Chapter 4

Resistance

Resistance Opposition to electron flow in a circuit.

Expressed by the symbol R. Measured in ohms. Abbreviated with the Greek symbol . Varies from material to material.

Silver is best. Copper is most common. Gold doesn’t tarnish. Affected by temperature. Affected by the size (diameter) of the conductor.

Resistivity The resistance of a material to current flow.

Resistivity is different for different materials. Even good conductors have different levels of

resistivity. In electric circuit, the larger the diameter of the

wire, the lower the electrical resistance to current flow,

One ohm is the resistance of a circuit, or circuit element, that permits a steady current flow of one amp when one volt is applied to the circuit.

Conductance The ability of a material to pass

electrons. Unit known as Mho (ohm backwards). Abbreviated with the inverted Greek

symbol Ω.

Resistors Components manufactured to possess a

specific value of resistance to the flow of current.

Come in two classifications: Fixed value Variable

Variety of shapes and sizes to meet specific circuit, space, and operating requirements.

Tolerance The amount that the resistor may

vary and still be acceptable. The larger the tolerance, the cheaper

it is to manufacture. Resistors are available with

tolerances of ±20%, 10%, 5%, 2%, and 1%.

Molded carbon resistor The most commonly used. Inexpensive Manufactured in standard resistor values.

Wire wound Used in high-current circuits. Resistance varies from a fraction of an

ohm to several thousand ohms.

Film resistors Becoming increasingly popular. Three types: carbon film, metal film,

and tin oxide film. Surface mount resistors

Ideal for small circuit applications. Available in both thick and thin films.

Variable resistors* Allow the resistance to vary. Vary linearly or logarithmically. Called a potentiometer when used to

control voltage. Called a rheostat when used to

control current.

Resistor identification Alphanumeric EIA (Electronic Industries Association)

Color Code The color bands are read from left to

right,

Resistors – Color Codes

Reproduced by permission of Tony van Roon, 2002 http://www.uoguelph.ca/~antoon

EIA Color Code

1st Band 2nd Band 3rd Band 4th Band

1st Digit 2nd Digit Number of Zero’s Tolerance

BLACK 0 0 N/A N/A

BROWN 1 1 0 1%

RED 2 2 00 2%

ORANGE 3 3 000 N/A

YELLOW 4 4 0,000 N/A

GREEN 5 5 00,000 0.5%

BLUE 6 6 000,000 0.25%

VIOLET 7 7 N/A 0.10%

GRAY 8 8 N/A 0.05%

GOLD N/A N/A X.1 5%

SILVER N/A N/A X.01 10%

NO COLOR N/A N/A N/A 20%

Example of EIA color code:* Orange = 3 White = 9 Red = 2 zeros Gold = 5%

= 3900 ohms or, Resistor is a 3.9K Ohm 5%

tolerance fixed resistor.

Resistors in Circuits Resistors are typically configured in a

circuit in one of three different ways: Series circuit configuration. Parallel circuit configuration. Compound circuit configuration.

Lab 1-4

Go to the classroom website and complete the Labs for Resistance. You can find Lab 1-4-A and 1-4-B in

the Handouts tab in the dashboard on the classroom website.

Type your answers in Microsoft Word and upload to the classroom website when completed.

Resistors in a Series Circuit

• A series circuit contains tow or more resistors and provides on path for current to flow.

• Current runs from the negative side to the positive side,

• The more resistors in the circuit, the more resistance to current flow,

• The total Resistance in a series circuit is the sum of the individual resistors in the circuit,

• Series circuit comprised of resistors:

• To calculate out the total resistance we use the formula RT = R1 + R2 + R3 + etc.

• The total resistance for the above figure is:• 60 OHMS

Resistors in Parallel

• A parallel circuit contains two or more resistors and provides two or more paths for current to flow,

• Each current path is called a “branch”,• The more “branches”, or paths, the less

opposition there is to current flow, (less resistance),

• When a resistor is added in parallel to a circuit, the total resistance in the circuit decreases.

• Parallel circuit comprised of resistors:

• Formula to calculate the total resistance

• The total resistance for the above figure is:• 5.45 OHMS

• Compound circuit comprised of resistors:

• To calculate the total resistance we use both the series and parallel formulas.

• Total resistance for the above figure is:• 66.7 OHMS

• In Summary:– Resistance

• Resistors• Resistivity• Conductivity

– Resistor identification• Fixed• Variable

– Circuit configurations• Series• Parallel• Compound