Measuring A Resistor | Understand Resistor Color Codes

Practical ways to measure resistance:

As a beginner, you might find it difficult. Believe me, it is simple and fascinating. You can measure resistance using a multimeter or with the help of colour codes.

Using Multimeter:

An ohmmeter is an instrument used to measure the value of resistance. Today, you might not find an ohmmeter. Because it is a part of a multimeter.

A multimeter is an instrument that combines several measuring instruments. Usually, you can measure voltage, current and resistance with a multimeter.

A multimeter is a handy instrument for measuring resistances. Adjust the multimeter knob such that it can measure resistance. There are various ranges available like kΩ, MΩ etc. Place the resistor in parallel with the multimeter. You can also check for open and short-circuit.

Touch the probes with resistor legs. If the meter reads “1” or “OL”, it means overload. Select higher ranges. If the meter reads “0.00”, then select lower ranges.

Keep in mind that the resistance may vary due to temperature and tolerance level.

Understand the colour coding:

Even if you are a beginner, you must have seen a resistor. You also observe the different colour bands painted on resistors. Each colour represents a number and every band has a different value. These colour bands help determine the resistance of resistors as well as the tolerance level. Now we will learn how to read the resistor value from these colour codes.

Colour-code is a system of standards for the identification of resistances of resistors. The colours painted on the resistor body are called colour bands.

There are three different types of marking standards for resistors. Some resistors are marked with four bands of colours, some are marked with five bands of colours. We will see each standard in detail in the next section.

The multiplier band is a decimal multiplier.

The tolerance band gives you accuracy. It indicates the difference between the actual value and theoretical value. It is measured in percent. You can measure the actual value with the help of a multimeter. While theoretical value can be determined from the colour codes. The gold band means +/- 5% tolerance. A 1000 ohm resistor with a gold band means its value is between 950-1050 ohms.

TCR stands for the temperature coefficient of resistors. It is defined as the rate of change of resistance of a resistor with the temperature change. It is available in high precision resistors only.

Digits	Colour Codes	Multiplier	Tolerance	TCR
0	Black	100
1	Brown	101	+/- 1%	100
2	Red	102	+/- 2%	50
3	Orange	103		15
4	Yellow	104		25
5	Green	105	+/- 0.5%
6	Blue	106	+/- 0.25%	10
7	Violet	107	+/- 0.1%	5
8	Gray	108
9	White	109
	Silver		+/- 5%
	Gold		+/- 10%

Four band resistors:

Four band resistors are the most commonly used. The placement of colour bands on the resistor is very important.  3 bands are painted on the left, while 4th is on the right. Put a resistor read it from your left, the tolerance band is on your right.

• 1st band represent the first significant value
• 2nd band represent the second significant value
• 3rd band is a multiplier.
• 4th band is a tolerance band. The tolerance band is separated from the others.

You can get your resistor value with the help of the following formula. This is applicable for +/- 5 % tolerance carbon film resistors.

R = (a*10 + b)m +/- tolerance

Where,

a and b are the values of the first and second band,

m is the multiplier band.  

Example:

Look at the figure above, it shows a 4 band resistor. Brown, black, red and gold colour band. How do you know the resistance value?

Brown >> 1 >> 1st significant digit

Black >> 0 >> 2nd significant digit

Red >> 2 >> multiplier

Gold >> +/- 5% >> tolerance

The theoretical resistance of above resistor is 10*102 = 10*100 = 1000 ohms.

The actual value may vary from 950 ohms to 1050 ohms. Evaluate 5% of 1000 ohm, which is 50. The 5% tolerance shows the precision of the resistor.

Five band resistors:

The placement of colour bands on a five-band resistor is such that 4 bands are painted closely while the 5th band is painted such that it is separated by a small distance.

• 1st band represent the first significant value
• 2nd band represent the second significant value
• 3rd band represent the third significant value
• 4th band is a multiplier
• 5th band is a tolerance band. The tolerance band is separated from the others

Example:

Look at the figure above, it shows a 6 band resistor. Brown, blue, black, yellow, brown, red colour band. How do you know the resistance value?

Yellow >> 4 >> 1st significant digit

Violet >> 7 >> 2nd significant digit

Black >> 0 >> 3rd significant digit

Red >> 2 >> multiplier

Brown >> +/- 1% >> tolerance

The theoretical resistance of above resistor is 470*102 = 470*100 = 47000 ohms = 47 Kilo ohms.

To evaluate tolerance level, calculate the 1% of 47000, which is 470. The actual value of the above resistor may vary from 46530 to 47470.

Six band resistors:

The placement of colour bands on a six-band resistor is such that 4 bands are painted closely while the 5th and 6th bands are painted such that they are separated by a small distance.

• 1st band represent the first significant value
• 2nd band represent the second significant value
• 3rd band represent the third significant value
• 4th band is a multiplier
• 5th band is a tolerance band. The tolerance band is separated from the others
• 6th band TCR

Example:

Look at the figure above, it shows a 6 band resistor. Brown, blue, black, yellow, brown, red colour band. How do you know the resistance value?

Brown >> 1 >> 1st significant digit

Blue >> 6 >> 2nd significant digit

Black >> 0 >> 3rd significant digit

Yellow >> 4 >> multiplier

Red >> +/- 2% >> tolerance

Violet >> 5 >> TCR

The theoretical resistance of above resistor is 160*104 = 160*10000 = 1600000 ohms = 1.6 Mega ohms

To evaluate tolerance level, evaluate 2% of 1.6*106, which is 32000. The actual value of the resistor may vary from 1.56*106 ohms to 1.63*106 ohms.

Frequently Asked Questions:

Why don't manufacturers print numerical values on resistors?

Preciously, it is very difficult to print numerical values on a tiny component like resistors. Modern printing technologies are also available to print numerical values on resistors. But the colour-coded resistors are still popular.

A disadvantage of resistor colour codes

Colour blindness is a common problem, colour-coded resistance value might be problematic for such people.

Why do we measure resistance when there is no power?

It is advisable to isolate the resistor you want to measure and switch off the supply. This is because, when you place the multimeter probes to a resistor present in a circuit, it provides a small voltage and current flows through the resistor. With the help of Ohm's law, the resistance multimeter calculates the resistance.

If there is a supply voltage, the measured value of resistance is wrong. Also, if a resistor is placed in a circuit, other components may affect the value of resistance.

Checking for a defective resistor

A resistor can burn easily with over-voltage and is responsible for malfunctioning. A defective resistor can either be short or open internally. Now, we aim to check whether a resistor is defective or not.

Place the multimeter probes to the resistor, if the reading is too high as compared to its rated value then it is open-circuited internally. If the resistance is too low and approaches zero, then it is shorted internally. 

Resistors & Resistances - Definitions, Composition, Construction

Resistors - Introduction, Effect of Temperature on conductors and Semiconductors

In this article I am going to introduce the most important and basic passive element, that is a resistor. Resistor is a tiny two terminal element, that is used in electrical and electronic circuits for variety of purposes. Like divide voltages, limit currents, adjust signal levels, bias active elements etc. It is mainly used to limit the current through each component. The larger the resistance, the smaller the current.

Mostly students think this is a redundant element or the resistance is responsible for signal degradation. The article will help you to understand the importance and practicality of resistors in different types of applications. This is a detailed article, I tried to cover every aspect and every question on this topic.

Outline:

• What is resistor and resistance?
• Resistance of metals and semiconductors
• Temperature dependence of resistance
• 2 practical ways to measure resistance
• What is inside the resistor (construction, advantages, disadvantages, applications of different types of resistors)

• Types of resistors

• Linear resistors
• Variable resistors
• Semi Fixed resistors
• Fixed resistors
• Non linear resistors
• Thermistors
• LDR (Photoresistor)
• Varistors
• Special type of resistors
• Zero ohm
• Fusible resistor

• Series connected resistors
• Voltage division rule for series connected resistors
• Parallel connected resistors
• Current division rule for parallel connected resistors

Resistor, Resistance & Resistivity :

What do you know about resistors? It decreases the rate of flow of charges through the circuit.

Resistance is the measure of flow of charges through the circuit. While the opposite quantity is conductance, which is defined as the ease with which charges flow through the circuit. Every component in a circuit offers some internal resistance, it could be an ohmic resistance or non ohmic resistance. 

The electrical resistance of a component can also be defined as the ratio of voltage applied to the current flows through it.

R = V/I

The electrical resistance of a wire is dependent on size and geometry. Larger (L) wires offer more resistance. For larger cross sectional area (A) of wires offer less resistance.

R = ρ * L/A

Where, ρ is the coefficient called resistivity of the material.

Resistivity can be defined as resistance offered by per unit length and per unit cross sectional area. It is measured in (Ω⋅m). It is the ability of material to conduct or resist the flow of charges through the material. The higher the resistivity, the lower the flow of charges through the conductor.

Resistance and resistivity are temperature dependent. So it's time to understand temperature dependence of resistance.

Resistance and temperature dependence:

While, learning basic electronics we come across two types of conducting materials that are metals and semiconductors. Connecting wires, contacts are made from metals whereas electronic components like diodes, transistors are made from semiconductors. We have to visualize the behaviour of both materials at higher temperatures.

Effect of temperature on resistance of a conductor:

In pure metals, the resistance increases with increase in temperature. They have positive temperature coefficient (α)

                    Rt = Ro (1+ αot)

In metals or good conductors, there are large number of free electrons available. As the temperature increases, molecular vibrations increases violently. Due to these vibrations, the mean free path of molecules decreases. The molecular vibrations disturbs the motion of free electrons, and the electrons can't not move freely. They experience more frequent collisions from vibrating molecules. This is the basic cause of resistance or hindrance in the flow of free electrons at higher temperature. Or in other words increased temperature decreases conductivity of the metals.

Effect of temperature on resistance of a semiconductor:

In semiconductors, the resistance decreases with increase in temperature. They have negative temperature coefficient.

In semiconductors, there are a few free electrons at room temperature. An increase in temperature, the loosely bound valence band electrons get enough energy and escape from their parent atom. It produce greater number of free electrons. Resistance of semiconductors decreases with an increase in temperature. That's why it has negative temperature coefficient. You can further read about temperature dependence of semiconductors here.

What is inside the resistor?

Have you ever thought about what is inside the resistor? A 10 ohm resistor and a 10k resistor both have same size, two terminals but different values.

Actually resistors are made up of different types of materials having different resistivity values. R = p*L/A where p is the resistivity. Resistance can vary by varying these three quantities.

• Resistivity (p) varies different for each material
• Length (L)
• Cross sectional area (A)

Frequently Asked Questions:

Difference between ohmic and non ohmic resistance:

Ohmic resistance, which obeys Ohm's law or voltage and current have linear relation. Or the IV characteristics graph is a straight line. Current and resistance are temperature dependent. It is static resistance. Example, a metal.

Ohmic and non-ohmic resistances (linear and non-linear resistance)

Non ohmic resistance, which doesn't obey Ohm's law or voltage and current doesn't have linear relation. Or the IV characteristics graph is not a straight line.Current and resistance are not temperature dependent. It is dynamic resistance.  Example, semiconductors.

Difference between resistance and resistivity:

Resistivity: Each material has different ability to resist the flow of current. Some materials allow more charges to flow, while other offers more resistance. It measures how strongly a material can oppose charges to flow.  This is known as resistivity or specific electrical resistance. It is the physical property of the material of the conductor, it doesn't depends on shape or geometry of the conductor. Resistivity depends on nature of material and temperature. A good conductor offers low resistance at lower temperatures.

Resistance: It is the measure of flow of charges. It is the property of the conductor. The resistance of a conductor depends upon its geometry, nature of material and temperature as well. For example, thicker wires have less resistance.

How does resistivity change with temperature? Temperature coefficient of resistance | resistivity

As you have seen, resistance and resistivity both are temperature dependent. Increase in temperature will increase in resistivity.

Temperature coefficient of resistivity can be defined as rate of change of resistivity per degree change in temperature. It is denoted by“alpha” (α).

The resistivity of metallic conductors linearly changes with temperature. As temperature increases resistivity increases. So they have positive temperature coefficient.

Similarly for semiconductors, resistivity decreases with an increase in temperature. So they have negative temperature coefficient.

Figure: Resistivity depends on temperature