Capacitors are used to store an electrical charge and are used in timer circuits. If the supply voltage is too high or the circuit has sudden voltage changes, the capacitors prevent voltage spikes. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F.
Reference by http://www.youtube.com/watch?v=zNpbdptwxlQ&feature=player_embedded
EXPERIMENT No. 5
The capacitor stores electric charge.
Types of capacitor:
Non-electrolytic capacitor
· No polarity requirements - they can be inserted either way into a circuit.
· Can take a fairly high voltage.
Variable capacitor
· 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
· 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
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.
* 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.
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.
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 1 500 500
2 100 0.1 50 50
3 100 0.47 235 200
4 330 1 1.65s 1.5s
Calculated Time, R(F) X C(Ω) X 5=T
1uF=0.000001F, 100uF=0.0001F
1. 0.0001F X 1000Ω X 5=0.5S(500ms)
2. 0.0001F X 100Ω X 5=0.05S(50ms)
3. 0.0001F X 470Ω X 5=0.235S(235ms)4. 0.00033F X 1000Ω X 5=1.65S
Label the axis of each graph:
Circuit 1:
Capacitance 100uF Resistance 1KΩ
2 : The supply voltage is charging through the capacitor. The voltage is significantly charged with higher speed at the beginning of charging time. However, when the capacitor is nearly fully charged, the changing voltage is small. The charging time reached at around 500ms.
3 : The supply voltage is fully charged in a capacitor at 12v.Circuit 2:
Capacitance 100uF Resistance 100Ω
2 : The supply voltage is charging through the capacitor. The voltage is significantly charged with higher speed at the beginning of charging time. However, when the capacitor is nearly fully charged, the changing voltage is small. The charging time reached at around 50ms.
3 : The supply voltage is fully charged in a capacitor at 12v.Circuit 3:
Capacitance 100uF Resistance 470Ω
2 : The supply voltage is charging through the capacitor. The voltage is significantly charged with higher speed at the beginning of charging time. However, when the capacitor is nearly fully charged, the changing voltage is small. The charging time reached at around 200ms.
3 : The supply voltage is fully charged in a capacitor at 12v.Circuit 4:
2 : The supply voltage is charging through the capacitor. The voltage is significantly charged with higher speed at the beginning of charging time. However, when the capacitor is nearly fully charged, the changing voltage is small. The charging time reached at around 1.5s.
3 : The supply voltage is fully charged in a capacitor at 12v. When the resistor values are higher, the charging time of a capacitor increases under same capacitance values.
How does changes in the capacitor affect the charging time? When the capacitance is higher with the same resistors, the charging time of the capacitors also goes up.
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