Capacitive Smoothing

Back to Topic 3

 

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 from a CRO.

 

 

 

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.

 

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.

 

Question 5  A power supply gives out a voltage of 8.4 V.  When it is connected to a load, the minimum voltage is 7.3 V.  What is the ripple voltage? ANSWER

 

 

The greater the RC time constant, the more marked the smoothing action.  Large values of capacitors are best.  However if the value of the capacitor is very large, there is a large current at switch-on which can damage the rectifier.  This is called a surge current.

 

We can show the ripple voltage on the CRO.  The photograph below shows the ripple voltage between the two dashed lines.

 

 

If we increase the load, the current increases.  This discharges the capacitor more, so the ripple voltage gets bigger, as shown in the photograph below.

 

 

If the current is very high, then the capacitor becomes completely discharged, and there is no smoothing effect at all.

 

 

When working out the value for a smoothing capacitor, you might find this strategy helpful:

  1. Find out the discharge current.  Assume this is constant, unless the discharge current is very large.

  2. Use Q = It to find out the charge lost.  The time is the time between two successive peaks, i.e. the period.

  3. Find out what the required ripple voltage is.

  4. Use Q = CV to work out the capacitance.

Question 6 A half wave rectified supply is used to obtain a pd of 340 V DC which must have a ripple of less than 2 V when supplying a current of 40 mA.  What value of capacitor is needed?  Mains frequency is 50 Hz.  Assume that the current remains constant.

 

ANSWER

 
 

Where it is important that there should be no ripple, a voltage regulator is used.

 

 

Summary

Alternating currents can be rectified using diodes;

 

A single diode will carry out half wave rectification;

 

Two diodes connected to a centre tapped transformer well carry out full wave rectification;

 

Four diodes in a bridge circuit form a bridge rectifier.

 

Capacitors are  used to smooth rectified AC.

 

Ripple voltage = peak voltage – minimum voltage.


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