Fundamental of Resistors and their characteristics

Introduction To Resistors

A package of material which exhibits a certain resistance made up into a single unit is called a resistor. Different res. having the same resistance value my be different in physical size and contruction depending on its power and applications.

When current flows through a res., it causes the conductor to become heated up. CFR that are needed to carry large current must be physically big enough so that the heat can be radiated quickly to the surronding air or else it may melt or burn. Therefore, the study of the power derating of the res. is important.

Figure above shows a typical carbon film res. which is commonly used in the market. Chip res. is becoming more common nowadays replacing carbon film res..

Figure below shows the typical form of the CFR.

Values and Tolerance

Unit of resistance is ohms, the symbol for ohm is an omega . Res. values are normally shown using coloured bands. Each colour represents a number as shown in the table below.

Color code of 5 color bands

a) The first band gives the first digit.
b) The second band gives the second digit.
c) The third band gives the third digit.
d) The fourth band indicates the number of zeros.
e) The fifth band is used to shows the tolerance (precision) of the res. Tolerance of res.(fourth band of colour code).Tolerance is the precision of the res. and it is given as a percentage. For example a 1000 ohm res. with a tolerance of �10% will have a value within 10% of 1000, between 1000 - 100 = 900 and 1000 + 100 = 1100 (100 is 10% of 1000).

Example

1st 2nd 3rd 4th 5th Color
Brown Green Yellow Orange Brown
(1) (5) (4) (1000) (�1 %)

This means that the value of the res. is 154k ohm with a tolerance of 1%.

Color code of 4 color bands

a)The first band gives the first digit.
b)The second band gives the second digit.
c)The third band indicates the number of zeros.
d)The fourth band is used to shows the tolerance (precision) of the resistor.

Example

1st 2nd 3rd 4th Color
Brown Green Orange Gold
(1) (5) (1000)(�5 %)

This means that the value of the res. is 15k ohm with a tolerance of 5%.

Power Ratings

Electrical energy is converted to heat when current flows through a res. Usually the effect is negligible, but if the resistance is low (or the voltage across the resistor high) a large current may pass making the res. become noticeably warm. The res. must be able to withstand the heating effect and res. have power ratings to show this.

The power, P, developed in a res. is given by: P = I� � R or

P = V� / R where: P = power developed in the res. in watts I = current through the res. in amps R = resistance of the res. in ohms V = voltage across the res. in volts

Examples:

A 390 res. with 12V across it, needs a power rating P = V�/R = 12�/390 = 0.37W. In this case a standard 0.5W res. would be suitable. A 27 res. with 9V across it, needs a power rating P = V�/R = 9�/27 = 3W. A high power res. with a rating of 5W would be suitable.

Figure below shows the typical power derating curve at different temperature of the resistor.

Figure below shows the ratings example of a carbon film resistor with its maximum voltage values for 0.5W and 0.25W. A jumper is used to connect circuits on the PCB generallly has a resistance of less than 50 mohm and current rating of 2A.

Recommendation when using resistors

a) Keep the rated power and ambient temperature within the specified derating curve. When positioning and mounting Carbon Film Resistors, make allowance for the effect of heat generated through close contact between the Resistors and neighboring components and for the temperature rise of adjacent heat-generating components.

b) If a transient load (heavy load in a short time) like a pulse is expected to be applied, check and evaluate the operations of the Resistors when installed in your products before use. When applying pulses to the Resistors, keep the pulse peak within the rated voltage.

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