DC Theory 5 Segment 2
Resistors and the Color Code
Now let's consider resistors and how they
are made.
A resistor limits current flow to safe values
for components
or reduces voltage to allow for proper operation
of circuits.
As you have seen before, the symbol for a
resistor is a jagged line
representing the fact that a resistor puts
up an opposition to the flow of current.
Let's take a look at how they are made.
Carbon is the most commonly used material
for a resistor.
There are several common types of fixed resistors.
At first most resistors were made of a simple
carbon composition.
At the core of these resistors are carbon
granules to which wire leads are attached.
More carbon increases the conductivity and
lowers the resistance value.
An insulating material is used to coat the
carbon for strength as well as insulation.
Now, the most common type of resistor is the
carbon film type.
A carbon film is deposited on an insulated
form and then spiral cut around the form.
The longer the strip cut by the spiral, the
higher the resistance.
For very high precision requirements some
resistors are manufactured using a
special metal film that doesn't change resistance
as temperature changes unlike carbon.
As with carbon film resistors, the deposited
film is cut, usually with lasers
to form the spiral ribbon of resistive material.
The 4th common type of resistor is the wire
wound resistor or power resistor.
Such resistors are made using nichrome resistance
wire
which allows high amounts of heat to be safely
generated within the resistor.
Resistors have 2 ratings, their Resistance
of course and their heat capacity.
The heat capacity of a resistor is called
its Power Rating.
Generally speaking the larger the size of
the resistor the higher the power rating.
Typical power ratings are 1/4 watt, 1/2 watt
and 1 watt for low power resistors
while wire wound resistors are rated for 5
watt 10 watt and 25 watts or more.
When designing electronic circuits, heat or
power generated in the resistors must be considered.
Otherwise the results can be catastrophic.
As a safety margin, engineers always select
a resistor with a power rating
twice the power to be generated in the resistor.
In other words, if a resistor will actually
consume 1/4 of a watt of power, a 1/2 watt
resistor would be placed in the circuit.
The Ohmic value of a resistor, its Resistance,
is often indicated using a color code system.
There are a couple of reasons for this.
The value could be printed on the body of
the resistor
but some resistors are so small that you probably
could not read the numbers.
The second reason for the color ring is that
since they go fully around the resistors body
the value is visible no matter which way the
resistor is mounted.
There are normally 4 colored bands around
the resistor.
To interpret the color code we need to know
2 things.
What color represents what number and how
the 4 bands represent the number of ohms.
Black represents zero.
Brown is 1, Red is 2, Orange is 3, Yellow
is 4
Green is 5, Blue is 6, Violet is 7
Gray is 8 and White is 9.
To remember the color code, try using a phrase
that has the first letter of each word
represent the first letter of each color in
order.
Here's one for you.
Bad boys run on your grass but Violet and
Greg won't.
Colors going from black to white are Bad = Black,
Boys = Brown
Run = Red, On = Orange, Your = Yellow, Grass
= Green, But = Blue
Violet = 7, Greg = Grey, Won't = White.
Whenever you are working with resistors it
is a good idea
to write down each letter and its number as
you begin
so you don't have to think about it over and
over.
To read the value of a resistor, always begin
with the color nearest the edge of the resistor.
The first color band represents the first
digit of the value.
On this resistor it is brown.
Looking at our table of colors we see that
brown represents 1.
So the first number of the resistor's value
equals 1.
The 2nd band represents the 2nd digit.
Black in this case represents 0.
The second number of the resistor's value
equals 0.
So the first two colors represent the first
two digits of the resistor's value, one zero.
Since the third band on the resistor is orange,
and orange represents 3
you might think that the 3rd digit of the
resistor's value equals 3.
In fact only 2 digits are normally use to
represent the value
with the third color representing a multiplier
factor or number of digits.
As a multiplier, the 3rd color represents
a power of 10.
Since 10 to the 3rd power is 1,000 or 1 Kilo
as multiplier,
the first two digits would be multiplied by
1,000 or 1K,
which equals a value of 10,000 or 10 K ohms.
Thinking of the 3rd band simply as the number
of zeros to add
the value would be read as Brown representing
1, Black representing zero
and orange for 3 zeros, giving us a value
of 10,000 ohms, or 10K Ohms.
Here's a similar resistor with the same first
two bands, but the third band is also black.
Since black represents zero, you might think
that the 3rd digit of the resistor's value
is zero,
but remember that only 2 digits are normally
used to represent the value
with the third color representing a multiplier
factor, or number of zeros.
Since the 3rd band represents the number of
zeros, black means to add zero zeros.
So the resistor's value would be 10 ohms.
Now what of the 4th color band on a resistor?
It represents an accuracy factor, or tolerance
for the resistor.
The tolerance band is most often silver or
gold
Silver represents +/- 10% accuracy.
Gold represents +/- 5% accuracy.
Our original resistor has a silver 4th band
representing a plus or minus 10% accuracy.
10% of 10,000 means to multiply it by .1,
or to move the decimal point one place to
the left
giving a tolerance value of 1,000.
The resistor's value could range from the
high range of the color code value
of 10,000 plus 1000 ohms, or 11,000 ohms
to a low value of 10,000 minus 1000 ohms,
or 9,000 ohms.
So our resistor, color coded brown black orange
silver
has a value of 10k ohms with a 10% tolerance.
Its value could range anywhere between 9,000
to 11,000 ohms
and be within acceptable tolerance.
Red, representing 2% accuracy, and brown for
1% accuracy
are sometimes seen as tolerance bands, but
they are unusual.
Most often, highly precise resistors have
their value stamped on them.
No color code is used at all.
This is particularly true of the highly precise
metal film resistors.
By the way, earlier we said to read the resistors
starting with the color closest to the edge.
Some resistors have their colors distributed
evenly across the body
In this case, look for the tolerance band,
virtually always silver or gold
and then work from the other end of the resistor.
