Transformer types

Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts.

Small appliance and electronic transformers may use a split bobbin, giving a high level of insulation between the windings.

Shields between primary and secondary may be fitted to reduce EMI (electromagnetic interference), or a screen winding is occasionally used.

Small appliance and electronics transformers may have a thermal cut-out built into the winding, to shut-off power at high temperatures to prevent further overheating.

Donut-shaped toroidal transformers save space compared to E-I cores, and may reduce external magnetic field.

This provides a transformer with an inherent current limitation due to the loose coupling between its primary and the secondary windings.

The output and input currents are kept low enough to preclude thermal overload under any load conditions — even if the secondary is shorted.

The transformer windings have either air or ferrite cores and the bandwidth can be adjusted by varying the coupling (mutual inductance).

When the short-circuit inductance of the secondary side of the transformer is Lsc and the resonant capacitor (or stray capacitance) of the secondary side is Cr, The resonance frequency ωs of 1' is as follows The transformer is driven by a pulse or square wave for efficiency, generated by an electronic oscillator circuit.

Applications: By arranging particular magnetic properties of a transformer core, and installing a ferro-resonant tank circuit (a capacitor and an additional winding), a transformer can be arranged to automatically keep the secondary winding voltage relatively constant for varying primary supply without additional circuitry or manual adjustment.

Manufacturers either use flat copper sheets or etch spiral patterns on a printed circuit board to form the "windings" of a planar transformer, replacing the turns of wire used to make other types.

Large transformers used in power distribution or electrical substations have their core and coils immersed in oil, which cools and insulates.

Some transformers were built to use fire-resistant PCBs, but because these compounds persist in the environment and have adverse effects on organisms, their use has been discontinued in most areas; for example, after 1979 in South Africa.

However, because the molds for casting the coils are only available in fixed sizes, the design of the transformers is less flexible, which may make them more costly if customized features (voltage, turns ratio, taps) are required.

Laminated steel used for power transformer cores is very inefficient at RF, wasting a lot of RF power as heat, so transformers for use at radio frequencies tends to use magnetic ceramics for winding cores, such as powdered iron (for mediumwave and lower shortwave frequencies) or ferrite (for upper shortwave).

The core material a coil is wrapped around can increase its inductance dramatically – hundreds to thousands of times more than “air” – thereby raising the transformer's Q.

Old RF transformers sometimes included an extra, third coil (called a tickler winding) to inject feedback into an earlier (detector) stage in antique regenerative radio receivers.

So-called “air-core” transformers actually have no core at all – they are wound onto non-magnetic forms or frames, or merely held in shape by the stiffness of the coiled wire.

Sometimes the windings are coaxial cable, sometimes bifilar (paired parallel wire); either is wound around a ferrite, powdered iron, or "air" core.

This style of transformer gives an extremely wide bandwidth but only a limited number of impedance ratios (such as 1:1, 1:4, or 1:9) can be achieved with this technique.

For upper VHF and UHF frequencies, where coil self resonance interferes with proper operation, it is usually the only feasible method for transforming line impedances.

"Balun" is a generic name for any transformer configured specifically to connect between balanced (non-grounded) and unbalanced (grounded) circuits.

"Hum" is a term commonly used to describe unwanted signals originating from the "mains" power supply (typically 50 or 60 Hz).

Good high-frequency response requires carefully designed and implemented windings without excessive leakage inductance or stray capacitance.

In a push–pull amplifier, an inverted signal is required and can be obtained from a transformer with a center-tapped winding, used to drive two active devices in opposite phase.

The ends of the iron wires are then bent around the electrical winding to complete the magnetic circuit, and the whole is wrapped with tape or string to hold it together.

It consists of two flat spiral coils suspended vertically facing each other, hinged at one side so one could swing away from the other to an angle of 90° to reduce the coupling.

It fed back some of the signal from the plate circuit into the input again, and this positive feedback increased the tube's gain and selectivity.

Unlike variable differential transformers, the coils, and not just the core, move relative to each other, so slip rings are required to connect the primary.

Variations in anode voltage supplied by the flyback can result in distortions in the image displayed by the CRT.