A relay coil symbol represents the electromagnetic winding that actuates a relay's mechanical contacts. In modern global schematics (IEC 60617), the coil is drawn as a simple rectangle, usually labeled with an alphanumeric prefix like K1 or CR1. In legacy US industrial drawings (NEMA/ANSI), it is often depicted as a circle. The coil is strictly the input device; the switches it controls (the contacts) are drawn separately elsewhere on the schematic and linked only by matching alphanumeric tags.
The Complete Relay Coil Symbol Reference Table
The table below maps the visual symbols you will encounter on schematics to their governing standards and practical meanings. Use this as your primary decoder when reading panel wiring diagrams or PLC I/O sheets.
| Symbol Shape / Visual | Governing Standard | Typical Prefix | What It Means in Practice |
|---|---|---|---|
| Rectangle (Blank or with diagonal line) | IEC 60617 (Global/EU) | K, KA | Standard auxiliary or control relay coil. The rectangle represents the physical winding housing. |
| Circle (Often with 'CR' inside) | NEMA / ANSI/IEEE 315 (US Legacy) | CR, M | Control relay or motor starter coil. Common in older US Motor Control Centers (MCCs) and legacy Allen-Bradley schematics. |
| Rectangle with internal 'X' or Clock | IEC 60617 | KT, TR | Time-delay relay coil. The 'X' or clock face indicates a timing function (on-delay or off-delay) before contacts change state. |
| Rectangle with mechanical latch hook | IEC 60617 | KL, SR | Latching (bistable) relay coil. Requires a pulse to set and a separate pulse (or polarity reversal) to reset. |
| Rectangle with optoisolator symbol | IEC / Modern Solid-State | SSR, K | Solid-State Relay input. Represents the internal LED and phototransistor rather than a physical copper winding. |
| Circle with internal loop / overlapping lines | Old UK (BS 3939) | C, R | Legacy British standard relay coil. Rarely used in new designs but frequently found in pre-1990s UK plant documentation. |
Regional Standards and 'Rows People Get Wrong'
Knowing which standard applies to your region prevents costly wiring errors. If you are working in the EU, Asia, or on modern equipment globally, IEC 60617 is the governing standard. If you are maintaining older industrial machinery in the US or Canada, you will frequently encounter NEMA / ANSI/IEEE 315 symbols. Modern US CAD software (like AutoCAD Electrical) defaults to IEC-style rectangles even for domestic projects, blending the two worlds.
When reading these schematics, beginners and intermediate technicians consistently trip over three specific 'rows' or concepts:
- Confusing the Coil with the Contacts: The coil symbol (rectangle/circle) is only the electromagnetic winding (the input). The actual switches (NO/NC contacts) are drawn on completely different pages or rungs of the ladder diagram. They are linked solely by the designation tag. If the coil is labeled K1, you must hunt through the schematic for contacts labeled K1-13/14 (Normally Open) or K1-11/12 (Normally Closed). Applying power to the contact pins expecting the relay to pull in will result in a dead short or no action.
- Assuming AC/DC is Shown in the Symbol: The geometric shape of the relay coil symbol rarely indicates whether it requires AC or DC voltage. You must look adjacent to the symbol for a voltage callout (e.g., '24VDC' or '120VAC ~'). On the physical relay, IEC standard coils are almost always terminated at A1 (positive/line) and A2 (negative/neutral). NEMA and older relays might use X1/X2 or simple numerical pins.
- Ignoring the Flyback Diode Symbol: In DC control circuits, you will often see a diode symbol drawn in parallel with the relay coil rectangle, with the cathode (stripe) pointing toward the positive supply. This is a flyback (freewheeling) diode. It is not part of the coil itself, but it is critical for suppressing inductive kickback that would otherwise destroy the PLC transistor output driving the coil.
Safe Interpretation When Markings Are Faded or Missing
On the jobsite, you will inevitably encounter a 30-year-old control panel where the relay stickers are baked off, or a physical relay (like a Finder 55.34 or Omron LY2) where the plastic housing has melted and obscured the coil voltage rating. Guessing the voltage by applying power is a fast way to pop a coil, arc a terminal, or destroy a downstream PLC card.
Follow this bench-test protocol to safely identify an unmarked relay coil:
- Locate the Coil Pins: Look for pins marked A1/A2. If unmarked, use the multimeter's continuity mode to find the two pins that show a continuous path but are not continuity-linked to any other pins (isolating them from the contact matrix).
- Measure DC Resistance: Switch your multimeter to the Ohms (Ω) setting. A standard 24VDC relay coil (like the ubiquitous Omron G2R-1) will typically read between 600Ω and 700Ω. A 12VDC coil will read much lower (around 150Ω to 200Ω). Conversely, a 120VAC coil will read significantly higher, often 3,500Ω to 5,000Ω, because the higher voltage requires more turns of thinner wire to limit current.
- Check for Internal Diodes: If you suspect it is a DC coil, use the multimeter's Diode Test mode across the coil pins. If you get a ~0.6V drop in one direction and an 'OL' (open loop) in the reverse direction, the relay has an internal flyback diode. This confirms it is strictly a DC coil and tells you the required polarity (the side that reads 'OL' on the red probe is your positive A1 terminal).
For deeper verification, consult the manufacturer's datasheet. Resources like the Electronics Tutorials relay guide provide excellent baseline resistance charts, while specific component datasheets from manufacturers like Omron or Schneider Electric will list the exact coil resistance and power dissipation (usually 0.5W to 1.0W for standard industrial relays).
Frequently Asked Questions
What is the difference between a relay coil symbol and a contactor coil symbol?
In modern IEC 60617 schematics, both use the exact same rectangle symbol. The difference is identified purely by the alphanumeric prefix and the context of the contacts. A prefix of K or KM usually denotes a power contactor (switching high-current 3-phase loads), while KA denotes an auxiliary control relay (switching low-current logic signals). In older NEMA diagrams, a contactor coil was sometimes drawn as a circle with a specific mechanical linkage line or a half-circle overlay, but this is largely obsolete in modern CAD libraries.
How do I identify a time-delay relay coil on a schematic?
Look for the standard IEC rectangle, but check the interior for an 'X', a small clock face, or a diagonal arrow. An arrow pointing up (or delayed on the top contact) typically indicates an On-Delay (the contacts change state a set time after the coil is energized). An arrow pointing down indicates an Off-Delay (the contacts change state a set time after the coil is de-energized). The time setting (e.g., 't = 5s') is usually written directly next to the symbol.
Why does my schematic show a resistor-capacitor (RC) symbol parallel to the relay coil?
While DC coils use a simple diode for flyback protection, AC coils cannot use a standard diode (it would block half the AC waveform and cause the relay to chatter or fail to pull in). Instead, AC relay coils are often protected by an RC snubber network (a resistor and capacitor in series) drawn parallel to the coil rectangle. This suppresses the high-voltage inductive spike when the AC waveform crosses zero and the contacts open, protecting sensitive triac outputs and preventing EMI from disrupting nearby microcontrollers. For more on AC vs DC switching protection, refer to application notes from major component distributors or standard references like Lessons in Electric Circuits.
