Concept of AC voltage – Learn in simple language

In this article, don’t forget to read the reference notes.

Power supply is the basic requirement of any working circuit or device. We know that the electronic circuit uses semiconductors and so the circuit requires only DC power supply for its smooth working and good performance.

So in this chapter, we shall study the conversion of AC voltage into proportional DC voltage, using the technique of rectification process.

Since AC mains voltage source is cheap and convenient rather than the DC source of batteries, we shall use AC mains voltage like 230V, 50Hz source used particularly in India. There varieties of mains voltage source on global level[i]. But the explanation of the circuits given here is equally applicable on global level.

Concept of AC voltage

AC voltage is generated on the principle of Faraday’s law of electromagnetic induction. Consider a simple model of generator as shown in following figure. Suppose a rectangular coil is rotating, in anticlockwise direction, on a fixed axis within the strong magnetic field of two permanent magnets (left figure). Then AC voltage is produced across its two end points A-B. The voltage thus, produced at each position is given in the graph (right figure).

Suppose the rotation of the coil starts from point ‘a’. At point ‘a’ the coil and the magnetic field are parallel to each other. So voltage produced in the coil is zero[i]. When coil rotates through an angle of 45°, at point ‘b’ the coil and the magnetic field are becoming perpendicular to each other. So the voltage in coil increases and attains the voltage known as r.m.s. voltage. When it comes to point ‘c’ by sweeping an angle of 90°, then it is exactly perpendicular to magnetic field. So maximum positive voltage is produced across it. In the same way, at ‘d’ the voltage is again r.m.s. voltage and at point ‘e’, its half rotation is complete and it is now parallel with the magnetic field. So again its voltage is zero. This completes positive half cycle of the AC voltage.

During next half rotation same process takes place, only negative half cycle of AC voltage is obtained. Because direction of induced e.m.f. in the coil is opposite to the previous direction. Hence, we get half cycle in IV quadrant. And lastly when the coil completes one rotation, it comes at point ‘a’ and the voltage induced in it will be zero, again.

In each case, the value of voltage generated depends on the number of turns of the coil, the strength of magnetic field and the speed at which the coil is rotating within the magnetic field. It we rotate the magnetic field by rotating magnets, keeping the coil steady, even though same effect is observed. It means that relative motion between magnetic field and coil is necessary.

Transformer is the main device used in DC power supply circuits. So it is essential to know more about transformer first. It is based on the principle of electromagnetic induction. A transformer generally consists of two coils, primary and secondary. The primary is supplied with AC supply, generally 230V AC mains having frequency 50-60Hz. When AC supply is connected to it, it produces strong alternating magnetic field which is cut by secondary coil and proportional e.m.f. is induced in it. It is important to note that the phase of secondary voltage is 180° out of phase[1] with respect to primary voltage.

Neglecting the internal losses, within the transformer, the ratio of number of turns of primary and secondary plays an important role in deciding the output voltage at secondary. Thus the two voltages and the number of turns are related as –

Considering the above equation, there can be three types of transformers: step-up, step-down and isolation transformer. When number of turns of secondary is less than the primary, it is called as step-down transformer. When secondary turns are greater than primary it is called step-up transformer and when both are equal it is called isolation transformer.

Every transformer provides isolation from electric shock, because the neutral terminal of mains supply is recreated at secondary, hence, it is isolated from earth. Now any person standing on earth, when touches the ‘phase’ terminal of secondary coil, he doesn’t feel any electric shock, as the neutral terminal of mains supply is recreated and hence isolated from earth.

Since secondary voltage of step-down transformer is already small, this factor is immaterial for such transformer. However, this factor becomes important for step-up transformer, as it protects the user and completely isolates him from electric shock.

This doesn’t mean that the user will never feel electric shock at the secondary of step-up transformer. If he accidently touches both the terminals of secondary, then he must feel the shock, because the secondary is in shunt with his body.

We know that transformer is an electrical device that has two coils: primary coil and secondary coil. When AC voltage is connected across primary, bidirectional current flows through it and alternating magnetic field is setup around it. Since the secondary coil is placed within the magnetic field of primary coil, proportional e.m.f. is induced in secondary coil and thus, we get AC voltage across it.

The e.m.f. induced in secondary is directly proportional to its number of turns. The number of turns of primary and secondary and their e.m.f. are related with an equation, as follows:

Now if we keep two identical secondary coils (sec-1 & sec-2) in alternating magnetic field of primary, then same AC voltage is produced in both the secondaries, as shown in the following Fig: 1.2(a). Note the polarity of AC voltages produced across each secondary. During positive half cycle of AC voltage, upper point of each secondary is positive and lower is negative. It means that the polarity of the two adjacent points of two secondaries is always opposite with respect to each other. Now if we join the adjacent terminals of two secondaries together, as shown in Fig: 1.2(b), the center-tap transformer is formed. When the two points are joined together, the opposite polarities cancel out each other and hence, the potential at the center-tap is always zero.

[1] It is a routine practice to ignore this phase inversion. I have read some books on applied electronics, which even don’t care to tell this fact to the readers. But it is important to note this point, because if you are analyzing waves on CRO, in particular, then phase inversion is a critical factor.

[i] According to Faraday’s law, when conductor is parallel to the magnetic field lines, then the voltage produced in the coil is zero. However, when it is perpendicular, then maximum voltage is produced in the coil.

[i] In larger regions of Asia, Africa and Europe 200V to 230V AC mains supply is used. However, this excludes some parts of Madagascar, Libya, Morocco and South East Asia, where the supply is 110V, 50Hz. The larger part of Northern America with Columbia, Venezuela and Ecuador use strictly 115V, 60Hz supply. However, almost entire part of Argentina uses 220V, 50Hz supply. Australia entirely uses 230V, 50Hz supply. The global variations in mains power supply are: 200V,50Hz; 220V,50Hz; 230V,50Hz, 240V,50Hz; 100V,60Hz, 110V,60Hz; 115V,60Hz; 120V,60Hz; 127V,60Hz; 220V,60Hz; 230V,60Hz; 240V,60Hz; 100V,50Hz; 110V,50Hz; 115V,50Hz. Only there are two peculiar cases of 127V, 50Hz and 127V, 60Hz. They are some parts of Libya, Indonesia, Vietnam, Malaysia, Madagascar, Papua (New Guinea) and Morocco (Standard Ref: World Plugs, retrieved 2012 Dec 19. Iec.ch. Retrieved on 2013-02-05).