You might want to ask what is considered a large value of resistance and what is considered a small value of resistance?
The answer to this question is very simple “it depends”. It depends on who is working. Electric power engineers who work with big hydroelectric power generators, electric power lines and electric power distribution stations would consider something like 0.1Ω a large resistance value. This is because large current exists in the power lines, hence, the resistance of these power lines should be kept very small to prevent energy loss via wasted heat.
(a) An electrician that does wiring in homes would consider 1Ω a large resistance.
(b) A linear circuit designer will consider a resistor larger than 500kΩ as large resistance value. On an IC chip, 50kΩ is considered a large resistance.
(c) Physicists and designers of special measurements equipment would consider 10MΩ and sometimes in special cases greater than 100MΩ as a large value.
(d) Resistor larger than 10MΩ are not common. Resistors larger than 100MΩ and larger are packaged in small glass tubes with the ends sealed. Such large-valued resistors are kept clean to prevent errors caused by current flow across the surface of the package.
RESISTOR COLOUR BANDS
Early, resistors were much larger in size than their modern counterparts, so it was easy to print the resistor’s value on its body.
However, as size reduced, doing so became a not so good idea, so colour codes were devised to indicate the component’s value. Before the advent of improved resistor manufacturing techniques, the tolerance of a color coded resistor was just 20%, there was no need of having a colour band to represent the tolerance of a colour coded resistor. The colour print on the resistor is as shown below
The colour codes are as follows:
Body > End > Stripe Body = 1st digit, End = 2nd digit, Stripe = Multiplier
Body > End > Dot Body = 1st digit, End = 2nd digit, Dot = Multiplier
We shall use the colour code system to give the value of this resistors later.
when resistors became much smaller, the Body-End-Stripe, Body-End-Dot pattern changed to small band painted around the resistors. By having the code in band all around the cylinder, it also made it easier to see the value even if a smaller component was mounted on a pcb.
The colour code has remained pretty much the same over the years with the easy to remember mnemonic big (black) boys (brown) Ravish (Red) our (orange) young (yellow) girl (green) but (blue) virgins (violet) go (grey) without (white).
Table showing the various resistor mnemonic, colour and value
HOW TO CALCULATE RESISTOR COLOUR CODE
The resistance value, tolerance, and wattage rating are generally printed onto the body of the resistor as numbers or letters when the resistors body is big enough to read the print, such as large power resistors. But when the resistor is small such as a 1/8W carbon or film type, this specification must be shown in some other manner as the print would be too small to read.
So, to overcome this, small resistors use colour painted bands to indicate both their resistive value and their tolerance with the physical size of the resistor indicating its wattage rating. This colour painting bands produced a system of identification generally known as a “resistors colour code.”
An internationally and universally accepted resistor colour code scheme was developed many years ago as a simple and quick way of identifying a resistor ohmic value no matter what its size or condition.
It consists of a set of individual coloured rings or bands in spectral order representing each digit of the resistor’s values.
The resistor colour code marking are always read one band at a time starting from the left to the right, you start from the band with colour neither Gold nor Silver, or the band closest to the end of the resistor body with larger width tolerance band oriented to the right side indicating tolerance. By matching the colour of the first band with its associated number in the digit column of the colour chat below the first digit is identified and this represents the first digit of the resistance value.
Again, by matching of the second band with its associated number in the digit column of the colour chat will get the second digit of the resistance value and so on. Then the resistor is read from the left to the right as illustrated in figure 3.0 below;
Figure 2.0 is a typical image of a colour coded resistor, to count the bands, you count from left to right. From the resistor colour code above, the first band is yellow, followed by violet, red and silver.
- The first band gives the first digit of the resistor value
- The second band gives the second digit of the resistor value
- The third band gives the power multiplier of the resistor value
- The fourth band gives the tolerance value of the resistor
Now, let’s calculate the resistance value of the resistor in figure 2.0,
First band: Yellow >>> 4 (first digit)
Second band: Violet >>> 7 (second digit)
Third band: Red >>> X10² (Multiplier)
Fourth band: Silver >>> 10% (Tolerance)
Hence, we have 47X10² = 4700Ω = 4.7KΩ
The tolerance value of the resistor above can be calculated with the tolerance percentage as shown below. For the given resistor above the tolerance is given as
4700 + 470 = 5170Ω
(10/100)X4700 = 470Ω,
4700 – 470 = 4370Ω
Value range 4370Ω to 5170Ω
What this means is that, even if the resistor fails, i.e. if for any reason the resistor did not give the accurate value given by the colour code when measured with an ohmmeter, it’s value should not be below 4370Ω or beyond 5170Ω. The value that can be tolerated from the resistor should be within the range 4370Ω to 5170Ω.
When the colour is not clear, an ohmmeter should be used to measure the accurate value of the resistor. note that, the value given by the colour code is the ideal value, the real value of the resistor is given by measuring with an ohmmeter, which is the instrument used to measure the resistance of a resistor. you can find ohmmeter embedded in a Multimeter.
Note: every 3 band colour coded resistor has a tolerance of ±20%
Resistors with smaller tolerance values are more expensive than resistors with larger tolerance values. Resistors with low tolerance value are more precise in value than their counterparts.
We cannot have resistors with any tolerance value. For example, there are standard multipliers for 5% tolerance value, these are called “preferred values” they are:
10,11,12,13,15,16,18,20,22,24,27,30,3336,39,43,47,51,56,62,68,75,82 and 91.
These standard multipliers are not used to indicate scale factor. They show what values are available for numerical part, not factors of 10. The list of standard multipliers can have any decimal (10) multiplier over the commonly supplied range of 10Ω to 10Ω.
So, we can have resistors of 5% tolerance having values like 100Ω, 130Ω, 150k, etc. but not 140k, 170k, etc.
5 AND 6 BANDS COLOUR CODED RESISTORS
For 5 bands, the 1st, 2nd, and 3rd bands give the digits while 4th band gives multiplier and the 5th band gives the tolerance value. Such resistors are very reliable and precise in value.
The 6 bands resistor has same properties as the 5 bands, only that the 6th band gives the temperature coefficient of the resistor which is measured in parts per million (ppm), this property tells how the resistance of the resistor will change with change in temperature.
in the next tutorial on resistor, we will discuss resistor power ratings, meanwhile consider reading other tutorials below for a better understanding of electronics.