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What is a Filter Capacitor?

By Paul Scott
Updated May 17, 2024
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A filter capacitor is an electronic component that removes voltage or signal spikes in electronic circuits. Capacitors are used as filter devices due to their ability to absorb and effectively store electrical charges at predetermined values. This characteristic is used in a filter capacitor application to soak up or buffer voltage values which exceed set parameters. Filter capacitors are typically placed across a load or as a path to ground in circuits. Several different filter capacitor types include electrolytic, ceramic, and tantalum.

One of the many hurdles electronic circuit designers have to overcome is the removal of voltage and signal anomalies. These may take the form of DC ripple in power supply outputs, audio interference, and switch induced arcing. These anomalies can create havoc especially in sensitive circuits or high-end audio applications. A capacitor's ability to absorb and hold electric charges of known values makes it an ideal filter component to remove these voltage and signal spikes.

A filter capacitor is typically placed across a load or connected in such a way that it offers a controlled path to ground. In this configuration, the capacitor effectively absorbs any voltages above its rating. The stored energy is either passed to ground or dissipated back into the circuit at an appropriate point. The known operational range of capacitors makes it possible for circuit designers to precisely match the filter capacitor to the circuit requirements.

Capacitors used for DC power rail and power supply filtering are typically polarized electrolytic types. This type of capacitor offers high capacitance values in relatively small packages and at low costs. Filter capacitors applied in radio frequency (RF) filtering applications remove signal interference in both signal and power supply lines in radios and other audio equipment. These capacitors are generally ceramic plate or silver mica types with low inductance values.

Filter capacitors of the wound plastic film type and rated for mains power use are commonly utilized as switch-arc suppressors. Ceramic disc capacitors are used on low voltage DC motors to filter the RF interference caused when the motor runs. Computers utilize many filter capacitors in their circuitry with the tantalum capacitor being the most commonly used type. High power cross-over speaker systems use non-polarized electrolytic/ceramic capacitor pairs to cut signal interference. In fact, few circuits do not employ a filter capacitor at some point and they remain one of the cheapest, most efficient, and widely used methods of voltage filtering.

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Discussion Comments

By ladyr — On Feb 14, 2014

What happens to the trace of voltage across the four diode bridge when the 100uF capacitor is put in parallel with the 330 Ohm resistor?

By anon326284 — On Mar 20, 2013

@Kentuckycat: The output voltage you see is obtained from the transformer. These either step up or step down the voltage, as determined by its windings on the primary and secondary sides. The power ratio will tell you if the voltage steps up or down.

By matthewc23 — On Jul 16, 2011

@kentuckycat - In regards to your last question, I think I can help. I had a friend five or six years ago who wanted to create a homemade stun gun. He never got it to work, and we finally had an electrician explain it to us later. I'll try to remember what I can.

My friend found that, while you can charge a capacitor and release the current, the capacitors in normal household objects usually do not carry enough current to be felt by a human, let alone disrupt our electrical system. In other words, you probably won't be shocked by touching the capacitor itself.

The way a stun gun works is by taking electrical current and passing it through transformers that can multiply the voltage. After this happens, the current can reach 10,000+ volts. This higher current can then be sent through capacitors and onto a conductor (a person). The much higher voltage can definitely be felt.

By kentuckycat — On Jul 15, 2011

So, when something like a phone charger says that it is X number of volts, is that because of a capacitor? I've always wondered how the wall voltage of 120 gets knocked down to whatever it say on the plug in unit. Would it be possible to get shocked by a capacitor if the extra current hadn't been dispersed yet?

Along the same lines, is it possible for there to be too much electricity for the capacitor to handle? If I found a way to connect a normal charger to a 240 volt outlet, would it overpower the capacitor? What would happen to the circuit and everything else in the device?

By jmc88 — On Jul 15, 2011

I'm not sure I understand how a motor capacitor is used. I don't really know anything about electricity or motors, so I could use a little more of an explanation.

When a motor runs, isn't its function to turn something like a belt that can power something like a fan? I know there are much more detailed examples, but that is the first one I thought of.

If the motor is turning the belt, how would a capacitor fit into the system? I didn't think that motors generated electrical power that needed to be limited by something like a capacitor. Please help me if you think you could explain it.

By JimmyT — On Jul 14, 2011

If I wanted to identify the capacitor in a piece of electrical equipment, what would it look like? I have seen quite a few circuit boards, but have never known what the different components are or what they do.

I know there is usually a round, cylindrical piece as well as a smaller, round, flat piece with two wires that come out of it. Are one of these the capacitor? If not, what are they, and what does the capacitor look like?

By Izzy78 — On Jul 13, 2011

I always hear the terms capacitor, transformer, and transistor used, but I can never remember which has what purpose. I didn't know that there were so many different types of capacitors for different applications. I assumed they were all relatively similar.

What would happen if you were to take the capacitor out of a circuit board or some other piece of electronic equipment? Would the extra electricity cause the circuit board to overheat and catch fire, or would it simply "fry" the circuit?

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