Variable Resistor Symbol
Definition: The Variable Resistor symbol represents a two-terminal passive component whose resistance can be adjusted continuously between zero and its maximum rated value, standardised in IEC 60617-04 as a rectangle with an adjustable arrow and in ANSI Y32.2 / IEEE 315 as a zigzag with an adjustable arrow, with terminals A (end 1) and B (end 2), measured in ohms (Ω) and designated R.
Also known as: rheostat, adjustable resistor, sliding resistor, variable resistance, rheostatic resistor, two-terminal variable resistor.
What the Variable Resistor symbol means
The Variable Resistor symbol represents a two-terminal resistor component whose resistance value is continuously adjustable by moving a wiper contact along the resistive element. Unlike a potentiometer (which has three terminals and functions as a voltage divider), the variable resistor — also called a rheostat — connects only two terminals: one end of the resistive track (A) and the movable wiper (B), so the current path passes through a variable portion of the track, changing the resistance in the circuit.
In schematic diagrams, the variable resistor symbol indicates that the resistance at that node is intentionally adjustable, allowing a circuit parameter such as current, brightness, motor speed, or heating power to be set manually. The arrow through or across the resistor body (rectangle in IEC, zigzag in ANSI) and pointing at the element indicates the adjustable wiper. Variable resistors appear in lamp dimmer circuits, fan speed controls, audio volume controls (older single-channel), laboratory bench power supplies, and anywhere a continuously adjustable resistance is needed.
How to identify the Variable Resistor symbol
The Variable Resistor symbol is drawn as the standard resistor body — a filled rectangle in IEC 60617-04, or a zigzag line in ANSI Y32.2 — with a diagonal arrow through or across the resistive element. The arrow indicates the movable wiper contact. Only two terminal connections exit the symbol: one from each end of the resistor body (A and B in the symbol), or one from an end and one from the wiper. The absence of a third wiper terminal (compared to a potentiometer, which has three terminals) is the key distinguishing feature of the two-terminal rheostat symbol. The arrowhead points toward the resistor body, not away from it.
Function in a circuit
A variable resistor (rheostat) varies the resistance in series with a circuit load. As the wiper moves from one end to the other, the active resistance between the two connected terminals changes from 0 Ω (wiper at the connected-end position) to the full rated resistance (wiper at the far end). By placing the rheostat in series with a load, current through the load varies inversely with the resistance setting (Ohm's Law: I = V / (R_load + R_rheostat)). This allows control of lamp brightness, motor armature current, heater power, and other current-controlled parameters. High-power rheostats (wirewound, ceramic) handle kilowatts; small carbon or cermet types handle milliwatts.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-04 (passive components): the variable resistor (rheostat) is drawn as a rectangle (IEC fixed resistor symbol) with a diagonal adjustable arrow passing through or alongside it, indicating the movable contact. Two terminals are used (end A and end B / wiper). |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315: the variable resistor uses a zigzag line (ANSI fixed resistor symbol) with a diagonal arrow indicating the adjustable wiper. Two terminals are connected — one end and the wiper — for rheostat operation. Designator: R (or VR for variable resistor). |
| Key difference | IEC 60617-04 uses a filled rectangle for the resistor body; ANSI Y32.2 uses a zigzag line. Both place a diagonal adjustment arrow through the symbol to indicate variability. The two-terminal rheostat connection (versus three-terminal potentiometer) is the same in both standards. Designator R or VR applies in both. |
Terminals / pins
| Pin | Name |
|---|---|
| a | A |
| b | B |
Typical values
Common resistance ranges: 1 Ω to 10 kΩ (small panel types), 10 Ω to 1 kΩ (wirewound power types). Power ratings: 0.1 W (carbon film), 2–200 W (wirewound ceramic/vitreous-enamel). Resolution: continuous (wirewound smooth) or step (tapped types). Temperature coefficient: 50–200 ppm/°C (carbon), <50 ppm/°C (wirewound precision).
Where the Variable Resistor symbol is used
- Series current limiting and motor speed control in DC motor circuits
- Lamp brightness control in incandescent and halogen theatre dimmer circuits
- Bias resistance adjustment in analogue amplifier and oscillator circuits
- Load resistance simulation in test and calibration bench setups
- Field current rheostat control in DC generators and synchronous machine excitation
- Heating element power control in laboratory furnaces and industrial process heaters
Example
In a DC motor speed control circuit, a 50 Ω / 10 W wirewound variable resistor symbol is placed in series with the motor armature winding. Sliding the wiper toward end B increases resistance, drops more voltage across the rheostat, reduces armature current, and slows the motor — a classical technique shown on motor control ladder diagrams and a direct application of Ohm's law.
Key facts
- The Variable Resistor symbol is a resistor body (IEC rectangle or ANSI zigzag) with a diagonal adjustable arrow, connected at two terminals: A (end of track) and B (the other end or wiper position); designator R or VR.
- IEC 60617-04 uses a rectangle body; ANSI Y32.2 / IEEE 315 uses a zigzag body — both add a diagonal arrow to indicate the variable (adjustable) wiper contact.
- Two-terminal connection distinguishes the rheostat from a three-terminal potentiometer: only one end of the track and the wiper are used, making the resistance continuously adjustable from 0 Ω to maximum rated value.
- Resistance measured in ohms (Ω); rated power in watts (W); adjustment by mechanical wiper movement along the resistive element.
- Wirewound variable resistors handle high power (up to 200 W or more); carbon-film and cermet types are used for low-power signal-level applications.
- In the rheostat (two-terminal) configuration, the unused third terminal should be connected to the wiper terminal to prevent open-circuit if the wiper contact fails on a carbon-track type.
- The variable resistor symbol's arrow points at the resistive element (into the rectangle or zigzag body), indicating a moving contact along the element — distinguishing it from a fixed resistor.
- Historical note: the term 'rheostat' (from Greek rheos = flow, statos = standing) was coined by Charles Wheatstone in 1843 to describe a device for varying electrical resistance.
Frequently asked questions
What does the variable resistor symbol look like?
The variable resistor symbol looks like a standard fixed resistor — a filled rectangle in IEC 60617 or a zigzag line in ANSI Y32.2 — with a diagonal arrow through or across the middle of the resistor body. The arrow represents the movable wiper contact. Only two terminals exit the symbol: one from an end of the resistive track and one from the wiper connection.
What does the variable resistor symbol mean in a circuit?
The variable resistor symbol means there is an adjustable resistance at that point in the circuit. The resistance value between its two terminals can be varied from 0 Ω up to the component's maximum rated value by moving the wiper, allowing manual control of current, brightness, speed, or other circuit parameters dependent on resistance.
What is the difference between a variable resistor and a potentiometer symbol?
A variable resistor symbol has two terminals and is used as a series current-control element (rheostat). A potentiometer symbol has three terminals — two end terminals and a wiper — and is used as a voltage divider. On a schematic, the potentiometer symbol shows three connection points; the variable resistor shows only two. In practice, a potentiometer component can be wired as a rheostat by connecting only one end and the wiper.
What is a rheostat?
A rheostat is the traditional name for a variable resistor connected in a two-terminal (series) configuration. It places an adjustable resistance in series with the circuit to control current. The word rheostat and variable resistor are often used interchangeably; modern usage tends toward 'variable resistor' while rheostat persists in older texts and power electronics contexts.
What is the designator letter for a variable resistor?
The reference designator for a variable resistor is R (general resistor designation per IEC 60617 and IEEE 315 / ANSI Y32.2) or VR (variable resistor — a common informal convention). The full resistance value and wattage rating are annotated beside the designator on the schematic.
What is the IEC vs ANSI difference for the variable resistor symbol?
IEC 60617-04 draws the resistor body as a rectangle and adds a diagonal arrow for variability. ANSI Y32.2 / IEEE 315 draws the resistor body as a zigzag line and adds the same diagonal arrow. The meaning, terminal count, and designator (R or VR) are identical in both standards; only the resistor body shape differs.
What standard defines the variable resistor symbol?
The variable resistor symbol is defined in IEC 60617-04 (passive components — resistors and capacitors) for IEC countries and in ANSI Y32.2 / IEEE 315-1975 for North American and international electronic engineering drawings. Both standards are widely used and mutually understood in global engineering documentation.
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