Crocodile Clips - Guides
Capacitors
Introduction
Capacitors can be
classed as mini-batteries, storing and discharging current. Find out all
about them here.
Types of Capacitor
There are two types
of capacitor, electrolytic ones and polyester/ceramic ones.
Electrolytic capacitors are polarized, which means that the direction that they face in a circuit matters; they must always have the plus sign connected towards the +ve end of the circuit. They look like the type on the left. In real life, they are the ones which look like little cylinders, often blue and black.
Polyester and ceramic capacitors are not polarized, which means that they can be placed in either direction. In real life they look like little brown circles or (often yellow) cuboids. They are the type on the right of the picture.
Sizes of Capacitors
Capacitors often come in sizes which are fraction of the Farad. A
typical size for say, a 555 astable circuit might by 40µF, thats
40 millionths of a Farad. Compare their sizes below:
|
Multiple
|
Symbol
(Name)
|
Example
|
Notes
|
| 1 millionth (10^-6) | µF (micro) | 40µF | Very common |
| 1 thousand millionth (10^-9) | nF (nano) | 100nF | Not-very common |
| 1 million millionth (10^-12) | pF (pico) | 68pF | Really, really, really rare |
If you have more than on capacitor in a row, you can calculate the total capacitance like this:
Time Constant
The time constant,
or 'RC' chain of a resistor and capacitor in series is calculated using
the formula T = R x C. What this means is that if a 100k resistor and
100µF capacitor were placed in a circuit like this,
you would calculate the time taken for the capacitor to charge up to 2/3s of the supply voltage like this:
|
T = R x C T = 0.1 x 100 |
R in ohms & C in Farads or R in Mohms & C in µF |
Note the units required for the equation.
You can also carry on with the sum. If it takes 10s to fill up the capacitor with 6v (2/3s of 9v) then it will take another 10s to reach 8v (2/3s of 9v-6v), and then another 10s to reach 8.66667v (2/3s of 9v-8v) and so on and so on. From this, you can also see that the capacitor will never quite fill up completely.
You can see a capacitor charging by placing a voltmeter around its two legs. To discharge a capacitor, place any resistor around its two legs.
Questions
1. Calculate the overall capacitance in this circuit:
2. Calculate the time constant for this circuit:
