Resistor Symbol: The Building Block of Circuit Diagrams

In order to read a circuit diagram, it is necessary to understand the various electrical components that it is made from. There are several types of resistors in use today, and they use different ways of representing the resistance symbol. As a result of reading this article, you will be familiarized with these resistor types and symbols. You will also know the principle of operation for these resistors and their most common applications.

What Does a Resistor Mean?

The core purpose of a resistor is to block the flow of electricity. Resistors are classified as passive electrical components. This means that they do not require an external power source to operate. Rather, they regulate the flow of current between their terminals.

Resistors are also used in signal processing where there is a need for increasing or decreasing the signal levels. Whenever current travels from one branch to another, or divides into multiple branches, resistors are used to regulate the amount of current flowing into each branch. Resistors are also used to terminate circuits, as a high resistance can act as a grounding source.

What is a Resistor Symbol?

A Resistor Symbol is a unique graphic that is used to represent a resistor on a circuit diagram. In order to represent the various kinds of resistors in modern electrical circuits, there are a number of resistor symbols.

There have been several international standards to describe the resistor symbols. Some of these are:

1.   IEC 60617 (International)
2.   ANSI Y32 / IEEE 315 (US)
3.   DIN 40900 (Germany)
4.   AS 1102 (Australia)

Resistor Types and Resistor Symbols

In the following sections, we will discuss the various kinds of categories that resistors are divided into. We will also look at the symbols for these resistors.

Linear Resistors

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

Fixed Resistor

In fixed resistors, you cannot adjust or modify the value of resistance they have. They offer constant resistance. As a result, they deliver a great amount of precision and sensitivity. You will find fixed resistors to generally have a simple construction, like Wire Wound Resistors. Carbon Film Resistors are another form of fixed resistors.

According to the new international standard, the symbol of a fixed resistor is a rectangle. The short sides of this rectangle represent the two terminals of the resistor. It should be noted that this symbol differs from the old one, in which the fixed resistor was denoted by a zig-zag line.

Variable Resistor

In variable resistors, you have the option to manually adjust the resistance they offer. The resistance of a variable resistor can be modified by a third terminal, which operates in the form of a slide or knob, as in a potentiometer. A rheostat is another variable resistor that finds use in lighting circuits.

According to the new international standard, the symbol of a variable resistor is a rectangle with an arrow pointing towards it representing the movable sliding contact. In the old standard, the resistor was represented by a zig-zag line whereas the sliding contact was the same.

 Non-Linear Resistors

Non-Linear Resistors are those in which the value of resistance may change depending upon external factors. They do not obey Ohm’s Law. This means that the current through the terminals of a non-linear resistor is not directly proportional to the voltage applied across it.

Some of the widely used non-linear resistors are Thermistors, Varistors and Photoresistors.

If you are looking for a resistor which alters its resistance with temperature, you want a thermistor. Similarly, a varistor is an example of a non-linear resistor which is voltage based. Its resistance value changes with applied voltage. Photoresistor is another non-linear resistor in which its resistance depends upon the amount of light incident upon it.

Thermistor

The semiconductor material used to make thermistors is highly sensitive to temperature changes. Therefore, thermistors are widely used in circuits requiring response to temperature changes.

In general, thermistors have a negative temperature coefficient (NTC). This means that with an increase in temperature, their resistance decreases. However, thermistors constructed using pid barium titanite have positive temperature coefficients, i.e. their resistance increases with temperature.

The symbol of a thermistor is a rectangle with a line drawn across it. A ‘+t’ or ‘-t’ sign below it signifies whether the thermistor has a positive or a negative temperature coefficient.

Varistors (VDR)

Varistors (VDR) are Voltage Dependent Resistors. They are used to eliminate high voltage transients in a circuit. Varistors are variable resistors that protect the electrical circuit from damage caused by voltage spikes.

When the voltage increases abnormally across a sensitive device or system, VDR increases its resistance, thereby reducing the flow of current in the circuit.

The old varistor symbol was two diodes. This is now used for a diac. The modern symbol for varistor is the same as that for thermistor, a rectangle with a line drawn across it.

Photoresistor

Photoresistors are also called Light Dependent Resistors (LDRs). It is a type of variable resistor which offers resistance that is dependent upon light rays. This is made possible due to its construction using photoconductive materials like lead sulfide and cadmium sulfide.

When light falls on the surface of a photoresistor, it transmits energy to free carriers in the semiconductor material. As a result, more current is allowed to flow across the terminals, decreasing the resistance. Hence, photoresistors have a negative temperature coefficient.

The symbol of the photoresistor is a rectangle with two arrows pointed towards it. The rectangle represents its role as a resistor, while the arrows indicate incident light beams.

Color Coding in Resistors

In resistors, color coding is used to determine their resistance value. The color code is displayed in strips upon the body of the resistor, and its resistance value in ohms can be calculated using the value assigned to each color as defined by the International Standard. The colors range from black to white and represent numbers between 0 to 9.

You can also tell the tolerance value of a resistor using its color-coded information. The tolerance of a resistor is the amount of deviation from its nominal value. It is measured in percentage and ranges from 0.0005% to 10%.

Applications of Resistors

Because of the fundamental nature of control that resistors offer over electrical circuits, they are widely used in many types of devices. Following are some common uses of resistors:

1.   In devices like ampere meters, resistors with high current control, increased sensitivity, precision and accuracy are required for shunting. This makes wire wound resistors an ideal choice for such applications.
2.   In home security and alarm systems, photoresistors are used for their high sensitivity.
3.   Resistors are used in heater and immersion rods.
4.   In case there is a sudden voltage burst, resistors can act as fuses and protect the circuit.
5.   Resistors play an important role in communication devices such as transmitters, modulators and demodulators.
6.   Resistors are used in signal processing devices like amplifiers, oscillators and multimeters.

Conclusion

Resistor Symbols are an important part of electrical circuit diagrams. They allow you to recognize the name and function of a resistor in a short and convenient way. A wide variety of resistors ranging from fixed value resistors to non-ohmic resistors are available in the market today.

While their symbolic representation might differ occasionally, their function in an electrical circuit remains constant. With an in-depth understanding of the principle upon which these resistors operate, we are able to carry out work on electrical equipment better.