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RECTIFICATION
In the exam you are expected to know about:
• Half wave rectification;
• Full wave rectification;
• The bridge rectifier;
• Effect of a capacitor on output waveform;
Rectification is the process by which alternating current, usually from the mains supply,
is turned into direct current. Alternating Current can be easily converted to direct current
using a diode as a rectifier.
Half Wave Rectification
The incorporation of a single diode is the simplest way of rectifying AC. Only the
forward half-cycle is passed. The reverse is blocked. The rectified current is a series of
pulses. This is adequate for a crude circuit, for example the low voltage fan motor for a
hair dryer.
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Full wave Rectification
In the case above we see that both forward and reverse half cycles are rectified.
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Two half-wave rectifiers are placed back to back.
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The load is connected to a centre tapping of the transformer.
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This is called a centre-tap full-wave rectifier.
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It always needs a transformer with a centre tap.
A centre tapping is where a connection is made to the secondary coil half way between
the outside terminals. This allows for an arbitrary zero point for the voltage. During the
forward half cycle, the top terminal will be positive relative to the centre tap, while the
bottom terminal will be negative relative to the centre tap, and vice versa for the reverse
half cycle.
A more common method to get full wave rectification is to use four diodes in a bridge
rectifier circuit, two allowing current from each half cycle to pass.
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Bridge Rectifier
The arrows show the forward and reverse half cycles:
In this circuit we have four diodes, A, B, C, and D.
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In the forward half-cycle, shown by the black arrows, we can see that both B and D
conduct. The rectified current goes clockwise through the load
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In the reverse half-cycle, shown by the grey arrows, we can see that both A and C
conduct. Because the two diodes are conducting on the reverse half cycle, the
current then passes through the load clockwise.
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The load current goes clockwise through the load regardless of whether it is being
driven by the forward or reverse half cycle.
Bridge rectifiers usually come in a ready made pack, rather than having to solder four
diodes together. On the pack the AC inputs and the positive and negative terminals are
marked.
When selecting a component, we need to assess:
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Working voltages.
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Current that will be passed.
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What kind of cooling will be needed.
Heavy-duty rectifiers have heat sinks, which are fins to conduct excess heat to the air.
Sometimes in high power applications, a fan may be used. We need to check the
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specifications for the diodes when we plan the circuits. Any electronics catalogue will
provide these.
When we get DC rectified from AC, we find that the voltage is far from steady, quite
unlike the steady DC from a battery. The diagram shows the trace we would expect.
For crude applications such as running a motor or lighting a bulb, this is quite adequate.
Electronic circuits need to have steady voltages. The ripple supplied by unsmoothed
DC can have an adverse effect on the function of electronic circuits. This can be an
intrusive mains hum, or the inability of the circuit to function properly at all.
Smoothing of the rectified AC is accomplished with a capacitor. The simplest way is to
have a large value electrolytic capacitor called a reservoir capacitor in parallel with the
load.
As the voltage rises from 0, the current passes through the load and there is also a current
that passes to the capacitor. The voltage almost reaches the peak (not the rms) value.
Once the waveform voltage drops below the peak value, the capacitor starts to discharge.
It cannot do this through the rectifier, instead it discharges through the load. The load
takes its current from the capacitor and the voltage drops, until the next pulse comes
along to raise the voltage again. There is a ripple, but it is much smaller in amplitude
than would be the case if there were no smoothing capacitor.
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We can see that even with a capacitor in the circuit, there is a variation in the voltage
level. This is called a ripple voltage.
The ripple voltage is the difference between the peak voltage and the minimum voltage
provided by a supply.
Read Only
PFC Page 334 – 335
• p – n junction diode
• using diodes to convert a.c. to d.c.
• half wave rectification and full wave rectification
• using four diodes (a silicon bridge diode)
• capacitor smoothing
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