EET 251

Electricity and Electronics

Lesson Three

ELECTRICAL CIRCUITS

To understand electricity and electronics, it is necessary to know how to apply basic electrical theory. Electricity and electronics is a somewhat mathematical discipline. The mathematics is easy to understand, because it has practical applications. The basic theory used is called Ohm’s law; which is important because it aaplies to the basic theory of electrical circuits. All examples in this lesson are direct-current (dc) circuits. Alternating-current (ac) circuits are more complex and will be discussed later.

3.1 Use of Calculators and Computers

Scientific calcultor: exponent, reciprocal, square, square root, trigonometry (sine, cosine, tangent), inverse trigonometry, logarithms, inverse logarithms, storage and recall functions.

Computer parts: Monitor, video card, disk drive controller card, random access memory (RAM) chips, motherboard, floppy disk drive, hard disk drive, CD-ROM drive, keyboard, mouse, printer, printer cable.

Circuit simulation Software: For example, Electronics Workbench and Pspice.

3.2 Ohm’s Law

Ohm’s Law explaining the relationship of voltage, current and resistance.

3.3 Troubleshooting

Steps in the process of troubleshooting:

(1) Determining from the operation of the equipment that a fault does exist

(2) Checking the source voltage for its proper value

(3) Determining the cause of zero, high, or reduced current

(4) Making repairs to the system

(5) Checking the system for proper operation

Symptoms of problems in power conversion (overheating)

3.4 Series Electrical Circuits

Use of subscripts to identify electrical components (ex: R_T or V₃)

Characteristics of a series circuit:

(1) The same current flows through each part of a series circuit.

(2) The total resistance of a series circuit is equal to the sum of the individual resistances.

(3) The voltage applied to a series circuit is equal to the sum of the individual voltage drops.

(4) The voltage drop across a resistor in a series circuit is directly proportional to the size of the resistor.

(5) If the circuit is broken at any point, no current will flow.

Troubleshooting: open circuits, short circuits, changed resistor values.

3.5 Parallel Electrical Circuits

Characteristics of a parallel circuit:

(1) There are two or more paths for current flow.

(2) Voltage is the same across each component of the circuit.

(3) The sum of the currents through each path is equal to the total current that flows from the source.

(4) Total resistance is found by using the formula: 1/R_T=1/R₁+1/R₂+1/R₃+...

(5) If one of the parallel paths is broken, current will continue to flow in all the other parts.

Troubleshooting: zero, high and low current.

3.6 Combination Electrical Circuits

Description and examples of combination circuits

Finding the total resistance, the total current of a combination circuit

3.7 Kirchhoff’s Laws

Kirchhoff’s voltage law (KVL)

3.8 Examples and Graphics

Meters: ohmmeter, multimeter (VOM), analog meter, digital meter

Resistance tests or continuity checks

3.9 Power in DC Electrical circuits

Formulas to determine voltage, current, resistance or power

To find...	Formulas	To find...	Formulas
V	I x R	R	V/I
	P/I		V^2/P
	%&PxR&&		P/I^2
I	%&P/R&&	P	I x V
	P/V		I^2 x R
	V/R		V^2/R

Watt, watt-second, kilowatt-hours (kWh)

Work= Force x distance (use appropriate units)

3.10 Maximum Power Transfer in Circuits

Maximum power transfer

3.11 Voltage-Division Circuits

Voltage-divided circuits

Tapped resistor

Voltage-divider design

Safety factor

Voltage-division equation or voltage-divider rule

Negative voltage derived from a voltage-divider circuit

Voltage division with a potentiometer

3.12 Problem-Solving Methods

(1) Kirchhoff’s voltage law

(2) Superposition

(3) Equivalent circuits: also called complex circuit theorems, network theorems. Thevinin equivalent circuit method, Norton equivalent circuit method

(4) Bridge circuit simplification

REVIEW

1. What is Ohm’s law?

2. What are the symbols used for (a) voltage, (b) current, and (c) resistance?

3. What is the relationship of voltage and current in a circuit?

4. What is the relationship of resistance and current in a circuit?

5. What is a series circuit?

6. What is a parallel circuit?

7. What is a combination circuit?

8. What are the voltage, current, and resistance characteristics of (a) series circuits and (b) parallel circuits?

9. How is total resistance of a series circuit measured?

10. How is total resistance of a parallel circuit measured?

11. How is total current of a series circuit measured?

12. How is total current of a parallel circuit measured?

13. How is voltage drop measured for a series circuit?

14. How is voltage drop measured for a parallel circuit?

15. What are Kirchhoff’s laws? Explain them.

16. Explain the three ways used to find total resistance of parallel circuits.

17. What are three ways to find the electrical power of a circuit?

18. What is meant by kilowatt-hour?

19. What is a voltage-divider circuit?

20. What is meant by a negative voltage?

21. Discuss each of the following problem-solving methods: (a) Kirchhoff’s voltage law; (b) Superposition; (c) Thenivin’s theorem; (d) Norton’s theorem; and (e) Bridge-circuit simplification.