RESISTORS

What is Resistor?
Resistor is defined as
A passive electrical component with two terminals that are used for either
limiting or regulating the flow of electric current in electrical circuits.
The main purpose of resistor is to reduce the current flow and to lower the
voltage in any particular portion of the circuit. It is made of copper wires
which are coiled around a ceramic rod and the outer part of the resistor is
coated with an insulating paint.

What is the SI Unit of Resistor?
The SI unit of resistor is Ohm.

Symbol of Resistor

Each resistor has one connection and two terminals. We will look at the three types of symbols that are used to represent the resistor.

The terminals of the resistor are each of the lines extending from the squiggle (or rectangle). Those are what connect to the rest of the circuit. The resistor circuit symbols are usually enhanced with both a resistance value and a name. The value, displayed in ohms, is obviously critical for both evaluating and actually constructing the circuit.

Types of Resistors

Resistors are available in different shapes and sizes. Common types that are available are through-hole and surface mount. A resistor might be static, standard resistor, special, or a pack of variable resistors.

There are two basic types of resistors as follows:

• Linear resistor
• Non-linear resistor

Linear resistors

The resistors whose values change with change in applied temperature and voltage are known as linear resistors. There are two types of linear resistors:

Fixed resistors: These resistors have a specific value and these values cannot be changed. Following are the different types of fixed resistors:

• Carbon composition resistors
• Wire wound resistors
• Thin film resistors
• Thick film resistors

Variable resistors: These resistors do not have a specific value and the values can be changed with the help of dial, knob, and screw. These resistors find applications in radio receivers for controlling volume and tone. Following are the different types of variable resistors:

• Potentiometers
• Rheostats
• Trimmers

Non-linear resistors

The resistor values change according to the temperature and voltage applied and is not dependent on Ohm’s law. Following are the different types of non-linear resistors:

• Thermisters
• Varisters
• Photo resistors

Resistor Color Code Chart

The chart below shows how to determine the resistance and tolerance for resistors. The table can also be used to specify the color of the bands when the values are known. An automatic resistor calculator can be used to quickly find the resistor values.

Tips for Reading Resistor Codes

In the sections below, examples are given for different numbers of color bands. But, first, here are some general tips for reading the color code:

• The reading direction might not always be clear. Sometimes the increased space between bands 3 and 4 provide an indication of the reading direction. Also, the first band is usually the closest to a lead. A gold or silver band (the tolerance) is always the last band.
• It is a good practice to check the manufacturer’s documentation to be sure about the color coding system used.
• When in doubt, measure the resistance with a ohmmeter. In some cases this might even be the only way to figure out the resistance; for example when the color bands are burnt off.

4 band resistor

The four band color code is the most common variation. These resistors have two bands for the resistance value, one multiplier and one tolerance band. In the example shown here, the 4 bands are green, blue, red and gold. By using the color code chart, one finds that green stands for 5 and blue for 6. The third band is the multiplier, with red representing a multiplier value of 2 (10²). Therefore, the value of this resistor is 56 · 10² = 56 · 100 = 5600 Ω. The gold band means that the resistor has a tolerance of 5%.  The resistance value lies therefore between 5320 and 5880 Ω (5560 ± 5%). If the tolerance band is left blank, the result is a 3 band resistor. This means that the resistance value remains the same, but the tolerance is 20%.

5 band resistor

Resistors with high precision have an extra band to indicate a third significant digit. Therefore, the first three bands indicate the significant digits, the fourth band is the multiplication factor, and the fifth band represents the tolerance. For the example shown here: brown (1), yellow (4), violet (7), black (x 10⁰ = x1), green (0.5%) represents a 147 Ω resistor with a 0.5% tolerance.

There are exceptions to this 5-band color system. For example, sometimes the extra band may indicate failure rate (military specification) or temperature coefficient (older or specialized resistors). Please read the subsection “Color Code Exceptions” below for more information.

6 band resistor

Resistors with 6 bands are usually for high precision resistors that have an additional band to specify the temperature coefficient (ppm/˚C = ppm/K). The most common color for the sixth band is brown (100 ppm/˚C). This means that for a temperature change of 10 ˚C, the resistance value can change 1000 ppm = 0.1%. For the 6 band resistor example shown above: orange (3), red (2), brown (1), brown (x10), green (1%), red (50 ppm/°C) represents a 3.21 kΩ resistor with a 1% tolerance and a 50 ppm/°C temperature coefficient.

Resistors in Series

Resistors are said to be in series when the current flowing through all the resistors is the same. These resistors are connected from head to tail in series. The overall resistance of the circuit is equal to the sum of individual resistance values.

Resistors in Series Formula

Where,

• R_total is the sum of reciprocal of all the individual resistances

Resistors in Parallel

Resistors are said to be in parallel when the terminals of resistors are connected to the same two nodes. Resistors in parallel share the same voltage at their terminals.

Resistors in Parallel Formula

Where,

• 1𝑅𝑡𝑜𝑡𝑎𝑙𝑖𝑠𝑡ℎ𝑒𝑠𝑢𝑚𝑜𝑓𝑎𝑙𝑙𝑡ℎ𝑒𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒𝑠.

Applications of Resistor

Following are the applications of resistors:

• Wire wound resistors find applications where balanced current control, high sensitivity, and accurate measurement are required like in shunt with ampere meter.
• Photoresistors find application in flame detectors, burglar alarms, in photographic devices, etc.
• Resistors are used for controlling temperature and voltmeter.
• Resistors are used in digital multi-meter, amplifiers, telecommunication, and oscillators.
• They are also used in modulators, demodulators, and transmitters.