Understanding The Functions Of Filter Capacitor In Electronic Circuit

Have you ever wonder why there are so many capacitors in electronic circuit board? They are all scattered through out the whole electronic board. You could see mica capacitors, ceramic capacitors, tantalum capacitors, aluminum electrolytic capacitors, paper capacitors and etc. All of them in the circuit have a function and you can find widespread applications in the electrical and electronics fields. Some of the common situations in which capacitors are employed are as follow:

Electric Field

-Power factor correction;

-Motor start and run; and

-Welding (stored energy in capacitor discharged rapidly)

Electronic Field

- Act as filter to remove ripple in power supplies;

- As inter stage signal coupling capacitors- A capacitor will not pass DC, but an AC signal will go through;

- Tuning resonant circuits, oscillator circuits;

- Decoupling in power supplies and amplifiers;

- As part of timing circuit; and

- As waveshapers and filters

In today’s article I’m going to talks about capacitor act as filter to remove ripple from power supplies. Once the AC signal from output transformer passed through diodes it would become an uneven DC. A filtering section is used to smooth out this uneven DC signal.

Usually, the filter includes one or more capacitors. The capacitors are then placed between the DC line and ground. The value of the capacitors is chosen carefully to give the filter circuit a certain timing constant or TC. This TC measures how fast the circuit responds to changes in the voltage level.

If the circuit has just the right timing constant, it can act to smooth the output voltage, and once the voltage leaves the filter stage, most of the variations or “ripple” should be removed. The AC has been converted into a smooth DC.

If the filter capacitors fail either increase in ESR reading or lose some of its capacitance, the AC signal will be allowed to remain with the DC, creating a signal which is noisier than it should it.

This unwanted noise signal can cause problems in electronic circuit. If it happen in computer monitor power supply, the bad capacitors can cause power to blink, low power, voltage shutdown, no display, intermittent display problem ands many more. Now you understand why every time when a monitor has the complaint like above, I would first check all the secondary side filter capacitor with my ESR meter. If all capacitors checked okay I would then move on to another circuit. That’s just one of my troubleshooting methods.

Next, have you seen all the little blue caps around a circuit board? Those small value caps (usually 0.1uf 50 v or 104) are also used to remove noise from electronic circuit. Then what about the function of big filter capacitors we have just mentioned above-aren’t they already removed the unwanted signal?

Why not one filter capacitor is enough for each DC line voltage? The answer is that the big filter capacitors (large value) at the secondary side of power supplies  is to smooth the DC and remove unwanted lower frequency noise. Meanwhile the small blue resin coated ceramic disc capacitor is more effective in shorting very high frequency noise spikes to ground and to preventing it from getting back into the power supply line and traveling to other circuits.

Hope you are now understand why there are two different types of capacitors in the DC supply line. By the way for the small blue capacitor you can check it with analog meter set to X10k ohms (charge/discharge) or just use a digital capacitance meter to check for the capacitance value. The small blue capacitor has very high chances to go fully shorted compare to electrolytic capacitor which is very rare.

A simple power supply circuit