The standard coil relay symbol in modern schematics is a simple rectangle (IEC 60617) or a circle/rectangle with internal diagonal lines or letters (NEMA ANSI/NFPA 70). The coil represents the electromagnetic input side of the relay, which is entirely distinct from the mechanical switch contacts it actuates. Below is the definitive reference for identifying, selecting, and troubleshooting relay coils across global standards.
The Master Coil Relay Symbol Reference Table
Use this table to decode the input side of your schematic. The coil symbol dictates the voltage, current type, and physical relay family you need to procure.
| Relay Type | IEC 60617 Symbol (Global/EU) | NEMA / ANSI Symbol (North America) | Standard Designator | Practical Meaning |
|---|---|---|---|---|
| Standard DC Coil | Rectangle with polarity marks (+/-) or a straight internal line | Circle or Rectangle with 'DC' or straight line | K, KA (IEC) / CR (NEMA) | Standard electromagnetic coil requiring DC voltage. Polarity matters if an internal flyback diode is present. |
| Standard AC Coil | Rectangle with an internal squiggly line (~) | Circle or Rectangle with 'AC' or squiggly line | K, KA (IEC) / CR (NEMA) | Coil designed for alternating current. Polarity does not matter. Includes a shading ring to prevent AC hum. |
| Latching (Bistable) Coil | Rectangle with a mechanical latch symbol (crossed lines) or two separate coils | Rectangle with 'L' or dual coil circles | KL (IEC) / LCR (NEMA) | Requires only a momentary pulse to change state. Maintains position without continuous power. |
| Time-Delay (On-Delay) | Rectangle with a small arrow pointing right or an 'ON' block | Rectangle with 'TD' and timing arrow | KT (IEC) / TR (NEMA) | Contains internal RC timing circuitry. Contacts change state only after a set duration of continuous coil power. |
| Solid State Relay (SSR) Input | Rectangle containing an LED/diode symbol and optical isolation line | Box with diode symbol and 'SSR' text | KSS (IEC) / SSR (NEMA) | Not a magnetic coil. It is an optocoupler LED input. Requires strict current limiting (usually 3-30V DC). |
Regional Standards: IEC vs. NEMA vs. Legacy UK
Schematic symbols are not universal. Misinterpreting a regional standard can lead to wiring a 24VDC coil to a 120VAC line, instantly destroying the component. Always check the title block of the drawing to confirm the governing standard.
- IEC 60617 (Global/EU/Modern): The dominant global standard. Coils are strictly drawn as rectangles. Designators typically start with 'K' (e.g., K1, K2 for standard relays, KA for auxiliary). The IEC standard separates the coil drawing from the contact drawing, linking them only by the alphanumeric designator (e.g., K1 coil actuates K1 NO/NC contacts scattered across the schematic). See the All About Circuits reference guide for deeper IEC context.
- NEMA / ANSI / JIC (North America): Common in US industrial and HVAC panels. Coils are often drawn as circles (especially in older JIC ladder logic) or rectangles, labeled with 'CR' (Control Relay) or 'M' (Motor Starter). NEMA schematics often draw the coil and its associated contacts in close physical proximity on the page, unlike the IEC scattered approach.
- Legacy UK (BS 3939): Largely superseded by IEC standards, but still found in older British industrial plants. Coils were sometimes represented by semi-circles or specific circle variants with internal alphanumeric codes. If you encounter BS 3939, treat all circular elements as potential coils and verify with a multimeter before energizing.
Table Rows and Symbols People Get Wrong
When reading the reference table above, field technicians and hobbyists consistently misinterpret specific rows. Here is how to avoid those traps:
People frequently wire continuous voltage to a latching (bistable) coil, assuming it operates like a standard relay. A true magnetic latching relay coil will overheat and burn out if held energized. If the symbol shows a latch mechanism, you must use a momentary push-button circuit or a microcontroller GPIO pin pulsed for 50-100ms, not a continuous HIGH signal.
An SSR input symbol looks like a coil but is actually an LED. If you apply 24V directly to an SSR input without checking the datasheet for an internal current-limiting resistor, you will pop the internal optocoupler. Always verify if the SSR requires an external series resistor (common in older Macromatic and Omron SSRs) or if it accepts raw 3-32VDC directly.
The Missing Flyback Diode: In IEC schematics, a DC coil symbol often lacks an explicitly drawn flyback diode, assuming the physical relay module has one built-in. In NEMA schematics, the diode is almost always drawn explicitly in parallel with the coil. If your physical relay lacks a diode and the schematic doesn't show one, you must add a 1N4007 diode in reverse-parallel across the coil terminals to protect your driving transistor from inductive kickback.
Decision Tree: Selecting the Physical Relay from the Symbol
Use this decision path to translate the schematic symbol into a physical part number for your BOM (Bill of Materials).
| If the Schematic Shows... | And the Voltage is... | Then Procure This Exact Part Family |
|---|---|---|
| Standard Rectangle (Row 1) | 12V or 24V DC | Omron G2R-1-E (PCB/Through-hole) or G2R-2-S (DIN rail socket) |
| Standard Rectangle with Squiggle (Row 2) | 120V or 240V AC | Schneider Harmony RXM4AB2BD (14-pin DIN) or Finder 55.34 (PCB) |
| Latching Symbol (Row 3) | 5V or 12V DC Pulse | Omron G6CK or Panasonic ALQ series bistable relays |
| SSR Box with Diode (Row 5) | 3-32V DC Input / 24-280V AC Output | Omron G3NA-210B or Crydom D2410 panel mount SSR |
Default Recommendation: If the schematic is ambiguous, lacks a specific BOM, or you are designing a generic control board, default to the Omron G2R-1-E DC24 for DC applications. It is a 10A, SPDT workhorse with a 650-ohm coil, available globally for under $3, and represents the safest, most universal baseline for modern DC control circuits.
Field Troubleshooting: Faded Markings and Unknown Coils
On legacy machinery, physical relay labels fade, peel off, or get painted over. Never guess the coil voltage based on the system's main power supply; a 240V AC machine frequently uses 24V DC for its control relays. Follow this safe interpretation protocol:
- Measure DC Resistance: Isolate the relay from the circuit. Place your multimeter in Ohms mode across the coil pins (usually A1/A2 or 13/14 on 14-pin relays).
- 150 - 400 Ω: Likely a 12V DC coil.
- 600 - 1200 Ω: Likely a 24V DC coil.
- 2000 - 10,000 Ω: Likely a 120V/240V AC coil (AC coils have high inductive impedance but lower DC resistance than expected; however, very high DC resistance usually points to higher voltage AC).
- Open Line (OL): The coil is burnt out. Replace the relay.
- Check for Internal Protection: Switch your multimeter to Diode Test mode. Probe the coil pins in both directions. If you get a ~0.6V drop in one direction and OL in the other, the relay has an internal flyback diode. You must observe polarity when wiring this relay, or the diode will short your power supply.
- The Safe Bench Test: If resistance readings are inconclusive, use a variable DC bench power supply with current limiting set to 100mA. Connect it to the coil. Start at 3V and increase by 1V increments. Listen for the mechanical 'click'. Note the voltage at which it actuates. The nominal voltage is typically 30-40% higher than the minimum actuation voltage. Never exceed 110% of the suspected nominal voltage, or you will melt the copper windings.
