Basic Idea of Resistor

Resistor

Resistor is a two terminal passive electrical device that acts to reduce current through itself. In electronic circuits resistors are most common component.

Symbols:

In PCBs and electronic schematic diagrams we can find various symbols of resistors. Schematic symbols may be different according to its type. Some symbols of resistors are shown below:

 

Types:

 

Connecting with Circuit:

A resistor has no polarity between it’s terminals so it can be connected in any direction with circuit.

Unit of Resistance:

In S. I. unit measuring unit of resistance is ‘Ohm’ and it is presented by greek letter , For example 20Ω, 50Ω, 100Ω etc. But when amount of resistance is very high it can be represented in kilo ohm range for example 20kΩ, 50kΩ, 100kΩ etc. Sometime we can find ‘R’ or ‘K’ with resistance value in many circuits, here ‘R’ means ohm. In this case 10R means 10 Ohm, 10R5 means 10.5 Ohm, 56k means 56 kilo ohm and 5k6 means 5.6 kilo ohm.

Value of Resistance:

Resistance is the property of resistor. A resistor limits current through itselt due to this property. When resistance is high, current flows minimum and when resistance is low, current flows maximum. The value of resistance of a specifie resistor is printed on it. Incase of small resistor such as carbon composition resistor, value of resistance can not be able to write on it. In this case color code system is used to present it’s value.

Calculating Resistance using Color Codes:

Every color code represents a specific numeric value that are shown in table below:

Calculation:

1st band as 1st digit, 2nd band as 2nd digit, 3rd band as multiplier and 4th band considered as tolerance value. Calculating procedure will be cleared from following example:

In the above example we can find a resistor with four color codes, green as 1st band which value is 5, blue as 2nd band which value is 6, brown as 4th band as multiplier which value is ×101 golden as 4th band which value is ±5%. Value of first 2 bands considered as first two digit of ohmic value and they make 56 together. Value of multiplier must be multiplied with first two digit and we get 56×101 = 560Ω. This is the standard approximate ohmic value of resistor, but the real ohmic value may be different ±5% from it.

Power Rating:

When a resistor is working in a circuit it becomes hot due to power loss in it. More power loss generates more heat and finally resistor will be burnt. To avoid this incident, manufacturer companies define a dissipated power limit for every resistor so that it can be used safely during long time. This power limit is called resistors’ power rating or wattage rating. A resistor can be used at any combination of voltages and currents but dissipated power (P=V×I) never be exceed resistors’ power rating otherwise it will be burnt. For example a 10Ω, 2 watt resistor can be used safely when a maximum current 0.333A through itself with 6V between two terminals, because dissipated power, P=6×0.33=2 watt is not more than watt rating. Resistors are found in market of various power ratings such as 0.25 watt, 0.5 watt, 1 watt, 5 watt etc. Resistors of various watt rating are shown below:

Uses:

Most common uses of resistor are current limiter, voltage divider, biasing. Following examples can be considered:

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