A control transformer wiring diagram is the schematic that shows how a control transformer’s primary winding connects to the incoming line and how its secondary winding feeds the low-voltage circuit that powers contactors, relays, PLC inputs, and pilot lights. In plain terms, it is the wiring map you follow so the transformer steps line voltage down to a safe, isolated control voltage — usually 110 V, 220 V, or 24 V — without damaging equipment or tripping protection.
Reading this diagram correctly is the difference between a dependable control circuit and one that blows fuses or burns out coils. This guide explains the terminal markings, the symbols, and a practical wiring sequence you can apply to a machine tool control transformer.
A control transformer wiring diagram is a simplified electrical schematic, not a physical layout. It represents the transformer as two coils — the primary winding on the line side and the secondary winding on the control side — separated by an iron core, with the primary drawn to the supply and the secondary drawn to the control load, such as a contactor coil.
The diagram normally includes the following details:
Terminal lettering follows a simple convention that makes wiring far less error-prone. H stands for the higher-voltage (primary) side, and X stands for the lower-voltage (secondary) side. On many machine tool control transformers, the primary is split into two windings marked H1–H2 and H3–H4, and the secondary is split into X1–X2 and X3–X4.
Beyond the terminals, expect to see standard electrical symbols: two parallel lines for the magnetic core, circles or rectangles for the windings, a fuse symbol on the primary feed, and a ground symbol where the core or panel must be bonded to earth.
A control transformer is a step-down transformer. When line voltage is applied to the primary winding, it produces a changing magnetic flux in the iron core. That flux induces a proportionally lower voltage in the secondary winding, according to the turns ratio. A transformer built with roughly a 3.5:1 turns ratio, for example, converts a 380 V input into about 110 V at the secondary, while still isolating the control circuit from the supply.
Two properties make it ideal for machine tools and automation. First, it provides galvanic isolation, so a line-side fault does not pass into the sensitive control electronics. Second, it delivers a stable, lower voltage that keeps relay and contactor coils within their design range even when the supply fluctuates. The BKJ5 Machine Tool Control Transformer, for instance, is built with a silicon steel core and copper windings and is rated for a 220 V or 380 V input with a 24 V, 110 V, or 220 V output, depending on the selected taps.
Most wiring mistakes happen because someone rushes to connect terminals before confirming the voltages. Follow this sequence and verify each step with a multimeter.
Units like the BKJ5 use screw terminal connections and support vertical or horizontal mounting, which makes this sequence straightforward even inside a crowded panel.
The same transformer can often be wired several ways, which is exactly why the diagram matters. The table below summarizes the configurations you will most often encounter on a machine tool control transformer.
| Configuration | Primary connection | Secondary output | Typical use |
|---|---|---|---|
| Single-voltage primary | 220 V across H1–H2 | 110 V | General machine tool control circuits |
| Dual-primary, series | 380 V across series windings | 110 V or 220 V | Three-phase-fed control panels |
| Dual-primary, parallel | 220 V across parallel windings | 24 V | PLC and low-voltage automation supplies |
| Multi-tap secondary | 220 V or 380 V | 24 / 110 / 220 V selectable | Mixed loads on a single transformer |
The right control transformer depends on capacity, mounting style, and how quiet and efficient the circuit must be.
| Model | Rated capacity | Input / Output | Best suited for |
|---|---|---|---|
| BK Series | 50VA–2500VA | 220/380 V → 6–220 V | Automation systems, electrical cabinets, lighting |
| JBK3 Series | 50VA–2500VA | 220/380 V → 6–220 V | CNC machine tools and industrial automation |
| BKJ5 Series | 2500VA | 220/380 V → 24/110/220 V | Machine tools and precision equipment (featured) |
| R-type | Customizable | 220/380 V → customizable | Low-noise applications such as medical and audio |
For heavy machine tool loads, the BKJ5 is a common starting point because of its 2500VA capacity and its selectable 24/110/220 V output, which covers the full range of relay, contactor, and PLC control voltages in a single unit.
Even experienced electricians make these mistakes when they skip the diagram.
A control transformer wiring diagram is not just a formality — it is the reference that keeps your control voltage isolated, stable, and safe. Read the terminal markings, confirm the voltages, protect both windings, and test with a meter before you power up. For more detail on selecting a unit for a specific machine, you can browse the full catalog or explore the range of our company’s control transformer products to compare capacities and mounting styles. If you are unsure which tap or capacity your application needs, contact us for a tailored recommendation, or return to the home page to see the complete product range.
It is a schematic that shows how the primary and secondary windings of a control transformer connect to the supply and to the low-voltage control devices, along with fuses and grounding points.
H terminals are the higher-voltage primary connections and X terminals are the lower-voltage secondary connections. H1–H2 receives the line, and X1–X2 supplies the control circuit.
Connect the two primary windings in series for 380 V input, then take 110 V from the secondary terminals X1–X2, adding fuses on the primary side before energizing.
Yes. A primary fuse protects the transformer itself, while a secondary fuse or breaker protects the control wiring and the devices it feeds. Both are recommended in most panels.
Technically a transformer can be back-fed, but it is not recommended for control transformers because voltage regulation, inrush, and terminal ratings are designed for the labeled direction. Always wire primary to the line and secondary to the load.
Most PLCs run on 24 V DC or 110 V AC depending on the model. Check the PLC supply rating and match it to the transformer’s secondary tap, which is why selectable outputs such as 24/110/220 V are useful.
