What is capacitor?

Capacitor is an electronic device for storing charge. The simplest type of capacitor: parallel plate capacitor shown as figure; two conducting components (plats) with area S separated by distance d and charged with charge, +q and -q.

 In electronic circuit, capacitor is shown like this

One way to visualize the action of a capacitor is to imagine it as a water tower hooked to a pipe. A water tower "stores" water pressure -- when the water system pumps produce more water than a town needs, the excess is stored in the water tower. Then, at times of high demand, the excess water flows out of the tower to keep the pressure up. A capacitor stores electrons in the same way, and can then release them later.

• Charging

 

 When connected to battery, Plate attached to positive pole of battery will loss electrons. Plate attached to negative pole of battery will receive electrons. Capacitor will have the same voltage as the battery.

• Discharging

 

When a bulb is hooked with the capacitor and the battery is replaced by wire, current will flow from one plat of the capacitor to the other and light the bulb until the capacitor completely discharge.

Capacitor has the same function as battery, but with very different way of work. The difference between a capacitor and a battery is that a capacitor can dump its entire charge in a tiny fraction of a second, where a battery would take minutes to completely discharge itself.

The use of capacitor in connection with other electronic devices:
-Reduces voltage fluctuation in electronic power supply
-A capacitor allows pulse signals to flow
-Made up an oscillator circuit if associated with inductive component
-Filtering specific frequency
-Provide electronic time delay

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Resistor - Color Coding and Standard Value

The flow of charge through any material encounters an opposing force similar in many respects to mechanical friction. This opposition, due to the collisions between electrons and between electrons and other atoms in the material, which converts electrical energy into another form of energy such as heat, is called the resistance of the material. Resistance is measured in Ohms (Ω), the capital Greek letter omega. The circuit symbol for resistance appears in figure below with the graphic abbreviation for resistance (R).

 Most resistors have four coloured band. The first three bands determine the resistance, the fourth band indicates its tolerance. Tolerance is how much the resistance varies drom its specified value, given as percentage : e.g 5% tolerance is indicated by a gold fourth band. It means that a 100 Ohm resistor could actually be between 95-105 Ohm.

E.g., the grey-red-brown-gold resistor is 820 Ohm Resistor.

There are also 5 band resistors. The first four bands determine resistance and the fifth indicates the tolerance generally these are a 1% tolerance resistor, much more accurate than 5% resistor. To determine the resistance, the first three rings are the significant value, the fourth ring is the number of zeros following or multiplier. Eg. 1kOhm is brown-black-black-brown-brown

 

Resource : My lecture note

                  Introductuory Circuit Analysis by Boylestad

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The 555 Timer - Monostable Multivibrator

A Monostable Multivibrator is a circuit that produces an output pulse of fixed duration each time the input of the circuit is triggered. This is useful in producing timing signals. We can use 555 as monostable multivibrator by arranging the 555 timer as shown at picture below.

When the trigger input is slightly less then +Vcc/3, the lower op amp has a high output and reset the flip-flop. This cuts off the transistor allowing capacitor to charge. When the threshold voltage is slightly greater than +2Vcc/3, the upper op amp has a high output, which sets the flip-flop. As soon as Q goes high, it turns on transistor and quickly discharge the capasitor.

The output pulsewidth can be calculated by this formula

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The 555 Timer - An Introduction

The 555 timer analog IC (Integrated Circuit) is very economical and convenient for use in mutivibrator circuits since few external components are required. It's packaged as a black box.

The black box has eight connections called ‘pins’ and there is little white dot and dimple denote pin 1. The 555 uses a 3 – 18V DC power supply.

• The 555 timer IC has two comparator, one SR flip-flop and one switching BJT transistor.
• Power supply voltage Vcc is applied to a series of three resistors so that almost 1/3Vcc and 2/3Vcc is established.
• The reset, threshold and trigger control the state of the flip-flop.
• If reset is low, Q output is low regardless of the input applied, and transistor is saturated in this case. So, reset has the highest priority in setting Q.
• When reset is high (connected to Vcc) it does not affect the output Q.
• If the trigger input is lower than 1/3Vcc, then the comparator output is high, setting Q to high and transistor is off.
• If  the threshold input becomes higher than 2/3Vcc, then the output of the comparator is high, resetting Q to low and transistor is in saturation.

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