Tutorial 1 Capacitor Properties

 

Learning Objective

  • Recall that a capacitor can store charge.

  • Understand the properties of capacitors.

  • Explain the choice of a capacitor for a particular application.

 

Capacitors are short term charge stores that hold electrical energy in the form of an electric field. They are used widely in electronic circuits. They are at the heart of all electronic timing devices. They also can act as back up power supplies to memory chips. Another use is to smooth out the ripples from a power supply; in effect they are electrical springs. They are also found in oscillators, signal generators, tone controls (filter circuits) to name a few. A variable capacitor is used to tune radios.

 

At its simplest a capacitor consists of two metal plates separated by a layer of insulating material called a dielectric.

Here are some capacitors. 

And the symbols are below:

There are two types of capacitor, electrolytic and non-electrolytic.

 

Electrolytic

Non electrolytic

Advantages:

  • High capacitance

  • Can have high working voltages.

Advantages:

  • Do not lose charge

  • Polarity does not matter

  • Stable up to 106 Hz (or more)

Disadvantages:

  • Polarity important

  • High leakage current

  • Not stable above 10 kHz

  • Can be damaged by AC

Disadvantages:

  • Low capacitance

 

Question 1

Why is an electrolytic capacitor likely to be damaged by AC?

Answer

 

 

Capacitance is measured in units called farads (F).  A farad is a very big unit, and we are much more likely to use microfarads (mF) or nanofarads (nF). You may even see picofarads (pF)

 

Question 2

1.      Convert these value of capacitors:

1 pF =                                                   F

4.7 mF =                                                F

1200 pF =                                             F  =                                                   nF

3.2 10-6  F =                                      mF =                                                   nF

4.7 mF =                                              mF

Answer

 

Properties of Capacitors

Leakage Current

In an electrolytic capacitor there has to be a current to maintain the aluminium oxide layer. This is about 1 mA. Over a period of time the charge leaks away. This is called the leakage current. Also it is important that the polarity of the capacitor is correct, otherwise the aluminium oxide layer is not made and the component will conduct. The resulting heating effect can result in the capacitor exploding.

 

Working Voltage

All capacitors have a maximum working voltage. All insulators have a maximum voltage at which they will retain their insulating properties. The breakdown voltage is quoted in units of volts per metre, so it is actually an electric field. The breakdown voltage of air is 3000 V/mm, so a 5 mm gap will insulate up to 15 000 V. The actual voltage at which the breakdown occurs depends on the thickness of the material. The thinner the material, the lower the voltage that is needed before sparking will occur. If sparking occurs over a dielectric, then a hole will be burned in the dielectric and that is the end of the useful life for the capacitor.

 

Picture by Ethanbrodsky, Wikimedia Commons

 

In this picture you can see some blown capacitors in a computer power supply. In the middle the case has actually blown off the component, showing its guts. Three of its neighbours are decidedly swollen and were about to go.

 

There are plenty of You-Tube video-clips of capacitors exploding.  Watch these to your heart's content, but don't try it yourself.  It is dangerous.                                                                Watch

 

Question 3

1.  An electronics engineer has a 2500 mF capacitor which has a working voltage of 28 V.  He needs it for an application where 90 % of the time the working voltage of the system is well below 28 V, but in 10 % of the time, the working voltage could rise to over 40 V. 

 

Advise him with reasons whether its a good idea to use this component.

Answer

 

Stability

As capacitors age, their values can change. This too can lead to poor stability in circuits.

 

 

Temperature Coefficient

Capacitors, especially electrolytic, can lose their capacitance, i.e. hold less charge, when they get hot. The decrease in capacitance can change the characteristics of the circuit so much that it will not work properly. Therefore it is essential that the temperature in which the circuit is going to operate at is taken into consideration when designing a circuit and choosing the components.

 

The experiment below shows what happens if a capacitor is cooled with a freezing spray.

 

 

The results are like this:

 

From this graph we can see that:

  1. Mica capacitors are very stable with temperature

  2. Ceramic bead capacitors have a linear relationship.

 

Other types of capacitor have a temperature at which their capacitance is at a maximum. It falls away either side of the optimum.

 

Question 4

1.      The same electronics engineer in Q3 has several 0.047 mF capacitors that he has recycled from some cannibalised equipment.  The system being built will be used in an industrial environment where there are large variations in temperature.  He recycles things to save money.

 

Advise him with reasons whether its a good idea to use these components.

Answer

Data Sheets

Electronic engineers need to know the specifications of the components they are going to use. They refer to data sheets in catalogues, which give them all the information that they need to make a choice. For capacitors, data sheets might include:

 

Value (pF)

Tolerance

( %)

Working Voltage (V)

Temperature Coefficient (ppm/K)

Size

Thickness diameter (mm)

Price (pence)

4.7

0.25

100

0

2.5 5

13

330

5

100

+350 to -1000

2.5 5

13

4700

10

100

100 000

2.5 8

13

22 000

-20 to +80

63

220 000

2.5 10

13

 

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