Rectification and Power Supplies

Rectification and Power Supplies
We now have the question of how do we turn an alternating voltage into a constant ‘direct’ voltage.   The crystal radio set operates by chopping off half of the alternating radio signal.   If we were to do this to the output from a mains transformer with an output of say, 12 Volts AC, the result is not very satisfactory:

Here, we have the situation shown in the upper diagram.   The output consists of isolated pulses at 50 per second.   You will notice that there is no output power for half of the time.   The negative part of the waveform is blocked by the high resistance of the diode while the positive part of the waveform is allowed through by the low resistance of the ‘forward-biased’ diode.   It should be remembered that the diode drops 0.7 Volts when conducting so the output of the half-wave rectified transformer will be 0.7 Volts lower than the transformer’s actual output voltage.

If four diodes are used instead of one, they can be arranged as shown in the lower diagram.   This arrangement of diodes is called a ‘bridge’.   Here the positive part of the waveform flows through the upper blue diode, the load ‘L’ and on through the lower blue diode.   The negative part flows through the left hand red diode, the load and then the right hand red diode.   This gives a much better output waveform with twice the power available.   The output voltage will be 1.4 Volts less than the transformer output voltage as there are two silicon diodes in the supply chain.

The output from even the full-wave rectifier is still unsatisfactory as there is a voltage drop to zero volts 100 times per second.   Only a few devices operate well with a power supply like that, an incandescent bulb as used in a car can use this output, but then, it could use the original AC supply without any rectification.   We need to improve the output by using a reservoir device to supply current during those moments when the voltage drops to zero.   The device we need is a Capacitor which used to be called a ‘condenser’.   The circuit of a mains unit using a capacitor is shown here:

This produces a much better result as the capacitor stores some of the peak energy and gives it out when the voltage drops.   If the load on the unit is light with not very much current taken from it, the output voltage is quite good.   However, if the current drain is increased, the output voltage gets dragged down 100 times per second.   This voltage variation is called ‘ripple’ and if the unit is supplying an audio system or a radio, the ripple may well be heard as an annoying hum.   The larger the capacitor for any given current draw, the smaller the ripple.

To improve the situation, it is normal to insert an electronic control circuit to oppose the ripple:

This circuit uses one new component, a new variety of diode called a ‘Zener’ diode.   This device has an almost constant voltage drop across it when its current-blocking direction breaks down.   The diode is designed to operate in this state to provide a reference voltage.   The circuit merely uses a tiny current from the top of the zener diode to drive the Darlington pair emitter-follower transistors used to provide the output current.

With this circuit, when the output current is increased, the resistance of the transistor pair automatically reduces to provide more current without varying the output voltage.   The 1K resistor is included to give the transistors a completed circuit if no external equipment is connected across the output terminals.   The zener diode is chosen to give 1.4 Volts more than the required output voltage as the two transistors drop 1.4 Volts when conducting.

You should note that the output transistor is dropping 6 Volts at the full supply current.   Watts = Volts x Amps so the power dissipated by the transistor may be quite high.   It may well be necessary to mount the transistor on an aluminium plate called a ‘heat sink’ to keep it from overheating.   Some power transistors, such as the 2N3055, do not have the case isolated from the active parts of the transistor.   It is good practice to use a mica gasket between the transistor and the heat-sink as it conducts then heat without making an electrical connection to the metal heat-sink.

A capacitor, being an electrical reservoir, can be used as part of a timer circuit.   If the current flow into it is restricted by passing it through a resistor.   The length of time between starting the flow on an empty capacitor, and the voltage across the capacitor reaching some chosen level, will be constant for a high-quality capacitor.

As the voltage increase tails off, it becomes more difficult to measure the difference accurately, so if the capacitor is to be used for generating a time interval, it is normal to use the early part of the graph area where the line is fairly straight and rising fast.

Electronics Tutorial

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