What is a Smoothing Capacitor?

10uf electrolytic capacitor

A Smoothing capacitor is a capacitor that acts to smooth or even out fluctations in a signal.

The most common and used application for smoothing capacitors is after a power supply voltage or a rectifier.

Power supply voltage can sometimes supply erratic and unsmooth voltages that fluctuate greatly.

When a steady DC signal is needed and is necessary, a smoothing capacitor is the right component needed in order to smooth out the fluctuating signal to make it more steady.

We'll go over an example of this now.

A prime example of when a smoothing capacitor is used is in conjunction with a rectifier circuit.

If you place a resistor in series with a diode and then input an AC signal into the circuit:
Half Wave Rectifiier Circuit

Now if you place a smoothing capacitor in parallel with the diode, like this, the resulting waveform will be:
Half wave rectifier with smoothing capacitor

You can see now how much smoother the waveform is. It no longer goes all the way down to zero and back up. The capacitor charges up from 0 to the top of the waveform and then discharges from 0 to the bottom of the waveform. This charging and discharging smooths out the waveform so that it doesn't hit the extreme ups and downs. Thus, a smoothing capacitor is extremely useful in cases of fluctuating signals that need to be more constant and steady.

Usually when choosing a smoothing capacitor, an electrolytic capacitor is used from anywhere from 10µF to a few thousand µF. The greater the amplitude of the fluctations and the greater the waveform, the larger capacitor will be necessary. Thus, if you're smoothing a 30mV waveform, a 10µF capacitor may suffice to smooth out the signal. However, if you're dealing with a much greater signal, you will need a much larger capacitor, say, maybe 3300µF in order to smooth it out to a near DC level. Experiment with the capacitors. Check the signal on an oscilloscope to see which capacitor suffices best and is best for the circuit at hand.

Related Resources

What is a Coupling Capacitor?

What is a Bypass Capacitor?

What is a Filter Capacitor?

Capacitor Equations

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