Resistor Symbols Explained: Fixed, Variable, Thermistor, and Photoresistor Symbols

In short, a resistor symbol represents resistance in a circuit. The basic symbol can look like a zigzag line or a rectangle. Further, additional marks to the resistor symbol represent parts whose resistance can be adjusted or changes with temperature, light, or voltage (thermistors, photoresistors, and varistors).

You do not need to memorize every variation before reading a schematic. Start with the symbol’s shape, then check its terminals, label, value, and connections. Together, those details tell you far more than the shape alone.

Standard Resistor Symbols: IEC vs ANSI

Two fixed-resistor symbols appear repeatedly in circuit diagrams:

The zigzag is commonly seen in North American schematics that follow American conventions. The rectangle is the resistor symbol used in the IEC 60617 graphical-symbol system, which is common in international documentation. The two shapes do not indicate different electrical behavior. Rather, a 10 kΩ resistor remains a 10 kΩ resistor regardless of how its schematic body is drawn.

For a new schematic, choose a convention and use it consistently. That makes the drawing easier to scan and helps reviewers distinguish intentional symbol variations from accidental inconsistency. If you are working within an existing design, follow its established convention unless your team has a reason to change the whole drawing.

Symbols for Different Types of Resistors

A fixed resistor has two terminals and a nominal resistance that is not intended to be adjusted during operation. Its schematic body is the zigzag or rectangle shown above. The value beside it—such as 330 Ω or 10 kΩ—usually matters more to circuit behavior than the choice of drawing convention.

A variable resistor adds an arrow or similar adjustment mark to the resistor body. The mark tells you the effective resistance can change. The exact symbol of a variable resistor can vary by symbol library, so inspect the number of terminals and the circuit connections as well.

A potentiometer has three terminals: both ends of a resistive element and a movable contact called the wiper. In a schematic, the wiper is normally shown as an arrow touching or pointing to the resistor body. When all three terminals are used, the part often forms an adjustable voltage divider. When laying out a physical circuit, confirm which footprint pad is the wiper; our potentiometer wiring guide walks through that mapping.

A rheostat describes a variation of a variable resistor. More specifically, using the wiper and one end terminal to create an adjustable two-terminal resistance. A schematic might use a distinct rheostat symbol or show an ordinary potentiometer with only two terminals connected. When reading the schematic, do not assume the function of the rheostat based on the arrow alone. Instead follow the wires to understand how it is wired.

A thermistor is a temperature-dependent resistor. Its symbol adds a temperature-related mark to the resistor body, although the precise notation varies among standards and libraries. The symbol alone may not tell you whether resistance decreases with temperature (NTC) or increases with temperature (PTC). Check the part label, specification, and manufacturer part number.

A photoresistor, also called a light-dependent resistor or LDR, is commonly drawn as a resistor with two arrows directed toward it. The arrows represent incoming light, not electrical current. For example, IEC’s photoresistor symbol entry uses this visual cue.

A varistor has resistance that changes with applied voltage. A common use case includes transient protection. The varistor symbol includes a voltage-dependent mark, but the exact form differs across libraries. Identify it by checking the label and part specification as well as the drawing. You can expect to see these often when reviewing a protection circuit.

How Resistors Are Labeled in Circuit Diagrams

The symbol identifies the component and the nearby text identifies the particular component in the design.

Most schematics use R as the reference-designator prefix. For example, R1, R2, and R3 are three separate resistors in the schematic, even if all have the same value. A potentiometer or other specialized part may use a different prefix in some libraries, so follow the project’s naming convention.

Resistance is measured in ohms (Ω). Common values follow the E12/E24 series, such as 10, 22, and 47 times a power of ten. You may also see notation that places the multiplier where the decimal point would be: 4K7 means 4.7 kΩ, while 1M0 means 1.0 MΩ.

Other properties may appear in the component details rather than beside the symbol. A resistor marked R1 10 kΩ could still require a particular tolerance, power rating, package, and temperature coefficient. For example, “10 kΩ, 1%, 0.1 W, 0603” (10,000Ω resistor, with 1% tolerance, 0.1W rating and 0603 package size) describes a more specific purchasing choice than “10 kΩ” alone. The schematic should make the electrical intent clear, while the bill of materials (BOM) should identify the actual part to source.

Consider this simplified LED circuit:

R1 is the resistor’s unique identifier. 330 Ω is its nominal value. The LED and resistor are in series, so the resistor limits current through the LED. If another 330 Ω resistor appears elsewhere, it has its own designator, such as R2. A schematic’s identifiers allow engineers to associate each symbol to the correct BOM line and physical board location.

For through-hole parts, a resistor color-code calculator can help check the markings on the component. It does not replace the schematic or BOM value.

How to Read Resistor Symbols in a Schematic

Once you recognize a resistor symbol, follow the nodes to other components, as the position and connectivity reveal the function of the resistor.

LED current limiting. If a resistor sits in series between a supply or output pin and an LED, it likely controls LED current. Its value must be chosen from the supply voltage, the LED’s forward voltage, and the desired current rather than the symbol’s appearance.

Pull-up resistor. If one end connects to a signal and the other to the logic supply, the resistor may hold an undriven input HIGH. A similar connection to ground may be a pull-down. Check what else drives the signal and whether the supply voltage is safe for the input. Our pull-up and pull-down guide shows both arrangements.

Voltage divider. Two fixed resistors in series between a voltage source and ground can result in an intermediate voltage (marked VOUT below) at their junction:

The symbol tells you there are two resistors and the junction tells you where to measure the divided voltage. Look at the values of both resistors and any load connected to VOUT before predicting the actual result.

Adjustable resistance. A variable resistor or potentiometer may let a user tune a threshold, gain, or reference voltage. Check whether the circuit uses all three potentiometer terminals or only the wiper and one end. That distinction tells you whether the part is functioning as an adjustable divider or rheostat.

Temperature sensing. A thermistor often appears in a divider whose midpoint feeds a microcontroller’s analog input. The changing resistance produces a changing voltage. To understand the reading, you need the thermistor’s NTC or PTC behavior and its resistance-versus-temperature data.

These examples show why reading a circuit diagram is about connections and labels as much as just understanding the symbols themselves.

Using Resistor Symbols in Flux

In Flux, start by searching the Parts Library for the resistor type you need and placing the component on the schematic canvas. Connect its pins to the other components by drawing nets between them. Flux’s schematic editor guidance describes this basic placement-and-connection workflow.

Set or confirm the resistance value and review the part’s other properties. For an early concept, a generic resistor can communicate the intended value. When the design approaches purchasing or PCB layout, choose a manufacturer part that meets the required tolerance, power rating, package, and availability constraints. For a thermistor, photoresistor, or potentiometer, also check that the selected schematic symbol and footprint represent the correct terminals.

Keep designators and values close enough to their symbols to read without crowding the wires. Give related parts a logical arrangement. For example, keep both resistors in a divider near the node they create. A readable schematic makes it easier for someone else to spot a misplaced connection or incorrect value.

Finally, share the Flux project with a reviewer and ask them to check both the component choice and the circuit’s function. A clear resistor symbol is the starting point; the right value, connections, and physical part complete the design.

FAQs

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