A Step-down transformer is one whose secondary voltage is less than its primary voltage. The step down transformer is designed to reduce the voltage from the primary winding to the secondary winding.

This kind of transformer "step down" the voltage applied to it. They often range in voltage sizes from 0.5 kva to 500 kva.

There is many uses for step-down transformer and the larger devices are used in electric power systems, and small units in electronic devices.  Industrial and residential power transformers that operate at the line frequency (60 Hz in the U.S.), may be single phase or three-phase, are designed to handle high voltages and currents.  Efficient power transmission requires a step-up transformer at the power-generating station to raise voltages, with a corresponding decrease in current.  Line power losses are proportional to the square of the current times the resistance of the power line, so that very high voltages and low currents are used for long-distance transmission lines to reduce losses. At the receiving end, step-down transformers reduce the voltage, and increase the current, to the residential or industrial voltage levels, usually 115 to 600 V.

In electronic equipment, transformers with capacities in the order of 1 kw are largely used ahead of a rectifier, which in turn supplies direct current (DC) to the equipment.  Such electronic power transformers are usually made of stacks of steel alloy sheets, called laminations, on which copper wire coils are wound. Transformers in the 1-to100-W power level are used principally as step-down transformers to couple electronic circuits to loudspeakers in radios, television sets, and high-fidelity equipment.  Known as audio transformers, these devices use only a small fraction of their power rating to deliver program material in the audible ranges, with minimum distortion.  The transformers are judged on their ability to reproduce sound-wave frequencies (from 20 Hz to 25 kHz) with minimal distortion over the full sound power level.

How does a step-down transformer work?

A transformer is a electrical device with one winding of wire placed close to one or more other windings, used to couple two or more alternating-current circuits together by employing the induction between the windings. A transformer in which the secondary voltage is higher than the primary is call a step-up transformer, if the secondary voltage is less than the primary, then its a step-down transformer. The product of current times voltage is constant in each set of windings, so that in a step-up transformer, the voltage increase in the secondary is accompanied by a corresponding decrease in the current.

 Factors in choosing a step-down transformer:

Transformers must be efficient and should dissipate as little power as possible in the form of heat during the transformation process. Efficiencies are normally above 99 percent and are obtained by using special steel alloys to couple the induced magnetic fields between the primary and secondary windings.  To increase transformer efficiency and reducing heat one of the most important considerations is choosing the metal type of the windings. Copper windings is more efficient than aluminum and other winding metal choices.  Transformers with copper windings cost more initially, but can save on electrical cost and maintenance over time and more than makes up for the initial cost. The dissipation of even 0.5 percent on the power transmitted in a large transformer generates a large amount of heat, which requires special cooling. Typical power transformers are installed in sealed containers that have oil or another substance circulating through the windings to transfer the heat to external radiator-like surfaces, where it can be discharged to the surroundings.

Information on a typical step-down transformer:

A transformer is a device for stepping-up or stepping-down electric signal. Without efficient transformers, the transmission and distribution of ac electric power over long distances would be impossible.

 Typical transformer

There are two circuits; the primary circuit, and the secondary circuit. There is no direct electrical connection between the two circuits, but each circuit contains a winding which links it inductively to the other circuit. In transformers, the two windings are wound onto the same iron core. The purpose of the iron core is to channel the magnetic flux generated by the current flowing around the primary windings, so that as much of it as possible also links the secondary winding. The common magnetic flux linking the two windings is conventionally denoted in circuit diagrams by a number of parallel straight lines drawn between the windings. In other words, the ratio of the peak voltages and peak currents in the primary and secondary circuits is determined by a the ratio of the number of turns in the primary and secondary windings; this latter ratio is usually called the turns ratio of the transformer. If the secondary winding contains more turns than the primary winding then the peak voltage in the secondary circuit exceeds that in the primary circuit. This type of transformer is called a step-up transformer, because it step us the voltage of an ac signal. Note that the peak current in the secondary circuit is less than the peak current in the primary circuit in  a step-up transformer (as must be the case if energy is to be conserved). Thus, a step-up transformer actually steps down the current. Likewise, if the secondary winding contains less turns than the primary winding then the peak voltage in the secondary circuit is less than that in the primary circuit. This type of transformer is called a step-down transformer. Note that a step-down transformer actually steps up the current (i.e., the peak current in the secondary circuit exceeds that in the primary circuit).

The use of step-up and step-down transformers in power distribution stations:

Electricity is generated in power stations at a fairly low peak voltage (sometime like 440V), and is consumed at a peak voltage of 110V to 220V for households and businesses in the U.S.  AC electricity is transmitted from the power station to the location where it is consumed at a very high peak voltage (typically 50,000V). As soon as a ac signal comes out of the generator in a power station it is fed into a step-up transformer and fed into a high tension transmission line, and transports the electricity over many miles, and once the electricity has reached its point of consumption, it is fed through a series of step-down transformers until its peak voltage is often reduced down to 110V.

If Electricity is both generated and consumed  at low peak voltages, why go to the trouble of stepping up the peak voltage to a very high value at the power station and then stepping down the voltage again once the electricity has reached its point of consummation? Why not generate, transmit, and distribute the electricity at a voltage of 110V? Consider an electric power line which transmits a peak electric power between a power station and a city. We can think of the number of consumers in the city and the nature of the electrical devices which they operate, as essentially a fixed number. Suppose that the peak voltage and peak current of the ac signal are transmitted along the line. We can think of these numbers as being variable, since we can change them using a transformer. However, since, the product of the peak voltage and the peak current must remain constant. The resistance of the line causes power loses that are greater at lower voltages over distance. The peak rate at which electrical energy is lost due to ohmic heating in the line is high.

If the power transmitted down the line is a fixed quantity, as is the resistance of the line, then the power lost in the line due to ohmic heating varies like the inverse square of the peak voltage in the line. It turns out that even at very high voltages, such as 50,000 V, the ohmic power losses in transmission lines which run over ten kilometers can amount to up to 20% of the transmitted power. It can readily be appreciated that if an attempt were made to transmit ac electric power at a peak voltage of 110V then the ohmic losses would be so severe that virtually none of the power would reach it destination.  It is only possible to generate electric power at a central location, transmit it over large distances, and then distribute it at its point of consumption, if the transmission is performed at a very high peak voltage (the higher, the better). Transformers play a vital role in this process because they allow us to step-up and step-down the voltage of a ac electric signal very efficiently. A well designed transformer typically has a power loss which is only a few percent of the total power flowing through it.

Step Down Transformers

Step down transformers are designed to reduce electrical voltage. Their primary voltage is greater than their secondary voltage. This kind of transformer "steps down" the voltage applied to it. For instance, a step down transformer is needed to use a 110v product in a country with a 220v supply. Step down transformers convert electrical voltage from one level or phase configuration usually down to a lower level. They can include features for electrical isolation, power distribution, and control and instrumentation applications. Step down transformers typically rely on the principle of magnetic induction between coils to convert voltage and/or current levels. Step down transformers are made from two or more coils of insulated wire wound around a core made of iron. When voltage is applied to one coil (frequently called the primary or input) it magnetizes the iron core, which induces a voltage in the other coil, (frequently called the secondary or output). The turns ratio of the two sets of windings determines the amount of voltage transformation. An example of this would be: 100 turns on the primary and 50 turns on the secondary, a ratio of 2 to 1. Step down transformers can be considered nothing more than a voltage ratio device. With step down transformers the voltage ratio between primary and secondary will mirror the "turns ratio" (except for single phase smaller than 1 kva which have compensated secondaries). A practical application of this 2 to 1 turns ratio would be a 480 to 240 voltage step down. Note that if the input were 440 volts then the output would be 220 volts. The ratio between input and output voltage will stay constant. Transformers should not be operated at voltages higher than the nameplate rating, but may be operated at lower voltages than rated. Because of this it is possible to do some non-standard applications using standard transformers. Single phase step down transformers 1 kva and larger may also be reverse connected to step-down or step-up voltages. (Note: single phase step up or step down transformers sized less than 1 KVA should not be reverse connected because the secondary windings have additional turns to overcome a voltage drop when the load is applied. If reverse connected, the output voltage will be less than desired.)