Power Transformers

A power transformer is a device that transfers electrical energy from one circuit to another through magnetic induction, with the ability to simultaneously change the voltage and current levels in the process. It sits at the very front of every linear power supply, taking the 120 V AC from your wall outlet and converting it to whatever AC voltage the rest of the supply needs. Transformers also appear throughout radio equipment in roles that have nothing to do with power supplies — audio coupling, impedance matching, RF isolation, balun construction — making a solid understanding of transformer behavior one of the most broadly useful things you can know in electronics.

Electromagnetic Induction: Why Transformers Work

To understand a transformer you first need to understand the relationship between electricity and magnetism discovered by Michael Faraday in 1831. Faraday's law states that a changing magnetic field induces a voltage in any conductor that the field passes through. The key word is changing — a static magnetic field induces nothing; it is only a field that is growing or shrinking that creates a voltage.

A transformer exploits this relationship directly. The primary winding is a coil of wire connected to an alternating voltage source. Because the voltage alternates — it increases, peaks, decreases, passes through zero, reverses, and repeats 60 times every second — the current flowing in the primary also alternates, and that alternating current creates a magnetic field that alternates in exactly the same pattern. That alternating magnetic field passes through the iron core and through the secondary winding wound around the same core. Because the magnetic field is continuously changing, it continuously induces a voltage in the secondary winding.

The iron core serves two critical functions. First, it concentrates and guides the magnetic flux so that almost all of it passes through the secondary winding rather than dispersing into the surrounding air. Second, iron has a much higher magnetic permeability than air, meaning it can carry far more flux for a given amount of magnetizing current. Without the core, a transformer would need thousands of turns and would still be very inefficient. With a good silicon steel core, a transformer can be over 98 % efficient even at modest sizes.

A power transformer: the primary winding creates an alternating magnetic flux in the iron core; the secondary winding picks up that flux and has a voltage induced across it. The ratio of turns determines the ratio of voltages.

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One important consequence of Faraday's law is that transformers work only with alternating current. If you connect a DC source to the primary, the current flows but does not alternate, the magnetic field is constant rather than changing, and no voltage is induced in the secondary. In fact, connecting DC to a transformer primary is dangerous — without the back-EMF that alternating operation provides, the primary draws far more current than it was designed for and will overheat rapidly.

The Turns Ratio and Voltage Transformation

The voltage induced in any coil wound on the core is directly proportional to the number of turns that coil has. If 100 turns of the primary winding produce a flux that induces 1 V per turn in any winding on the core, then a 200-turn secondary will have 200 V induced in it, and a 50-turn secondary will have 50 V. This is the turns ratio principle, expressed as:

Rearranging gives us the secondary voltage directly:

The ratio N_s/N_p is called the turns ratio. A transformer with a turns ratio greater than 1 is a step-up transformer — the secondary voltage is higher than the primary. A turns ratio less than 1 is a step-down transformer — the secondary voltage is lower. A turns ratio of exactly 1 is an isolation transformer, which produces the same voltage on the secondary but provides complete electrical isolation between the two circuits.

The turns ratio governs both voltage and current transformation. More turns means more voltage; fewer turns means less voltage but more current capability.

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Current Transformation and Power Conservation

If a transformer increases voltage, what happens to current? The answer comes directly from the law of conservation of energy: in an ideal transformer with no losses, the power input must equal the power output. Since power equals voltage times current (P = V × I), if the secondary voltage is higher than the primary voltage, the secondary current must be proportionally lower, and vice versa.

Stated another way: a transformer is a power-conserving device. What goes in as VA (volt-amperes of apparent power) comes out as VA. It cannot create energy. It can only transform the voltage/current relationship at constant power.

Real transformers are not perfectly ideal. The difference between ideal and real behavior manifests as heat — the transformer's core and winding losses appear as power dissipated as warmth in the transformer body. Good quality power transformers for linear power supplies typically achieve 90–98 % efficiency at full load, meaning 2–10 % of the input power is wasted as heat.

VA Rating and Power Dissipation

Transformers are rated in VA (volt-amperes) rather than watts, because their heating depends on the product of voltage and current regardless of whether the load is resistive (in-phase), capacitive, or inductive. A 50 VA transformer can supply any combination of voltage and current whose product does not exceed 50 VA — for example, 25 V at 2 A, or 12.6 V at 3.97 A, or 6.3 V at 7.9 A.

When selecting a transformer for a power supply, you must account for the fact that a rectifier with a capacitor filter draws current in narrow pulses rather than as a smooth sinewave. The peak current in each pulse is much higher than the average DC current. As a rule of thumb, you should use a transformer rated for at least 1.5 to 2 times the average DC power the supply will deliver. A supply delivering 13.8 V at 10 A (138 W) needs a transformer rated for at least 200–280 VA.

Overloading a transformer causes it to run hotter than its design temperature. Heat degrades the insulation on the windings, ultimately causing a short circuit. A correctly rated transformer should run warm (40–60 °C above ambient) but never hot enough to be uncomfortable to touch for more than a moment.

Core Types and Frequency Response

The core material has a profound effect on transformer performance. Three core types are commonly encountered in amateur radio equipment:

Laminated Silicon Steel (E-I Core)

The most common type for power supply transformers operating at 60 Hz. The core is built up from thin stampings of silicon steel (typically 0.014 to 0.025 inches thick) that are interleaved to form the familiar E and I shapes. The laminations are coated with an insulating varnish or oxide layer that prevents eddy currents — circulating currents induced in the core itself by the changing magnetic field — from flowing through the full cross-section of the core. Without laminations, eddy currents would cause enormous heating losses in a solid iron core.

Silicon steel cores work well from about 25 Hz to 400 Hz but become very lossy at higher frequencies. They are heavy and bulky but are inexpensive and highly reliable. Most linear ham shack power supplies use laminated steel core transformers.

Toroidal Core

A toroidal transformer uses a donut-shaped core (a torus) wound continuously with wire. Because the core forms a closed magnetic loop, flux leakage to the outside is extremely low — typically 10 times lower than an equivalent E-I transformer. This makes toroidal transformers much easier to mount near sensitive circuits without causing hum interference. They are also smaller and lighter for a given VA rating, and they hum less acoustically because the core vibration is more symmetric.

The downsides are higher cost and the fact that they can saturate severely if the power is applied at the peak of the AC cycle (in-rush current). Quality toroidal supplies include a soft-start circuit to ramp up the voltage slowly. Many commercial ham transceivers and quality station power supplies use toroidal transformers.

Ferrite Core

Ferrite is a ceramic magnetic material with very low eddy current losses even at high frequencies. Ferrite cores are used in switching power supplies (which operate at 20 kHz to several hundred kHz), in RF transformers, in baluns, and in common-mode chokes. Ferrite cores cannot support the large flux required for 60 Hz power supply transformers in any reasonable size, but at switching frequencies they allow tiny, lightweight transformers to handle hundreds of watts.

When you open a switching power supply and see a small black rectangular transformer about the size of a matchbox, you are looking at a ferrite-core transformer operating at tens or hundreds of kilohertz.

Three transformer core types: laminated E-I steel (left) for 60 Hz linear supplies, toroidal silicon steel (centre) for low-hum and low-leakage applications, and ferrite (right) for high-frequency switching supplies.

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Practical Transformer Specifications

When you read a transformer datasheet or catalog listing, several specifications appear beyond just VA and secondary voltage:

Voltage Regulation

Transformer regulation describes how much the secondary voltage drops when the transformer is loaded from no-load to full load. It is expressed as a percentage:

A typical E-I transformer has regulation of 5–15 %. This means if the secondary is rated "15 V" (measured at full load), the no-load voltage might be 16.5–17.3 V. This matters when calculating filter capacitor peak voltages — you must use the no-load voltage, not the rated voltage, when calculating peak rectified voltage.

Toroidal transformers generally have better regulation (3–6 %) because their tighter coupling produces lower leakage inductance.

Frequency Range

Power transformers for 60 Hz operation are specified at 60 Hz. Do not operate a 60 Hz transformer at 50 Hz without derated loading — the core flux will be about 20 % higher at 50 Hz for the same applied voltage, potentially causing saturation and excessive heating. This is relevant if you travel with equipment internationally; most of the world runs at 50 Hz.

Insulation Class and Temperature Rating

Transformers are rated by temperature class: Class A (105 °C), Class B (130 °C), Class F (155 °C), Class H (180 °C). A Class B transformer can operate with its windings reaching 130 °C without degrading its 20-year insulation life. The ambient temperature plus the temperature rise must stay below the class rating.

Center-Tapped Transformers

A center-tapped transformer has a secondary winding with a wire brought out from its exact midpoint. This gives two equal half-secondaries, each producing exactly half the total secondary voltage. The center tap is connected to the DC return (ground) in the power supply, and each half-secondary feeds one of the two diodes in a full-wave center-tap rectifier circuit.

For example, a transformer labeled "30 V CT" (center-tap) produces 30 V total across the full secondary, and 15 V from either end to the center tap. When rectified with a full-wave center-tap circuit, the peak DC voltage is approximately 15 × √2 − 0.7 = 21.2 − 0.7 ≈ 20.5 V peak.

Center-tap transformers are the traditional choice for full-wave rectifier power supplies because they require only two diodes (versus four for a bridge rectifier) and the diodes have a lower PIV (peak inverse voltage) requirement. However, they require more transformer copper (two separate half-windings instead of one full winding) and cannot deliver as much current as a bridge rectifier from the same VA transformer.

A 30 V CT (center-tapped) transformer produces 15 V from either end to the center tap. The center tap connects to ground in the DC circuit, giving two equal AC sources that feed a full-wave rectifier.

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Transformer Secondary Voltage/Current Calculator

Use this calculator to find the secondary voltage or current given the primary values and turns ratio, or to find what turns ratio you need for a target secondary voltage.

Ham Radio Applications of Transformers

Power supply transformers are the most obvious application, but ham radio uses transformers in many other roles:

Station Power Supplies

The classic ham shack linear power supply uses a step-down transformer rated at 28–32 V AC CT (or 14–18 V for a simpler unregulated supply), followed by a bridge rectifier, filter capacitors, and a linear regulator. The transformer must supply enough VA to handle transmit peaks — a 100 W HF transceiver drawing 20 A at 13.8 V on transmit needs about 276 W, so a transformer rated 400–500 VA is appropriate with headroom for intermittent operation.

Antenna Baluns and Impedance Matching

A balun (balanced-to-unbalanced transformer) matches a coaxial feedline (unbalanced, one conductor at RF ground potential) to a balanced antenna such as a dipole (neither conductor at ground). A 1:1 current balun suppresses common-mode currents on the coax braid without changing impedance. A 4:1 voltage balun steps the impedance by a factor of 4, for example transforming a 200 Ω dipole feedpoint impedance to 50 Ω for the coax.

Intermediate Frequency Transformers

Inside every superheterodyne receiver (virtually every HF and VHF radio made since the 1930s) are IF transformers — small, tuned transformers that provide selectivity and impedance matching between stages in the intermediate frequency amplifier. These operate at frequencies typically between 455 kHz and 10.7 MHz and use ferrite or powdered-iron cores in a small metal can.

Audio Output Transformers

Older tube-type transceivers and amplifiers used audio output transformers to match the high impedance of the output tube's plate circuit (typically 2,000–8,000 Ω) to the low impedance of a loudspeaker (4–16 Ω). Modern solid-state equipment generally handles this matching with amplifier topology rather than a transformer, but the principle appears in some high-quality audio designs.