Identifying the correct circuit symbol for a solenoid depends entirely on whether you are reading an IEC electrical schematic, an ANSI fluid power diagram, or a legacy British wiring plan. Misinterpreting these symbols on the bench or jobsite leads to blown drivers, miswired PLCs, and stalled hydraulic presses.
Solenoid Circuit Symbols Reference Chart
The table below maps the most common solenoid representations across the three dominant standard bodies. Use this as your primary lookup when tracing schematics or designing control panels.
| Component / State | IEC 60617 (International) | ANSI/IEEE 315 (US) | ISO 1219 (Fluid Power) | Practical Meaning & Application |
|---|---|---|---|---|
| Standard Solenoid Coil | Rectangle with a single diagonal line from top-left to bottom-right. | Rectangle or circle, often labeled 'SOL' or 'CR' (Control Relay). | Square actuator box attached to the end of a valve envelope. | Standard discrete On/Off actuation. Common in ASCO 8210 series or Parker D1VW hydraulic valves. |
| Proportional Solenoid | Rectangle with a diagonal line, bisected by a diagonal arrow. | Rectangle with an arrow pointing through it, or labeled 'PROP'. | Square actuator box with a diagonal arrow crossing the box. | Variable force/stroke control. Requires a PWM driver or proportional amplifier card, not a simple dry contact. |
| Latching / Bistable Solenoid | Standard coil symbol with a small mechanical latch hook or cross-hatched box adjacent. | Two opposing coils or a coil with a 'L' and mechanical interlock symbol. | Two square actuator boxes on opposite ends of the valve spool envelope. | Pulse to actuate, pulse to release. Draws zero holding current. Common in battery-powered irrigation or safety shutoffs. |
| Solenoid with Manual Override | Standard coil symbol with a small push-button symbol attached. | Coil symbol with a manual switch drawn in parallel or adjacent. | Actuator box with a diagonal line extending outward (representing the override pin). | Allows manual actuation during power loss or system commissioning. Crucial for troubleshooting hydraulic directional valves. |
Regional Standards and the 'Rows People Get Wrong'
Standards dictate not just the drawing, but how the control system is expected to behave. In the US and Canada, ANSI/IEEE 315 and NEMA standards dominate electrical panels, while fluid power systems strictly adhere to ISO 1219 (adopted by NFPA as T3.22.1). In Europe and most international markets, IEC 60617 is the universal language for electrical schematics. The old UK standard, BS 3939, is largely obsolete but still appears on legacy machinery imported before the late 1990s; it typically represented solenoids as simple circles with 'S' inside, which modern techs frequently confuse with motors.
The Rows People Get Wrong
- Confusing Proportional with Discrete: The most costly mistake on the bench. If you wire a 24VDC proportional solenoid (indicated by the arrow through the IEC rectangle) directly to a standard PLC relay output, you will destroy the relay contacts due to the high-frequency PWM inductive kickback. Proportional symbols demand dedicated proportional amplifier cards or high-frequency PWM transistor outputs.
- Misreading the ISO 1219 Actuator Box: In hydraulic diagrams, the square box on the end of the valve is just an actuator indicator. It does not show internal electrical wiring, coil voltage, or flyback diode orientation. Techs often look at a fluid power diagram expecting to see the electrical circuit symbol for a solenoid's suppression diode, which will only be found on the separate electrical schematic.
- Assuming AC/DC Distinction in the Symbol: Neither IEC nor ANSI standard symbols inherently differentiate between an AC and DC coil. A rectangle with a diagonal line means 'solenoid coil' regardless of whether it's a 120VAC ASCO Red-Hat or a 24VDC pneumatic valve. The voltage is always specified in the adjacent tag (e.g., 'Y1 - 24VDC'), not the geometry of the symbol itself.
Safe Interpretation When Markings Are Faded or Missing
On older industrial equipment, sun-faded panel labels and peeling schematic envelopes mean you often have to verify the coil type without relying on the printed circuit symbol for a solenoid. When the diagram is illegible, use your multimeter and physical inspection to determine the coil's nature.
1. Measure Coil Resistance (The Dead Test)
De-energize the circuit and lock out the panel. Disconnect the coil leads and measure resistance. A standard 120VAC industrial solenoid typically reads between 10 and 40 ohms. A 24VDC coil usually reads between 10 and 50 ohms. If your meter reads 'OL' (Open Line), the internal thermal fuse has blown or the winding is severed. If it reads less than 1 ohm, the coil has shorted internally and will trip your breaker or blow your PLC output fuse upon energization.
2. Identify AC vs. DC Physically
If the schematic is entirely missing and you need to source a replacement, you must know if the coil is AC or DC. Look at the top of the solenoid armature (the pole face). AC solenoids require a copper shading ring (a visible embedded loop of wire) to prevent the armature from chattering at the zero-crossing of the AC sine wave. DC solenoids do not have shading rings. Furthermore, AC coils generally use fewer turns of thicker wire, while DC coils use many turns of very fine wire.
3. Check for Internal Suppression
Many modern 24VDC solenoids (like those from Festo or SMC) have built-in flyback diodes or varistors molded directly into the DIN connector (the little black rectangular plug on top). If you measure resistance in one direction and get a reading, but get 'OL' when you swap the multimeter probes, the coil has an integrated polarity-sensitive diode. This means the physical wiring must match the +/- markings on the connector, even if the basic circuit symbol for a solenoid on the drawing doesn't explicitly show the diode.
Frequently Asked Questions
What is the standard schematic symbol for a solenoid valve?
There is no single universal symbol because electrical and fluid power diagrams serve different purposes. On an electrical schematic, the circuit symbol for a solenoid is simply the coil (an IEC rectangle with a diagonal line, or an ANSI box labeled 'SOL'). However, on a pneumatic or hydraulic P&ID, the symbol represents the entire valve assembly: a series of square envelopes showing the flow paths, with the solenoid coil drawn as a square actuator box attached to the end of the spool envelope, per ISO 1219-1 standards.
How do I read solenoid coil symbols on hydraulic diagrams?
Hydraulic diagrams abstract away the electrical wiring entirely. The solenoid is represented as a square actuator box on the side of the valve's directional envelope. If you see a small diagonal line extending outward from that square box, it indicates a manual override pin (usually a small button or screw you can push with an Allen key to shift the spool manually during commissioning). To find the actual wiring, voltage, and PLC routing, you must locate the corresponding electrical ladder diagram using the component tag (e.g., 'SOL1' or 'Y1').
What does a diagonal line through a solenoid symbol mean?
A standard diagonal line (top-left to bottom-right) inside the IEC rectangle simply identifies the component as an inductive coil/solenoid. However, if that diagonal line is crossed by a second diagonal arrow, it designates a proportional solenoid. This tells the panel builder and programmer that the device requires a variable voltage or PWM signal to modulate the spool position, rather than a simple 0V/24V discrete digital output.
Is there a specific symbol for a latching or bistable solenoid?
Yes. In IEC schematics, a latching solenoid is typically drawn as a standard coil symbol with a small mechanical latch indicator (like a hook or a cross-hatched box) drawn adjacent to it. In fluid power diagrams (ISO 1219), a bistable valve is usually shown with two separate solenoid actuator boxes on opposite ends of the valve envelope, or a single box with a mechanical detent symbol, indicating that a brief electrical pulse shifts the valve and it remains in that position mechanically until a reverse pulse is applied.
