When tracing a hydraulic or pneumatic control circuit, misidentifying the electrical schematic symbol for a solenoid valve can lead to catastrophic miswiring or blown PLC output cards. Below is the complete reference for standard solenoid valve symbols across major global drafting standards, followed by critical interpretation rules for legacy prints.
Solenoid Valve Schematic Symbols Reference Table
The table below maps the geometric representations used in modern schematics. Note that the electrical symbol (the coil) is almost always drawn adjacent to the fluid power symbol (the valve body squares and arrows), connected by a dashed or solid mechanical linkage line.
| Valve Type / Function | IEC 60617 / ISO 1219 Symbol | ANSI/IEEE 315 Symbol | Typical Application & Notes |
|---|---|---|---|
| 2/2 Way, Normally Closed (NC) | Rectangle with internal diagonal line pointing to closed port; adjacent square box for coil. | Similar rectangle, but coil box may feature a specific bow-tie or solenoid winding hash mark. | Emergency shut-offs, drain valves. De-energized = blocked flow. |
| 2/2 Way, Normally Open (NO) | Rectangle with diagonal line pointing away from closed port; adjacent coil box. | Identical fluid body to NC, but internal arrow shows default open path. | Cooling loops, venting. De-energized = flow allowed. |
| 3/2 Way (Single Coil) | Two adjacent squares (positions); one square shows 3 ports (P, A, R). Coil box on one end. | Standard 3-port envelope diagram with single solenoid box and spring return on opposite side. | Single-acting pneumatic cylinders. Energizing shifts spool to exhaust or supply. |
| 5/2 Way (Single Coil) | Two adjacent squares; 5 ports (P, A, B, R1, R2). Coil box on left, spring on right. | Identical envelope logic; ANSI may use specific exhaust port triangles instead of open lines. | Double-acting cylinders. Spring returns cylinder to home position on power loss. |
| Proportional Solenoid | Standard coil box, but with a diagonal arrow striking through the center of the box. | Same diagonal arrow through the coil hash marks. | Flow/pressure control via 4-20mA or 0-10VDC. Requires a dedicated amplifier card. |
| Bistable / Latching (5/2) | Two coil boxes, one on each end of the valve body squares. No spring symbol. | Two solenoid boxes on opposite ends of the envelope diagram. | Memory circuits. Pulses to shift, holds position without continuous power. |
| Coil with Manual Override | Small solid triangle or circle protruding from the top or side of the coil box. | Small circle or push-button symbol attached to the coil enclosure. | Used for commissioning and troubleshooting without PLC power. |
Regional Standard Variants & Faded Markings
While the ISO 1219-1:2012 standard has largely unified fluid power graphic symbols globally, the electrical coil representations still vary based on whether the drafting engineer followed European IEC 60617 or North American ANSI/IEEE 315 conventions.
- IEC 60617 (Europe/Global): Represents the solenoid coil as a simple square or rectangle. An AC coil is denoted by a squiggly line (tilde) inside the box, while a DC coil is represented by a solid straight line or a '+' sign. This is the dominant standard in modern PLC-controlled machinery.
- ANSI/IEEE 315 (North America): Often uses a 'bow-tie' or a rectangle with internal diagonal hash marks to represent the electromagnetic winding. It is heavily integrated with NFPA fluid power drafting standards.
- Old UK BS 3939 (Legacy): You will still find this on machinery built in the UK prior to the late 1990s. It uses a semi-circle or a distinct looped wire symbol for the coil, which can easily be mistaken for a capacitor or inductor by modern technicians.
If you are troubleshooting a 20-year-old schematic where the internal diagonal lines of a 3/2 or 5/2 valve are faded, do not guess the Normally Open (NO) or Normally Closed (NC) state based on the drawing. Miswiring an NO valve as NC in a pneumatic press can cause immediate actuation upon power-up. Instead:
1. De-energize the main control panel and verify zero voltage at the coil terminals.
2. Physically locate the valve (e.g., an SMC SY3000 series or Festo VTUG).
3. Press the manual override button (indicated by the triangle symbol on the schematic).
4. Observe the mechanical actuator. If the actuator moves when you press the override, the valve is NC (you are manually forcing it open). If the actuator does not move or vents air, it is NO.
The 'Rows People Get Wrong' Notes
Even experienced controls technicians make specific errors when reading solenoid valve schematics. Watch out for these three common traps:
1. Confusing the AC/DC Coil Indicator
In IEC schematics, a small zigzag line inside the coil box means AC voltage (typically 120VAC or 230VAC). A solid straight line or a '+' means DC voltage (typically 24VDC). Technicians frequently wire 24VDC PLC outputs to 120VAC coils because they ignore the tiny zigzag, resulting in a weak magnetic field, a loud 60Hz hum, and eventual coil burnout due to the lack of a shading ring on a DC-designed armature.
2. The Bistable (Latching) Valve Trap
When a schematic shows a 5/2 valve body with two coil boxes (one on each side) and no spring return symbol, it is a bistable valve. A common mistake is wiring both coils to the same PLC output relay to 'ensure it shifts'. Doing so will energize both coils simultaneously, creating opposing magnetic fields that stall the spool in the center, overheat the coils, and potentially weld the relay contacts. They must be wired to two separate, interlocked PLC outputs.
3. Ignoring the DIN 43650 Connector Pinout
The schematic symbol rarely shows the physical connector pinout, but 90% of industrial solenoid valves use a DIN 43650 Form A (large square) or Form C (small flat) connector. For Form A: Pin 1 is VCC (+), Pin 2 is GND (-), and Pin 3 is PE (Earth Ground). Technicians often wire Pin 1 and Pin 3, ignoring Pin 2, which leaves the circuit open. Always verify the physical valve connector pinout against the manufacturer's wiring guide, as LED indicator modules inside the connector can alter the expected current draw.
Solenoid Valve Symbol FAQ
What does the small triangle mean on a solenoid valve schematic symbol?
The small solid triangle protruding from the side or top of the electrical coil box indicates a manual override. This is a mechanical button or screw that allows a technician to physically shift the valve spool without applying electrical power to the coil. A triangle usually denotes a non-locking push-button (momentary), while a circle with a line through it often denotes a locking twist-and-turn override.
How do I wire a 5/2 solenoid valve if the schematic only shows one coil?
If a 5/2 valve symbol shows only one coil box on one side and a zigzag or spring symbol on the opposite side, it is a spring-return, single-solenoid valve. You only need to wire the single coil to your PLC output or relay. When the PLC output turns ON, the coil energizes and shifts the spool. When the PLC output turns OFF, the internal mechanical spring forces the spool back to its default (de-energized) position. Do not attempt to install a second coil on the spring end.
Why does my solenoid valve symbol have a zigzag line next to the coil box?
A zigzag line drawn in series with the coil box represents an integral suppressor, varistor, or RC snubber network built into the valve's electrical connector. This component absorbs the inductive voltage spike (back-EMF) generated when the coil is de-energized. If you see this symbol, you do not need to install an external flyback diode or snubber module on your PLC output card; doing so is redundant and can actually slow down the valve's drop-out time, causing sluggish shifting.
Is there a difference between the electrical symbol and the pneumatic symbol for the same valve?
Yes. A complete schematic will show two distinct but linked symbols. The pneumatic/hydraulic symbol (governed by ISO 1219) shows the fluid paths, ports (P, A, B, R, S), and shifting envelopes using squares and arrows. The electrical symbol (governed by IEC 60617) shows only the electromagnetic coil, its voltage type (AC/DC), and any integrated electronics. They are linked on the drawing by a dashed line or a shared alphanumeric tag (e.g., 'Y1' for the coil and 'V1' for the valve body) to indicate they are part of the same physical assembly.
