November is Transformer month, and we’ll be featuring blog posts about those handy components used for transforming voltage. The following is an excerpt from Charles Platt’s definitive Encyclopedia of Electronic Components, Vol. 1:
What It Does
A transformer requires an input of alternating current (AC). It transforms the input voltage to one or more output voltages that can be higher or lower.
Transformers range in size from tiny impedance-matching units in audio equipment such as microphones, to multi-ton behemoths that supply high voltage through the national power grids. Almost all electronic equipment that is designed to be powered by municipal AC in homes or businesses requires the inclusion of a transformer.
The one at the rear of the image above is rated to provide 36VAC at 0.8A when connected with a source of 125VAC. At front, the miniature transformer is a Radio Shack product designed to provide approximately 12VAC at 300mA, although its voltage will be more than 16VAC when it is not passing current through a load.
Transformer schematic symbols are shown below:
The different coil styles at left and right are functionally identical. Top: A transformer with a magnetic core—a core that can be magnetized. Bottom: A transformer with an air core. (This type of transformer is rare, as it tends to be less efficient.) The input for the transformer is almost always assumed to be on the left, through the primary coil, while the output is on the right, through the secondary coil. Often the two coils will show differing numbers of turns to indicate whether the transformer is delivering a reduced voltage (in which case there will be fewer turns in the secondary coil) or an increased voltage (in which case there will be fewer turns in the primary coil).
How It Works
A simplified view of a transformer is shown below:
Alternating current flowing through the primary winding (orange) induces magnetic flux in a laminated core formed from multiple steel plates. The changing flux induces current in the secondary winding (green), which provides the output from the transformer. (In reality, the windings usually consist of thousands of turns of thin magnet wire, also known as enameled wire; and various different core configurations are used.)
The process is known as mutual induction. If a load is applied across the secondary winding, it will draw current from the primary winding, even though there is no electrical connection between them.
In an ideal, lossless transformer, the ratio of turns between the two windings determines whether the output voltage is higher, lower, or the same as the input voltage. If Vp and Vs are the voltages across the primary and secondary windings respectively, and Np and Ns are the number of turns of wire in the primary and secondary windings, their relationship is given by this formula:
Vp / Vs = Np / Ns
A simple rule to remember is that fewer turns = lower voltage while more turns = higher voltage.
A step-up transformer has a higher voltage at its output than at its input, while a step-down transformer has a higher voltage at its input than at its output.
The Core The ferromagnetic core is often described as being made of iron, but in reality is more often fabricated from high permeability silicon steel. To reduce losses caused by eddy currents, the core is usually laminated—assembled from a stack of plates separated from each other by thin layers of varnish or a similar insulator. Eddy currents tend to be constrained within the thickness of each plate.
Because a DC voltage would cause magnetic saturation of the core, all transformers must operate with alternating current or pulses of current. The windings and geometry of a transformer are optimized for the frequency range, voltage, and current at which it is designed to operate. Deviating significantly from these values can damage the transformer.
The following are some types of transformers commonly encountered:
Power Transformer Typically designed to be bolted onto a chassis or secured inside the case or cabinet housing a piece of electrical equipment with solder tabs or connectors allowing wires to connect the transformer to the power cord, on one side, and a circuit board, on the other side.
Plug-in Transformer Usually sealed in a plastic housing that can be plugged directly into a wall power outlet. They are visually identical to AC adapters but have an AC output instead of a DC output.
Isolation Transformer Also known as a 1:1 transformer because it has a 1:1 ratio between primary and secondary windings, so that the output voltage will be the same as the input voltage. When electrical equipment is plugged into the isolation transformer, it is separated from the electrical ground of AC power wiring. This reduces risk when working on “live” equipment, as there will be negligible electrical potential between itself and ground. Consequently, touching a grounded object while also touching a live wire in the equipment should not result in potentially lethal current passing through the body.
Autotransformer This variant uses only one coil that is tapped to provide output voltage. Mutual induction occurs between the sections of the coil. An autotransformer entails a common connection between its input and output, unlike a two-coil transformer, which allows the output to be electrically isolated from the input. Autotransformers are often used for impedance matching in audio circuits, and to provide output voltages that differ only slightly from input voltages.
Audio Transformer When a signal is transmitted between two stages of a circuit that have different impedance, the signal may be partially reflected or attenuated. (Impedance is measured in ohms but is different from DC electrical resistance because it takes into account reactance and capacitance. It therefore varies with frequency.)
A device of low input impedance will try to draw significant current from a source, and if the source has high output impedance, its voltage will drop significantly as a result. Generally, the input impedance of a device should be at least 10 times the output impedance of the device that is trying to drive it. Passive components (resistors, and/or capacitors, and/or coils) can be used for impedance matching, but in some situations a small transformer is preferable.
Split-Bobbin Transformer This variant has primary and secondary coils mounted side by side to minimize capacitive coupling.
Surface-Mount Transformer May be less than 0.2″ square and is used for impedance matching, line coupling, and filtering.
When selecting a power transformer, its power handling capability is the value of primary interest. It is properly expressed by the term VA, derived from “volts times amps.” VA should not be confused with watts because watts are measured instantaneously in a DC circuit, whereas in an AC circuit, voltage and current are fluctuating constantly. VA is actually the apparent power, taking reactance into account.
The relationship between VA and watts will vary depending on the device under consideration. In a worst-case scenario:
W = 0.65 VA (approximately)
In other words, the averaged power you can draw from a transformer should be no less than two-thirds of its VA value.
Transformer specifications often include input voltage, output voltage, and weight of the component, all of which are self-explanatory. Coupling transformers may also specify input and output impedances.