Transformers
A transformer transfers electrical energy from one circuit to another using magnetic coupling — without any direct electrical connection between the two. Think of it like a magnetic gearbox: you can trade voltage for current or current for voltage, keeping power roughly constant. This ability to change voltage levels safely and efficiently makes transformers essential in power supplies, audio equipment, chargers and measuring instruments.
- How a transformer works: mutual inductance
- The turns ratio
- Voltage and current transformation
- Impedance transformation
- Transformer efficiency and losses
- Types of transformers in practice
- Isolation, audio and current transformers
How a Transformer Works: Mutual Inductance
A basic transformer consists of two coils wound on a shared core. When alternating current flows through the first coil (the primary), it creates a constantly changing magnetic field in the core. That changing field passes through the second coil (the secondary) and, by Faraday's law of electromagnetic induction, induces a voltage across it. No electrical connection between the two circuits is needed — the energy transfer is entirely magnetic.
This only works with AC or changing currents. A steady DC through the primary creates a static magnetic field — no change, no induction, no output from the secondary. Transformers are fundamentally AC devices.
The primary winding drives alternating flux through the shared core. The secondary winding intercepts that flux and has a voltage induced across it proportional to its number of turns.
View LargerThe Turns Ratio
The turns ratio (n) is the ratio of the number of turns on the primary winding (Np) to the number of turns on the secondary winding (Ns). This single number determines everything: the voltage ratio, the current ratio, and the impedance ratio.
n = Np / Ns
- n > 1: step-down transformer (secondary voltage lower than primary)
- n < 1: step-up transformer (secondary voltage higher than primary)
- n = 1: isolation transformer (same voltage, but circuits isolated)
Turns Ratio Calculator
Enter the number of primary and secondary turns to find the turns ratio, or enter the turns ratio directly.
Voltage and Current Transformation
The secondary voltage is proportional to the turns ratio. If you have twice as many turns on the secondary as the primary, you get twice the voltage. Current behaves inversely: double the voltage means half the current, because power is conserved (in an ideal transformer).
- Vs = Vp × (Ns / Np)
- Is = Ip × (Np / Ns)
- Power in = Power out: Vp × Ip = Vs × Is (ideal)
Secondary Voltage and Current Calculator
Enter primary voltage, primary current, and turns ratio (Np/Ns) to find secondary values.
Impedance Transformation
One of the most useful jobs a transformer can do is transform impedances. A valve (tube) guitar amplifier's output stage needs to see a load of several thousand ohms, but a loudspeaker is only 8 Ω. Connected directly, the speaker would deliver almost no power. A transformer with the right turns ratio bridges the gap. The impedance ratio changes as the square of the turns ratio:
Zp / Zs = (Np / Ns)² = n²
- A 4:1 impedance transformer has a turns ratio of 2:1
- A 9:1 impedance transformer has a turns ratio of 3:1
- A 625:1 impedance transformer has a turns ratio of 25:1
Required impedance ratio: 5,000 / 8 = 625:1
Required turns ratio: √625 = 25:1
An output transformer with 25 primary turns for every secondary turn makes the 8 Ω speaker look like 5,000 Ω to the valves.
Transformer Efficiency and Losses
Real transformers are not perfectly efficient. Energy is lost through:
- Copper losses (I²R): Heat generated in the resistance of the winding wire. Heavier gauge wire reduces this.
- Core losses (eddy currents and hysteresis): Circulating currents induced in the core material produce heat. Laminated iron cores and ferrite materials reduce this.
- Flux leakage: Not all the magnetic flux links both windings, especially in loosely coupled designs.
Good power transformers achieve 95–99% efficiency. Ferrite-cored transformers in switching power supplies also achieve high efficiency at their much higher operating frequencies. Efficiency matters most in high-power applications, where every lost watt becomes heat that has to be removed.
Types of Transformers in Practice
| Type | Purpose | Where found |
|---|---|---|
| Mains power transformer | Step 120/230 V AC down (or up) to working voltages; isolation from the mains | Linear power supplies, battery chargers, audio amplifiers |
| Switching supply transformer | Small ferrite-cored transformer working at tens to hundreds of kHz | Phone chargers, laptop adapters, LED drivers |
| Audio transformer | Impedance matching between stages; isolation; balanced/unbalanced conversion | Microphone inputs, valve amplifier outputs, DI boxes |
| Isolation transformer | 1:1 transformer that separates equipment from the mains earth | Safe servicing of mains equipment on the bench |
| Current transformer | Produces a small current proportional to the current in a conductor passing through it | Clamp meters, energy monitors, overload protection |
| Pulse / signal transformer | Passes fast pulses or data while blocking DC and isolating grounds | Ethernet ports, gate drivers, isolated interfaces |
Isolation, Audio and Current Transformers
An isolation transformer has a 1:1 ratio, so it does not change the voltage. What it does is break the connection between the equipment and the mains earth. With the equipment powered through an isolation transformer, touching one point of the circuit and earth no longer completes a path for current. This makes servicing mains-powered equipment much safer — but it is not a licence to be careless, because touching two points of the circuit at once is still dangerous.
An audio transformer matches impedances and isolates grounds in audio systems. A DI (direct injection) box uses one to turn a guitar's high-impedance, unbalanced output into a low-impedance balanced signal for a long cable to the mixing desk, and to break ground loops that would otherwise cause hum.
A current transformer uses the conductor carrying the load current as a one-turn primary. A secondary of many turns produces a small current in proportion. Clamp meters and home energy monitors use this principle to measure large currents without breaking the circuit.
- Step-down ratio: a current transformer with 1,000 secondary turns produces 1 mA of secondary current for every 1 A in the conductor.
- Never open-circuit a current transformer in use: without a load resistor on the secondary, it can develop a dangerously high voltage.
- Ground-loop isolators: small 1:1 audio transformers in car audio and PC sound setups break the ground loop that causes engine whine and computer noise.
Frequently Asked Questions
Why does a transformer only work with AC, not DC?
Electromagnetic induction only occurs when the magnetic field is changing. A steady DC current through the primary creates a constant magnetic field — no change means no induction in the secondary and no output voltage. Alternating current continuously changes direction and magnitude, so the magnetic field is always changing, continuously inducing voltage in the secondary. Some DC transformer equivalents (such as inverter-based SMPS) rapidly switch DC on and off to create a changing field.
Why do some audio transformers cure hum?
Hum often comes from a ground loop: two pieces of equipment connected to each other by a signal cable and also to the mains earth at different points. A small voltage difference between the earths drives current around the loop, and some of it ends up in the audio. A transformer passes the audio signal by magnetic coupling only, with no direct connection between its windings, so it breaks the loop while letting the audio through.
How does impedance transformation relate to maximum power transfer?
Maximum power is transferred when the source impedance equals the load impedance. A transformer allows you to match mismatched impedances — effectively making the load appear to be the correct impedance from the source's perspective. In audio work, an amplifier delivers its rated power only into the load it was designed for, so an output transformer is used to make the loudspeaker present that load. In high-speed and high-frequency circuits, the same idea keeps cables and inputs matched so that signals are not reflected.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.