Rectifiers and Battery Chargers - Part A

Question 1

How do you find the instantaneous value of an AC sine wave?

Answer 1

you take the maximum or peak value of that sine wave and multiply it by the “sin” of the angle you are trying to find the voltage at Vi = Vmax X sin (angle)

Question 2

How do you find the Effective or RMS value of an AC sine wave?

Answer 2

you multiply the maximum or peak value of that sine wave by 0.707…or… divide by the square route of 2 Veff = Vmax X 0.707

Question 3

What two things does an Oscilloscope display on a graph?

Answer 3

Voltage and Time

Question 4

How many Valence electrons do Conductors have?

Answer 4

1-3

Question 5

How many Valence electrons do Insulators have

Answer 5

5-8

Question 6

How many Valence electrons do Semiconductors have?

Answer 6

4

Question 7

What is “doping” and how does it work?

Answer 7

The process of introducing impurities into a pure intrinsic material Only about one atom of impurity is introduced for every twelve million atoms of pure intrinsic material, and this is enough to reduce the resistance of the material The result is “Extrinsic Material” which is either N-type or P-type

Question 8

What is a Semiconductor?

Answer 8

Semiconductor’s are neither good conductors or good insulators. The possess four valence electrons are referred to as “tetravalent elements”

Question 9

What are the two most common elements used in Semiconductor devices?

Answer 9

Silicon (Si) is the most common Germanium (Ge) has also been used

Question 10

What happens when you dope Silicon with Pentavalent atoms/materials?

Answer 10

Doping Silicon with Pentavalent materials creates N-type semiconductor material, which, means there is a loosely bonded free electron available as a charge carrier

Question 11

What is a Pentavalent atom or material? And what are some examples of these materials?

Answer 11

Pentavalent atoms or materials have five valence electrons Some examples are arsenic, phosphorous, and antimony

Question 12

What happens when you dope Silicon with Trivalent atoms/materials?

Answer 12

Doping Silicon with Trivalent materials makes P-type semiconductor material, which means there is a deficiency of one electron which results in a hole in the crystal lattice structure of the material

Question 13

What is a Trivalent atoms/material? And what are some examples of these materials?

Answer 13

Trivalent atoms/materials have three valence electrons Some examples are Boron, Aluminum, and Indium

Question 14

What are the majority charge carriers in P-type semiconductor material?

Answer 14

The majority charge carriers in P-type semiconductor material are holes

Question 15

What are the majority charge carriers in N-type semiconductor material?

Answer 15

The majority charge carriers in N-type semiconductor material are electrons

Question 16

What are Light-Dependent Resistors (LDR) or photoresistors?

Answer 16

They are photoconductive cells(photocells), which means when they are exposed to light some of their electrons break their covalent bonds, which reduces their resistance These semiconductor devices are composed of materials like lead-sulphide, indium-antimonide, and cadmium-sulphide.

Question 17

What is a Diode?

Answer 17

A Diode is a basic semiconductor device composed of a PN Junction It allows current to flow in one direction and prevents current flow in the opposite direction It behaves like a switch The lead attached to the P-type material is called the Anode(A) and the lead attached to the N-type material is called the Cathode(K)

Question 18

What is a Metal-Oxide Varistor(MOV)?

Answer 18

A Metal-Oxide Varistor is a common type of surge suppressor

It filters out voltage spikes that occur on an AC supply

Question 19

What is a Forward-Biased Diode?

Answer 19

• A diode only allows current to flow if the Anode(A) is positive with respect to the Cathode(K) with sufficient voltage
• Ignoring the Barrier Potential, a forward-biased diode behaves like a closed switch
• When a voltage of the correct polarity is gradually increased from zero volts, the free electrons in the N region, repelled by the negative cathode lead, start to move toward the PN junction. At the same time, the holes in the P-type material are drive away from the positive Anode lead toward the PN junction. Because the depletion region of the PN junction no longer has charge carriers it presents a barrier to current flow
• A minimum amount of applied voltage must be applied to the PN junction. This allows the charge carriers to be pushed across the depletion region and current to flow. This applied voltage is called the “Barrier Potential”
• When the applied voltage equals the Barrier Potential, the depletion region is overcome and current flows from the Anode(A) to the Cathode(K)
• At room temperature, the Barrier Potential for a Silicon(Si) PN junction is approximately 0.7V
• At room temperature, the Barrier Potential for a Germanium(Ge) PN Junction is approximately 0.3V

Question 20

What is Barrier Potential?

Answer 20

• a Diode will not allow current to flow if the Anode(A) is negative with respect to the Cathode(K), this is called reverse bias
• When an external voltage is applied where the Cathode(K) is positive with respect to the Anode(A), the diode is reverse biased. Under this condition the electrons in the N-type material are attracted away from the PN junction towards the positive terminal, while the holes in the P-type material are attracted to the negative terminal. The charge carriers are attracted to both ends of the diode and almost no current flows through the Diode
• There is a small amount of reverse current referred to as “leakage current”, but this is insignificant in most cases
• A Reverse-Biased Diode can be thought of as an open switch
• The ideal reverse resistance of a Silicon diode is infinity; in more practical terms it is approximately 30 Mega Ohms

Question 21

What is the Forward Bias Limit of a Diode?

Answer 21

• It is the rated current or maximum continuous forward current of the diode - If(max)
• If the current flowing through the diode exceeds the the forward bias limit of that diode, it can cause overheating which may destroy the diode

Question 22

What is the Reverse Bias Limit of a Diode?

Answer 22

• It is the maximum reverse voltage that can be appled to the diode
• this limit may also be called the “Voltage Repetitive Reverse Maximum”(Vrrm), “Peak Inverse Voltage”(PIV), or “Peak Reverse Voltage”(PRV)
• “Breakdown Voltage” or “Avalanche Voltage” are a voltage above the Reverse Bias Limit which causes the diode to breakdown and allow high values of reverse current to flow and destroy the diode. This is called the “Avalanche Current”

Question 23

What is the “Breakdown Voltage” or “Avalanche Voltage” of a Diode?

Answer 23

• Voltages above the safe maximum or Reverse Bias Limit, that cause the diode to breakdown and allow high reverse currents to flow, which destroy the diode. This high current is called the “Avalanche Current”

Question 24

What is the rating of a Low Current Diode?

Answer 24

A typical low-current diode has a current rating of 3A or less

Question 25

How can you identify the Cathode of an Axial-type Diode?

Answer 25

The Cathode is usually the end identified by a coloured or silver bar

Question 26

How are Medium Current Diodes usually mounted?

Answer 26

• Usually stud mounted with a pig-tail but may also be press fit diodes
• Diodes in the 150A to 500A range are usually stud mounted with a pig-tail

Question 27

What is a Heat Sink

Answer 27

• Usually a piece of metal a Diode is mounted on to help dissipate the heat produced by the diode
• usually configured to produce as much surface area as possible in as small of an area as possible
• The heat energy produced by the diode is readily conducted into the heat sink and dissipated to the surrounding air

Question 28

What are High Current Diodes and what do they look like?

Answer 28

• the largest current rating diodes are available in a shape called a “Hockey Puk”

Question 29

How do you perform a Forward-Bias check on a diode?

Answer 29

• You need a multimeter with a “diode check” range, usually indicated by a diode symbol on the multimeter
• Touch the positive lead to the Anode(A) and the negative lead to the Cathode(K), the display will read the forward bias voltage that the meter had to produce in order to detect the small amount of test current (usually 0.5mA)

Question 30

How do you perform a Reverse Bias check on a diode?

Answer 30

• You need a multimeter with a “diode check” range, usually indicated by a diode symbol on the multimeter
• Touch the positive lead to the Cathode(K) and the negative lead to the Anode(A), the display should be the same as an open circuit on the ohmmeter range, typically OL. This means the meter can’t produce the small test current through the diode. The diode acts like an open switch

Question 31

How would you identify if a diode is faulty and is open?

Answer 31

The diode will read OL(open circuit) for both forward and reverse bias checks

Question 32

How would you identify a shorted out diode?

Answer 32

The multimeter display will be the same for both the forward and reverse bias check

Question 33

What is a Freewheeling Diode?

Answer 33

• Also referred to as suppression or protection diodes
• They are used to control the destructive effect of the CEMF when the switch is opened in a DC circuit
• They are connected in parallel to the inductive load, in reverse bias to the supply
• When the switch in a DC circuit supplying an inductive load is opened, the inductor CEMF opposes the change and attempts to keep the current flowing through the inductor in the same direction. Since the current cannot flow through the switch to the supply, it then flows through the diode because the diode is foward biased in this direction of current flow. This diverted current flows until the flux collapses and no longer produces voltage. No significant arcing appears at the switch and the circuit is opened effectively and safely

Question 34

What is a Direction or Blocking Diode?

Answer 34

• Referred to as Directional, Blocking, Steering or Isolating Diodes
• These diodes only allow current to flow in one direction and not the other
• Commonly used in battery charger circuits, prevents current from flowing from one battery to another, or to feed back into the charger supply

Question 35

What are Isolating Diodes?

Answer 35

• Diodes that block current and provide battery isolation to prevent one load from discharging both batteries
• Commonly used in vehicle charging systems

Question 36

What are Steering or Directional Diodes?

Answer 36

• Commonly found in low-voltage switching circuits because they only require two wires to the switch which reduces costs
• These diodes steer the current through either the off or on coil of the relay in a low voltage switching application

Question 37

What is a photodiode?

Answer 37

• A photodiode is a PN Junction that is connected in reverse biased to the circuit and is only used in DC applications
• This diode allows a small current to flow when no light is applied, this current is called “dark current” and is negligable is more applications
• When light energy is applied to this diode, its resistance decreases and allows current to flow. Current increases as the light increases

Question 38

What is a Light Emitting Diode(LED)?

Answer 38

• When an electron moves across a PN junction, it releases some energy as it goes from a higher energy level to a lower energy level. This energy is mainly radiated as heat.
• LED’s are usually made of Gallium Arsenic Phosphate(GaAsP) or Gallium Phosphate(GaP)
• In these materials the energy released by the electrons that move across the PN Junction is sensed as light rather than heat
• LED’s emit light when a forward current flows
• Typical forward operating voltages are 1.8V to 2.2V
• Typical forward operating currents are 15mA to 30mA
• Maximum power dissipating is approximately 80mW to 100mW
• Commonly used in optocouplers, optoisolators, indicator lamps, digital displays and lighting

Question 39

What is the most common application for a diode?

Answer 39

• Rectifiers are the most common application for diodes
• Rectifiers convert AC in DC by conducting in one direction and not the other

Question 40

What is the Barrier Potential of a Silicon PN junction?

Answer 40

0.7V

Question 41

What is the Barrier Potential of a Germanium PN junction?

Answer 41

0.3V