When reading industrial schematics or wiring a fluid power system, correctly identifying the electrical symbols for solenoids is critical. A solenoid is an electromechanical actuator that converts electrical energy into linear mechanical motion, typically to shift a hydraulic or pneumatic valve spool. However, the way these components are drawn on a blueprint changes drastically depending on whether the schematic follows North American NEMA/JIC standards or international IEC standards. Misinterpreting a proportional solenoid symbol as a standard on/off coil, or wiring a latching solenoid for continuous duty, will result in burned-out coils and blown fuses.
Solenoid Symbol & Valve Reference Chart
The table below maps the most common solenoid and solenoid-valve symbols across the two dominant global standards. Use this as your primary desk reference when cross-referencing OEM schematics.
| Component Type | NEMA / JIC (US/Canada) | IEC 60617 (Global/EU) | Practical Meaning & Application |
|---|---|---|---|
| General Solenoid Coil | Rectangle with 'SOL' inside | Rectangle with diagonal lines or letter 'Y' | The basic electromagnetic coil. Converts electrical current into a magnetic field to pull a plunger. |
| 2-Way Solenoid Valve | Coil box attached to a square with 2 port lines | Two adjacent squares (positions) with flow arrows and port numbers (1, 2) | Simple on/off flow control. Used for basic fluid shut-off or venting. |
| 3-Way Directional Valve | Coil box on a rectangle with 3 port lines (P, A, Exh) | Two squares with internal arrows showing P to A, and A to R (exhaust) | Used to actuate single-acting pneumatic cylinders. Provides pressure and exhaust paths. |
| Proportional Solenoid | Coil rectangle struck through with a diagonal arrow | Coil rectangle with a diagonal arrow (often labeled with a PWM or mA signal) | Valve position is proportional to the input current. Requires a dedicated electronic driver, not just a relay. |
| Latching Solenoid | Coil rectangle adjacent to a U-shaped permanent magnet symbol | Coil rectangle with a small permanent magnet symbol or 'L' designation | Uses a permanent magnet to hold position without power. Requires only a brief voltage pulse to shift. |
Solenoid Connector Pinouts & Terminal Designations
Most industrial solenoid valves do not use hardwired flying leads; they utilize standardized DIN connectors to ensure IP65/IP67 environmental sealing. The MAC Valves technical documentation and other fluid power standards heavily rely on the DIN 43650 standard. If you are wiring a replacement coil, you must match the physical form factor and the internal pinout.
| DIN 43650 Form | Spacing / Size | Pin 1 | Pin 2 | Ground / Earth | Typical Voltage Range |
|---|---|---|---|---|---|
| Form A | 18 mm (Large) | Line / Phase (+) | Neutral / Return (-) | Center mounting screw to housing | 12V to 240V AC/DC |
| Form B | 11 mm (Medium) | Line / Phase (+) | Neutral / Return (-) | Center mounting screw to housing | 12V to 120V AC/DC |
| Form C | 8 mm or 9.4 mm (Micro) | Line / Phase (+) | Neutral / Return (-) | Center mounting screw to housing | 12V to 24V DC (Rarely AC) |
| Hardwired (ISO 6952) | N/A (Flying Leads) | Brown or Black wire | Blue or White wire | Green/Yellow wire | Any (Depends on wire gauge) |
Regional Standards: NEMA vs. IEC vs. Legacy UK
Understanding which standard applies to your region—and more importantly, which standard the original equipment manufacturer (OEM) used—is vital for accurate interpretation.
NEMA / JIC (North America)
Governed largely by NEMA ICS 16 and JIC (Joint Industrial Council) electrical standards, North American schematics favor explicit text labels. Solenoid coils are typically labeled SOL, SOL1, or sometimes CR (Control Relay) if the solenoid is acting as a pilot relay for a larger contactor. Terminal markings on the coil itself are often simply 1 and 2, or left unmarked because AC coils are non-polarized.
IEC 60617 (Global / European)
The IEC 60617 standard relies heavily on standardized graphical shapes and letter codes rather than spelling out 'SOL'. In IEC schematics, solenoids and electromechanical actuators are designated by the letter Y (e.g., Y1, Y2 for directional valve solenoids). Coil terminals are strictly designated as A1 (positive/line) and A2 (negative/neutral), which is critical when wiring DC coils with integrated flyback diodes or LED indicators, as reversing polarity on these specific coils will prevent the indicator light from working or blow the internal suppression diode.
Legacy UK (BS 3939)
If you are troubleshooting older machinery in the UK or Commonwealth nations, you may encounter BS 3939 symbols. This superseded standard used the letter S for solenoids and drew the coil as a simple circle with an 'S' inside, rather than the modern IEC rectangle. Treat these exactly as you would a modern IEC 'Y' designation, but verify the coil voltage manually, as legacy systems often utilized unusual control voltages like 50V AC or 110V DC center-tapped earth.
Rows People Get Wrong & Interpreting Faded Markings
Even experienced technicians make specific errors when reading solenoid symbols or dealing with degraded field equipment. Here is how to avoid the most common pitfalls.
The 'Rows People Get Wrong' Notes
- Confusing Proportional with On/Off: If you see the diagonal arrow striking through the coil rectangle, do not wire it to a standard mechanical relay or PLC discrete output. Proportional solenoids have very low internal resistance and require a specialized PWM (Pulse Width Modulation) amplifier card. Applying raw 24VDC will instantly draw excessive current, potentially destroying your PLC output channel.
- Continuous Duty vs. Latching: A latching solenoid (indicated by the adjacent permanent magnet symbol) only requires a 20ms to 50ms voltage pulse to shift states. If you wire it to a standard maintained switch and leave it energized, the coil will overheat and melt within minutes because latching coils are designed for high-inrush, low-duty-cycle operation.
- Missing the Flyback Diode Symbol: On DC schematics, look for a small diode symbol drawn in parallel with the coil rectangle (cathode pointing toward the positive terminal). If the schematic shows it, but your physical coil doesn't have one built into the DIN connector, you must add an external flyback diode (like a 1N4007) across the coil terminals to prevent inductive voltage spikes from destroying your switching transistor.
Safe Interpretation When Markings are Faded or Missing
In harsh industrial environments, solenoid nameplates fade, peel, or become coated in hydraulic oil. If you cannot read the voltage rating, do not guess and apply line voltage. Follow this bench-test procedure to safely identify the coil:
- Measure DC Resistance: Use a multimeter on the ohms (Ω) setting. A 24VDC coil typically reads between 10Ω and 50Ω. A 120VAC coil usually reads between 100Ω and 500Ω. A 240VAC coil will read above 800Ω. (Note: AC coils have lower DC resistance than their impedance, but this baseline still separates low-voltage DC from high-voltage AC).
- Test for Internal Suppression: Switch your multimeter to 'Diode Test' mode. Place the probes across the coil terminals, then swap them. If you get a reading in one direction (e.g., 0.6V) and an 'OL' (open loop) in the other, the coil has an internal flyback diode or rectifier. This confirms it is likely a DC coil or an AC coil with an integrated surge suppressor.
- The Low-Voltage 'Click' Test: If resistance measurements are ambiguous, connect the coil to a current-limited bench power supply set to 12VDC. Slowly increase the voltage. A 24VDC coil will usually produce a weak but audible 'click' around 14V-18V. A 120VAC coil will not actuate at 12VDC, but the coil will remain cool. If the coil becomes hot to the touch within 10 seconds at 12VDC, it is likely a 12V coil—disconnect immediately to prevent thermal damage.
By cross-referencing the schematic symbols with physical terminal measurements, you can accurately replace, wire, and troubleshoot solenoid circuits without relying on legible nameplates or single-standard assumptions.
