Converting Ac into Dc

Converting Ac into Dc

Basics:

To understand Rectifiers, first you need to know what are alternating and direct current signals.

Alternating Current:

AC1         AC2

Direct Current:

AC4          AC3

 

RECTIFIERS:

Rectifiers are simple circuits for conversion of Alternating Current (AC) to Direct Current (DC). This process of conversion is called ‘Rectification’.

The function of a rectifier is well explained from the figure below:

AC5

When a rectifier is connected to an AC supply, a DC signal or supply is received at the load.

Types OF Rectifiers:

  • Half wave rectifier:

It is a circuit which only uses one diode connected in series with the load. A simple AC supply or a transformer can be used at the input and a half wave rectified signal is received at the output/load.

It operates at low power levels.

AC6

How it works:

During the positive half cycle of the input signal, the diode goes forward biased because the anode receives positive potential, thus it conducts and a half cycle is received at the output/load. The signal’s peak at the output is 0.7 v less than the input because of the occurrence of diode drop at the diode.

Then during the negative half cycle of the input signal, the diode goes reverse biased as the anode receives negative potential and thus do not conduct. Therefore, the negative half cycle is grounded and no signal is received at the output/load during the second half cycle. In this way we are able to receive a rectified signal at the output which has only one polarity.

Note: The power is half reduced at the ouput because half of the cycle is grounded so this rectification is not suitable for power applications. Suppose if 220v peak to peak is given at the supply, we receive 110v of single only and the other 110v are vanished (including diode drop).

V(out)= Vp(sec)/2-diode drop.

  • Full wave Center-tapped Rectifier:

In this rectifier circuit the drawback of halfwave rectifier to eliminate one peak of the input is somehow reduced by using a center-tapped transformer at the supply which splits the input signal into two halves, however the power issue remains there. Moreover two diodes are used for rectification activating alternately one after one depending on the transformer.

AC7

How it works:

When input signal goes positive, the polarity at first end of transformer is positive and at the second end is negative which tends the diode 1 to get forward biased and diode 2 to get reverse biased. Thus the output is received for the first positive cycle of the input. The polarities are shown in figure (a):

AC8

Note: Diode drop is again considered in both the cycles.

When input signal goes negative, the polarity reverses. Thus diode 1 goes in reverse biasing and the diode 2 gets forward biased and the output is again received in the positive direction at the output. Hence both the peaks of input are consumed resulting in full wave rectification (Pulsating DC):

AC9

As you can see the power issue is still there. Suppose of we are supplying 200v at at the input, the first peak of 100v is splited into +50v and -50v

So a 50v peak is received at the ouput subtracting the diode drop and for the other half process remains same. So this means that we receive 100v peak to peak at the ouput but this time the diode drop is subtracted twice as we are using two diodes.

V(out)=Vp(sec)/2-2(diode drops)

  • Full wave bridge rectifier:

This is also a full wave rectifier which uses bridge combination of 4 diodes. Transformer is used as a supply.

AC10

How it works:

It is indicated by the two figures given above. When the input goes positive the path of current is shown in the first figure. This shows that during the first cycle diodes D3 and D1 becomes forward biased as cathode of D1 receives negative potential from the second end if the transformer and the anode of diode D3 receives the positive potential from the first end of the transformer. These two diodes are in series with the load so a positive peak is received at the output.

Similarly, for the second half negative cycle diodes D2 and D4 gets forward biased as the cathode of D2 receives negative potential so again a peak is received at the output resulting in fully rectified signal (pulsating DC).

Also this time two diode drops are subtracted and the output becomes:

V(out)= Vp(sec) – 2(diode drops).

This was the basics from rectifiers which I tried ti explain in least difficult words. J

Stay tuned Guys!

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