Transformers

Definition: A transformer is a device that transfers electrical energy between circuits by electromagnetic induction, typically changing voltage level in the process.

How It Works

Two or more coils of wire (windings) share a common magnetic core, usually laminated iron, without any direct electrical connection between them
A changing AC current in the primary winding creates a changing magnetic flux in the core, which induces a voltage in the secondary winding (Faraday’s Law of induction)
The voltage ratio between windings matches their turns ratio, so more turns on the secondary steps voltage up, and fewer turns steps it down
Mutual inductance between the primary and secondary windings governs how efficiently energy couples from one coil to the other
A transformer’s coupling coefficient describes how much of the primary’s magnetic flux actually reaches the secondary, with tightly wound cores approaching a value of 1 (near-perfect coupling)
Toroidal (donut-shaped) cores are popular in compact designs because their closed-loop geometry minimizes stray flux leakage compared to older E-I laminated cores
High-frequency transformers used in switch-mode power supplies use ferrite cores instead of laminated steel, since ferrite handles the much higher switching frequencies with far lower core losses
Insulation class ratings determine the maximum operating temperature a transformer’s winding insulation can safely tolerate before degrading
Power (minus small losses) stays roughly constant across an ideal transformer, so current changes inversely to voltage: step voltage up, and current steps down proportionally, and vice versa
Because the coupling is magnetic, not a direct wire connection, transformers also provide electrical isolation between primary and secondary circuits
Transformers only work with AC (or changing DC); a steady DC current produces no changing flux, so it induces nothing in the secondary
The core material (typically laminated silicon steel, or ferrite for high-frequency designs) is chosen to maximize magnetic coupling while minimizing eddy current and hysteresis losses
Laminating the core into thin insulated sheets specifically blocks large-scale eddy currents from circulating within the core itself, cutting one of the two major loss mechanisms

Types of Transformers

Step-up transformer — increases voltage, decreases current; used at power plants to prepare for transmission
Step-down transformer — decreases voltage, increases current; used near end users for safe delivery
Isolation transformer — 1:1 ratio, breaks a direct electrical path for safety without changing voltage
Autotransformer — a single tapped winding shared between primary and secondary, compact but with less isolation
Current transformer (CT) — used to measure large currents safely by scaling them down for a meter
Power transformer vs. distribution transformer — power transformers handle bulk transmission-level voltages; distribution transformers step down to end-user levels near homes and businesses

Illustration

Under the Hood

Turns ratio and voltage relationship:

Vs / Vp = Ns / Np

Current relationship (ideal transformer, power conserved):

Is / Ip = Np / Ns

Ideal power balance:

Vp × Ip = Vs × Is

Real-world efficiency, accounting for core and copper losses:

η = P_out / P_in × 100%

Worked Problem 1: step-down transformer voltage Given: a transformer has 1000 turns on the primary and 100 turns on the secondary, primary voltage is 240 V. Step 1: Vs = Vp × (Ns / Np) = 240 V × (100/1000). Answer: Vs = 24 V, a 10:1 step-down ratio matching the 10:1 turns ratio.

Worked Problem 2: current in a step-down transformer Given: using the transformer from Problem 1, the secondary supplies a load drawing 5 A at 24 V. Step 1: Ideal power balance: Vp × Ip = Vs × Is. Step 2: 240 V × Ip = 24 V × 5 A = 120 W. Step 3: Ip = 120 W / 240 V. Answer: Ip = 0.5 A on the primary side, ten times smaller than the secondary current, consistent with the inverse turns ratio.

Worked Problem 3: efficiency and power loss Given: a transformer has 500 W input and delivers 470 W output. Step 1: η = P_out / P_in × 100% = 470 / 500 × 100%. Step 2: η = 94%. Step 3: Power lost = 500 W − 470 W = 30 W, dissipated as heat from core losses (hysteresis, eddy currents) and copper losses (I²R in windings). Answer: the transformer is 94% efficient, losing 30 W as heat.

Worked Problem 4: sizing a transformer’s VA rating Given: a design needs a secondary that supplies 15 V at up to 4 A. Step 1: VA rating needed = Vs × Is = 15 V × 4 A. Step 2: VA = 60 VA. Answer: a transformer rated for at least 60 VA (commonly rounded up to a standard 75 VA or 100 VA part for margin) should be selected to safely supply this load without overheating.

Why It Matters

Transformers let power grids transmit electricity at very high voltage and low current to cut resistive losses (which scale with I²R), then step voltage back down to safe levels near homes and devices
Without transformers, long-distance power transmission would waste enormous amounts of energy as heat in the wires
Isolation transformers protect equipment and people by breaking a direct electrical path between circuits while still transferring power
Impedance matching transformers are used in audio and RF circuits to maximize power transfer between mismatched source and load impedances
Current transformers let utilities and electricians measure very large currents safely, without breaking the main circuit or exposing meters to dangerous voltages

Common Pitfalls

Assuming a transformer works with DC; a steady DC current produces no changing flux and induces no secondary voltage, so transformers only function with AC (or switching DC as in switch-mode supplies)
Ignoring that a transformer’s power rating (VA) sets the maximum safe current, not just voltage, and that a step-up in voltage means less available current, not more
Forgetting core saturation: driving a transformer’s core into saturation (from excessive voltage or DC offset) distorts the output and can rapidly overheat the windings
Miswiring center-tapped or multi-winding transformers, resulting in unexpected voltages or shorted turns
Neglecting that real transformers have losses (copper and core losses) so output power is always somewhat less than input power
Touching a supposedly “isolated” secondary while forgetting an isolation transformer only isolates from the primary’s ground reference, it doesn’t guarantee the secondary is safe to touch
Leaving a current transformer’s secondary open-circuited while current still flows in the primary, which can generate dangerously high voltage spikes across the open secondary
Underestimating inrush current when a transformer is first energized, which can briefly spike several times higher than steady-state current and trip protective breakers

Comparison

Type	Turns Ratio	Voltage Change	Typical Use
Step-up	Ns > Np	Increases	Power transmission from generating stations
Step-down	Ns < Np	Decreases	Distribution to homes, device power adapters
Isolation	Ns = Np	Unchanged	Safety isolation, medical equipment, test benches
Autotransformer	Single tapped winding	Either	Compact voltage adjustment, lower isolation
Current transformer	N/A (current scaling)	N/A	Safely measuring high currents for metering/protection

Quick Reference: Loss Mechanisms

Loss Type	Cause	Mitigation
Copper loss (I²R)	Resistance in the windings	Thicker wire, fewer turns where possible
Hysteresis loss	Energy lost magnetizing/demagnetizing the core each cycle	Core material with a narrow hysteresis loop
Eddy current loss	Circulating currents induced within the core itself	Laminated core construction
Flux leakage	Magnetic flux not fully coupling both windings	Tighter winding geometry, better core design

History

Michael Faraday demonstrated electromagnetic induction in 1831, laying the physics foundation for transformer operation
Practical transformer designs emerged in the 1880s, with William Stanley Jr. building a commercially usable transformer for Westinghouse around 1885-1886
Transformers were central to the “War of the Currents” in the late 1880s and early 1890s, where AC power (championed by Westinghouse and Tesla), which transformers could efficiently step up and down, won out over Edison’s DC distribution for long-distance power delivery
The ability to transform voltage is precisely why AC became the standard for power grids worldwide, since practical DC-to-DC voltage conversion at scale didn’t exist until much later with power electronics
Nikola Tesla’s polyphase AC system design work in the late 1880s complemented transformer technology, together forming the basis of modern power distribution

Example

A utility substation transformer steps 138 kV transmission voltage down to around 12 kV for local distribution, and a small transformer inside a wall adapter steps that down further to charge electronics at 5-24V. Audio equipment sometimes uses small impedance-matching transformers to couple a low-impedance amplifier output to a higher-impedance speaker line efficiently.

FAQ

Why don’t transformers work with DC? Induction requires a changing magnetic flux; a steady DC current creates a constant flux with no rate of change, so Faraday’s Law predicts (and observation confirms) zero induced voltage on the secondary.

What does “VA rating” mean on a transformer? It’s the transformer’s apparent power rating (volts times amps), which accounts for reactive loads better than a simple watts rating, and sets the safe maximum current at a given voltage.

Can a transformer step voltage up and down at the same time? Yes, transformers with multiple secondary windings, each with a different turns ratio, can provide several different voltages from a single primary simultaneously.

Why do transformers hum? Magnetostriction, tiny physical expansion and contraction of the core material as the magnetic field alternates, vibrates at twice the line frequency (100/120 Hz), producing an audible hum.

Why are power transformers often filled with oil? Mineral oil in large transformers serves as both an insulator between windings and a coolant, carrying heat away from the core and windings to external radiators far more effectively than air alone.

Can a transformer’s turns ratio be changed after it’s built? Some transformers include tap changers, selectable connection points on a winding that adjust the effective turns ratio in discrete steps, used to compensate for voltage variation on a grid without needing a separate device.

Referenced by

Dig deeper

Spot an error in this note?