Analog Electronics

What is a capacitor, uses of capacitors and more

A capacitor is an electronic device that stores energy in form of electrical charges.

Capacitors are one of the most important basic electronic devices used in most electronic circuits. Capacitors have many functions which can be explained on the basis of their characteristics:

  • Charge storing capacity: this is the basic characteristic of a capacitor. Capacitor consists of two metal conducting plates and a dielectric sandwiched between it. Capacitors when provided with the supply voltage get charged and thus can be used as a charge storage device.
  • DC blocking: Capacitor blocks DC and allows the AC signal to pass. Because of this, it is used as blocking capacitors in various circuits, like transistor amplifiers to allow only the AC component of the signal for amplification and block any DC signals.
  • Signal decoupling: Decoupling capacitor is used where we have to decouple a signal in a particular part of a circuit. In other words, the noise generated by one circuit can be grounded using a decoupling capacitor and this will not affect the performance of other parts of the circuit. A decoupling capacitor’s job is to suppress high-frequency noise in power supply signals. They take tiny voltage ripples which could otherwise be harmful to delicate ICs out of the voltage supply.
  • Signal filtering: capacitors can be used as filters in some circuits as they are used to filter the output signal of a rectifier.

   Capacitor construction

A capacitor is constructed anytime two electrical conductors are separated by a dielectric (very poor electrical conductor), so, you and your friend standing opposite you forms a capacitor, cos, you and your friend constitute two electrical conductors or conducting plates, while the air passing in-between you is a poor electrical conductor.

When two conducting plates are separated by a dielectric, a capacitor is formed. See image below:

capacitor construction
figure 1: Capacitor construction

The capacitance of a capacitor can be calculated with the following parameters:

1. The area of the conducting plates (A)

2. The distance of separation between the plates (d)

3. The dielectric constant of the material, which can also be called the permittivity of the dielectric (ε)

capacitance formula

From the formula above, the capacitance of a capacitor increases with increase in the permittivity of the dielectric material, increase in the area of the conducting plates and decrease in the separation distance between the conducting plates.

Voltage rating of a capacitor

The three commonest circuit elements in electronics, resistor, inductor and capacitor cannot be overemphasized in electronic circuit design. Resistor is a heat dissipating element and the other two elements inductor and capacitor are energy storing elements. Inductors and capacitors also dissipate some power due to what we call capacitance and inductive reactances, I will be making a tutorial on this in the future.

The voltage rating on a capacitor is the maximum amount of voltage a capacitor can safely be exposed to.

This means that for a capacitor to remain healthy, there is a particular amount of voltage it is expected to store and not more than that. Just as a person should consume a certain amount of food to remain healthy. Also, alongside the voltage rating of a capacitor is the rated charge value. Hence, a capacitor has designated voltage and charge values. This information is printed on the body of most capacitors.

So, when you go to buy a capacitor, you have to bear in mind the voltage rating of the capacitor you want to buy and its charge value. As a matter of fact, when you go to an electronic shop to buy a capacitor, once you tell the seller: give me 1000µF capacitor ( µF = micro farad the unit of measuring the capacitance of  capacitor) the response you will get will be “at what voltage?” hence, before you use a capacitor in electronic circuit design, you must define the voltage and the capacitance values of the capacitor, you must know what you are working with. Capacitance here refers to the amount of charge the capacitor can take at the rated voltage value, and it is related to voltage in the formula, Capacitance = Charge/Voltage, C = Q/V

capacitor
Figure 2: a capacitor showing voltage and charge ratings

It is important to note that capacitors come in different sizes and shapes, this is because when designing electronic circuit as we have discussed in our series of videos on how to design electronic circuit from scratch, you can watch one of the videos in the series below:

video teaching how to design electronic circuit from scratch

the designer should select components that will accurately suit the shape and size of the design. I will discuss the various types of capacitors in future tutorial.

various types of capacitors
Figure 3: various types of capacitors

Capacitors behave like batteries, in that they store charges, the difference being that capacitors discharge faster than batteries

Note that, capacitor’s voltage rating is not the voltage that the capacitor charges up to, but the maximum amount of voltage a capacitor should be exposed to while charging it.

 Capacitance measurement

The electronic tool used in measuring a capacitor is called a capacitance meter, it is often embedded on a Multimeter.

capacitance meter
figure 4: Capacitance meter

You can as well read our tutorial on how to use a multimeter to measure current and voltage here.

The unit of capacitance measurement is called Farad, F. It is rare to find a capacitor used in electronic circuit board that measures up to a farad. But supercapacitors can measure up to thousands of farads. Capacitors measured in sub-units like microfarad, nano farad and picofarad are used most in electronic circuit design.

1 microfarad abbreviated as µF is equivalent to 0.0000001F, while 1 nano-farad abbreviated as nF is equivalent to 0.000000001F and 1picofarad abbreviated as pF is equivalent to 0.0000000000001F

1 µF= 0.000001F

1 nF = 0.000000001F

1 pF = 0.000000000001F

Capacitor connections

Just like a resistor, a capacitor can be connected both in series and in parallel in a circuit, or the both together. Unlike resistors, when capacitors are connected in series, their capacitance value as a unit go down or is reduced, while this value increases when they are connected in parallel.

two capacitors in series
two capacitors in series
N number of capacitors connected in series
2 capacitors connected in parallel



To calculate the equivalent value of capacitors connected in series, we use the formula:

2 capacitors connected in parallel
2 capacitors connected in parallel
N number of capacitors in parallel
N number of capacitors in parallel

Problem 1: calculate the equivalent capacitance of the capacitors connected in (a) series and (b) in parallel, C1 = 3µF, C2 = 5µF.

solution

Uses of a capacitor

  1. Energy storage  
  2. Motor starting and running
  3. Quick Lighting as seen in cameras
  4. Rectifier output soothing
  5. High voltage pulse circuits
  6. Automobile ignition
  7. Filters of all types
  8. Coupling circuit
  9. Transient voltage damping
  10. Leading reactive power
  11. Power factor correction
  12. Safety through X and Y capacitors
  13. Timing circuits
  14. Biasing circuit
  15. Oscillator
  16. Sensors and measurement
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