From Bare PCBs To A Tested Control Cabinet — One Box-Build Job, Step By Step

Last month a control cabinet left our workshop on the way to a machine builder in Europe. It is not a giant panel - about the size of a shoebox - but it controls a production-line station, and the kind of box that lives or dies on how the inside is put together. This is how it went, from bare boards to a tested cabinet on a pallet.

It started with a schematic, a bill of materials and a rough mechanical drawing from the customer. Before a single component was ordered, our engineer sat down with the customer's team and went through the layout - where the power section should sit, how the signal wiring should run, what clearance the terminal blocks needed. That early review is where most box-build problems get killed cheaply, before any metal is cut.

The heart of the box is a set of custom PCBA control boards assembled in-house. For power, this build uses a BYQ 120T toroidal transformer (0–120 V input, 0–22 V output) feeding an EMC power filter - model PX3-10A-T(03), rated 115/250 VAC, 10 A, 50/60 Hz, working range −25 °C to +100 °C, CE/RoHS. The toroid keeps the stray magnetic field low, the filter keeps conducted noise off the supply line, and together they give the controller a clean, stable rail to run on.

Fig. 1 - Power section: toroidal transformer, EMC filter, relay and terminal block layout

Fig. 2 - Close-up of the BYQ 120T toroidal transformer and PX3-10A-T EMC filter

Everything is mounted on a metal backplate: the filter, the transformer, a DIN rail for the terminal blocks, and the boards. One detail worth pointing out - the relays are SONGLE SLC-12VDC-SL-C types rated 30 A (250 VAC / 30 VDC), comfortably over what the motor circuit actually pulls. Cable ducts keep the power wires separate from the signal wires, so noise does not travel along a shared path and a later technician can trace a wire without pulling the box apart.

Fig. 3 - Internal layout on the metal backplate, boards and DIN-rail terminals

The wiring is where the box either looks like a professional job or like a hobby project. Every wire here is cut to length, fitted with a number sleeve, and landed on a spring-cage terminal block, so no screw ever loosens on a vibrating machine. On the terminal rails the function of each point is marked - N1, N2, L2, PE-1A/2A, S1, S2, OV, SLV and a bank of ±VE returns - so a maintenance engineer on site can work from the drawing without guessing. Grounding is done properly: a dedicated PE return, and a yellow-green earth wire tied off on the cabinet door so the metal enclosure stays at chassis potential.

Fig. 4 - Terminal-block wiring with numbered sleeves and function markings

The door itself carries a small adapter board and the external communication port for this controller, so the end customer can plug in a network cable for remote monitoring and parameter setup without opening the box. Two locking clamps hold the door shut against dust.

Fig. 5 - Cabinet door: adapter board, earth bonding and locking clamps

Before it shipped, the cabinet went through a continuity and safety check - every terminal to its correct function, insulation, and a power-on test of the full circuit under load. That is standard here: no cabinet leaves without being powered up and checked against the wiring diagram first.

Fig. 6 - Finished cabinet, fully wired and ready for test

This is the part of box build most factories quietly dislike - the sourcing, the coordination, the rework when a drawing does not match a part. For our customers the value is simpler: one supplier handles the PCBA, the component sourcing, the mechanical assembly, the wiring and the final test, so both project lead time and vendor-management load come down. Prototype runs and full production use the same line, and ISO9001 / IATF16949 keep the process repeatable.

Table 1 - Key components in this build

If you build machines and would rather spend engineering time on the machine than on the control box, send us your schematic and BOM - happy to quote a box-build project from prototype to volume.