Relay Control Box for a Truck

Project Info


The client requested a design for controlling five 40Amp relays for a heavy truck. The system was to be used for connection of 5 high power consuming loads. Some other requirements set by the client included the followings:
– The Relays were to be controllable from a distance of 3m so that the user could easily change them.
– There were supposed to be an indicator for each relay.
– The size was to be compatible with the given requirement.


The main challenge with the design was with the high power accessories that were to be connected to the board. Each connection was requested to endure 40 Amps and the overall board feeding for all 5 accessories was to be capable of taking 5 x 40 = 200 Amps.
While the board was to withstand a high current consumption when all accessories were switched on, the final PCB design was to be sufficiently small to be located inside the dashboard or the truck’s front space.


The design took about one week to be ready for the delivery to the client for the prototyping.  The block diagram was made and after considering all requirements of the client, all the details including the connections and shape were checked with him. The final block diagram was prepared as the following.

Also, the model was chosen by the client for the final product.

In order to overcome the main challenge of this board, i.e., the high current of the accessories, the width of the copper traces and their thickness were calculated. Based on the result, the thickness of the traces was chosen to be 2cm. Also, a two-layered PCB was considered with two connected face to face traces of 1Oz copper both carrying the current for each of the accessories.

The final size of the board was made 14cm x 18cm which was compatible with the size constraint.
For connections of control wires to the switchboard which had 5 switches, an RJ45 socket was used as the connector to easily connect the PCB to the main control switchboard.



The final design was prototyped and checked by the client and he was fully satisfied with the design as it met all his requirements.

T. Cohen
May 28, 2017
US, Los Angeles
Electrical and Electronic Engineering