Introduction to electricity

TEXT FOR READING 4.5.

DIODES AND TRANSISTORS

Part 1

Introduction to electricity

Direct current

Direct current is the type of electricity that comes from batteries. It moves from negative to positive.

Alternating current

Alternating current is when the polarity switches back and force between positive and negative. The speed at which the polarity changes is measured in hertz. AC can be switched to DC using a transformer or diode. The process is known as rectification.

Additional terms

Capacitance is the ability to store a quantity of electricity in an electric field. Inductance is the ability to store electricity in a magnetic field. The impact of capacitors and inductors on an alternating current is known as reactance. Impedance is a way of assessing how easily electricity passes through an electric device.

Ex.1.1

Mark the following statements as true (T) or false (F)

1. _____ Alternating current changes the polarity.

2. _____ Capacitance is the ability to store electricity in a magnetic field.

3. _____ Transformer is used to switch AC to DC.

Ex.1.2

Match the words (1-5) with the definitions (A-E)

1.______direct current

2.______inductance

3. ______negative

4. ______positive

5.______reactance

A. Having the electrical charge of a proton

B. The opposition to a charge in voltage or electrical current, as a result of capacitance and inductance

C. The ability to store electricity in a magnetic field

D. A flow of electrical current moving in one direction

E. Having the electrical charge of an electron

Part 2

Diodes

A diode is a one way valve that allows current to flow through it. Each diode has a cathode and anode for allowing the current to pass in the right direction. The cathode can be founded by locating the band on the diode.

Larger diodes are found in power supplies and are called rectifiers. Several types of rectifiers are common, including a bridge rectifier and a double rectifier.

It is common to see the peak inverse voltage (PIV) indicated. The PIV is important because exceeding this maximum can destroy the diode. Monitoring the pulse is also important, in order to notice any changes in a current.

Both smaller diodes and rectifiers are commonly found in electrical items. Diodes assist in the function of these items by directing voltage. They can also be used as biasing elements for transistor and LED panels. A light- emitting diode emits light on the control panels of most modern electronics.

Ex.2.1

Choose the correct answers:

1. What is the purpose of this passage?

A. To describe different types of diodes

B. To explain how to measure currents

C. To show the use of LEDs in electronics

D. To discuss when to use transistors

2. Which of the following is NOT a part of a diode?

A. Anode

B. Band

C. Cathode

D. Bridge

3. What do diodes help do?

A. Maximize pulse

B. Direct voltage

C. Destroy current

D. Control light

Ex.2.2

Match the words (1-8) with the definitions (A-H)

1.____vale

2.____rectifier

3.____anode

4.____cathode

6.____double rectifier

7.____light-emitting diode

8.____peak inverse voltage

A. The highest value of the input voltage that a diode can handle before allowing it to pass.

B. The place where current flows in from the outside

C. The place where current flows out from the inside

D. Four rectifiers arranged with a diamond like configuration

E. Emits light when a current passes through it

F. A device that controls the flow of liquid, gas or other substances

G. A large diode that forces an alternating current to become direct

H. Two rectifiers that share one terminal

Part 3

All about transistors

A transistor is a device that determines the flow and quantity of current. Transistors have the ability to switch and amplify electronic signals.

There are several different types of transistor: the bipolar transistor, the junction field effect transistor (JFET) and the metal-oxide semiconductor field-effect transistor (MOSFET). Both FET and bipolar transistor need a bias voltage in order to turn on.

A bipolar transistor has three terminals. An electrical current flows into the first terminal, known as the base. The base changes the current flow between the emitter and collector.

Field effect transistors also have three terminals. However, the terminals have different names: the gate, source and drain. The other difference is that voltage applied to the gate changes the current flow between the source and drain. MOSFET has two operating modes: enhancement mode and depletion mode. Depletion mode transistors are “normally ON” whereas enhancement mode transistors are “normally OFF”

Ex. 3.1

Match the words (1-7) with the definitions (A-G)

1.____amplify

2.____bias

3.____drain

4.____enhancement mode

5.____gate

6.____JFET

7.____source

A. an operating mode where the channel is free of charge carriers when the gate voltage is zero

B. the part of a transistor where charge- carrying holes originate

C. a type of a transistor that has a gate, source and drain

D. a fixed voltage, applied to a device, in order to control its operation

E. the controlling terminal that alters the current flow between the source and drain

F. to increase the power of a signal

G. the part of a FET that does the same job as the collector

Ex.3.2

Read the sentence pair. Choose where the words best fit the blanks.

1. base/depletion mode

A. ______________ transistors are devices that are “normally ON”

B. The ___________ changes the flow of current between two terminals.

2. MOSFET/emitter

A. ______________ is a kind of transistor that requires the least amount of signal current in order to turn on.

B. The ___________ is the section of a transistor where charge-carrying holes originate.

3. Bipolar transistor/collector

A. Once charge carriers have left the base, they flow into the ________.

B. A ___________ is a piece of equipment used to amplify and switch electronic signals.

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