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Halfwave Rectifier with Capacitor Filter and Ripple Factor Calculation

The ability of diode to conduct current in one direction and block it in another direction, it can be used as a rectifier. Rectifiers are the electrical circuit which converts the AC voltage to DC voltage. All the electronic appliances are working on DC voltage rather than AC, so rectifiers are an essential part of all electronic appliances. The simplest rectifier is half wave rectifier.

The following diagram shows the half-wave rectifier circuit where the diode, load and sinusoidal AC source are connected together.

Half wave rectifier circuit diagram

Working of half wave rectifier:

For the positive half cycle of the input sinusoidal voltage, the anode of the diode is connected with the positive side of source and cathode is connected with the negative side of source and diode become forward bias. So, for the positive half cycle, the output is the same as input ideally. The current will pass through the load resistor during the positive half cycle. For practical purposes, the output voltage will be less by 0.7 volts. Before the diode becomes forward bias the input must overcome the barrier potential of the PN junction, that’s why the output in practical diode will be less by 0.7 volts.

For the negative half cycle, the anode of the diode will connect with the negative side of source and cathode will connect with the positive side of source and diode become reverse bias. Ideally, the diode will act as an open switch and no current will pass through the load resistor. But practically there will be small leakage current. Another thing is that diode can withstand up to breakdown voltage. So the reverse blocking voltage must be in the range of the withstanding voltage. For the safer operation, the maximum input voltage must be 20% less than that of PIV (Peak Inverse Voltage) rating of the diode.

The output of the half wave rectifier does not change the direction of current in the load resistor, that’s why it is called DC voltage. But the magnitude of the voltage varies with time so it is called pulsating DC voltage. A steady-state DC can be achieved by using a filter circuit.

The average value of the input sinusoidal voltage is zero because of the same area above and below the axis line. Where the average value of the output can be calculated as follows

$v_{avg}=\frac{V_{p}}{2\pi }(\int_{0}^{\pi }{\sin t dt}+\int_{\pi }^{2\pi }{0 dt} )$

$v_{avg}=\frac{V_{p}}{2\pi }(2+0)$

$v_{avg}=\frac{V_{p}}{\pi }$

$v_{avg}=0.318 V_{p}$

Half Wave Rectifier Average Output Voltage

The DC voltmeter will measure the average value of the half wave rectifier.

Filtered Output of Half Wave Rectifier:

The output of the Half Wave rectifier is pulsating DC instead of steady-state. Where the electronic devices work on steady-state DC and some device may response unexpectedly for such type of pulsating DC. A filter circuit may be required to convert the pulsating DC to steady-state DC, where a simple filter circuit can be capacitor input filter. In the capacitor input filter circuit, the output of Half Wave rectifier is passed through a capacitor as the following circuit shows.

Half wave rectifier Circuit with Capacitor in filter

For the first quarter of the positive cycle of the input voltage, the capacitor will charge up to the supply maximum voltage Vp. For the second quarter of the positive cycle, the diode will become reverse bias because of the cathode at a higher potential than the anode. So, for the rest of the cycle, the capacitor will provide current to the load and discharge until the supply voltage becomes more than that of capacitor voltage. As the input voltage increased from the capacitor voltage the capacitor will again start charging and the chain will remain. The discharging time of the capacitor depends upon the RC time constant.

Filtered Output of Half wave Rectifier

In the filtering action, the capacitor charges quickly and discharge slowly because of load resistance. That cause a change in voltage across the capacitor, which is undesirable and called ripple voltage.

$v_{r(pp)}\approx (\frac{1}{fR_{L}C})v_{p}$

$v_{DC}\approx 1-(\frac{1}{2fR_{L}C})v_{p}$

A measure of the effectiveness of the filter can be judged by the parameter called ripple factor. The formula of the ripple factor is the ratio between ripple voltage (peak to peak) and DC voltage.

$r=\frac{v_{r(pp)}}{v_{DC}}$

Conclusion:

  • A rectifier converts AC voltage to DC voltage.
  • Half wave Rectifier only passes current through load during the positive half cycle of sinusoidal.
  • The output of half wave rectifier is pulsating DC voltage, to convert it to a steady state, a filter is used.
  • The effectiveness of the filter can be measured by the ripple factor.
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About Shahab Yousafzai

Shahab Yousafzai is a professional electrical engineer, who has a history with the field since 2008. He is happy to share his experience and thoughts over the forum. He is interested in Power Electronics, Analog Electronics, and Electric Machines.

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