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  UNDERSTANDING ELECTRONIC CIRCUIT PARTS:

RESISTORS

Resistors are fundamental and a common part in electronic circuits. Defined as a device that limits the current flow or reduce the voltage in an electronic circuit, a resistor is used for setting biases, controlling gain, fixing time constants, matching and loading circuits for voltage division, heat generation, and other related functions.

 
  Two common types of resistors:

Classification of resistors:

Photo showing some common types of resistors.
 
  The basic unit of resistance is ohm Ω. In electronic circuits, the resistance of a resistor is not the only thing being considered. The tolerance and the power rating of a resistor should also be considered when selecting a resistor.

The tolerance of a resistor indicates how precise the resistance would be close to the designated value. A ±1% tolerance resistor denotes that it is ±1% closer to the indicated value. A ±1% tolerance resistor is usually expensive than an ordinary ±5% tolerance resistor. An example of a ±1% tolerance resistor is shown below. Here in the my country a ±1% resistor cost P5.00@ and an ordinary ±5% tolerance resistor cost P0.25@, imagine the difference, however, we only use ±1% resistors on precise analog/linear circuits.

An example of a 1% tolerance resistor   A closer look to a ±1% tolerance resistor

A typical ±1% tolerance resistor actually has five(5) color bands. The image(above right) shows a closer look to the five bands of a ±1% tolerance resistor. A typical ±5% tolerance resistor has four(4) color bands. A resistor color code chart for reading and identifying a ±5% tolerance resistor is shown in this page.

The power rating of a resistor is also an important factor in selecting a resistor. The maximum power that can be dissipated by a resistor is expressed in Watt. Power is the product of multiplying the square of the current ( I2, expressed in ampere ) by the resistance ( R, expressed in ohm ). With this statement we can now show a proper way of equating power, as shown below.

P = I2 x R
where:
P = Power dissipated by the resistor in watts (W)
I = The current that passes through the resistor in ampere (A)
R = The actual resistance of the resistor in ohms (Ω)



back to top FIXED RESISTORS

Fixed resistors are resistors whose resistance characteristic value does not change and cannot be changed.

Fixed value resistors comes in different composition.
All fixed resistor have the same schematic symbol, shown below.

Schematic symbol of a fixed resistor
Schematic symbol for a fixed resistor


back to top WIRE-WOUND RESISTORS
This kind of resistor is constructed using a special type of wire called a resistance wire. The resistance wire is wrapped around an insulating core. The length of wire and its specific resistivity determines the resistance value of the part. Common types of resistance wires used are tungsten and manganin. The insulated core is made up of porcelain, cement or just a plain press paper. Wire-wound resistors are commonly used in high-current applications with power ranging from 5W to 100W or more and resistance value can be less than 1ohm to several thousands of ohms.

example of a wire wound resistor (ballast resistor)
An example of a wire wound resistor
(top: 0.2R 5W, bottom: 0.1R 5W)


back to top CARBON-COMPOSITION RESISTORS
Carbon-composition resistors are typically made of finely divided carbon or graphite, mixed with a powered insulating material as a binder, in the proportions needed for the desired resistance value. The resistor element is enclosed in a plastic case for insulation and mechanical strength. Joined to the two ends of the carbon resistance element are metal caps with leads of tinned copper wire for soldering the connection into an electronic circuit. These are called axial leads because they come straight out from the ends. Carbon resistors are commonly available in resistance values of 1ohm to 2MΩ.

an example of a 33k carbon-composition resistor
An example of a carbon-composition resistor


back to top FILM TYPE RESISTORS
There are two types of film-type resistors: carbon-film resistors and metal-film resistors.

back to top CARBON-FILM RESISTOR:
The carbon-film resistor is usually made by depositing a thin layer of carbon on an insulated substrate. The carbon-film is then cut in the form of a spiral to form the resistive element. The resistance is value is controlled by varying the proportion of carbon to insulator. As compared to carbon-composition resistors, carbon-film resistors have the following advantages: tighter tolerances, less sensitivity to temperature changes and aging, and less noise generated internally.
A carbon-film resistors tolerance is usually ±5% and has a power rating of 1/8W, 1/4W, 1/2W, 1W and 2W. They are the most commonly used resistor in electronic circuit making and are usually cheap.


carbon-film resistors From the top of photo:
2k /2W
3.3k /1W
100R / 1/2W
33k / 1/4W
280R / 1/8W


back to top METAL-FILM RESISTOR:
Metal-film resistors are constructed in a manner similar to the carbon-film type. However, in a metal-film resistor a thin film of metal is sprayed onto a ceramic substrate and then cut in the form of a spiral. Usually, metal-film resistors are used in electronic circuits where a more accurate tolerance is required.

It was tough for me to find a metal-film type resistor because I seldomly, never have used a metal-film type before. Luckily I found one on our old betamax system. Don't worry guys it's old and it's not working, I just scrapped everyrhing that I could from that, once was famous entertainment system.


metal-film resistor
Metal-film resistor which I found on the power supply part of a betamax system.

back to top SURFACE MOUNT RESISTORS
Surface mount resistors are also called chip resistors. Surface mount resistors are constructed by depositing a thick carbon film on a ceramic base. The exact resistance value of the resistor is determined by the composition of the carbon itself, as well as the amount of trimming done in the deposit.


back to top VARIABLE RESISTORS

A variable resistor is a type of resistor in which its resistance can be set during operation or preset at a certain resistance for accuracy and control. Variable resistors usually has their maximum resistance value marked on them. A typical type of variable resistor used as a volume control adjustment in an amplifier is a potentiometer (POT). For circuits that requires only to set a resistance once or during maintenance to a certain value, a trimmer potentiometer (TRIM POT) is used.The schematic symbol for a variable resistor is the figure shown below.



schematic symbol of a variable resistor


Photo showing two common types of potentiometer.
The photograph on left, shows two common variable resistors. From left, a typical 25kohm potentiometer, right, a typical 100kohm dual potentiometer. The two resistors can be used in volume control. Their values can be adjusted by rotating its shaft either clockwise or counter clockwise. The maximum allowable value for a certain potentiometer is marked on its casing.

  
Photo showing maximum resistance.(In this case 100k)

A trimmer potentiometer is used in applications where its value will only be set to a fixed resistance and would not require regular changing of the resistance. Trimpots, like the photo below are used inside electronic equipments (preferably on the circuit board itself).

trimmer potentimeters

The value is these trimpots are indicated either on the bottom or on the control knob. Reading its value is easy. An example of a marking value is shown on the right photograph. The value 103 is read as 10k, how?

1 -> First digit
0 -> Second digit
3 -> Number of zeros

writing them as digits would be: 1 0 000
and then read it in ohms 10,000 ohms or 10kΩ





back to top RESISTOR COLOR CODE CHART

Example:
1st digit = BROWN = 1
2nd digit = BLACK = 0
Multiplier (number of zeros) = RED = 102 or 100
Tolerance = #FFCC00 = ±5%

10 x 100 = 1000 or 1kΩ with a ±5% tolerance.
RESISTOR COLOR CHART
Color 1st
Significant Digit
2nd
Significant Digit
Number of
zeros (multiplier)
Resistance
Tolerance
Black 0 0 100 -
Brown 1 1 101 1%
Red 2 2 102 2%
Orange 3 3 103 -
Yellow 4 4 104 -
Green 5 5 105 0.5%
Blue 6 6 106 0.25%
Violet 7 7 107 0.1%
Grey 8 8 108 0.05%
White 9 9 109 -
Silver - - 10-2 10%
GOLD - - 10-1 5%