Chapter 6: Capacitor - Filtering

CapacitorPlanes1

Basically a capacitor consists of two parallel flat conductive plates, separated by a dielectric (insulating) material.

When a voltage is applied to its terminals, the current cannot flow trough the dielectric, so the electric charges accumulate in the plates surface.

 

Cap Ceram PictureUnlike resistors and diodes, the capacitor has the ability to store electric charges at a given time, and deliver those charges to the circuit after a while.

CapElec Picture

 

One of the main applications of a capacitor is power supply filtering. For a better understanding about how a capacitor performs this function, we can make an analogy to a buffer tank.

Let's say that we want to obtain a stream of water rather constant , but our water comes from a very rough sea, plenty of waves.

If we try to connect a pipe directly to the sea, depending of the level of the waves we will get a very variable stream through the pipe, and there will some moments that there will not come any water at all.

Capacitor Analogy 0

Such a poor quality service!

But what if we interpose a buffer tank between the raging sea and our pipe? Waves will keep the tank full, and a decent stream of water will pass through the pipe.

Capacitor Analogy 1

The bigger the tank, the less variation in the level and in the stream.

Ok, do you remember the full wave rectifier from previous chapter? Its output voltage was really rough.

RECTIF

It is not feasible to feed an electronic circuit with such a pulsating voltage, we need to smooth that signal. Here is where the capacitor comes, it goes in parallel with the output voltage:

Rectif Filt 1

Better? The blue signal in the diagram it is the filtered voltage. As you can see, it has a little ripple, but much less than before.

The thing is that when the rectifier output reach the peak value, the capacitor gets charged at this voltage. But when the rectifier output goes down, the capacitor keeps charged, although it falls a little voltage because it is delivering current to the load (a resistor in this case). The difference between Vpeak and the lowest tension that the capacitor reaches is called "ripple" voltage. We can reduce Vripple by increasing the the capacitor's size.Filtering 4

Nice story so far, but ... what kind of capacitor should I choose? what  value? how much ripple will I have? no problem with the voltage?

Well, first of all let's introduce the concept of "capacitance" which is the main magnitude that defines a capacitor. Form Cap 1

It means that if you charge a capacitor with a charge "Q", the tension on its terminals will reach a value "V", and the ratio between those values is the capacitance "C" which is measured in Farads. It defines the capacitance concept but you don't have to remember this equation, it is not of practical use.

As Farad is a unit too large for electronic applications, it is commonly used the microfarad, which is a millionth of a farad.

Form Farad 1

For supply tension filtering purposes mainly electrolytic capacitors are used, they have terminals "+" and "-", so you have to be careful to connect them to the corresponding "+" and "ground" in the circuit.

Knowing the current intensity that is going to be delivered to the circuit, we can calculate the ripple that will result for a given capacitor, or the other way round, if you have decided  the level of ripple, you can calculate the needed capacitor value.

Form Ripple 1

         or the same:Form Ripple 2

 

 

Where:
C: capacitance in Farads
I: current in Amper
f: frequency in Hertz (60Hz for USA and 50Hz for Europe)
Vr: ripple in Volts

Look at the example circuit below:

Rectif Filt 2

Here: Form Filt Examp 1

Then the resulting ripple is:

Form Filt Examp 2

To be continued ...

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