# Understanding resistors

Resistors are great electrical components used to reduce current flow, adjust signal levels, divide voltages, bias active elements, and other uses. It is a passive two-terminal electrical component that implements electrical resistance as a circuit element. High-power resistors are designed to dissipate many watts of electrical power as heat, may be used as part of motor controls, in power distribution systems, or as test loads for generators. There are various types of resistors out there, but fixed resistors have resistances that only change slightly with temperature, time, or operating voltage. the variable resistors are used to adjust circuit elements such as volume control or a lamp dimmer. Also, it is used as a sensing device for heat, light, humidity, force, chemical activity, etc.

In this comprehensive article, you’ll get to know the definition, symbol, applications, diagram, characteristics, components, color code, types, and materials of resistors. You’ll also learn the advantages and disadvantages of resistors, series and parallel designs, standards, etc.

Contents

## What is a resistor?

A resistor is an electrical component, used in virtually all electronic circuits and many electrical ones. Just as the name indicates, resistors, resist the flow of electricity, which is the key function to the operation in most circuits. They are common elements of electrical networks and electronic circuits and are ubiquitous and in electronic equipment. Practical resistors as discrete parts can be composed of various compounds and forms, and they are implemented within integrated circuits.

Being a key factor used in electrical and electronic circuits, resistance is the property of materials to resist the flow of electricity. That’s the effect of resistors and is governed by ohms law. This is to say, the behavior of an ideal resistor is dictated by the relationship specified by ohm’s law.

Ohm’s law states that the voltage (V) across a resistor is proportional to the current (I), where the constant of proportionality is the resistance (R). Let take for example, if a 500 Ohm resistor is attached across the terminals of a 12 Volt battery, then a current of 12 / 500 = 0.024 amperes flows through the resistor. You should know that practical resistors also have some inductance and capacitance that affect the relation between voltage and current in alternating current circuits.

## Symbols

The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. An ohm is equivalent to a volt per ampere. Since resistors are specified and manufactured over a very large range of values, the derived units of milliohm (1 mΩ = 10−3 Ω), kilohm (1 kΩ = 103 Ω), and megohm (1 MΩ = 106 Ω) are also in common usage.

Two circuit symbols can be used for resistors, the oldest one is still used in North America and consists of a jagged line representing the wire used in a resister. The other symbol is a small rectangle, which is termed the international resistor symbol and is more widely used in Europe and Asia.

The international IEC symbol of a resistor is rectangular with leads at each end just as shown in the figure below. In the United States, the ANSI standard is very common and represents a fixed resistor as a zigzag line.

 IEC fixed resistor symbol ANSI fixed resistor symbol

## Applications of resistors

There are wide applications of resistors in the electrical field, all kinds are used in vast quantities in manufacturing electronic equipment. A resistor is probably one of the most common types of electronic components used in electrical and electronic circuits. Because there are a large number of different types, the properties and applications of resistors can vary, and it ensures its availability for a specific requirement. Below are the most common uses of resistors:

### Resistors in series and parallel:

Although this is another topic on its own, which we’ll, later on, talk about. In electronic circuits, resistors are often connected in series or parallel to reach or obtain a specific resistance value. In a series connection, the current through each resistor is the same and the equivalent resistance is equal to the sum of the individual resistors. Whereas for parallel connections, the voltage across each resistor is equal. The inverse of the equivalent resistance is the same as the sum of the inverse values for all the parallel resistors.

### To measure electrical current (Shunt Resistor):

Resistors are used to calculate electrical current by measuring the voltage drop over a precision resistor with a known resistance, which is connected in series with the circuit. This current can be calculated using ohm’s law, which is also known as ammeter or shunt resistor. Usually, it is a high precision manganin resistor with a low resistance value.

### Resistors for LEDs:

Because LED lights require a specific current to operate, resistors are required. A too low current will not power the LED, and too high a current might burn the device. Thus, LEDs are often connected in series with resistors to set the current, which are also known as ballast resistors. They can passively regulate the current in the circuit.

### Blower motor resistors:

This resistor is common in cars, where the air ventilation system is actuated by a fan that is driven by the blower motor. Special resistors are used to control the fan speed which is called blower motor resistors. Different designs are available, one design is a series of different sizes for each fan speed. It is known as wire wound resistors. Another design incorporates a fully integrated circuit on a printed circuit board.

Different types of resistors can serve their purpose. For instance, carbon-composition resistors are used for all general-purpose circuits including entertainment applications such as radio, television, etc. The carbon film resistors are used in good high-frequency performance and stability circuits such as computers, telephone circuits, and high-fidelity amplifiers.

Furthermore, the wire-wound types of resistors are used in power supplies control circuits, as loads in television receivers. Precision wire wound resistors are used in bridges, voltmeters, and other instruments.

## Components of resistors

The components of resistors may differ since there are different types, properties, and materials. Below are the major components of resistors and their functions describe in the below diagram:

Endcap

Ceramic

Metal film

Empxy coating

Color bands

Insulation

## Resistors Characteristics

Below is the characteristic of resistors.

• Resistance value
• Long term stability
• Temperature coefficient
• Mechanical design
• Material resistive
• Parasitic reactance
• Electric noise
• Power rating
• Maximum voltage
• Pulse stability
• Mechanical strength
• Failure rate, etc.

## Types of resistors

The common types of resistors are categorized into fixed and variable resistors. Although there are various other types used for different applications.

### Fixed resistors:

Fixed resistors are the most common and widely used types of resistors. They are used in electronic circuits to set the right conditions, and their values are known during the design phase of the circuit. Also, they never require to be changed to adjust the circuit, just as their name has indicated. Many other types of fixed resistors will be discussed below.

### Variable resistors:

These types of resistors are designed with a fixed resistor element and a slider that taps into the main resistor element. This makes the component achieve three connections; two connections to the fixed element and the third is the slider. In this manner, it acts as a variable potential divider if all three connections are used. It is possible to connect to the slider and one end to provide a resistor with variable resistance.

Carbon film preset potentiometer Variable resistors and potentiometers are used for all forms of control. Starting from the volume controls on radios to sliders in audio mixers to a hot of areas where variable resistance is needed. On the other hand, a potentiometer and variable resistor strictly a potentiometer is a component where fixed resistors that have a slider are employed to provide a potential division from the voltage at the top. This is the same with a variable resistor, but the slider linked to one end of the resistor helps to provide a true variable resistance.

## Other types of resistors

The majority of resistors are standard fixed resistors or variable resistors. Other kinds of resistors are used in some specialized applications such as light-dependent resistor/photoresistor, Thermistor, varistors, etc.

### Light-dependent resistor / Photoresistor:

These types of resistors are also known as LDR or photoresistors as they change their resistance with the level of light. They are mostly used in sensor applications and they provide a very cost-effective solution in most cases. Normally, a leaded light-dependent resistor has a lag in the time taken to respond to light changes. However, they are cheap and easy to use.

### Thermistor:

The thermistor is a heat-sensitive resistor whose resistance changes with temperature. Some are designed with a negative temperature coefficient, which is known as the NTC thermistors. Other designs have a positive temperature coefficient, PTC thermistors.

### Varistor:

These types of resistors are available in various forms. Their resistance varies with the applied voltage and as a result, they find uses for spike and surge protection. Often, they are described as Movistors gotten from the words Metal Oxide Varistors. The selection of leaded varistors is the device that is used in surge or transient protected mains extension leads. It is also used to protect computers. Note that whenever a varistor receives a spike, its properties change slightly.

## Materials

Resistors are made from different materials depending on the types and properties it contains such as cost, precision, power, and other requirements. The manufacturing of all kinds of resistors is made from carbon composition, carbon film, metal film, metal oxide film, wirewound, Foil, etc.

The carbon composition method is a very old technique that produces a low precision resistor. Although it’s still used for specific applications where high energy pulses occur. Carbon composition resistors are made from a mixture of fine carbon particles and non-conductive ceramic.  On our list, the carbon film materials create resistors with a better tolerance (less resistance value variation) than carbon composition resistors. They are made of a non-conductive rod with a thin carbon film layer around them. This layer is treated with a spiral cut to increase and control the resistance value.

Metal and metal oxide film is the most common and widely used material nowadays. This is because they have better properties for stability and tolerance. Also, they are less impacted by temperature variations. Like a carbon film resistor, they are constructed with a resistive film around a cylindrical body. Resistors made of these materials are generally more durable.  Finally,

Wirewound resistors are probably the oldest type and are used for both high precision and high-power applications. Their construction is made by winding a special metal alloy wire such as nickel-chrome around a non-conductive core. They are durable, accurate, and have a very low resistance value. The limitation is that they suffer parasitic reactance at high frequencies.

In addition, the highest requirements on precision and stability, metal foil resistor should be used. They are constructed from a special alloy cold-rolled film cemented onto a ceramic substrate.

#### Resistor  color code:

Below are the advantages and disadvantages of the various types of resistors:

• Smaller in size compared to others
• Wide resistance range
• Cheap
• Good RF performance

• No precision and have a very high tolerance
• They got heat easily and can crackdown during soldering.
• They vary with aging
• Used on specific applications.

### Advantages of carbon film resistors:

• They are available in all resistor value
• Miniature sizes are available.
• Carbon film resistors can be used in ICs
• They can replace wire wound resistors in high-voltage applications.
• Cost is low
• They have good high-frequency properties.

• They cannot withstand high temperatures
• Also, they cannot withstand mechanical shocks
• Atmospheric moisture and humidity may also damage the device.
• They are unstable and are chemically reactive.

### Advantages of wire wound resistors

• These types of resistors can produce accurate resistance values with a very low tolerance.
• Capable of withstanding large power dissipation
• Wire wound resistors can be used in high-temperature applications.
• They can carry extremely high currents.
• This resistor can withstand mechanical shock and vibration.
• They can be used in high-voltage resistors
• They have stable resistance values which do not change much with aging.