24-Channel Relay Board for a Diesel Controller

Overview

This relay board is the companion of the diesel controller that is described in another project of this set. The brain of the system is the mainboard, which has 24 outputs; but these outputs do not switch the load by themselves, and relays must connect them to the circuits of the panel. The relay board does that: 24 relays on one board, each driven by one output of the controller, with the three ends of every relay (COM, NC and NO) brought to a screw terminal so it can be wired straight to the panel.

I had the original relay board in my hands, but there was no schematic and no PCB file for it. So I reverse-engineered it and rebuilt the schematic and the PCB file myself, then built the board (2021). My board has the same layout as the original one, and the same board can be used in both the MV panel and the diesel panel.

What the board does

• 24 relays (K01 to K24) with 6 V coils, each with one changeover contact. That is why the board has 72 terminals.

• The control signals come from the mainboard over a ribbon cable. Each line activates one relay.

• The coil supply is 6 V and comes from the battery. The board has its own supply input.

• The terminals sit in several rows along the edges of the board and each has its own number (01 to 66 and one six-pin block), so wiring the panel and tracing the channels is easy.

The diagram below shows the signal path.

Reverse engineering

The relay board has no complicated electronics, but its PCB is full of current-carrying tracks. My work was:

• I scanned both sides of the board and made clean black-and-white images of the top and bottom copper from the scans (the copper images are in the gallery).

• I worked out the circuit from these tracks and the parts, and drew its schematic myself.

• I rebuilt the PCB in Altium Designer, keeping the same placement of relays and terminals, so the board fits in the panel in place of the old one and its connectors match the mainboard.

• The board I built can be used in the MV panel and in the diesel panel.

PCB

The board has two layers. The relays sit in the middle in two columns and several rows, and the screw terminals take the edges of the board. The power tracks (coil and contacts) are wider and the control signals are thin. On the top layer the tracks run from the terminals to the relays and the bottom layer covers the rest; the two copper images in the gallery show both. The design is RoHS compliant. I blurred logos and company names in the images (the board scan and the schematic).

Specifications

• 24 relays with 6 V coils

• 72 screw terminals (COM, NC and NO for each relay)

• Control input: ribbon cable from the mainboard

• Two-layer PCB, designed in Altium Designer

• Usable in the MV panel and in the diesel panel

Result

A 24-channel relay board that works next to the diesel controller mainboard and can be used in the MV panel as well, with a schematic and PCB that I built from scratch. Only the pictures and a general description are shown on this site.