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Wednesday, December 5, 2012

CHAPTER 6 PRINCIPLES OF RECTIFICATION





6.1       DIODES








FIGURE 6.1
SOME USES OF DIODES

An element which is constantly used in control circuits, especially d.c. circuits, is the diode.  It is a device which allows current to flow freely in one direction but presents a high resistance to it if it flows in the other.  It does not necessarily completely stop the reverse flow, but in many applications it is regarded as blocking it completely.  It acts in much the same way as a mechanical non-return valve such as is fitted to a motor-car tyre; air may pass freely in, but it cannot get out. 
 

The first diode was the original Fleming thermionic tube, the forerunner of the electronic valve.  Here the electrons passed easily from the heated filament or cathode to the positive anode, but they could not return to the cathode even if the anode were made negative.  It was this one-way action and its likeness to the non-return valve which gave its name to the Fleming tube: the ‘valve’.

Nowadays the same action can be obtained from solid-state material, and in a much simpler, cheaper and more compact manner.  Solid-state diodes are widely used in electronic and control circuits, chiefly for their one-way blocking facility.  The action of a diode on an a.c. circuit and in some d.c. circuits is shown in Figure 6.1.

In Figure 6.1(a) an a.c. generator is feeding a load through such a diode.  In the positive halves of the cycle the current passes freely, giving a half-sine waveform.  In the negative halves, where the current would normally flow back, it cannot do so because it is blocked by the diode.  So the current waveform is a series of positive half-sine waves, with zero value in the gaps in between, and the current is unidirectional, though by no means constant.  It might be called ‘direct current’, but that would be misleading.  If the load were a lamp, there would be bad flicker, but if it were a battery to be charged, the pulsing nature would not matter and the battery would receive charging current in a series of pulses, and no discharge would take place in the negative parts of the cycle.  It would therefore receive a net charge.

In Figure 6.1(b) two batteries are shown feeding a common d.c. load in parallel.  Each has a charger, and a diode is placed in each battery output.  So long as both batteries have approximately the same voltage, each will be contributing to the load, though perhaps not quite equally.  But if one battery were discharged its voltage might be such that not only would it contribute nothing to the load but the healthy battery might try to feed current into it.  This would also occur if one charger failed.  The diodes prevent this by blocking any reverse current into either battery.

In Figure 6.1(c) is another classic example of the use of a diode.  A highly inductive load - for example a solenoid - is being fed from a d.c. source with a main switch or contactor.  When the switch is opened the current cannot immediately stop in the inductor, and it carries on to ‘pile up’ on the switch contacts.  This causes a very high voltage to appear there and usually severe arcing.

Suppose now a diode is placed in reverse to shunt the inductor as shown in Figure 6.1(d).  It will not pass any current while the switch is closed because it is placed so as to block it.  If now the switch is opened, the inductive current in the coil, instead of ‘piling up’ to put a charge on the switch contacts, will have an easy path through the diode and back into the coil.  It will circulate in this manner until eventually it is damped out by the resistance of the coil/diode circuit.  But it will have prevented high voltage and arcing at the switch contacts. Here the diode is used as an arc-suppressor.

6.2       RECTIFICATION - SINGLE-PHASE


It has already been shown in Figure 6.1(a) how a diode can change an alternating into a unidirectional current - this action is called ‘rectifying’ - and it was mentioned that such an arrangement could be used, for example, to charge a battery.

Figure 6.2(a) repeats Figure 6.1(a).  This arrangement is wasteful of time, as useful current flows for only half the available time.  It is called ‘half-wave’ rectification.  If the unidirectional current pulses are ‘smoothed’ to give a mean direct current, the d.c. level will be the line (shown dotted) where the areas above and below it (shaded) are equal.  It is in fact 0.318 times the amplitude, or 0.45 (= 2 x 0.318) times the rms value of the current.

This can be improved by the arrangement of Figure 6.2(b), where four diodes are connected in the form of a bridge.  It turns the negative half-wave into a positive instead of blocking it, so that each half-cycle has its quota of unidirectional current.  This arrangement is called ‘full-wave’ rectification.  It is more efficient and gives less flicker if used for lighting.  The ‘smoothed’ mean d.c. level is higher than in the half-wave case, as shown by the dotted line in Figure 6.2(b).  It is in fact 0.635 times the amplitude, or 0.90 (= 2 x 0.635) times the rms value of the current.  Thus an a.c. current of rms value 10A (peak 14.1A) will be converted to 9A d.c. (apart from losses).


A full-wave bridge is sometimes drawn in the alternative manner shown in the centre of Figure 6.2(b).

 
FIGURE 6.2
DIODES USED AS RECTIFIERS

6.3       RECTIFICATION – 3-PHASE


The idea can be extended to 3-phase, as shown in Figure 6.3.


                                                                              FIGURE 6.3
3-PHASE FULL-WAVE RECTIFIER

Here a six-diode bridge is connected to receive a 3-phase supply and to produce a unidirectional output.  The arrangement shown is full-wave, and it reverses the three negative halves each cycle to produce a unidirectional current with six peaks each cycle.  This is much less ‘peaky’ than the single-phase case, and it is more readily smoothed to produce a good, low-ripple direct current.

As before, the smoothed mean d.c. level is the line, shown dotted, where the shaded areas above and below it are equal.  The level is much higher than even the full-wave single-phase case, being equal to 0.955 times the amplitude, or 1.35 (2 x 0.955) times the rms value.  Thus an a.c. current of rms value 10A (peak 14.1A) will be converted to 13.5A d.c. (apart from losses).  It should be particularly noted that with 3-phase full-wave rectification the d.c. level is higher than the rms a.c. value.

6.4       CONTROLLED RECTIFICATION


It has been shown that the d.c. output from a 3-phase full-wave rectifier with six diodes is fixed at approximately 1.35 times the rms a.c. input.  The d.c. voltage output can be controlled by substituting three thyristors for three of the diodes - see Figure 6.4.  A thyristor is a solid-state device like a diode but with a third electrode which prevents the device passing even forward current until the third electrode is ‘triggered’.



FIGURE 6.4
CONTROLLED RECTIFIER (3-PHASE FULL-WAVE)

Since the thyristor will not conduct until signalled to do so on the third electrode, the ‘firing’ can be deliberately delayed.  An electronic circuit provides a firing pulse with a variable delay, so that the waveform appears as in Figure 6.4.  The mean d.c. level - that is, the line where areas above and below it are equal - will be different with differing delay times, so that the bridge can be used to give different d.c. output levels simply by controlling the electronic delay circuit.

6.5       SUMMARY


Rectifiers, which in modern practice are normally solid-state diodes or thyristors, are used in many applications of power work.  They all have a ‘one-way’ or blocking function, like a non-return valve, which can be used to convert a.c. into d.c. as well as preventing reverse flow of current, providing a means of spark suppression and similar applications.  They are described more fully in the manual ‘Control Devices’.

When diodes are used for a.c.-to-d.c. conversion, the d.c. level is fixed and depends entirely on the level of the a.c. applied, but if thyristors are used, the d.c. level can be controlled within broad limits.  In this form they are widely used for battery charging and, in the drilling world, for applying a variable d.c. voltage to the drilling motors in order to regulate their speed; they are there referred to by their earlier name ‘silicon controlled rectifiers’ (SCR).

The d.c. levels when rectified from a.c. are as follows:


Type of Rectification
D.C. Level
Fraction of a.c. peak

Fraction of a.c. rms


Single-phase, half-wave
Single-phase, full-wave
Three-phase, full-wave


0.318
0.635
0.955

0.45
0.90
1 .35


Thus a single-phase a.c. rms supply of 250V will give:

 112V d.c. with half-wave rectification
225V d.c. with full-wave rectification

and a 3-phase a.c. rms supply of 440V will give:

594V d.c. with full-wave rectification

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