What Is a Transformer? How It Works and How to Choose One

A transformer transfers electrical energy between windings through a changing magnetic flux. Depending on its design, it can step voltage up or down, provide electrical isolation, or couple measurement and signal circuits. A conventional transformer does not convert AC to DC or change the supply frequency. A 230/24 V transformer supplied with 230 V at 50 Hz therefore produces approximately 24 V AC at 50 Hz.

Matching the output voltage is only the starting point. VA rating, frequency, insulation, starting loads and operating conditions all matter. A contactor coil and a PLC may both carry a 24 V marking while requiring quite different supplies.

What do the primary, secondary and core do?

The primary winding receives power from the source; the secondary supplies the load. Applied primary voltage establishes a changing magnetic flux, which induces voltage in the secondary. Connecting a load allows secondary current to flow, and the primary draws additional current to supply that load. Even with the secondary disconnected, the primary draws magnetizing current and current associated with losses. See primary and secondary windings for a closer look.

The core helps link the magnetic flux to the windings. Power-frequency transformers commonly use laminated electrical steel, while high-frequency power circuits often use ferrite. Thin, insulated laminations reduce eddy-current losses. Winding insulation, the bobbin, terminals and cooling arrangement are also essential parts of the design.

How does the turns ratio affect voltage and current?

For an ideal two-winding transformer, V₁ / V₂ = N₁ / N₂ and I₁ / I₂ = N₂ / N₁, where V is voltage, I is current and N is the number of turns. For a given transferred power, stepping voltage up reduces the available current; stepping it down increases the available current. A transformer does not create power. In a real unit, winding and core losses make output real power lower than input real power.

Example: Reducing 230 V to approximately 24 V requires an ideal turns ratio of 230 / 24 ≈ 9.58:1. A real transformer's unloaded output may exceed its stated full-load voltage. As load increases, winding resistance and leakage reactance cause a voltage drop. The amount depends on the design and the load power factor.

Why are transformers rated in VA rather than watts?

For a single-phase transformer, apparent power is S = V × I, using RMS voltage and current. For a balanced three-phase system, using line values, S = √3 × V_line × I_line. Real power is P = S × PF. PF is the power factor; for nonlinear loads, it cannot always be represented by cos φ alone. See the difference between VA and watts.

A 100 VA transformer with a single 24 V secondary has a rated secondary current of approximately 100 / 24 = 4.17 A AC under its specified operating conditions. It does not continuously consume 100 W or force 4.17 A through every connected load. The load determines the current drawn; exceeding the rating can cause overheating.

Sizing example: A 24 V AC load drawing 3 A continuously requires 72 VA. A 100 VA transformer is a candidate for that steady load, but selection must also account for coil pickup current, simultaneous starting loads, ambient temperature and acceptable voltage drop. A single percentage margin is not a reliable sizing rule for every application.

Why a 24 V AC transformer is not a 24 V DC supply

Producing DC requires a rectifier, filtering and, where needed, regulation. The sinusoidal peak of 24 V RMS is 24 × √2 ≈ 33.9 V. With a bridge rectifier and reservoir capacitor, the unloaded DC voltage can approach that peak minus diode drops. Transformer regulation and supply tolerance can raise it further. A 24 V transformer and bridge rectifier therefore do not automatically produce 24 V DC.

A capacitor-input rectifier draws pulses of winding current, so the AC current rating cannot simply be treated as an equal continuous DC output rating. Supply sizing must account for the rectifier and filter arrangement. Before powering a controller, check its required AC or DC input and permitted voltage range.

Transformer types and what they are used for

Power and distribution transformers: Power transformers serve generation, transmission and major industrial systems. Distribution transformers deliver power through distribution networks toward end users. Their distinction is not defined solely by physical size or one universal power threshold; system role and design requirements also matter.

Step-up and step-down transformers: These terms describe the direction of voltage transformation. A unit can also be classified as a distribution, isolation or other application-specific transformer. Reverse feeding should not be assumed acceptable just because the turns ratio appears suitable; the manufacturer's connection and application limits must permit it.

Autotransformers: An autotransformer shares part of its winding between input and output. It can offer size and material savings, particularly when the two voltages are relatively close. However, it provides no galvanic isolation between input and output. The name does not mean that the device automatically regulates its output.

Isolation and control transformers: An isolation transformer uses an appropriate insulation system to separate circuits electrically; its ratio need not be 1:1. A control transformer is selected for loads such as relays and contactor coils, including their pickup demands. Medical applications and safety extra-low-voltage supplies require designs suitable for those duties; an ordinary transformer should not be assumed to qualify.

Instrument transformers: Current transformers (CTs) and voltage transformers (VTs or PTs) supply measurement and protection circuits. Ratio, accuracy class and connected burden matter as well as electrical ratings. Never leave the secondary of a conventional 1 A or 5 A CT open while primary current is flowing. Hazardous voltage can develop. By contrast, shorting a power transformer's secondary creates a fault. The connection rules for these devices are different.

High-frequency and pulse transformers: Switching circuits use cores and windings designed for their operating frequency. Pulse transformers can transfer pulses, drive switching devices or provide isolation. A transformer designed for a particular frequency range does not itself convert frequency; the associated switching or conversion circuit establishes it.

Audio, output and impedance-matching transformers: Audio transformers couple signals within an audio band. Output transformers connect the output stage of certain amplifiers to their load. In impedance matching, the ideal reflected impedance follows the square of the turns ratio: Z₁ = (N₁ / N₂)² × Z₂. Bandwidth, distortion and tolerance of DC bias depend on the application.

EI, toroidal, dry-type and oil-filled: different ways to describe a transformer

EI and toroidal describe core construction, while dry-type and oil-filled describe insulation and cooling arrangements. These are separate from the unit's application. A well-designed toroidal transformer can be compact and have low stray magnetic fields, but energization inrush may be a significant selection factor. A small ferrite transformer is not a replacement for a 50 Hz mains transformer simply because it fits the available space.

Can a transformer run on DC or convert 50 Hz to 60 Hz?

Do not apply continuous DC to a conventional transformer winding. After the initial transient, steady DC does not create the changing flux needed for sustained secondary voltage. Saturation, excessive primary current and winding damage can result. In a DC-fed switch-mode supply, electronics switch the DC to establish changing flux. That is different from connecting a transformer directly to a steady DC source. See transformer-based adapters and switch-mode supplies.

A transformer cannot turn 50 Hz into 60 Hz or vice versa. A separate issue is whether it is rated to operate at the available frequency. Using a 60 Hz-only unit at the same voltage on 50 Hz increases core flux and may be unsuitable. Check the voltage-to-frequency ratio and manufacturer limits. The output voltage alone does not establish 50 Hz and 60 Hz compatibility.

What to check on the nameplate and connection diagram

Input and connections: Check primary voltage, frequency, phase count and terminal arrangement. Dual-primary 115/230 V models and tapped primaries must be connected according to their own diagram. Wire colours or resistance readings alone are not a substitute for that diagram.

Output and duty: Check secondary voltage, current, total VA, duty cycle and the load's AC/DC requirement. “2 × 12 V” may indicate two separate windings; the actual arrangement must be confirmed from the diagram. “12-0-12 V” identifies a centre-tapped output. Series or parallel operation requires compatible windings, correct phasing and manufacturer approval. Incorrect paralleling can create a short circuit.

Environment and protection: Check ambient temperature limits, temperature rise, insulation class, ventilation, mounting and enclosure requirements. The transformer can draw a brief magnetizing inrush when energized, and the load may have its own starting current. Fuse selection therefore requires manufacturer guidance and protection coordination, not just a VA/voltage calculation. Repeated fuse operation is a fault to investigate, not a reason to fit a larger fuse.

Transformer, voltage regulator or power supply?

A transformer works through magnetic coupling and a winding ratio; its output normally changes when its input changes. A voltage regulator maintains an output within specified limits. A DC power supply combines rectification, filtering and usually regulation, and may contain a transformer. These terms describe different functions.

Why does a transformer get hot or hum?

Windings produce I²R losses, while the core has hysteresis and eddy-current losses. High load current, restricted airflow, the wrong input tap, an unsuitable frequency or a winding fault can increase temperature. Whether a transformer feels hot is not a reliable pass/fail test; use its specified temperature limits and suitable measurements.

Small dimensional changes in the magnetized core, together with mechanical vibration, can produce audible hum. A 100 Hz component may be present on a sinusoidal 50 Hz supply. However, a sudden increase in noise, a burning smell, discolouration, repeated protective-device operation or severe voltage collapse under load warrants investigation. See why transformers hum.

Continuity does not prove that a transformer is healthy. A multimeter may identify an open winding, but it may miss a short between a few turns or insulation that fails at operating voltage. Checks on a de-energized circuit with stored energy safely removed serve a different purpose from professional insulation, ratio and load testing. Never bypass a thermal protector or improvise measurements on exposed mains connections.

Common questions

Will a higher VA rating damage the load? At the correct output voltage and frequency, extra VA capacity does not force extra current into the load. However, fault current, fuse coordination, energization inrush and unloaded voltage may change. VA capacity alone is not enough to approve a replacement.

Does an isolation transformer replace an RCD? No. Protective measures depend on the circuit's earthing and separation arrangement. An RCD on the primary must not be assumed to detect every secondary-side fault.

Does an unloaded transformer use electricity? Yes. Core losses and primary winding losses consume real energy even with no load connected. The VA nameplate rating is not its no-load power consumption.