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:
-Power factor correction;
-Motor start and run; and
-Welding (stored energy in capacitor discharged rapidly)
- 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
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
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