Resistance and Resistors

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Resistance is the restriction of the flow of electrons. Resistance is the opposite of current. As an analogy, think of a river flowing fast with no obstructions (current through a wire). When the water comes to a point where there are huge rocks and trees (resistance), the water loses its speed and energy.

If the resistance in a circuit increases, the current will decrease.

Resistance is designated with R and its unit is the ohm (&#;).

A resistor is a device designed to produce resistance. Resistors can be used to limit current, divide voltage, or generate heat.

There are two main types of resistors: fixed and variable.

Fixed resistors have a certain amount of resistance and cannot be changed. They are available in a wide range of varying resistances. Different types of fixed resistors include carbon-composite, metal film chip, chip array, resistor network, and radial-lead for PC board. The most commonly used resistor is the carbon-composite resistor, pictured below.

Each fixed resistor has its own unique value, as well as a tolerance value of either 5% or 10%. Depending on the type of resistor you have, you can check its value by the coding on the outer shell.

The colour-coded carbon-composite is the most popular. There are four colour bands.

1. Start with the band closest to one of the terminals of the resistor. The first colour is the first digit of the value.
2. The second colour represents the second digit of the value.
3. The third band represents how many zeros follow the second digit (in other words, the multiplier).
4. The fourth band represents tolerance in percentage. This is usually gold or silver. If a fourth band does not exist, then the tolerance would be 20%.

On a resistor using a four-band code, the first two bands are the numbers used, the third band is the multiplier, and the fourth band is the tolerance.

On a resistor using a five-band code, the first three bands are the numbers used, the fourth band is the multiplier, and the fifth band is the tolerance.

Examples!

As an example, the four-band resistor below would have a value of 54 Ω ±2%

The five-band resistor below would have a value of 3.17 MΩ ± 5%

Variable resistors are used when it is necessary to change the values of a resistor easily. The two main uses of a variable resistor are as a potentiometer and a rheostat.

A potentiometer is a device with three terminals used to divide voltage in a circuit. Two of the terminals have a fixed resistance between them, while the third is connected to a wiper, or moving contact. Pictured below is a cutaway of a potentiometer.

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A potentiometer is often used to tap off different voltages for measurement. They are sometimes used for control on the variable speed drives of DC machines or for adjustments in electronics.

A rheostat is similar in that it has a varying resistance; however, a rheostat is generally beefier and is used to control current in a circuit.

 

Resistance is a measure of the opposition to current flow in an electrical circuit.

Resistance is measured in ohms, symbolized by the Greek letter omega (Ω). Ohms are named after Georg Simon Ohm (-), a German physicist who studied the relationship between voltage, current and resistance. He is credited for formulating Ohm's Law.

All materials resist current flow to some degree. They fall into one of two broad categories:

• Conductors: Materials that offer very little resistance where electrons can move easily. Examples: silver, copper, gold and aluminum.
• Insulators: Materials that present high resistance and restrict the flow of electrons. Examples: Rubber, paper, glass, wood and plastic.

Gold wire serves as an excellent conductor

Resistance measurements are normally taken to indicate the condition of a component or a circuit.

• The higher the resistance, the lower the current flow. If abnormally high, one possible cause (among many) could be damaged conductors due to burning or corrosion. All conductors give off some degree of heat, so overheating is an issue often associated with resistance.
• The lower the resistance, the higher the current flow. Possible causes: insulators damaged by moisture or overheating.

Many components, such as heating elements and resistors, have a fixed-resistance value. These values are often printed on the components' nameplates or in manuals for reference.

When a tolerance is indicated, the measured resistance value should be within the specified resistance range. Any significant change in a fixed-resistance value usually indicates a problem.

"Resistance" may sound negative, but in electricity it can be used beneficially.

Examples: Current must struggle to flow through the small coils of a toaster, enough to generate heat that browns bread. Old-style incandescent light bulbs force current to flow through filaments so thin that light is generated.

Resistance cannot be measured in an operating circuit. Accordingly, troubleshooting technicians often determine resistance by taking voltage and current measurements and applying Ohm's Law:

E = I x R

That is, volts = amps x ohms. R stands for resistance in this formula. If resistance is unknown, the formula can be converted to R = E/I (ohms = volts divided by amps).

Examples: In an electric heater circuit, as portrayed in the two illustrations below, resistance is determined by measuring circuit voltage and current, then applying Ohm's Law.

Example of normal circuit resistance Example of increased circuit resistance

In the first example, total normal circuit resistance, a known reference value, is 60 Ω (240 ÷ 4 = 60 Ω). The 60 Ω resistance can help determine the condition of a circuit.

In the second example, if circuit current is 3 amps instead of 4, circuit resistance has increased from 60 Ω to 80 Ω (240 ÷ 3 = 80 Ω). The 20 Ω gain in total resistance could be caused by a loose or dirty connection or an open-coil section. Open-coil sections increase the total circuit resistance, which decreased current.

Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers.

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