The standard solenoid electrical symbol is a rectangle representing the electromagnetic coil, typically bisected by two diagonal lines in IEC standards or drawn as a plain loop in NEMA schematics, paired with mechanical actuator arrows. Whether you are reading a hydraulic manifold print, troubleshooting an HVAC contactor, or designing an Arduino-driven relay shield, correctly identifying the exact coil variant dictates your flyback diode selection, PWM frequency, and power supply sizing. Misinterpreting a latching solenoid symbol as a continuous-duty coil will burn out the winding in seconds.
Complete Solenoid Electrical Symbol Reference Chart
The table below maps the most common solenoid coil symbols to their physical components, typical bench measurements, and actuator behaviors. Use this as a quick-reference when reading electrical schematics or fluid power prints.
| Symbol Visual Description | Standard Ref. | Component Type | Typical Coil Resistance (20°C) | Actuator Motion & Duty |
|---|---|---|---|---|
| Rectangle with two internal diagonal lines (IEC) or plain rectangle with A1/A2 terminals (NEMA) | IEC 60617 / NEMA ICS 2 | Standard On/Off Solenoid Valve or Relay | 12V DC: 10-20Ω 24V DC: 40-80Ω 120V AC: 400-800Ω |
Linear push/pull. Continuous or intermittent duty depending on thermal class. |
| Rectangle with a diagonal line featuring a small curve or step-function jog | IEC 60617 | Proportional Solenoid (PWM Driven) | 24V DC: 3-8Ω (Low impedance for high current PWM) | Linear stroke proportional to current. Requires high-frequency PWM (1000-2000 Hz) and dither. |
| Rectangle containing a permanent magnet symbol or two opposing polarity arrows | IEC 60617 | Latching (Bistable) Solenoid | 12V DC: 2-5Ω (High current pulse winding) | Moves to position A on positive pulse, position B on negative pulse. Zero holding power required. |
| Rectangle with an internal curved arrow or rotary arc symbol | NEMA / IEC | Rotary Solenoid Actuator | 24V DC: 15-30Ω | Shaft rotates (typically 25° to 90°) against a spring return when energized. |
| Standard coil rectangle intersected by a small zigzag line or manual push-button line | IEC 60617 | Solenoid with Manual Override | Same as standard on/off coil | Allows mechanical actuation via a physical pin or twist-cap when electrical power is lost. |
Regional Standard Variants: IEC vs. NEMA vs. Legacy UK
Electrical schematics are not universally standardized. The symbol you see on a Siemens PLC print will look different from an Allen-Bradley ladder logic diagram. Understanding these regional dialects prevents dangerous miswiring.
IEC 60617 (Global / European Standard)
The IEC graphical symbols database emphasizes function over physical form. The solenoid coil is almost always a rectangle with two diagonal slashes. IEC prints separate the electrical control circuit (the coil rectangle) from the power/fluid circuit (the valve boxes) using dashed mechanical linkage lines. Terminals are strictly labeled A1 and A2.
NEMA ICS 2 & ANSI/IEEE 315 (North American Standard)
NEMA standards, widely used in US industrial controls, often depict the solenoid coil as a simple rectangle or a circle with a letter designator (e.g., 'SOL' or 'M' for motor starter) inside. NEMA control device standards focus heavily on the physical device representation. In US ladder logic, you will frequently see the coil represented by parentheses ( ) on the right-hand power rail, with the specific solenoid identifier (e.g., SOL1) written next to it rather than drawn as a physical box.
BS 3939 (Legacy UK Standard)
While largely superseded by IEC 60617 in the UK, you will still encounter BS 3939 symbols on older machinery and marine panels. In this legacy standard, solenoid coils were often drawn as a circle with a single horizontal line through it, or as an inductor symbol (a series of semicircular humps) when representing heavy-duty linear actuators. If you are retrofitting a 1980s British CNC machine, expect to see these inductor-style coils.
The 'Rows People Get Wrong' Field Guide
Even experienced technicians misread specific solenoid symbols, leading to blown drivers or stalled machines. Here are the most common schematic traps.
Applying a continuous 24V DC load to a proportional solenoid (symbolized by the stepped diagonal line) will overheat and destroy the coil. Proportional solenoids have very low DC resistance (often under 5Ω) and rely on high-frequency Pulse Width Modulation (PWM) to limit average current. Always check for the step-function jog inside the IEC rectangle before applying bench power.
Trap 1: The Latching Solenoid Burnout
A latching (bistable) solenoid symbol features opposing arrows or a magnet icon inside the coil box. These devices require a brief, high-current polarity pulse to shift states. If a junior tech sees the coil symbol, assumes it is a standard continuous-duty relay, and applies constant 12V DC, the low-resistance winding will draw 5+ amps continuously and melt the insulation in under 10 seconds. Rule: If the symbol has bidirectional arrows, it needs an H-bridge motor driver, not a simple MOSFET switch.
Trap 2: Mixing Up the Coil and the Valve Body
In fluid power (hydraulic/pneumatic) schematics, the solenoid coil is the electrical symbol (the rectangle). The valve body is represented by adjacent square boxes containing internal arrows showing flow paths. Beginners often look at the 3-position valve boxes and think they are looking at the electrical symbol. The electrical symbol is always attached to the extreme ends of the valve body boxes, connected by a small push-pin line.
Trap 3: Ignoring the Manual Override Zigzag
A small zigzag line or push-button intersecting the main coil rectangle indicates a manual override. In automated systems, if a solenoid fails to shift, operators use a 2mm Allen key or a small screwdriver to push this override pin. Failing to recognize this symbol on a print leads to unnecessary machine downtime while waiting for a replacement coil, when a simple manual actuation could keep the line running.
Safe Interpretation When Markings Are Faded or Missing
On jobsites and in aging industrial panels, solenoid nameplates fade, burn off, or get painted over. When you have an unmarked solenoid coil on your bench and no schematic, you must safely deduce its electrical identity using a digital multimeter (DMM) like a Fluke 87V before wiring it to a power supply.
Step 1: Determine AC vs. DC via the Shading Ring
Look closely at the exposed metal core (plunger) of the solenoid. If you see a thin copper or aluminum ring embedded in the face of the steel core (a shading ring or shading coil), it is an AC solenoid. This ring prevents the AC magnetic field from dropping to zero at the 50/60Hz crossover, eliminating severe 120Hz buzzing. If the core face is solid, uninterrupted steel, it is almost certainly a DC solenoid.
Step 2: Measure DC Resistance and Polarity
Set your DMM to the Ohms (Ω) range and measure across the two coil terminals.
- Reading 10Ω to 100Ω: Likely a 12V or 24V DC continuous duty coil.
- Reading 400Ω to 2000Ω: Likely a 120V AC or 240V AC coil.
- Reading OL (Open Line): The internal thermal fuse has blown or the wire is broken. The coil is dead.
- Reading < 1Ω: The winding is shorted. Do not apply power.
Step 3: The Flyback Diode Test
Many modern 24V DC industrial solenoids (like those from Festo or SMC) have a built-in flyback suppression diode or LED indicator module plugged into the DIN connector. Switch your DMM to Diode Test mode. Measure across the terminals in one direction, then swap the probes. If you get a voltage drop reading (e.g., 0.6V) in one direction and 'OL' in the other, there is an internal diode. Critical: You must wire the positive supply to the anode side. If you reverse the polarity on a DC solenoid with an internal diode, you will create a dead short across your power supply, instantly blowing the panel fuse or tripping the breaker.
