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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