Transcript Resistors - ClassNet

Resistors
• Resistors are used to limit current or
divide voltage, and in some cases,
generate heat
• Resistors come in 2 main categories
Fixed and Variable
• Common resistors are carboncomposition or carbon film, metal film,
and wirewound
Color-code for Resistors with 3
or 4 Bands
• This coding is used for 5%, 10%, and 20%
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tolerance resistors
The first three bands are used to denote the
rated value
The fourth denotes how much the device may
vary from the rated value
1st band is the first digit of the resistance value
2nd band is the second digit of the resistance
value
3rd band is the multiplier (number of zeros)
4th band indicates the tolerance
Precision Resistors
• Precision resistors will have a tolerance of 2%,
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1%, 0.5%, 0.25% or 0.1%
Some precision resistor values are identified
using the color code with 5 color bands,
the first 3 bands indicate the rated resistance
value
the 4th band is the multiplier
the 5th band indicates the tolerance
Color Coded Precision
Resistors
Surface Mount Resistors
• are available as small resistor chips
• They use Alphanumeric Labeling
• Two or three digits, and one of the letters
R, K, or M are used to identify a
resistance value
• The letter is used to indicate the multiplier,
and its position is used to indicate decimal
point position
Surface Mount Resistors
Variable Resistors
• Variable resistors are designed so that
their resistance values can be changed
• A potentiometer is a three terminal
variable resistor used to divide voltage
• A rheostat is a variable resistor used to
control current
Variable Resistors
Power Resistors
• Used when the device must accommodate
high currents
• Physically large surface so it can dissipate
heat into the environment
• Heats the air around the device – may
become very hot (heat is a byproduct of
resistance)
Power Resistors
The Power Rating of Resistors
• The power rating is the maximum amount of
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power that a resistor can dissipate without being
damaged by excessive heat buildup
Power rating is determined by physical
composition, size, and shape of the resistor
The power rating is not related to the ohmic
value of the resistor
The Power Rating of Resistors
• Carbon film and Metal-film resistors are
available in standard power ratings from
1/8 Watt to 1 Watt
• When a resistor is used in a circuit, its
power rating should be at least 25%
greater than the maximum power that it
will have to handle
Relative Sizes of Resistors from 1/8
Watt to 1 Watt
As you can see a resistors ability to handle
power is proportional to its size
The Power Rating of Resistors
• When the power dissipated in a resistor is greater than
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its rating, the resistor will become excessively hot
– the device should be expected to prematurely and
unexpectedly fail
– the resistor may burn up
– its resistance value may be greatly altered
– damaged resistors may be visually detected by charred
or altered appearance of the surface,
– When a resistors power is exceeded, the suspect
resistor should be removed from the circuit and checked
with an ohmmeter