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DIY relay board

DIY relay board

In this tutorial, you’ll learn how to make a power automotive DIY relay board. This board allows the control of DC motors or other power devices for robotic, industrial, automotive or on-board electronics applications.

Printed circuit with the 12 volt relay on its plug-in support


When designing the board, it was intended for the production of an industrial test bench in the automotive world.
The forty power controls needed were made with relays mounted on brackets. Control is performed by the logic outputs of a measurement PC with Labview® type software. The relays actuate DC motors used in cars and also perform the necessary galvanic isolation function between computer equipment and power devices.

Other possible applications

  • Current inverter: using a N.O.-N.C. contact relay. (5 pins).
  • Control of: fog lights, additional headlights, 12-volt spotlight, siren.
  • Control for robot motors: robot mower, R2D2 robot, exploration robot.

Diagram and electrical design

Circuit diagram of the board

The relay

Detailed view of the relay terminals

According to the final application, you can choose the quality of the relay. The breaking capacity of the contacts (in amperes), the rise or fall times of the contacts (in milliseconds), the number of operations and the operating temperature (°C) are parameters that directly influence the service life of the relay (coil, mechanics and contacts). The relay terminals are of the male “6.35 faston terminal” type.
The relay used is form C and identified by five terminals. The coil has numbers 85 (+) and 86 and contacts 30 (common), 87 (NO) and 87a (NC). See the pinout and terminal identification in the following illustration.

Identification and numbering of relay terminals

The orthographic reform of 1990 proposed to return to the spelling relay (on the model of delay), but this one did not impose itself and relay remains despite everything the most common.

The relay support

The relay soldering support.

You can use this “6.35 faston” relay support to avoid soldering your relay directly to the board. Using this support, you can plug and unplug your relay anytime easily.

Protective devices

Across the coil terminals, you must wire a free-wheeling diode (therefore pay attention to the polarization of the wiring). It acts as an “energy absorber” when de-energizing the coil, which avoids propagating an overvoltage on the component that drives the relay (transistor or dry contact) and also limits the emission of transients on the cable which connects the logic output of the measurement PC to the relay card. As a precaution, we recommend protecting the output transistors of the logic board of the PC. You must find out about the equipment from the manufacturer or the instructions provided.
You can use other components to protect the relay contact controlling the inductive load. For example, if you choose to use a diode, it must be able to carry the same nominal current of the motor (or of the load in general). A varistor or an RC circuit (100 ohms + 0.1 mF for example) may be suitable in some cases.
The motor must also be protected from overcurrent with a fuse to be inserted in the power circuit (which will also protect the relay contact).


Here you can find the list of the electronic components:

  • 1N4005 diode (D).
  • MR751 diode (D2).
  • Automotive relay 12V (inverter) – form C (RL1).
  • 5-pin relay support – form C PCB version (RL1) plus the 5 terminals.
  • 4-point terminal block, 5 mm pitch (J1).
  • 5-point terminal block, 5 mm pitch (J2) with copper cages.
  • printed circuit REL12 70 to 105 μm, single-sided FR4, solder-side varnish, 40×60 mm.

The circuit board “PCB”

The recommended thickness of copper traces is 70 to 105 µm with a minimum trace width of 2.5 mm for a current of 20 amps. Additional tinning of the copper tracks may be necessary in the case of higher currents. Solder side varnish is preferable. The circuit board has four fixing holes, so you can fix it in a horizontal or vertical position.

The printed circuit artwork.

The wiring

It will be necessary to respect the direction of implantation of the diodes and the cage connectors. Solder the components in the following order:

  • Diode D1.
  • Diode D2.
  • Terminals J1 and J2.
  • The relay support.
Top view of the fully wired board.

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