Electrical Relay Diagram: How Relays Work and How to Read Relay Circuits

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An electrical relay diagram shows the coil, contacts, and switching action of an electromechanical relay in schematic form, illustrating how a small control signal switches a larger load circuit through galvanic isolation between the two circuits.

An electrical relay is an electrically operated switch. It uses an electromagnet to mechanically move one or more sets of contacts, allowing a low-power control circuit to switch a higher-power load circuit. The two circuits are electrically isolated from each other — the control circuit never makes direct electrical contact with the load circuit.

The relay diagram is a standardised schematic representation showing the two functional sections:

1. The coil (electromagnet): drawn as a rectangle (IEC) or as a coil symbol (older ANSI style) with two terminals. When the rated voltage is applied across these terminals, current flows through the coil winding, generating a magnetic field that pulls the armature. When the voltage is removed, a return spring restores the armature to its rest position.

2. The contacts: drawn as switching symbols adjacent to or below the coil symbol, with a dashed line indicating mechanical linkage. Three contact types are represented: - Normally Open (NO): drawn as two line segments with a gap between them. Open at rest; closes when coil is energised. - Normally Closed (NC): drawn as two line segments with a diagonal line bridging them. Closed at rest; opens when coil is energised. - Changeover (CO, or Common-NO-NC, also called SPDT): includes all three terminals — Common, NO, and NC.

Contact configuration naming: SPDT: Single Pole Double Throw — one common, one NO, one NC contact. DPDT: Double Pole Double Throw — two poles, each with CO contacts, ganged to one coil. 4PDT: Four Pole Double Throw — four ganged poles.

Relay schematic symbols vary between international standards: IEC 60617 standard: coil shown as rectangle; contacts shown as standardised switch symbols. ANSI/IEEE standard (older): coil shown as a circle or coil spring symbol; contacts shown similarly. Automotive diagrams: often use both, with PIN numbering per DIN 72552 (85, 86 for coil; 30, 87, 87a for contacts).

Timers and latching relays: An ON-delay timer relay energises the coil but delays contact operation for a set time. An OFF-delay relay operates contacts immediately but delays release. A latching (bistable) relay remains in its last switched position after the coil is de-energised, requiring a second coil pulse (or a separate release coil) to return to rest. These are shown in diagrams with additional symbols indicating their timing or latching function.

How to wire electrical relay diagram

Identify the relay type and draw the coil symbol Determine whether the relay is electromechanical or solid-state, and what coil voltage it requires. In a schematic, draw the coil as a rectangle (IEC) labelled with the relay designation (e.g., K1) and coil voltage. Mark the coil terminals A1 (positive) and A2 (negative or neutral). If using a time-delay relay, add the appropriate timing symbol inside or adjacent to the coil rectangle.
Draw the contact symbols linked to the coil Draw the contact switch symbol(s) adjacent to the coil or in the circuit where they are functionally connected. Use a normally open symbol (two separated lines) for each NO contact, and a normally closed symbol (two lines joined by a diagonal) for each NC contact. Draw a dashed line from the coil symbol to each set of contacts to indicate mechanical linkage. Label each contact block with the relay designation and a contact number (e.g., K1:1, K1:2).
Draw the control circuit connecting to the coil Wire the control circuit: connect the control supply positive through a stop switch (NC) in series with a start switch (NO) and any interlock contacts to terminal A1 of the coil. Connect A2 to the control supply neutral or negative. If a holding circuit is required, draw a normally open auxiliary contact of the relay in parallel with the start switch.
Draw the load circuit connecting to the contacts Draw the load circuit separately from the control circuit. Connect the load supply through the appropriate relay contacts (NO for a circuit that should activate when the relay is energised; NC for a circuit that should be active at rest). Show the load (motor, lamp, solenoid, etc.) between the contact output and the load supply return. Label all conductors and identify whether they are part of the control circuit or the load circuit.
Add suppression and protection components Add the coil suppression device: a freewheeling diode (for DC coils) shown in reverse-bias across A1 and A2, or a varistor/snubber (for AC coils). If the contact circuit switches an inductive load, add a snubber (RC network) or varistor across the contact output to suppress contact arcing. These components extend contact life and protect the control circuit.
Annotate with component values and ratings On the completed schematic, add annotation for all components: relay coil voltage and part number/designation, contact current rating, supply voltages, wire reference numbers, and terminal block numbers. This information is necessary for panel building, testing, and maintenance. Add a title block with the drawing number, revision, and date.

Specifications

IEC schematic symbol for relay coil	Rectangle with designation label; terminals labelled A1 and A2 per IEC 60617
Normally open (NO) contact symbol	Two horizontal lines with gap between them (open switch symbol)
Normally closed (NC) contact symbol	Two horizontal lines joined by a diagonal bridge (closed switch with actuating bar)
Relay contact current ratings (general purpose)	5 A, 10 A, 16 A, 30 A — resistive at rated voltage; derate for inductive loads
Standard coil voltages	5 V DC, 12 V DC, 24 V DC (low voltage); 24 V AC, 110 V AC, 120 V AC, 230 V AC, 240 V AC
Coil operate voltage (typical)	70–80% of rated coil voltage (minimum to pull in)
Coil release voltage (typical)	10–30% of rated coil voltage (voltage at which contact returns to rest)
Relevant standards	IEC 60947-5-1 (low-voltage control circuit devices), IEC 60617 (schematic symbols), DIN 72552 (automotive pin numbering)

Safety warnings

Relays used to switch mains-voltage (120 V AC, 230 V AC, or higher) load circuits must have contacts rated for that voltage and current. All mains wiring, including relay load circuits, must be installed and tested by a licensed electrician in compliance with NEC/NFPA 70, BS 7671, IEC 60364, AS/NZS 3000, or the applicable national standard. Isolate and verify dead before working on any relay circuit.
Control circuits at low voltage (24 V DC, 24 V AC) do not eliminate the safety hazard if the relay contacts are connected to a mains-voltage load. Both circuits coexist in the same panel or wiring harness. Treat all wiring in a panel containing mains-voltage load circuits as potentially live, and follow full lockout/tagout procedures.
Relay contacts that are overloaded or subjected to excessive switching duty can weld closed, creating a dangerous condition where the load circuit cannot be switched off. Size contacts correctly and derate for inductive or capacitive loads as specified by the relay manufacturer.
DC coil relays with internal suppression diodes must be connected with correct polarity. Reversed polarity puts the diode in forward conduction, creating a near-short-circuit across the coil supply and potentially damaging the power supply, control card, or relay coil within seconds.

Tools needed

Multimeter (resistance mode for coil and contact testing; voltage mode for circuit verification)
Insulated screwdrivers (for relay socket and terminal connections)
Approved voltage indicator (for verifying circuit is de-energised before work)
Schematic drawing software or pencil and engineering paper (for designing the circuit diagram)
Wire stripper and crimping tool (for wiring)
Ferrule crimping tool (for stranded wire terminations in screw terminals)
Lockout/tagout kit

Common mistakes

Using a relay with contact voltage and current ratings below the load requirements: this is the most common cause of premature contact wear, welding, and relay failure.
Omitting coil suppression: a DC relay coil without a flyback diode generates a back-EMF spike on de-energisation that can exceed ten times the supply voltage for a microsecond, destroying transistors, PLC outputs, and control ICs.
Confusing the coil terminals with the contact terminals on a PCB-mount relay: these are close together and easy to mix up. Always verify with the datasheet pinout diagram before soldering.
Using a normally-closed contact where a normally-open contact was intended (and vice versa): the logic of the circuit will be inverted. The load will be on at rest and off when the relay is triggered.
Designing a circuit with a relay holding itself energised indefinitely through a holding contact without a means of de-energising it: if the stop switch is omitted or fails open-circuit, the relay cannot be de-energised. Always include a means of breaking the holding circuit.

Troubleshooting

Relay coil energises but returns to rest position immediately (chatters): Cause: Control circuit supply voltage too low; intermittent contact in the holding circuit; coil supply unable to sustain the current; marginal coil voltage Fix: Measure the supply voltage at the coil terminals while the relay is attempting to hold in. If voltage drops below the relay's minimum hold-in voltage when the coil energises (because the control supply has insufficient current capacity), the relay will chatter. Increase the control supply capacity or replace with a lower-power coil relay. Check holding circuit auxiliary contact and wiring for intermittent open circuit.
Relay contacts show continuity in both energised and de-energised states (welded NO contact): Cause: Contacts welded closed due to overcurrent or switching inrush current exceeding contact rating Fix: Isolate and verify all circuits dead. Replace the relay. Identify the root cause of overcurrent: verify the load current against the contact rating; add a snubber or current-limiting device if motor inrush or capacitive load is the cause; uprate to a higher contact-rated relay.
Solid-state relay (SSR) output fails to switch off (output always on): Cause: SSR output device (TRIAC or MOSFET) failed in the on-state due to overcurrent, overheating, or voltage transient Fix: Isolate the load circuit and replace the SSR. SSRs require heatsinking proportional to their output current — verify heatsink thermal resistance and ambient temperature are within specification. For AC loads, verify that a voltage snubber is fitted across the SSR output if the load is inductive, to prevent voltage spikes that can destroy the output semiconductor.
Control electronics (PLC output, microcontroller) damaged when relay coil is de-energised: Cause: Missing or reversed flyback diode on DC relay coil; back-EMF spike exceeding control output device voltage rating Fix: Fit a flyback diode (cathode to positive supply, anode to negative) directly across the relay coil terminals. For AC coil relays, fit a varistor (MOV) rated above the peak coil supply voltage. Verify correct polarity if the diode is already fitted — a reversed diode conducts continuously and will burn out rapidly.

Frequently asked questions

What is the purpose of the dashed line in a relay schematic diagram?

The dashed line in a relay schematic represents the mechanical linkage between the coil and the contacts. It shows that the contacts are physically moved by the electromagnet, not electrically connected to it. The dashed line makes clear that the coil circuit and the contact circuit are separate, galvanically isolated circuits operated by the same physical relay body.

What is the difference between a normally open (NO) and normally closed (NC) relay contact?

A normally open contact is open (no current can flow through it) when the relay coil is de-energised, and closes when the coil is energised. A normally closed contact is closed (current flows through it) when the coil is de-energised, and opens when the coil is energised. 'Normal' always refers to the rest state of the relay with no power applied to the coil.

Why is galvanic isolation between the coil and contacts important?

Isolation allows a low-voltage, low-current control circuit (such as a 5 V PLC output or a 24 V control bus) to safely switch a high-voltage, high-current load circuit (such as a 230 V AC mains device or a 48 V DC motor). Without isolation, the high-voltage load would back-feed into the control circuit and damage sensitive components. Isolation is also required for safety when the load circuit operates at hazardous voltages.

What is a solid-state relay (SSR) and how does its diagram differ from an electromechanical relay?

A solid-state relay performs the same switching function as an electromechanical relay but uses semiconductor devices (typically thyristors, TRIACs, or MOSFETs) instead of mechanical contacts. Its schematic symbol shows a box containing both the control input (often an LED symbol representing an optocoupler) and the switching output (a TRIAC or MOSFET symbol). SSRs have no moving parts, switch silently, and have much longer cycle life than EMRs but cannot easily switch DC at high power.

What does a relay holding circuit (latching circuit) look like in a wiring diagram?

A holding circuit (also called a seal-in circuit) uses a normally-open auxiliary contact of the relay wired in parallel with the momentary start pushbutton. When the start button is pressed, the relay energises; its auxiliary NO contact closes and bypasses the start button, keeping the relay coil energised after the button is released. A stop button (NC) in series with the coil circuit breaks the seal-in current and de-energises the relay.

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