Direct current (DC or "continuous current") is considered as the constant flow of electrons in the single direction from low to high potential. This is typically in a conductor such as a wire, but can also be through semiconductors,insulators, or even through a vacuum as in electron or ion beams. In direct current, the electric charges flow in the same direction, distinguishing it from alternating current (AC). Aterm formerly used for direct current was Galvanic current.
Long after the usage of this term had been established, physicists realized that electrons actually flow to a negative potential (pole) and so-called "holes" flow to the opposite one. However, the established usage of the term prevailed.
The first commercial electric power transmission (developed by Thomas Edison in the late nineteenth century) used direct current. Because of the advantage of alternating current over direct current in transforming and transmission, electric power distribution today is nearly all alternating current. See War of Currents
A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts. A transformer comprises two or more coupled windings, or a single tapped winding and, in most cases, a magnetic core to concentrate magnetic flux. A changing current in one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings.
The oppsite polarity, diodes D1 to D4 that rectify the AC appearing across the secondary with D1 and D3 providing full wave rectification to produce a positive output, D2 and D4 providing full wave rectifiction to produce a negative output.
Coupling by mutual induction
A simple transformer consists of two electrical conductors called the primary winding and the secondary winding. Energy is coupled between the windings by the time-varying magnetic flux that passes through (links) both primary and secondary windings. Whenever the amount of current in a coil changes (including when the current is switched on or off), a voltage is induced in the neighboring coil. The effect, called mutual inductance, is an example of electromagnetic induction. [1]
A practical step-down transformer showing magnetising flux in the core
If a time-varying voltage is applied to the primary winding of turns, a current will flow in it producing a magnetomotive force (MMF). Just as an electromotive force (EMF) drives current around an electric circuit, so MMF tries to drive magnetic flux through a magnetic circuit. The primary MMF produces a varying magnetic flux in the core, and, with an open circuit secondary winding, induces a back electromotive force (EMF) in opposition to . In accordance with Faraday's law of induction, the voltage induced across the primary winding is proportional to the rate of change of flux:
and
where,
1.vP and vS are the voltages across the primary winding and secondary winding,
2.NP and NS are the numbers of turns in the primary winding and secondary winding,
3.dΦP / dt and dΦS / dt are the derivatives of the flux with respect to time of the primary and secondary windings.
Saying that the primary and secondary windings are perfectly coupled is equivalent to saying that . Substituting and solving for the voltages shows that:
where
1.vp and vs are voltages across primary and secondary,
2.Np and Ns are the numbers of turns in the primary and secondary, respectively.
Hence in an ideal transformer, the ratio of the primary and secondary voltages is equal to the ratio of the number of turns in their windings, or alternatively, the voltage per turn is the same for both windings. The ratio of the currents in the primary and secondary circuits is inversely proportional to the turns ratio. This leads to the most common use of the transformer: to convert electrical energy at one voltage to energy at a different voltage by means of windings with different numbers of turns. In a practical transformer, the higher-voltage winding will have more turns, of smaller conductor cross-section, than the lower-voltage windings.
The EMF in the secondary winding will cause current to flow in a secondary circuit. The MMF produced by current in the secondary winding opposes the MMF of the primary winding and so tends to cancel the flux in the core. Since the reduced flux reduces the EMF induced in the primary winding, increased current flows in the primary circuit. The resulting increase in MMF due to the primary current offsets the effect of the opposing secondary MMF. In this way, the electrical energy fed into the primary winding is delivered to the secondary winding. Also because of this, the flux density will always stay the same as long as the primary voltage is steady.
For example, suppose a power of 50 watts is supplied to a resistive load from a transformer with a turns ratio of 25:2.
1.P = EI (power = electromotive force × current)
50 W = 2 V × 25 A in the primary circuit if the load is a resistive load. (See note 1)
2.Now with transformer change:
50 W = 25 V × 2 A in the secondary circuit.
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