EXPERIMENT No.5 The Capacitor

The Capacitor

The capacitor stores electric charge.

A capacitor consists of two metal plates very close together, separated by an insulator. When connected to a battery or power source electrons flow into the negative plates and charge up the capacitor. The charge remains there when the battery is removed. The charge stored depends on the “size” or capacitance of the capacitor, which is measured on Farads (F).

Types of capacitor:

Non-electrolytic capacitor
	·          Fairly small capacitance - normally about10pF to 1mF ·          No polarity requirements - they can be inserted either way into a circuit. ·          Can take a fairly high voltage.
Variable capacitor
	·          Adjustable capacitor by turning a knob - similar to variable resistors. ·          The maximum capacitance available is about 200pF. ·          Used in radios.
Electrolytic capacitor
	·          Large capacitances - 1mF to 50000mF ·          Warning: These must be corrected the right way round (polarity) or they can explode - the white terminal on the diagram above signifies positive. ·          Black stripe with “-“ shows which terminal is the negative (usually the short one) ·          Low voltage rating – from 25 ~ 100V DC ·          They have a significant leakage current - this means that they will lose the charge stored over time.
Tantalum capacitor
	·          These have the same properties as the Electrolytic capacitor, but they are physically smaller & have lower leakage. As a result, though, they are more expensive.

Identifying Capacitor “Size”

If the Farad “size” is not printed on the capacitor, you may find an EIA code listed. Use the table below to figure out the capacitance

μF	nF	pF	EIA Code
0.00001*	0.01	10	100
0.0001*	0.1	100	101
0.001	1.0 (1n0)	1,000	102
0.01	10	10,000*	103
0.1	100	100,000*	104
1.0	1000*	1,000,00*	105
10.0	10,000*	10,000,00*	106

* Values with asterisk are not usually expressed in this form

RC Time Delay or “Charging Time”

Capacitors take time to charge. It doesn’t happen instantly. The charge time is dependent on the resistor in the circuit and the size of the capacitor. And it is expressed in the equation: R x C x 5 = T.  This is the time it takes to charge up to the applied voltage.

For example, 1,000,000 Ω x 0.000001 F x 5 = 5 seconds to charge to applied voltage. This can also be expressed as 1 MΩ x 1 μF x 5 = 5 seconds.

Capacitors are often used for timing when events take place. And often the voltage only has to get up to about 2/3 the applied voltage, and this happens at about 1/5 the time of their charging. So this is why the 5 is built into the equation. The concept of “time constants” is used here, where whatever the time it takes for a capacitor to build up to the full charge, it takes about 1/5 of that time to build up close to 2/3 of the charge. So you can divide the charge time into 5 segments, and the first time segment is often the time you are interested in.

Practice watching the capacitors charge up in the exercise below.

 

Fig 8-Capacitor Charging Circuit

Components: 1 x resistor, 1 x capacitor. 1 x pushbutton N/O switch.

Exercise: First, calculate how much time it would take to charge up the capacitor. Then, connect the circuit as shown above. Measure the time taken by the capacitor to reach the applied voltage on an oscilloscope. Fill in the chart below. Also draw the observed waveforms in the graphs below, filling the details on each one.

Note: you will need to adjust the time base to enable you to observe the pattern.

Circuit number	Capacitance (uF)	Resistance (KΩ)	Calculated Time (ms)	Observed Time (ms)
1	100	1	500ms	500ms
2	100	0.1	50ms	50ms
3	100	0.47	235ms	235ms
4	330	1	1650ms	2000ms

Label the axis of each graph:

Circuit 1:

Capacitance 100mF                     Resistance 1000

Circuit 2:

Capacitance 100mF                      Resistance 100

Circuit 3:

Capacitance 100mF                     Resistance 470

Circuit 4:

Capacitance 330mF                      Resistance 1000

How does changes in the resistor affect the charging time?

The charge time of the capacitor depends on its size and the size of the resistor in the circuit. The lower the resistance of the resistor, for example 100W the faster it would take to charge the capacitor. A resistor with a higher resistance like 1000W will take a longer time to charge then a 100W resistor using the same capacitor because the resistor limits the current flow which charges the capacitor at a slower rate.

How does changes in the capacitor affect the charging time?

Capacitors take time to charge up depending on the value of the resistor and size of the capacitor. A 330mf capacitor with a 1kW resistor will take a longer time to charge than a 100mf capacitor and same size resistor because the capacitor is determined by the size of the plates, the distance between them and the type of dielectric material used.

   

http://www.techitoutuk.com/knowledge/electronics/components/capacitors/capac.html (reference for the above picture)