Sheet N° 66

Car type relay connection

Electrical training


The relay is regularly used in on-board electrical installations. (Light vehicle, heavy goods vehicles, public works equipment, agricultural equipment, etc.

The relays separate the control circuits from the power circuits. The objective is to use  a control switch   deported to the dashboard to control a power relay. The relay, once excited by the switch, powers a high-powered receiver (such as a headlight).

The relay can also be used to create automatisms.


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The diagram above includes:

- A battery.                                                - A 30A 4-pin relay.

- F1 and F2: fuses.                                        - A lamp.

The switch is connected to the (+) terminal after the F1 fuse which protects the control circuit from short circuits. The switch exit is connected to the terminal (86) of the relay. The terminal (85) of the relay is connected to the vehicle earth or battery (-).

The terminal (30) of the relay is connected to the terminal (+) after the fuse F2 which protects the power circuit from short circuits. The relay output terminal (87) is connected to the lamp. The other terminal of the lamp is connected to the vehicle earth or battery (-).

The relay is composed of a coil (terminals 85 & 86) which attracts and closes a contact (terminals 30 & 87).

3- Operation.

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When the switch closes, the relay coil is subjected to a voltage difference of 12 V. The current flowing through the coil creates a magnet which attracts the core and closes the     * contact (NO) of the relay. The lamp is powered and lights up.

* NO contact: Normally Open (normally open when idle).

     NC contact: Normally Close (normally clos when idle).

4- Pinout.

The relays can have 4 or 5 pins.

86: Coil input (+).

85: Coil output (-).

30: Contact input (common).

87: NO contact output.

87a: NC contact output.


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Classic relay not polarized.

No protection from self-effects.

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Non-polarized relay with a resistance in parallel with the coil.

Protection from self-effects.

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Warning: Polarized relay with a freewheel diode parallel to the coil.

Protection from self-effects.

Note: The current induced in a coil creates a large reverse voltage in the electrical circuit when the excitation is stopped. This current is called self effect.  A freewheel diode or resistance installed in parallel with the coil cancels this self current and protects the control contacts of electrical arcs.
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The pinout of relays changes according to the standards used by the manufacturer (ISO, DIN, SAE...) and according to the associated contact. (NO, NC, inverter).

Car type relay connection : training course / polarized relay with freewwheel diode.

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