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Resistor Basics: Resistor Symbol

Author: Apogeeweb
Date: 30 Sep 2019
symbol for resistor

I Introduction

The resistor symbol as a kind of identification is unique to each type of resistor. There are many types of resistors, which can be divided into fixed resistors, variable resistors, special resistors, high-power resistors, low-power resistors, etc. They basically have different symbols for representation. This article mainly explains the symbol of some common resistors, including text symbols and graphical ones in different standards.

Schematic Diagrams & Symbols, Electrical Circuits - Resistors, Capacitors, Inductors, Diodes, & LEDs


I Introduction

II Overview of Resistor Types and Symbols

 2.1 Types of Resistors

 2.2 The Unit of The Resistor and Its Symbol

 2.3 Resistor Symbols and Marking Symbols

 2.4 Resistor Types and Identification Symbols Table

 2.5 Technical Characteristics of Commonly Used Resistors

III Symbols of Three Main Types of Resistors

 3.1 Fixed Resistor

 3.2 Variable Resistor

 3.3 Preset Resistor

IV Symbols for Potentiometer

V Symbols of Special Resistors

 5.1 Fuse Resistor

 5.2 Thermistor

 5.3 Photoresistor

 5.4 Varistor

 5.5 Humidity Sensitive Resistor

 5.6 Network Resistor

VI The Standard of Resistor Symbol

VII One Question Related to Resistor Symbol


II Overview of Resistor Types and Symbols

2.1 Types of Resistors

Resistors can be divided into general-purpose types resistors and special resistors based on their operating characteristics and their role in the circuit. General resistors are divided into fixed resistors and variable resistors.


Resistors with fixed resistance are called fixed resistors, mainly including carbon film resistors, metal film resistors, oxide film resistors, cement resistors and wire-wound resistors.


The rated power of the carbon film resistor is relatively small, generally 1/8 W to 2 W, and it is suitable for working at temperatures below 70 ℃.

Metal film resistors have high-temperature resistance and can work for a long time below 125 ℃. The resistor has a small temperature coefficient, good stability, high precision, low noise, and the power is generally 1/8 W to 3 W.


The oxide film resistor has a good heat resistance and pressure resistance and can replace the metal film resistor. Cement resistors and wire-wound resistors have large power and large volume.


A resistor whose resistance is continuously adjustable within a certain range is called a variable resistor or potentiometer. The prescribed resistor has two terminals and the potentiometer has three terminals.


Figure1. Resistor

2.2 The Unit of The Resistance and Its Symbol

The basic unit of resistance is ohm (Ω). In order to facilitate the writing of larger values of resistance, this unit is often abbreviated like current and voltage. The units of resistance from large to small are: MΩ (megaohm), kΩ (kiloohm), Ω (ohm), mΩ (milliohm), μΩ (microohm), the smaller units like “milliohm, micro "Europe" is not common. The conversion relationship between them is:

1 megohm = 1000 kohm

1 thousand ohms = 1000 ohms

1 ohm = 1000 milliohms

1 milliohm = 1000 microohm

Tip: When calculating the value in the actual work of an electrician, K usually means 1000 (especially when it is commonly used in component labeling).


2.3 Resistor Symbols and Marking Symbols

In general, resistors are usually represented by symbols such as R, RN, RF, and FS. In the circuit, the symbol of the fixed resistor and trimming resistor is R, and the symbol of the potentiometer is RP.


In order to distinguish between different types of resistance, several Latin letters are commonly used to indicate the resistor type, as shown in the figure below. The first letter R represents the resistor, the second letter represents the conductor material, and the third letter represents the shape and performance.

Carbon Film Resistor and Precision Metal Film Resistor

Figure2. Carbon Film Resistor and Precision Metal Film Resistor

2.4 Resistor Types and Identification Symbols Table





The First Letter

Main title




The Second Letter

Conductor material

Carbon film

Metal film

Metal oxide film

Wire wound


The Third Letter

Shape, performance, etc.




High power


2.5 Technical Characteristics of Commonly Used Resistors

Resistor Types

Rated Power

Nominal Resistance Range

Temperature Coefficient

Noise Potential

Operating Frequency

RT Type

Carbon film resistors





Below 10 MHz

RU Type

Silicon Carbon Film Resistor





Below 10 MHz

RJ Type

Metal Film Resistor





Below 10 MHz


Wire Wound Resistor





Low Frequency

WTH Type

Carbon Film Potentiometer





Below a few hundred KHz

WX Type

Wire-wound Potentiometer





Low Frequency

III Symbols of Three Main Types of Resistors

3.1 Fixed Resistor

The symbol of the fixed resistor in the circuit diagram represents the resistor body with a long square, and the short lines on both sides respectively indicate the two lead-out lines of the resistor. Whether it is a carbon resistor or a metal film resistor, all resistors with a fixed resistance are represented by this symbol.


It should be noted that the graphic symbols of fixed resistors commonly used in some countries are different from the internationally recommended symbols. As shown in the figure below, the right side is the symbol of the fixed resistor of an international standard:

Fixed Resistor Symbol

Figure3. Fixed Resistor Symbol

The concept of a fixed resistor is the same as its literal meaning. It refers to a resistor with constant resistance, and its literal symbol is R.

3.2 Variable Resistor

The symbol of the variable resistor in the circuit diagram still uses a long square to represent the resistor body, and an arrow to visually represent the movable sliding contact. Because the variable resistor is a variable resistance component with only two terminals, it is only represented by a lead wire and a broken line with an arrow, or by a fixed resistor symbol with an arrow.

Symbols For Variable Resistors

Figure4. Symbol For a Variable Resistor

3.3 Preset Resistor

The semi-adjustable resistor is also called a preset resistor, which can continuously change within a certain range of resistance. It is mainly used in circuits that do not require frequent changes in resistance. The text symbol is R, which is the same as the fixed resistor. Its graphical symbol is shown below:

Preset Resistor Symbol

Figure5. Preset Resistor Symbol

IV Symbols for Potentiometer 

The potentiometer is an adjustable resistor, and its text symbol is RP (P means that it has adjustability). The potentiometer can be used as a three-terminal element or a two-terminal element.


The short lines on both sides of the potentiometer symbol indicate the lead pads at both ends of the resistor, and the broken line with an arrow represents the sliding contact on the resistor.  


The graphical symbols are as follows:

Potentiometer Symbol

Figure6. Potentiometer Symbol

V Symbols of Special Resistors

5.1 Fuse Resistor

The fuse resistor has a dual function and has ordinary resistance characteristics under normal circumstances.  Once the voltage rise and current increase due to damage of a certain component in the circuit, the fuse resistor will fuse within a fixed time, so as to protect other components.

The Structure of Fuse Resistor

Figure7. The Structure of Fuse Resistor

At present, the fuse resistor in the power supply circuit of the CRT display is usually a black cylinder. The fuse resistors in the power supply circuit of the LCD display are generally green patch type, black row group, gray in-line type and PTC. The fuse is usually expressed by F / FB / L or R, and sometimes it is also marked as "0", "000" or rated current value. The symbol is as follows:

Fuse Symbol

Figure8. Fuse Symbol

5.2 Thermistor

The resistance of the thermistor is also variable, but it is controlled by the temperature of the external environment. The text symbol of the thermistor is RT (T-Thermistors indicate that its resistance is affected by temperature).


The thermistors can be divided into two categories according to their resistance value increasing or decreasing with increasing temperature, namely NTC thermistor and PTC thermistor. Their textual symbols are NTC (N means that their resistance increases with increasing temperature) and PTC (P means that their resistance decreases with increasing temperature).


The graphical symbols are shown in the following figure:

NTC Thermister and PTC Thermister Symbol

Figure9. NTC Thermister and PTC Thermister Symbol

5.3 Photoresistor

The circuit symbol of the photoresistor (LDR) is shown in the figure. The graphic symbol is an arrow line with two arrows inward added to the graphic symbol of the ordinary fixed resistor-indicating the acceptance of external light, to visually reflect the resistance value of the photoresistor can change with the intensity of the incident light And change.

Photoresistor Symbol

Figure10. Photoresistor Symbol

Note: A circle (indicating a case) was added to the circuit symbol of the photoresistor before, and this circle has now been discarded. When I checked some early circuit diagrams, I encountered graphic symbols of photoresistors with circles, which are no different from the meanings of graphic symbols of photoresistors without circles.

The graphical symbol of the new photoresistor is shown below:

New and Old Symbol for Photoresistor

Figure11. New and Old Symbol for Photoresistor

The photoresistor is similar to the thermistor, but its resistance is affected by the intensity of light. The text symbol of the photoresistor is "RL" (L means that its resistance is affected by light intensity; the old symbol is "RG") or "R". If there are many similar components in the circuit diagram, the natural numbers are marked behind the letters or the lower right corner as usual to show the difference, such as RL1 and RL2.


Due to the non-polarity of the photoresistor pins, the arrangement of its two-pin lines is unordered. In practical application, it is enough to connect the two pins into the circuit without reversal. This is more convenient to use than photodiodes and phototransistors. But it should be noted that some photoresistors have 3 pins, which actually assemble two photoresistors in the same housing. In actual use, the common foot must not be mistaken.


5.4 Varistor

(1) Varistor

A varistor is a variable resistor whose resistance depends on the applied voltage. The name was created by mixing two words: "variable" and "resistor". It is also called VDR (Voltage-dependent Resistor) and has non-ohmic characteristics. Therefore, they are non-linear type resistors.


In the early days, the varistor was represented as two diodes placed antiparallel to each other, as shown in the figure below because it has diode-like characteristics in both current directions.

Varistor Circuit Symbol

Figure12. Varistor Circuit Symbol

However, this symbol is now used for DIAC (diode for alternating current). In modern circuits, the symbol of the varistor is shown below.

Varistor Standard Symbol

Figure13. Varistor Standard Symbol

(2) Metal oxide varistor

Metal oxide varistor is a kind of voltage-sensitive element made of nonlinear volt-ampere characteristics of semiconductor materials, referred to as MOV for short, is a nonlinear overvoltage protection semiconductor element sensitive to voltage. It is represented by the text symbol "MOV" in the circuit, and its symbol is shown in the figure below:

Varistor Symbol

Figure14. Varistor Symbol

5.5 Humidity Sensitive Resistor

The humidity thermistor is similar to the thermistor affected by temperature, the photoresistor affected by light intensity, and the varistor affected by voltage, but it is affected by the humidity of the external environment. The text symbol is RS (S indicates that its resistance is related to humidity).

The graphical symbols of the humidity sensitive resistor are as follows:

Humidity Resistor Symbol

Figure15. Humidity Resistor Symbol

5.6 Network Resistor

The network resistor is a combined resistor that integrates discrete resistors arranged according to a certain rule, also called an integrated resistor. It is a kind of resistance network, which has the characteristics of small size, regularization and high precision. It is more common in computer motherboard circuits and LCD circuits. Network resistor is generally represented by RN. There are two main types of row resistors in the circuit of the computer motherboard: 8-pin and 10-pin. Among them, 8-pin is used more.

Network Resistor

Figure16. Network Resistor

VI The Standard of Resistor Symbol

There are different standards for resistor symbols, and different countries may use different resistor symbols. Some common standards are as follows:


(1)IEC 60617 (also known as British Standard BS 3939).

(2)There is also IEC 61131-3 - for ladder-logic symbols.

(3)JIC (Joint Industrial Council) symbols as approved and adopted by the NMTBA (National Machine Tool Builders Association). They have been extracted from the Appendix of the NMTBA Specification EGPl-1967

(4)ANSI Y32.2-1975 (also known as IEEE Std 315-1975 or CSA Z99-1975)

(5)IEEE Std 91/91a: graphic symbols for logic functions (used in digital electronics). It is referenced in ANSI Y32.2/IEEE Std 315.

(6)Australian Standard AS 1102. (Based on a slightly modified version of IEC 60617, Withdrawn without replacement with a recommendation to use IEC 60617)

The Standard of Resistor Symbol

Figure17. The Difference Between US-style Resistors and IEC-style Resistors

For example, many Americans are used to seeing "squiggle" -styled line segments. However, in many other countries, the normal resistor symbol is just a simple outlined rectangle (the IEC-standard symbol choice). And the IEC 60617 standard is the international standard.

A Question Related to Resistor Symbol and the Answer

Figure18. A Question Related to Resistor Symbol and the Answer



1. What is the symbol for a resistor?

The ohm symbol Ω represents the unit of electrical resistance.


2. What is the basic unit for the resistor?

The ohm (symbol: Ω) is the SI-derived unit of electrical resistance, named after German physicist Georg Ohm.


3. What is a fixed resistor?

A resistor having a fixed, defined electrical resistance that is not adjustable. In an ideal world, a perfect resistor would have a constant ohmic resistance under all circumstances. This resistance would be independent of for example frequency, voltage or temperature.


4. What is resistor function?

A resistor is a passive electrical component with the primary function to limit the flow of electric current.


5. Do resistors affect voltage?

A resistor has the ability to reduce voltage and current when used in a circuit. The main function of a resistor is to limit current flow. Ohm's law tells us that an increase in the value of a resistor will see a decrease in current. To reduce the voltage, resistors are set up in a configuration known as a 'voltage divider'.


6. Are resistors positive or negative?

If you think of the wire as being pipe electricity flows through, you can think of a resistor as being a narrow part of that pipe, that chokes off the flow. Resistors don't have positive and negative sides -- you can hook them up in either direction and they work just the same.


7. What is the working principle of the resistor?

The main purpose of a resistor is to maintain specified values of voltage and current in an electronic circuit. A Resistor works on the principle of Ohm's law and the law states that the voltage across the terminals of a resistor is directly proportional to the current flowing through it. The unit of resistance is Ohm.


8. What happens when you use the wrong resistor?

The equipment will cease to function in short order either by the resistor itself burning up, or by the heat, it generates burning up an adjacent component or possibly the circuit board is mounted on or even foil pattern on the circuit board, or maybe all three.


9. What is resistor value?

A resistor is a device that opposes the flow of electrical current. The bigger the value of a resistor the more it opposes the current flow. The value of a resistor is given in ohms and is often referred to as its 'resistance'.


10. How do we classify resistors?

• Carbon Composition Resistor – Made of carbon dust or graphite paste, low wattage values.

• Film or Cermet Resistor – Made from conductive metal oxide paste, very low wattage values.


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