Chapter 2: Capacitor
Jan 26th 2001 , Naveen P N
Capacitors in electronics are like storage tanks. They store charges. Basically a capacitor is made up of 2 parallel plates separated by a very small gap, but there are other ways of making a capacitor too. A capacitor has two terminals.
It is charged by applying a voltage across its terminals, and discharged by shorting the two terminals. When a capacitor is charged, a voltage develops across its terminals even when the charging source is removed. The voltage across the terminals of a capacitor is related to the amount of charge stored in it by the relation:
Voltage = Charge/Capacitance
V = Q / C
Where C is the capacitance of the capacitor and is measured in a unit called ‘farad’, denoted by F. If 1V is developed across the terminals of a capacitor by the storage of 1C(coloumb) of charge then its capacitance is 1 F.
Usually a farad is a large value and for most of the applications the value is expressed in terms of microfarads, nanofarads or picofarads.
1 microfarad ( uF ) = 10e-6 or 1/1000000th of a farad.
1 nanofarad ( nF ) = 10e-9 farad OR 1/1000th of a mic
1 picofarad ( pF ) = 10e-12 farad
( in uF above, ‘u’ is the greek letter mu and not the english letter ‘u’ )
Types of capacitors:
There are many types of capacitors but the two main types are:
Non-Electrolytic capacitors are non-polarised, i.e they can be connected either way in a circuit without having to worry about + & -. The most common is the disc-type capacitor that we normally use in electronics. The other types are ceramic, mica etc. In almost all applications we use the disc-type capacitor which is brown in color and has the shape of a disc. Its value ranges between about a few pF to as high as 1uF. ( You also get non-polarised capacitors of higher values and such capacitors have ‘NP’ written on them indicating Non Polarised)
Electrolytic capacitors are polarised and they are supposed to be connected in a specific way in the circuit. Their + and – terminals have to coincide with that specified in the circuit. They are much bulkier than the non-electrolytic type and hence have to be avoided when possible. They are used only if very high capacitance values are needed. Also the electrolytic capacitors are not very stable regarding their value i.e. their values change slightly with the temperature and other physical parameters. The non-electrolytic capacitors are relatively stabler. Electrolytic capacitors are available usually 1uF and upwards upto about 4700uF!
They are much costlier than the non-electrolytic capacitors.
CAUTION: Connecting an electrolytic capacitor in the wrong polarity may lead to an explosion! ( electrically controlled firecrackers? )
The symbol used for electrolytic and non-electrolytic capacitors are different.
The symbol for non-electrolytic capacitor is:
where the dark lines indicate the two plates, and the thin lines represent the two terminals.
The symbol for electrolytic capacitor is:
The terminal marked as + is the positive (or +) terminal and the other (unmarked) terminal is the negative or – terminal. When + is not indicated, the terminal near the curved line is assumed to be negative. On the actual electrolytic package, the negative terminal is usually indicated by a black line with arrows pointing towards the negative terminal.
When you buy a capacitor at a store, you have to specify 3 things:
1) Electrolytic/ non-electrolytic.
3) Max. tolerable voltage.
1) Electrolytic/non-electrolytic: This depends usually on the value of the capacitance. If its less then 1uF then go for non-electrolytic and for higher values use electrolytic.
2) As mentioned above, the value of a capacitor or its capacitance id specified in uF / nF / pF
3) All capacitors have a max. voltage specified and which is the max. voltage that can be applied across its terminals. If a higher voltage is applied, it may damage the capacitor.
The max. voltage for a non-electrolytic capacitor is usually a few hundred volts and is not specified in circuit diagrams since this voltage is much higher than the supply voltage of many electronic circuits.
For the electrolytic capacitors, the max. voltage is almost always specified in the circuit. If its not specified, assume it to be a little higher than the supply voltage to the circuit. For example if circuit operates at 12V then the electrolytic capacitors can be purchased with a max. voltage of about 16V.Note here that as the max. voltage increases the cost of the capacitor also increases.
How to read a capacitor’s value?
Some capacitors have their values printed in them. Unfortunately, there are various formats for printing the values and only a few can be discussed here:
1) If the printed value is like 101,102,103,204 etc then the value of the capacitor= (first 2 digits X 10 raised to the 3rd digit) pF.
For example if the value is 104 then capacitance = ( 10X 10e4) pF = 10e5 pF = 10e-7 F = 0.1uF
Remember a few of them: 104 = 0.1uF , 224=0.22uF , 103= 0.01uF, 102= 0.001uF
2) If the printed value is like 1K5, 100,220,10K etc,
Then capacitance = (printed value) pF
For example if the value is 10K then capacitance = 10K pF = 10X10e3 pF= 10e-8 F= 0.01uF
1K5 means 1.5K pF and so on.
Fortunately, both the capacitance and the max. voltage are both printed on the electrolytic in plain English!
There are a few capacitance available with color band coding like in the resistors, but their value is in pF and has to be multiplied by 10e-12 after de-coding the value.
Capacitors in Series and Parallel:
(Please read the chapter on resistors before proceeding)
This is very similar to the resistors except the formula for series and parallel connections are interchanged.
If Cab is the effective capacitance of a series or parallel combination then, it is given by.
1/Cab(series) = 1/C1 + 1/C2 + 1/C3 where C1,C2,C3 are individual capacitances.
Cab(parallel) = C1 + C2+ C3
Variable capacitors are available only for very small values like pF and should be normally avoided. They, like variable resistors have three terminals for the same reasons as discussed in the chapter on resistors.
The main use of variable resistors are in the radio and are used for tuning.
The circuit symbol of variable capacitor is:
Applications of capacitors:
Capacitors are as indispensable as the resistor in electronics. You can find them in almost every electronic circuit. They are used mainly in delay circuits like timers, noise suppression(smoothing) , oscillators to name a few.
The capacitor is used to:
1. Block DC
2. Pass AC
3. Store charges.
Formulae to memorize:
2) C(parallel)= C1+C2
3) C(series)= (C1*C2)/(C1+C2)
by Naveen P N
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