Transformers Power supplies contain two main circuits: a primary side and a secondary side. The primary side connects to the power source, and the secondary side connects to the load. The interface between the two main circuits is the heart of the supply: the transformer.

Transformers convert the voltage available on the primary side to the required voltage level on the secondary side. Energy transfers from the primary side to the secondary by the continuous building up and collapsing of a magnetic field. Alternating current passing through the primary winding generates this field. The transfer of energy, from the primary to the secondary, takes place during the build-up and collapse phase of the magnetic field. This electromagnetic energy gets picked up by the secondary winding to generate the required voltage on the secondary side.

The voltage generated on the secondary side is generally proportional to the ratio of number of wire turns between the primary and the secondary windings. A transformer is normally made of a primary winding of copper wire, which is isolated from a secondary winding, and a core, which is made from a ferrous material such as iron or ferrite. Design and construction of a transformer requires consideration of such things as input and output current, voltage, core cross-sectional area and materials, insulation materials and methods, physical size and style, and temperature rise caused by core and wire losses. A transformer that has not been designed correctly may have less efficiency and may be electrically unsafe.

Basic principles

A simple single phase(1f) transformer consists of two electrical conductors called the primary coil and the secondary coil. The primary is fed with a varying (alternating or pulsed continuous) electric current which creates a varying magnetic field around the conductor. According to the principle of mutual inductance, the secondary, which is placed in this varying magnetic field, will develop an electromotive force or EMF. If the ends of the secondary are connected together to form an electric circuit, this EMF will cause a current to flow in the secondary. Thus, some of the electrical power fed into the primary is delivered to the secondary. In practical transformers, the primary and secondary conductors are coils of conducting wire because a coil creates a denser magnetic field (higher magnetic flux) than a straight conductor.

Transformers alone cannot do the following:

• Convert DC to AC or vice versa
• Change the voltage or current of DC
• Change the frequency (the "cycles") of AC.

High frequency transformers

The universal transformer emf equation indicates that at higher frequency, the core flux density will be lower for a given voltage. This implies that a core can have a smaller cross-sectional area and thus be physically more compact without reaching saturation. It is for this reason that the aircraft manufacturers and the military use 400 hertz supplies. They are less concerned with efficiency, which is lower at higher frequencies (mostly due to increased hysteresis losses), but are more concerned with saving weight. Similarly, flyback transformers which supply high voltage to cathode ray tubes operate at the frequency of the horizontal oscillator, many times higher than 50 or 60 hertz, which allows for a more compact component.

Uses of transformers

• Electric power transmission over long distances. The simplicity, reliability, and economy of conversion of voltages by stationary transformers was the principal factor in the selection of alternating current power transmission (see War of Currents).
• High-voltage direct-current HVDC power transmission systems.
• Large, specially constructed power transformers are used for electric arc furnaces used in steelmaking.
• Rotating transformers are designed so that one winding turns while the other remains stationary. These can pass power or radio signals from a stationary mounting to a rotating mechanism, or radar antenna.
• Sliding transformers can pass power or signals from a stationary mounting to a moving part such as a machine tool head. See linear variable differential transformer,
• Some rotary transformers are precisely constructed in order to measure distances or angles. Usually they have a single primary and two or more secondaries, and electronic circuits measure the different amplitudes of the currents in the secondaries. See synchro and resolver.
• Small transformers are often used to isolate and link different parts of radio receivers and audio amplifiers, converting high current low voltage circuits to low current high voltage, or vice versa. See electronics and impedance matching. See also isolation transformer and repeating coil.
• Balanced-to-unbalanced conversion. A special type of transformer called a balun is used in radio and audio circuits to convert between balanced circuits and unbalanced transmission lines such as antenna downleads. A balanced line is one in which the two conductors (signal and return) have the same impedance to ground: twisted pair and "balanced twin" are examples. Unbalanced lines include coaxial cables and strip-line traces on printed circuit boards. A similar use is for connecting the "single ended" input stages of an amplifier to the high-powered "push-pull" output stage.

More on Power Supplies:

• Transformers Power Supplies
• Switching Transformers
• Linear Supplies
• Switching Supplies
• Advantages and Disadvantages