BASIC ELECTRONICS - 1

 

 

BASIC ELECTRRONICS



 

 Electric current

Electric current has two types D.C. or direct current and A.C. or alternating current. D.C. or direct current flows in one direction only, and does not change its path. A.C. or alternating current flows in both direction and changes its direction after a certain period of time. The number of time it changes its direction in one second is called frequency. In India the power line come to our homes has a frequency of 50 Hz. In some of countries it has frequency of 60 Hz.

Voltage

 

Voltage is a force or source of energy to push –ve charge substances from +ve terminal to –ve terminal of battery or power supply. For example, chemical reaction inside a battery provide energy required for current flow from +ve terminal to –ve terminal. It can be understood by an example, a fan while rotating produce a force to flow air. Higher the rotation of fan the more pressure will be produced to air flow. In an electric circuit this pressure is called voltage. The higher the voltage (pressure) the more current will flow into the circuit.

Resistance
 
Resistance is a basic component in electronics. It reduces flow of electric current (A.C. or D.C.) in a circuit, if connected in series resulting in drop of voltage across it. How much current or voltage will drop depends on the value of resistor. Each resistor has its certain value printed on it in colour code. However, the value printed on wire wound resistor is in numeric term. The resistor comprises three types carbon film, metal film and wire wound. The vlaue of resistors is measured in ohm and displayed as Ω. The unit ohm named after German scientist Georg Simon Ohm. In A.C. circuits, electrical impedence is also measured in ohms (example - loud speakers) By nature or by character the value of a resistor remains constant under a certain range of voltages, temperatures and other paremeters too. This is the one type of resistor which is used frequently in electronic ciruits. The other type of resistor is thermal resistor or thermistor. The important point is its value changes significantly with temperature. Thermistors have two types, NTC or Negative Termperature Coefficient and PTC or Positive Temperature Coefficient. The value of NTC decreases with the rise of temperature whereas the value of PTC increases with the rise of temperature.  NTCs are mainly used in inrush current limiter circuits, temperature sensor circuits etc.  PTC are used in Colour Television Degaussing circuit.

 
How to read the colour code of resistor.

 

The above figure shows the value of each colour and how to read the value of resistor. Hold the resistor in your hand so as the group of three colour is on your left hand side and the tolerance colour code is on your right hand side. The first two colours have most significant value i.e. the actual value of each colour and the third colour represents number of zeros to be placed after the value of first two colours i.e. most significant value. The fourth colour ring is for tolerance, gold represent 5% and silver represent 10% tolerance.

 

 

For example, in the above figure, at right hand side, first two colours are brown and black, the most significant value is 10 and third colour is orange which has the value of 3, therefore 3 zeros will be placed after 10. It means the value of resistor will be 10,000 ohms or 10 Kilo Ohms or 10K (in practical term). At left hand side the value of resistor is 100 ohm since the third colour is brown and brown has its value of one, so one zero will be placed after 10 (value of first two colours).



Another example is given in above figure, at left hand side, the value of resistor is 10 ohm, since the third colour is black and it has its value of zero, so no zero will be placed after 10 (value of first two colours). At right hand side the value of resistor is 5.6 ohm, in this case the third colour is golden and if the third colour is golden the value of first two colours (the most significant value) will be divided by 10 to work out the actual value of resistor.

How to use resistor in a circuit

A resistor can be connected either in series or parallel in a circuit. If a resistor connected in series with load (lamp) will reduce the current flowing in the circuit (Fig-2) and if connected in parallel with load (lamp) will increase the current flowing in the circuit (Fig-3).  See figure below.

How to calculate resultant value of resistors

Below in Fig-1 resistors R1, R2 and R3 are connected in series, the resultant value between points A and B will be derived from the equation mentioned below.

In Fig-2 resistors R1, R2 and R3 are connected in parallel, the resultant value between points A and B will be derived from the equation mentioned below.