6.1 DIODES
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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’.
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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
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Fraction of a.c. peak
|
Fraction of a.c. rms |
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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:
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