we get a pulsating DC output from rectifier circuit but electronics devices need a constant supply of DC voltage. Because a pulsating DC affects the performance of electronic devices and in some cases it will not work. To make the constant supply from rectifier we use filters in power supply circuits.
An ideal power supply filter removes large fluctuations from the output of rectifier circuit and producers almost constant DC power supply. Fig-1 shows a comparison between rectifier output waveform and a filtered output waveform
Fig-1: Waveform before and after filter
Small fluctuations in the output waveform of a power supply filter are known as ripples.
In capacitor-input filter, a capacitor is used in parallel to the load of rectifier circuit. We will start understanding the function of capacitor input filter using a half wave rectifier circuit. See Fig-2 below where capacitor is connected after the diode but parallel to the load resistance.
Fig-2: Capacitor-input filter circuit
To understand how capacitor reduces the ripples we should concentrate on the positive cycle of AC voltage see figure-3
Fig-3: Positive Half cycle of AC voltage
First Half of Positive Cycle
Just below 0.7 volt diode is off and no current is passing through a circuit (See Fig-4). At 0.7 V to Vp (peak voltage) diode is forward biased till the peak value. During this, current starts flowing and capacitor’s voltage goes on increasing with increasing voltage from 0.7 V to peak.
Fig-4: Capacitor Charging
Second Half of Positive Cycle
When the wave starts decreasing from peak to minimum diode is reverse biased. Because the capacitor doesn’t allow sudden change in the voltage. Due to this the cathode of diode is more positive than anode. Now the capacitor discharges through the load. (See Fig-5).
Fig-5: Capacitor Discharges
The discharge time of the capacitor is determined by RC time constant which is usually greater than time period of the input AC waveform. Refer to the fig-6 you can see the capacitor continuous to discharge until the next positive wave reaches. The diode again forward biased and capacitor starts charging again. This process continues from one positive half cycle to next positive half cycle so the circuit produces small ripples.
Fig-6: Small ripples produced at the output
Now in case of full wave rectifier where there is a less time between peak to peak voltage. We get more smooth wave as compared to half-wave rectifier. See Fig-7
Fig-7: More small ripples in full wave
The variation in the capacitor voltage due to forward and reverse biasing of the diode in the capacitor-input circuit is called ripple voltage. In Fig-8, Vr is the ripple voltage, Vdc is the average DC value of the filtered output and Vp is the peak voltage after rectifier output.
Fig-8: Ripple Voltage
Ripple factor determines the performance of the filter. It tells how much smooth is the output waveform of the filter.
Ripple Factor = Vr/Vdc
How can ripple factor decrease?
You can observe from above equation that if Vr decreases then ripple factor can decrease. We can decrease the Vr by two ways
- Increase Load Resistance
- Increase Capacitance
By increasing one or both things, the capacitor will take more time to discharge but on the other hand next positive cycle reaches. So capacitor starts charging early as compared to before.
Complete Lecture on Capacitor Input-filter in Urdu/Hindi