5-Pin Relay Pinout: Understanding Pins 30, 85, 86, 87, and 87a

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A 5-pin relay has five standardised terminals — 30 (common), 85 (coil ground), 86 (coil positive), 87 (normally open contact), and 87a (normally closed contact) — forming a single-pole double-throw switch driven by an electromagnetic coil.

The five-terminal automotive and general-purpose relay is one of the most widely used switching components in vehicle wiring and low-voltage control systems. Its five-pin layout follows an international convention originally defined by DIN 72552 for automotive electrical connections, and the same pin numbering is widely adopted outside the automotive sector for low-voltage DC relays.

Pin 30 is the common (COM) input terminal. This is the pivot point of the switching function and is typically connected to the power supply that will be switched — for example, the 12 V positive supply feeding a headlight circuit or a fuel pump.

Pins 85 and 86 are the coil terminals. The electromagnetic coil that operates the relay connects between these two pins. In a 12 V DC relay, a voltage of approximately 12 V across pins 85 and 86 energises the coil, drawing typically 150–200 mA. DIN convention assigns pin 85 as the coil negative (ground) terminal and pin 86 as the coil positive (switched or trigger) terminal. In practice, the coil is not polarity-sensitive unless a built-in flyback diode is present inside the relay body — if a diode is integrated, reversing the polarity renders the diode forward-biased in steady state, which will burn out the diode and possibly the relay.

Pin 87 is the normally open (NO) contact. When the coil is de-energised, pin 87 is disconnected from pin 30. When the coil is energised, the armature closes and pin 87 connects to pin 30, completing the switched circuit.

Pin 87a is the normally closed (NC) contact. When the coil is de-energised, pin 87a is connected to pin 30 — current flows without the relay being energised. When the coil is energised, pin 87a disconnects from pin 30 and the circuit is broken.

In most simple switching applications only pins 30, 86, 87, and 85 are used (the relay is used as a normally open switch), and pin 87a is left unconnected. When the relay is used as a changeover switch — for example, switching between two circuits depending on whether a condition is met — all five pins are used.

The five-pin relay is available in standard footprints that fit the Bosch mini relay socket (the most common automotive type) and numerous panel-mount, PCB-mount, and DIN-rail socket variations.

How to wire 5 pin relay pinout

Identify all five pins using the relay diagram Turn the relay over to view the terminals from the underside (pin side). The diagram moulded on the case or printed on a label shows the pin layout from this view. Locate pins 30, 85, 86, 87, and 87a. On the most common Bosch mini relay footprint, the four corner pins form a square and the fifth pin (87a) is offset. Note: always verify against the specific relay's datasheet, as there are variants.
Connect pin 86 to the trigger source Pin 86 (coil positive) receives the signal that activates the relay. This may be a switched positive from an ignition circuit, a push-button, a PLC output, a transistor collector, or any control signal capable of supplying the coil current (typically 100–200 mA for a 12 V relay). If the trigger signal is from a microcontroller or PLC output limited to 20–50 mA, a transistor driver stage is required between the output and pin 86.
Connect pin 85 to ground Pin 85 (coil negative) connects to the circuit ground or negative supply rail. In an automotive circuit this is chassis ground. Ensure a solid, low-resistance connection — a poor ground on pin 85 is a common cause of relay chattering or failure to energise. Never share the relay ground with a high-current load return path that could produce voltage drops causing false triggering.
Connect pin 30 to the load power supply Pin 30 (common contact input) connects to the positive supply that will be distributed to the load when the relay switches. In an automotive application this is typically a fused positive supply from the battery or fuse box. Always fuse this conductor between the supply and pin 30 to protect the wiring in the event of a contact failure or downstream short circuit.
Connect pin 87 to the load (normally open circuit) Pin 87 connects to the load — the device you want switched on when the relay is energised. When pin 86 is triggered and the coil energises, the armature closes, connecting pin 30 to pin 87 and powering the load. The wire from pin 87 to the load must be rated for the full load current; size it to the load's maximum current draw with an appropriate safety margin.
Connect pin 87a to the alternative load (if using changeover function) If using the relay as a changeover switch (SPDT), connect pin 87a to the circuit you want active when the relay coil is de-energised. This could be a default state (e.g., normally running fan speed) that switches to an alternative state (higher speed via pin 87) when the coil is triggered. If not using pin 87a, leave it unconnected and insulated.
Add a flyback suppression diode across the coil Fit a diode (a general-purpose switching diode rated at 1 A / 100 V minimum) in reverse-bias across pins 85 and 86: cathode to pin 86 (positive), anode to pin 85 (negative). This clamps the back-EMF spike when the coil de-energises, protecting the driving transistor, switch contact, or control module. Many relay sockets and relay modules include this diode internally; verify before adding an external one.

Specifications

Pin 30	Common (COM) contact input — connects to load supply
Pin 85	Coil negative (ground) terminal
Pin 86	Coil positive (trigger) terminal
Pin 87	Normally open (NO) contact — connects to pin 30 when coil is energised
Pin 87a	Normally closed (NC) contact — connects to pin 30 when coil is de-energised
Typical 12 V relay coil resistance	70–200 Ω (draws approximately 60–170 mA at 12 V)
Typical contact current ratings (automotive mini relay)	20 A or 30 A at 12 V DC (resistive); derate for inductive or motor loads
Pin numbering standard	DIN 72552 (automotive and general low-voltage relay convention)

Safety warnings

Before working on any automotive or vehicle wiring, disconnect the negative battery terminal and wait at least 60 seconds to allow any stored charge in capacitors or control modules to dissipate. Verify the circuit is de-energised with a multimeter before handling wiring.
Always fuse the load circuit (pin 30 supply) as close to the power source as possible. The fuse protects the wiring from overload or short circuit; a fuse rated above the wire's ampacity provides no protection. Use the correct fuse type for the wire gauge.
For relays used to switch mains AC voltage — where the load circuit (pins 30, 87, 87a) carries 120 V AC or 230 V AC — the relay must be rated and certified for the voltage, current, and load type. All fixed mains wiring must be performed by a licensed electrician in compliance with local codes (NEC/NFPA 70, BS 7671, IEC 60364, AS/NZS 3000). Isolate and verify dead before working.
A relay coil without a flyback suppression diode on a DC circuit can generate back-EMF spikes of several hundred volts on de-energisation, which can permanently damage transistors, microcontrollers, or PLC outputs driving the coil.

Tools needed

Multimeter (DC voltage and resistance measurement)
Wire stripper
Crimping tool (for insulated female connector terminals)
Soldering iron and solder (for diode installation if not using a module)
Test light or multimeter for circuit continuity verification
Heat-shrink tubing and heat gun (for insulating splices)

Common mistakes

Reversing the coil polarity when using a relay with an integrated flyback diode: pin 86 must be positive, pin 85 must be negative. Reversing them puts the diode in forward conduction continuously, drawing excessive current and burning out the diode.
Connecting the load to pin 87a (normally closed) instead of pin 87 (normally open) when a normally-open function is required: the load will be on at rest and off when triggered.
Using undersized wire on the pin 30 to load circuit: the contact circuit carries the full load current; using coil-circuit-gauge wire on the contact circuit causes overheating and voltage drop.
Omitting a fuse on the pin 30 supply: without a fuse, a short circuit in the load wiring can start a fire, since the relay contact may be rated far above the wire's current-carrying capacity.
Sharing the coil ground (pin 85) with a high-resistance or high-current return path: voltage drops in shared return conductors can cause the relay to chatter, fail to hold in, or trigger spuriously.

Troubleshooting

Relay clicks but load does not activate: Cause: Load connected to wrong pin (87a instead of 87), or open circuit in load wiring Fix: With relay energised, measure voltage between pin 87 and chassis ground. If 12 V is present at pin 87 but not at the load, the fault is in the wiring between pin 87 and the load. If voltage is absent at pin 87, confirm the relay is fully seated in its socket and the contact is not welded open. Swap the relay with a known-good unit.
Relay does not click or energise when trigger is applied: Cause: Insufficient voltage or current at pin 86; open coil; poor ground at pin 85 Fix: Measure voltage between pin 86 and pin 85 directly at the relay socket with the trigger applied. It should be within ±15% of the rated coil voltage. If voltage is correct, measure coil resistance between pins 85 and 86 (with trigger removed and relay disconnected). An open circuit indicates a failed coil — replace the relay. If voltage is too low, trace the trigger circuit for a wiring fault or insufficient driver current.
Relay contacts weld closed — load stays on even when trigger is removed: Cause: Load current exceeds contact rating; severe inrush current from motor or capacitive load welded the contacts Fix: Replace the relay. Uprate to a relay with a higher contact current rating. For inductive or motor loads, check if a current inrush limit resistor or soft-start circuit is needed. Ensure the fuse on the load circuit is the correct rating and type (fast-blow versus slow-blow for motor loads).

Frequently asked questions

What is the difference between pin 87 and pin 87a on a 5-pin relay?

Pin 87 is the normally open (NO) contact: it connects to pin 30 (common) only when the coil is energised. Pin 87a is the normally closed (NC) contact: it connects to pin 30 when the coil is de-energised and disconnects when the coil is energised. Together with pin 30, they form a changeover (single-pole double-throw) switch.

How do I know which relay pins are 85 and 86 (coil) versus 30, 87, 87a (contacts)?

Most relays print a diagram on the case showing the pin arrangement viewed from the terminal side. The coil pins are connected to a symbol representing the coil winding (two parallel lines or a rectangular box). Contact pins show switching symbols. If the diagram is missing, use a multimeter in resistance mode: the coil pins show a fixed resistance (typically 60–200 Ω for a 12 V relay); contact pins show open circuit or near zero depending on their rest state.

Can I use a 5-pin relay as a simple on/off switch by ignoring pin 87a?

Yes. In the most common application, pins 30, 85, 86, and 87 are used and pin 87a is left unconnected. Pin 86 receives the trigger signal, pin 85 goes to ground, pin 30 receives the load power supply, and pin 87 outputs to the load. The relay acts as a normally open switch.

Why use a relay to switch a load instead of switching it directly with a transistor or switch?

A relay provides complete galvanic isolation between the control circuit and the load circuit, switches AC or high-current DC loads that solid-state devices cannot handle directly, and its contacts have a very low voltage drop compared to many transistors. Relays are preferred for high-current loads (fans, fuel pumps, horns, solenoids) where a switch or transistor would need heatsinking and protective circuitry.

What is the coil resistance of a typical 12 V 5-pin relay and why does it matter?

A typical 12 V automotive-style relay coil has a resistance of 70–200 Ω, drawing 60–170 mA at 12 V. This matters because the control circuit (switch, relay output, or transistor) must be capable of supplying this current reliably. It also determines whether a suppression diode is needed — a coil with a stored energy of ½LI² releases this as a back-EMF spike on de-energisation.

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