC TECHNICAL INSTITUTE 2.2.3 The Bridge Rectifier The full-wave rectifier using a centre tapped transformer is not popular today because transformers cost a lot more than diodes. Figure 2-3 uses a diode bridge to produce a full-wave output without the use of a transformer. -+
A + D3 D2 D1 D4

+ B

+
R

+-

+ + +
PULSED DC OUTPUT

-

Figure 2-3 Bridge Rectifier Circuit
Operation When A is positive and B is negative, diodes D1 and D2 conduct. The current which flows through the load as shown (………………….)  When B is positive and A is negative, diodes D3 and D4 conduct. The current flows through the load again in the same direction as shown (  ).

A full-wave positive going output flows through the load. Reverse all the diodes and the output is negative going.

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ADNOC TECHNICAL INSTITUTE 2.2.4 Smoothing Circuits The pulsating D.C. output from a rectifier circuit is useless so a smoothing circuit must be added to produce a stable D.C. A typical smoothing circuit added to a bridge rectifier is shown in Figure 2-4.

+

RESERVOIR CAPACITOR

SMOOTHING CIRCUIT

Figure 2-4 Bridge Rectifier and Smoothing Circuit
Operation  The reservoir capacitor charges to the peak value of the positive going input wave and discharges through the load resistor.  The smoothing circuit acts as a filter for the A.C. ripple (wave) which is produced as the reservoir capacitor charges and discharges. capacitor is a high resistance to D.C. and a low resistance to A.C. The inductor is a high resistance to A.C. and a low resistance to D.C. The

DIODE APPLICATION DATE OF ISSUE 8-DEC-09

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ADNOC TECHNICAL INSTITUTE Therefore the D.C. appears across the load resistance and the unwanted A.C. across the inductor. This action is shown by the graph in Figure 2-5.
OUTPUT
RESERVOIR CAPACITOR AND LOAD ONLY + A.C. RIPPLE

D.C. LEVEL TIME

OUTPUT
WITH L.C. SMOOTHING

OUTPUT D.C. LEVEL

INSIGNIFICANT A.C. RIPPLE

VOLTAGE
LOST ACROSS INDUCTOR

Figure 2-5
 Smoothing is easier using a full wave rectifier as the frequency of the ripple is twice that of a half-wave rectifier. This makes the action of the inductor twice as effective (XL = 2fL).  It is possible to smooth the output without a reservoir capacitor but this requires a large inductor and is unlikely to be used in instrument systems.

DIODE APPLICATION DATE OF ISSUE 8-DEC-09

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ADNOC TECHNICAL INSTITUTE 2.2.5 The Integrated Circuit Regulator The use of a smoothing circuit is becoming rare in instrumentation as inductors are expensive and produce a loss resistance in the circuit. This loss resistor causes the output voltage to fall as the current through the load increases. This leads to poor regulation. Figure 2-6 shows what is meant by regulation. r
VOLTAGE LOSS = Ir I LOSS RESISTANCE IN COIL R LOAD

OUTPUT VOLTAGE

D.C. SUPPLY (VS) LOAD CURRENT

OUTPUT VOLTAGE (VO)

Figure 2-6 Regulation
From Ohm’s law, we know that the voltage across the loss resistor will be the load current times its resistance (Ir). The larger the voltage across the loss resistor the larger the fall in output voltage (VO=VS - Ir). The graph shows a typical regulation curve, the smaller Ir the better the regulation. A modern instrumentation system needs a DC supply which remains stable when there are changes in load. So, the regulation must be almost perfect. The circuit below shows a modern system which gives good smoothing and near perfect regulation within the stated range.

A.C. SUPPLY

3 LEG REGULATOR

RESERVOIR CAPACITOR 1000 F

10 F

TANTALUM CAPACITOR

CONSTANT OUTPUT VOLTAGE

Figure 2-7 The 3-Leg Regulator

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ADNOC TECHNICAL INSTITUTE Figure 2-7 shows a typical D.C. rectifier circuit using a 3 leg regulator. The regulator is an integrated circuit which smoothes and regulates the D.C. output. A typical example is the Motorola MC 7805 CT. .This can provide 5V, from no load to a full load of 1A with a maximum loss of 40mv. The output capacitor is usually made of tantalum. A tantalum capacitor is very small but provides high capacitance and efficiently removes any output ripple.

2.2.6 The Zener Diode The ZENER diode makes use of the reverse breakdown effect of a PN junction. It is specially doped to break down at a fixed voltage and is used as a cheap voltage reference. The regulation of the device is good when there are only small changes in current through the load. A typical circuit using a Zener diode is given below.

R

+
FIXED OUTPUT VOLTAGE

ZENER DIODE

Figure 2-8 Using a Zener Diode

Figure 2-8 shows a typical bridge rectifier and smoothing circuit providing a D.C. voltage to a Zener diode. The Zener diode breaks down at a fixed voltage to provide a fairly constant output voltage to the load. Zener. This can be very high if there is no load. The resistor R is included in the circuit to limit the reverse current through the

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ADNOC TECHNICAL INSTITUTE Note : The Zener diode must always be connected in the reverse direction for correct operation. The Zener breakdown voltage is fixed for each device within a range of about 2.7V to 75V. The set voltage for each Zener is written on the device.

2.2.7 Light Emitting Diodes and Photo-Diodes Light Emitting Diodes (LED) These are specially doped PN junctions. When they receive a small

forward D.C. voltage (around 1.5V), they produce (emit) light. The light at the junction is focused by a lens to produce a bright spot. They come in many different colours e.g. red, green, yellow, orange and blue, depending on the semi-conductor used. For example, a red LED uses gallium with arsenic and phosphorus doping. The sketch below shows a typical LED with its symbol (see Figure 2-9)
PLASTIC LENS

CONNECTORS

LIGHT EMITTING JUNCTION

LONG LEG ANODE

SHORT LEG CATHODE

+

LED CONSTRUCTION

Figure 2-9 Enlarged view of an LED

LED SYMBOL

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ADNOC TECHNICAL INSTITUTE The Photo-Diode This device is the opposite of an LED. Light is focused onto a PN junction and the light energy is used to produce a current through the junction. The photo-diode is reverse biased so that very little current flows. The light which is shone onto the PN junction produces extra hole/electron pairs and the conductivity goes up. A typical photo-diode is shown in Figure 2-10, together with a typical circuit
FOCUSIN G LENS PN JUNCTION

METAL CAN

GLASS/METAL SEAL

PHOTO-DIODE SYMBOL

PHOTO-DIODE CONSTRUCTION (ENLARGED)

+
R DETECTOR AMPLIFIER

-

SIMPLE PHOTO-DIODE CIRCUIT

Figure 2-10 The Photo-Diode
The photo diode is used to detect changing light levels. As the light falls on the photo-diode, the reverse current increases. This increases the voltage across the resistor, (R). The increased voltage is detected by an amplifier to show increasing light levels. If the voltage falls the amplifier shows falling light levels.

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ADNOC TECHNICAL INSTITUTE 2.2.8 The Opto-Coupler This is a popular device in instrumentation electronics. It's used for

isolating one circuit from another one. It consists of an LED and photodiode together in one unit, the opto-coupler. A typical circuit to show its operation is shown below.
CURRENT DETECTOR LED PHOTO-DIODE

+

+

OPTO-COUPLER (OPTO-ISOLATOR)

-

Figure 2-11 Basic Opto-Coupler Circuit
Figure 2-11 shows the basic use of an opto-coupler. If the switch is

closed the LED lights and shines onto the photo-diode which operates the current detector. The current detector will then light up to indicate the switch has closed and the current is flowing in the other circuit. This happens although there is no electrical connection between the two circuits. The insulation between the LED and the photo-diode must be transparent so that light can pass through it. With suitable insulation between the two diodes, voltages of over 10,000 V can be isolated. The opto-coupler acts as a safety barrier between the two circuits. It separates control board circuits from field devices eg, solenoid valves.

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ADNOC TECHNICAL INSTITUTE 2.3 OTHER DIODE APPLICATIONS 2.3.1 The Voltage Clamp

INPUT SIGNAL

AMP

INPUT SIGNAL

+

+ AMP

+ -

+

+ -

+

EARTH

EARTH

POSITIVE CLAMP
6.9 V

NEGATIVE CLAMP

AMP
15 V 6.9 V

CLAMPING TO A LEVEL

Figure 2-12 The Voltage Clamp
Figure 2-12 shows the use of a diode to clamp (hold) a signal line at one level. In a positive clamp the diode will conduct if the input signal tries to go positive. Therefore only a negative going signal is sent to the amplifier input. In a negative clamp the diode is reversed so that only positive signals are sent to the amplifier input. A Zener diode is used to clamp to a set level. In the diagram, if the input signal goes above 6.9V positive, the Zener conducts to hold at this voltage.

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ADNOC TECHNICAL INSTITUTE 2.3.2 Switch Protection +
SWITCH D.C. COIL OF SOLENOID VALVE OR RELAY

BACK EMF

Figure 2-13 Switch Protection
Figure 2-13 a diode used to protect a switch when a D.C. solenoid circuit is broken. The back EMF produced is shorted out by the diode and does not appear across the switch.

2.3.3 Reverse Current Protection
+ +
BATTERY CHARGER CHARGING CURRENT BATTERY PACK

-

Figure 2-14 Reverse Current Protection
Figure 2-14 shows a diode used for reverse current protection. Normally the battery charger drives current into the battery pack. If there is a fault in the battery charger (for example, reversed connections) the diode stops reverse current from the batteries damaging the charger.

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Description: Instrumentation Course