What is a Capacitor?

Capacitor

A capacitor works as a small reservoir to store electric charge in it and provide small current in circuit whenever required. It can be also treated as a very small rechargeable battery. It’s a combination of two metallic parallel plates separated by dielectric material between them. The dielectric material can be ceramic, paper, mica, glass, distilled water, dry air etc. 

Capacitor Construction

If a voltage is applied across a capacitor, it charges it and the charge is stored till any load is connected across it which can give a discharge path. There are broadly two types of capacitor i.e. fixed capacitor and variable capacitor. Any capacitor has two specifications i.e. its Capacitance in micro-farad (µf) and maximum voltage which can be applied across it without destroying it. Capacitance shows how much charge it can hold in it, larger the capacitance value, larger the charge it can hold and vice-e-versa. So, any capacitor will have rating as 100 µf / 12V means we can apply a maximum charging voltage of 12V across it without creating any defect. Usually we do not give full rated charging voltage; rather for circuit design purpose we use half the rated voltage I.e. 6V in this example.

Every dielectric material has a dielectric constant denoted by epsilon (ε). So, if we know the area of parallel plates of a capacitor (A), distance between two plates (d) and the dielectric constant, then we can use the formula C = ε × A/d to calculate it’s capacitance. Where C is in Farad, A is in square meter, e is in Farad per meter and d is in meters. And the charge stored in a capacitor is calculated as q = C x V, where q is charge stored in Coulomb, C is capacitance in Farad and V is voltage applied across capacitor in Volts.

So, in a fixed capacitor, area of plates and distance between the plates are fixed. But in variable capacitor, distance between plates are fixed and one plate is fixed the other plate can be moved from external shaft.  As we rotate the shaft of variable capacitor, the overlapping area of two plates varies which changes its capacitance. Variable capacitors are used in radio receivers where we manually tune the radio station by rotating the knob in front of it. Today in age of digital music systems, these variable capacitors no more used.

Fixed capacitor can be divided further in two categories, electrolytic capacitor and non-electrolytic capacitor.  All electrolytic capacitors have polarity marked on body. One terminal is mostly marked as Negative so the other terminal should be considered as Positive. We should connect correct polarity of capacitor in circuit else it will go defective. Electrolytic capacitors have value starting from 1µf and above, and non-electrolytic capacitors have value below 1µf. Also the non-electrolytic capacitors don’t have polarity, so any terminal can be connected to any point in circuit. The symbol in circuit diagram indicates where we need to connect positive and negative terminal. Non electrolytic capacitor can have value as low a 1pico-farad which is equal to 1/1000,000,000,000 Farad.

Capacitor Symbol

Symbol of Capacitor is as shown here, as electrolytic capacitors have polarity hence the two plates are denoted by straight line and a curve line. Straight line denotes positive terminal and curved line indicates negative terminal. For a non-electrolytic capacitor, we use only two parallel straight lines, as it has no polarity.

Now let us have a look at this circuit, when switch S1 is closed, the loop-1 between cell, resistor R1 and capacitor C is completed hence a current flow through resistor and charges the capacitor. As the capacitor charges, the voltage across the capacitor starts increasing and the moment the voltage across capacitor reaches 1.5V, the difference in potential between point A and B becomes same, the charging current stops as current flow only if there is potential difference. 

RC Circuit

That means the flow of charging current through capacitor stop once the capacitor is completely charged. Now let us switch off S1 and close switch S2, this connects the load resistor R2 with capacitor and loop-2 completes and capacitor finds a path to discharge it’s stored charge through resistor R2 hence the discharging current flows through R2 until the capacitor is fully discharged i.e. the potential difference between point A and B is equal I.e. 0V. You can see the graph how voltage across capacitor increases while charging and how voltage decreases while it discharges through R2. It neither charges linearly nor discharges linearly that’s why the charge and discharge graph is also not linear rather curve.

This charging and discharging time T is time period if this charge and discharge cycle. The higher there value of R1 and C, the longer is the charging time. Similarly, higher the value of C and R2 the longer is the discharging time.

Charge-Discharge Graph

Time of any resistor and capacitor circuit can be calculated as t = R x C, where t is in second, R in Ohms and C in farad. So, the charging time t1 = R1 x C and discharge time to = R2 x C. So total time period if this circuit is T = t1 + t2 seconds. So, in any circuit if we want to create a wave of time period of any value, we can decide value of Capacitor and Resistor in circuit and get desired time period. We will see its use, when we make our blinking LED circuit.

Parallel and series connection of Capacitors

Let us see what happen if we connect capacitors in parallel and series, if there Capacitance increases, decrease or remain same. The net Capacitance is sum of 

Parallel Capacitor

all capacitors value connected in parallel, so it is reverse to parallel connection of resistor.  While connecting electrolyte capacitors in parallel we should ensure to connect same polarity together, we cannot reverse the polarity. For non-electrolytic capacitor we can ignore polarity as they do not have polarity. So, net capacitance of parallel connected capacitors is calculated as C = C1 + C2 + C3 + ……….+ Cn.

If capacitors are connected in series, the net capacitance is lower than the 

Series Capacitor

lowest capacitor in series, this again reverse to series connection of resistor where resistance increases in series. We should again note here that positive terminal of one capacitor should be connected to negative terminal of other capacitor and so on, so that at the both ends of series chain finally we have one negative terminal and one positive terminal. If we don’t maintain the polarity, the series capacitor will still work, but will act as a non-polar electrolytic capacitor.

Value is clearly mentioned on electrolytic capacitor; hence it is easy to read. For non-electrolytic capacitor we should know how we can read their value as their size is small and not easy to read. If it is written 1 or 1P, it means 1 Pico-farad. Similarly up to 100 or 100P means 100 Pico-farad. Above that if we have 3^rd digit other than ZERO, work as multiplier. For example, 102 means 10x100 = 1000pf = 1kpf. Similarly, if we have 104 mentioned on capacitor, it means 10x10000 = 100,000pf = 100kpf can also be read as 100nf (Nano-farad).

So, that’s all about capacitors, next we will learn about Inductors or Coils, keep watching.

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