Relay Pin Out: Understanding DIN 72552 Terminal Numbers 30, 85, 86, 87 and 87a

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A relay pin out diagram identifies each terminal by its DIN 72552 number so you can connect a relay correctly in any automotive or industrial control circuit without guessing.

A relay is an electromagnetically operated switch. When a small control current energises the relay's coil, it generates a magnetic field that attracts a ferrous armature, mechanically closing (or opening) a set of electrical contacts that carry a completely separate, typically higher-current, load circuit. The essential value of this arrangement is that a low-current signal — from a microcontroller, a logic circuit, or a switch — can safely control a high-current device such as a motor, lamp, fuel pump, or solenoid.

The most widely encountered relay in automotive applications is the five-pin ISO mini relay, and its terminal numbering follows DIN 72552, the German standard for automotive electrical terminal designations that has become effectively universal in vehicle wiring worldwide.

The five terminals are assigned as follows. Terminal 30 is the relay's common contact — the power input to the switching side of the relay. It connects to the load supply (typically battery positive or a fused supply rail) and is always connected to the normally open or normally closed contact regardless of coil state. Terminal 85 is one coil terminal (ground in most common configurations — it is the terminal connected to the switching negative, i.e. the signal from the control device, which grounds the coil circuit). Terminal 86 is the other coil terminal (supply positive — connected to battery positive or ignition-switched positive to energise the coil). The relationship of 85/86 is symmetric — the coil is a simple inductance and has no inherent polarity, though some relays include a transient suppression diode across the coil, which makes polarity relevant: 86 must then be positive. Terminal 87 is the normally open (NO) contact — it is connected to terminal 30 only when the coil is energised. Terminal 87a is the normally closed (NC) contact — it is connected to terminal 30 when the coil is de-energised (at rest), and disconnects when the coil is energised.

On a four-pin relay, terminal 87a is absent — the relay has only the common (30), coil (+/– 85/86), and normally open (87) contacts. The five-pin version adds the normally closed function.

How to wire relay pin out

Identify the relay type and terminal layout Confirm whether the relay is a four-pin (NO only) or five-pin (NO and NC) type. Locate the terminal numbers moulded into the relay body or printed on the datasheet. The ISO mini relay has a standardised footprint: terminal 86 and 85 (coil) typically occupy one pair of positions, and 30, 87, and 87a (contacts) the other positions. The number positions on the housing identify each terminal.
Connect the coil circuit (terminals 85 and 86) Connect terminal 86 to the positive supply that will energise the relay — this may be the ignition-switched positive, a microcontroller transistor output stage, or a timer output. Connect terminal 85 to ground (chassis earth in automotive applications, or 0 V rail in electronic applications). If the relay has a built-in transient suppression diode, ensure polarity is correct: 86 = positive, 85 = negative.
Connect the common (terminal 30) to the supply being switched Terminal 30 receives the voltage that will be switched to the load. This is typically the battery positive or a fused supply rail. This terminal must be fused to protect the wiring to the load. Use a fuse rating matched to the load current, not the relay's maximum contact rating.
Connect the load to terminal 87 (normally open) or 87a (normally closed) For a load that should activate when the relay is energised, connect it from terminal 87 to ground. For a load that should be active at rest and deactivate when energised, connect to terminal 87a. For switching between two loads, connect one load to 87 and the other to 87a, sharing the common supply at terminal 30.
Install a flyback (freewheeling) diode across the coil if not integrated When the relay coil is de-energised, the collapsing magnetic field generates a voltage spike (back-EMF) that can damage transistors or other switching devices. A diode placed across the coil (anode to terminal 85, cathode to terminal 86 in a conventional positive-switched coil) clamps this spike. Many modern relays include this internally — check the datasheet.
Test the relay before finalising the installation Apply the coil supply voltage and confirm you can hear the relay click (the armature closing). Measure continuity between terminal 30 and terminal 87 with coil energised (should be near-zero resistance). Measure between 30 and 87a with coil de-energised (should be near-zero). No continuity where expected indicates an internal contact fault.

Specifications

Terminal designation standard	DIN 72552 (ISO 1724 for some applications)
Terminal 30 function	Common contact — switching circuit supply input
Terminal 85 function	Coil terminal (conventionally connected to ground / negative / switching control output)
Terminal 86 function	Coil terminal (conventionally connected to positive supply)
Terminal 87 function	Normally open (NO) contact — connects to terminal 30 when coil is energised
Terminal 87a function	Normally closed (NC) contact — connects to terminal 30 when coil is de-energised
Typical coil resistance (ISO mini relay, 12 V)	50–200 Ω (resulting in 60–240 mA coil current at 12 V)
Typical contact current rating (ISO mini relay)	20–30 A resistive; lower for inductive and motor loads (verify with relay datasheet)

Safety warnings

Always fuse the load circuit connected to terminal 30. The relay's contact current rating is not a substitute for a properly rated fuse — without a fuse, a short circuit in the load wiring cannot be safely interrupted.
In automotive applications, disconnect the vehicle battery before making permanent wiring connections to avoid short circuits from tool contact with the battery positive terminal.
Relay coils generate a back-EMF spike when de-energised. In circuits driving relay coils from solid-state outputs (transistors, microcontroller GPIO via driver), fit a flyback diode across the coil or use a relay with an integrated suppressor. Failure to do so damages or destroys the driving transistor.
Do not exceed the relay's rated contact current. Contact arcing at overcurrent degrades the contacts rapidly, causes contact welding (the relay sticks closed), and presents a fire risk.
When using relays in enclosed control panels or near flammable materials, ensure the relay's IP rating and arc containment are appropriate for the environment and comply with the applicable installation standard (IEC 60947-4, IEC 61810, or equivalent).

Tools needed

Digital multimeter (continuity, DC voltage, and resistance functions)
Ratchet crimping tool and insulated spade terminal crimps
Wire strippers
Flathead screwdriver (for screw-terminal relay bases)
12 V DC test supply or bench power supply (for pre-installation relay testing)
Inline fuse holder and appropriate fuses

Common mistakes

Connecting the load to the relay's coil terminals (85/86) instead of the contact terminals (30/87), which applies load current through the coil and either fails to switch the load or damages the relay.
Omitting a fuse on the load circuit, so a wiring fault in the downstream load circuit cannot be safely interrupted.
Reversing 85 and 86 on a relay with an internal suppression diode, which creates a short circuit through the forward-biased diode when the coil is energised.
Using a relay's maximum contact rating as the basis for fuse sizing — the maximum rating is a worst-case limit, not a recommendation. Size the fuse for the actual load current.
Not considering the relay's coil current draw when calculating total current on a control circuit supply — a relay coil draws 100–200 mA, which must be accounted for in the control circuit's current budget.

Troubleshooting

Relay does not click when coil is energised: Cause: No voltage at terminal 86; no ground path at terminal 85; open coil winding; supply voltage below relay's minimum pickup voltage Fix: Measure voltage across terminals 85 and 86 with the control circuit activated. Should be close to nominal coil voltage (12 V for automotive). Measure coil resistance directly (disconnect coil supply, measure 85–86 with multimeter) — should be 50–200 Ω typical. An open coil reads infinite resistance and requires relay replacement.
Relay clicks but load does not activate: Cause: Open circuit in load wiring; blown load fuse; failed contact inside relay; load not grounded; terminal 30 not connected to supply Fix: With relay energised, measure voltage between terminal 87 and chassis ground — should equal supply voltage. If no voltage at 87 with relay energised, the relay contacts have failed (replace the relay). If voltage is present at 87, the fault is downstream in the load circuit or its earth connection.
Relay contact welds and load cannot be switched off: Cause: Sustained overcurrent through the contacts; contact arcing from inductive load without suppression; relay contacts undersized for the load Fix: Replace the relay. Identify the root cause: measure the load current and compare to the relay contact rating. Add a freewheeling diode or varistor across inductive loads (motors, solenoids) to suppress the back-EMF that causes arcing at switch-off. Upgrade to a relay with a higher contact current rating if the load current was excessive.

Frequently asked questions

What does terminal 30 on a relay connect to?

Terminal 30 is the common (COM) contact — the power input to the relay's switching section. In automotive circuits it almost always connects to the positive supply that needs to be switched: battery positive, a fused supply rail, or a source that should be present whenever the relay is in use. Current flows through terminal 30 and exits at terminal 87 (when energised) or 87a (when de-energised).

Is there a polarity requirement for terminals 85 and 86 on the relay coil?

A plain relay coil without a suppression diode has no polarity requirement — 85 and 86 can be connected either way and the coil will energise. However, if the relay includes a flyback (transient suppression) diode across the coil, polarity is critical: terminal 86 must receive positive supply and terminal 85 must connect to ground. Reversing polarity forward-biases the diode, creating a short circuit. Check the relay datasheet.

What is the difference between a four-pin and a five-pin relay?

A four-pin relay has terminals 30, 85, 86, and 87 only — providing a normally open (NO) switching function. A five-pin relay adds terminal 87a, giving both normally open (87) and normally closed (87a) contacts sharing the common input (30). The five-pin version allows the relay to switch between two separate circuits depending on coil energisation state.

Why do automotive relay diagrams use DIN 72552 numbers instead of descriptive labels?

DIN 72552 terminal numbering is language-independent and standardised across the international automotive industry. Rather than translating labels such as 'normally open' or 'coil positive' into every language, a single number (87, 85, 86, etc.) is immediately understood by any technician trained in automotive electrical systems, regardless of nationality.

Can I use a relay to protect a microcontroller output from a high-current load?

Yes, and this is one of the most common relay applications in electronics. A microcontroller's output pin typically sources only 20–40 mA. A relay's coil draws 100–200 mA at 12 V, which exceeds most GPIO limits directly — so a transistor driver stage (e.g. NPN transistor with base resistor) is usually inserted between the GPIO and the relay coil. The relay's contacts then switch the high-current load completely independently of the control circuit.

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