Basic Electronics On The Go - Varistor, the Voltage Dependent Resistor

http://www.electronicshub.org/varistor/

Introduction

Varistor is the  portmanteau of variable resistor. It is a passive non linear two terminal solid state semiconductor device.

Varistor provide over voltage protection to electrical and electronic circuits in contrast to circuit breakers or fuse which provide protection to circuits from over current. Varistor provide protection by voltage clamping method which is similar to that in a Zener Diode.

Even though the name varistor is derived from the terms variable resistor, the resistance in varistor can’t be varied manually unlike potentiometer or rheostat where the resistance can be varied manually between their maximum and minimum values.

The resistance of a varistor is varied according to the voltage applied to it. The change in voltage across a varistor will result in change of its resistance, making it a voltage dependent device. Hence the varistor is also called Voltage Dependent Resistor (VDR).

Generally , varistors are made from semiconductor materials. The voltage and current characteristics of a varistor are non linear in nature. Also the voltage and current characteristics of a varistor are suitable for both DC and AC supplies.

Physically, a varistor looks like a capacitor in many ways. Because of the resemblance, a varistor is often confused to a capacitor. However, application wise, a capacitor can’t prevent from voltage surges that a varistor can.

The results of an accidental high voltage surge to any circuit can be catastrophic. Hence the use of varistor in the protection of delicate and sensitive electrical or electronic circuits from high voltage surges and switching spikes is very important.

Resistance of a Varistor

Even though the aim of the varistor is to provide resistance, the operation of a varistor is different from a potentiometer or rheostat. The resistance of a varistor is very high under normal operating conditions.

The functionality of varistor is similar to that of a Zener diode where it allows voltages of lower threshold to pass unaffected.

The functionality of a varistor changes at high operating voltages.When the voltage applied across a varistor is larger than that of its rated value, the effective resistance of the varistor falls drastically and it continues to decrease as the voltage applied to it increases.

V-I Characteristics

According to Ohm’s law, the current-voltage characteristics curve of a resistor is a straight line, assuming the value of the resistor is kept constant. In this case, the current flowing through a resistor is directly proportional to the voltage applied across the ends of resistor.

In case of a varistor, the current-voltage characteristics curve is not a straight line. This is because of the unusual resistance behavior of the varistor. In the case of a varistor, a small change in the voltage applied to it will cause a sufficiently great change in the current flowing through it.

A varistor has bidirectional symmetrical characteristics. This means that the varistor can operate or function in either directions or polarities of a sine wave. This functionality of a varistor is similar to that of back to back connected Zener diodes.


The current-voltage characteristic curve of a varistor shows a linear relationship between current and voltage when the varistor is not conducting. This is because the current flowing through the varistor will remain constant and the value is very low.

This is the leakage current in the varistor and the value of this current is in the order of very few milli amperes. The reason for this is the high resistance of the varistor. This small current will remain constant until the voltage applied across the varistor reaches the rated voltage of the varistor.

The rated voltage of the varistor is also called as Clamping voltage. The rated voltage of a varistor is the voltage across  which is measured with a specified DC current of 1mA .This further can be explained as the DC voltage applied across the terminals of the varistor that allows a current of 1 milli Ampere to flow through it.

The current flowing through the varistor body is dependent on the material used for the construction of the varistor. At this rated voltage level, the functionality of the varistor begins to change.

Until the rated voltage, the varistor acts as an insulator. If the applied voltage of varistor reaches its rated voltage, the behaviour of the varistor changes from insulating state to conducting state.

The resistance of the varistor becomes very small when the transient voltage applied across the varistor is greater than or equal to the rated voltage of the varistor. This is because of a phenomenon called Avalanche Breakdown in semiconductor materials.

Avalanche breakdown is a form of current multiplication that allows large currents in the materials which are previously acting as insulators. Due to this situation the small current flowing through the varistor which is the leakage current will quickly rise.

Even though the current flowing through varistor rises, the voltage across it is limited to a value just near to the rated voltage of the varistor. This means that the varistor acts as a self-regulator to the transient voltages applied across it by passing or allowing more current to flow through the varistor.

Hence, after crossing the rated voltage of the varistor, the current-voltage curve becomes a steep nonlinear curve. Due to this feature, a varistor can pass extensively varying currents over a very narrow range of voltages by clipping off any spikes in voltage.

Capacitance in Varistor

When the applied voltage across the varistor is less than the rated or clamping voltage, a varistor acts as a capacitor rather than a resistor. The reason for this conclusion is that the  main conducting area of the varistor behaves  as a dielectric between the two terminals of the varistor.

The two terminals and the dielectric form a capacitor. This is valid until the voltage reaches the clamping voltage. Every varistor that is made up of a semiconductor material will have a capacitance value. This value depends on the area of the varistor and is inversely proportional to its thickness.

The capacitor behaviour of the varistor is different in DC and AC circuits. In DC circuits, the capacitance of a varistor exists when the applied voltage is below the rated voltage of the varistor and it decreases abruptly when the applied voltage is near to the  rated voltage.

When a varistor is used in AC circuits, the frequency plays an important role. In AC circuits, when the varistor is operated in its non-conductive leakage region, the capacitance of the varistor will affect its body resistance.

The varistors are normally connected in parallel to electrical or electronic devices to protect them from over voltages.

Due to this, the leakage resistance of the varistor drops with an increase in the frequency. The relation between the frequency and the resulting parallel resistance is approximately linear. The AC reactance X_C can be calculated using the formula

X_C = 1 / (2 ×π × f×C) = 1/(2 πfC)

Here C is the capacitance and f is the frequency.

Hence as the frequency increases, the leakage current also increases.

Metal Oxide Varistor (MOV)

To overcome the limitations of semiconductor based varistors like Silicon carbide varistors, the Metal Oxide Varistors (MOV) have been developed. Metal oxide varistor is a voltage dependent resistor. It is also a non linear device and provides very good transient voltage surge protection.

The resistance material in a metal oxide varistor mainly consists of Zinc Oxide grains that are pressed as a ceramic mass. The mixture consists 90 % of Zinc Oxide grains and the other 10 % is made with, other metal oxides like cobalt, bismuth and manganese.

This mixture is sandwiched between two electrodes (metal plates). The filler material acts as a binding agent to zinc oxide grains so that the component is kept intact between two metal plates. The connecting leads of a metal oxide varistor are radial leads.

Metal oxide varistors are the most commonly used components that are used as voltage clamping devices to protect small or heavy devices from transient voltage surges. Since a metal oxide is used in its construction, the ability to absorb short voltage transients and the energy handling capabilities are extremely high.

The operation of metal oxide varistor and silicon carbide varistor are very similar. A metal oxide varistor starts conducting current at rated voltage and it stops the conduction if the applied voltage is below the threshold value.

The main difference between a silicon carbide varistor and a metal oxide varistor is the amount of leakage current. The leakage current in MOV is very small at normal operating conditions.

The reason for lesser leakage currents can be explained as follows. In a metal oxide varistor, the two immediate neighbouring  zinc grains will form a diode junction between their borders.

Hence a metal oxide varistor can be considered as a collection of huge number of diodes that are connected in parallel. Because of this, when a tiny voltage is applied between the electrodes, the reverse leakage current that appears across the diode junction is very small.

When the applied voltage increases and reaches the clamping voltage, the diode junction breaks due to avalanche breakdown and electron tunneling , and allows a huge current through it. Metal oxide varistors have high levels of non linear current voltage characteristics.

The maximum surge current a varistor can take will depend on the width of transient pulse and number of pulse repetitions. Typical widths of transient pulse are in the range of 20 microseconds to 50 microseconds.

There is a chance of overheating if the rated peak pulse current is insufficient. Hence, to avoid overheating of the circuit, it is important to quickly dissipate the energy that is absorbed from transient pulse.

High voltage surge protection

Whether the supply is AC or DC, transient voltage surges originate from many electrical sources and circuits regardless of the supply. This is because the transients are generated in the circuit or transmitted from external sources into the circuit.

Transients that are generated within the circuit can increase rapidly and may cause the voltage to increase to a value of several thousand volts. These voltage spikes may cause severe problems to sensitive electrical or electronic devices and hence must be prevented from appearing across them.

Some of the common sources of voltage transients are as follows:

• The voltage effect L di / dt (Ldi/dt) caused in inductive circuits. This effect is due to switching of inductive coils and magnetizing currents in transformers.
• Power supply surges.
• DC motor switching.

A varistor is connected across the mains supply to avoid voltage transients. This connection can be either between phase and neutral or between phase and phase in case of AC supply.

In case of DC supply, the varistor is connected across the supply between positive and negative terminals. In DC electronic circuits, the varistor can be used for voltage stabilization to protect from over voltage pulses.

Varistor Specifications

The following are the specifications of a typical varistor.

Maximum working voltage: It is the peak steady state DC voltage or sinusoidal rms voltage that can be applied continuously at a specified temperature.

Varistor voltage: It is the voltage between the terminals of the varistor with a specified DC measuring current applied.

Clamping voltage: It is the voltage between the terminals of varistor with specified impulse current applied to obtain peak voltage.

Surge current: The maximum current that flows through the varistor.

Maximum Energy: The maximum energy that is dissipated when an impulse of transient is applied.

Surge shift: The variation in voltage after giving the surge current.

Capacitance: Measured when the voltage is less than the varistor voltage.

Leakage Current: The current flowing through the varistor when it is in non-conducting state.

Response Time: The time between the application of rated voltage and transition from non-conducting state to conducting state.

Varistor Applications

Varistors are used in almost all heavy electrical circuits to small electronic designs. Varistors provide high voltage surge protection for both AC and DC circuits.

Some of the applications are

1. To protect electrical circuits from over voltages. 

     2. In electronic circuits, the devices are very sensitive to changes in voltage. Hence a                  varistor is used. 

     3. To provide surge protection to AC or DC motors.

Varistor Limitations

When a varistor is used in a  transient voltage surge suppressor, it may not provide power protection to the device. This is because the presence of a varistor in this situation will cause problems to the  equipment and the device itself.

Varistor cannot provide protection from the following

1. Current surges during device start up
2. Current from short circuit.
3. From voltage sags or brownouts.

Basic Electronics on the Go - Resistors in AC Circuits

From http://www.electronicshub.org/resistors-in-ac-circuits/

Introduction

.In Direct Current (DC), the flow of electric charge is unidirectional. In DC, the voltage and current maintain a constant polarity and direction. The source of direct current is the battery. On the other hand, in Alternating Current (AC), the flow of electric charge reverses direction periodically. In AC, the voltage changes polarity from positive to negative and vice versa over a period of time.This change in the polarity of the voltage is because of the change in the direction of current.AC is the supply used to power up the households, offices, industries, etc.Even though sine wave is the most common form of AC supply, some applications use different wave forms like triangular wave, square wave and sawtooth wave.

The most common form of AC supply is sinusoidal wave. The mathematical function describing a typical AC voltage is

V (t) = V_Max sin ωt.

V (t) is the voltage in the function of time. The voltage changes with time.

t is the variable time in seconds.

VMax is the peak value that the sine wave can reach in both positive and negative directions. For positive cycle it is V_Max and for negative cycle it is -V_Max.

ω is the angular frequency. ω = 2πf.

f is the frequency of the sine wave.

In DC circuits, the calculation of current, voltage and power is carried out using Ohm’s law. Here the polarities of both voltage and current are assumed to be constant.

In case of pure resistive AC circuits, the values of inductance and capacitance are negligible. Hence the calculation of current, voltage and power will follow the same principles of Ohm’s law and Kirchhoff’s Circuit laws. The difference is in the use of instantaneous peak to peak value or rms value.

Resistor with DC and AC Supplies

Resistor is a passive device. It does not consume or produce any energy. Energy here is electrical energy. But resistor dissipates electrical energy in the form of heat.

In DC resistive circuits, the resistance which is the ratio of voltage to current is linear.

In AC circuits, the voltage to current ratio is mainly dependent on supply frequency f and phase angle or phase difference φ. Hence the term Impedance is used in AC circuits to denote the resistance as it possess both magnitude and phase in contrast to the resistance in DC circuits where is possessed only magnitude. The symbol of impedance is Z.

V-I Phase Relationship in a Pure Resistive AC circuit

The resistance value of the resistor in both AC and DC circuits is  the same irrespective of the frequency of the AC supply voltage. The change in direction of current in AC supply does not affect resistors behavior. So the current in the resistor will rise and fall according to the voltage as it rises and falls.

The voltage and current in AC resistive circuit reach maximum, then fall to zero and reach minimum at the same time. They are said to be “in phase” as they rise and fall at exactly the same time.

The current for a purely resistive circuit is I (t) = I_Max sin ωt.

The voltage V (t) = V_Max sin ωt. => V (t) = I_Max R sin ωt.

As the circuit is purely resistive, the effects of inductance and capacitance are negligible and the phase difference is 0.

The instantaneous values of currents and voltages are “in phase” along the x- axis of the curve. They rise and fall at the same time and reach their maximum and minimum values exactly at the same time. This implies that their phase angle is θ = 0.

AC Power, Voltage and Current Calculations

The instantaneous values of current and voltage in an AC resistive circuit can be utilized to give the resistance in its Ohmic form by using Ohm’s law.

Let the supply voltage be V (t) = V_Max sin ωt connected to a resistor R.

Let the instantaneous voltage across the resistor be VR.

Let IR be the instantaneous current which is flowing through the resistor.

As the above circuit is pure resistive in nature, the principles of Ohm’s can be applied.

From Ohm’s law, the voltage across the resistor at an instant t is

VR = VMax sin ωt.

Similarly, the current flowing through the resistor at an instant t can be determined using Ohm’s law as

IR = VR / R

But VR = VMax sin ωt.

Hence IR = (VMax * sin ωt) / R

But the value VMax / R is nothing but the maximum current in the circuit denoted by IMax..

Therefore IR = IMax sin ωt.

In a pure resistive series AC circuit, the total circuit voltage is equal to the sum of voltages of individual resistors because all the individual voltages are in-phase in pure resistive circuit.In the similar way,the total current in a pure resistive parallel AC circuit is the sum of the individual branch currentsof all the parallel resistive branches.
To calculate power in an AC circuit, the power factor plays an important role. Power factor is defined as the cosine of the phase angle between current and voltage. The phase angle is denoted by the symbol φ.

If P is the real power in a circuit measured in Watts and S is the apparent power of the circuit measured in Volt Amps, the relation between real power and apparent power is given by

P = S Cos φ.

In case of pure resistive AC circuits, the phase angle between current and voltage is 00. Therefore φ = 00.  Hence the power factor Cos φ is Cos 00 = 1.

Therefore the real power is equal to the apparent power which is the product of voltage and current.
In pure resistive AC circuits, the power at any instant in the circuit can be found out by calculating the product of voltage and current at that instant.

The power consumed by above mentioned circuit can be calculated using

P = VRMS * IRMS * Cos φ.

As φ = 00 in this case, the power is

P = VRMS * IRMS

Power in a Pure Resistance

In case of pure resistive AC circuits, the power consumed by the circuit is simply the product of voltage and the current as there is no phase angle between current and voltage.

The power waveform consists of a series of positive pulses. This is because when both the voltage and current are positive in the first half cycle, their product which is the power is also positive. And when both the voltage and current are negative in the second half cycle, their productive power isagainpositive (-V x -I = +P). Hence the value of power is always greater than or equal to zero.

In the case of a pure resistive circuit with an AC RMS power supply, the power dissipated is the same as that in case of a resistor connected to DC power supply.

P = V_RMS * I_RMS = I^2_RMS * R = V^2_RMS / R.

V_RMS and I_RMS are rms values of voltage and current respectively.

P is power in Watts.

R is resistance in Ohms (Ω)

To compare the heating effects caused by AC and DC, the DC current should be compared to the RMS value of AC current but not to the maximum or peak current I_MAX¬.

Basic Electronics on the Go - Potential Difference in Resistor Networks

From  http://www.electronicshub.org/potential-difference-in-resistor-networks/

Potential Difference Definition

Consider a task of moving a charge from A to B in a uniform electric field. Let this movement be against  the electric field. Some work will be done by an external force on this charge and this work will change the potential energy to a higher value. The amount of work done is equal to the change in potential energy. This change in potential energy will result in a difference in potential between the two points A and B. This difference in potential is called Potential Difference and it is measured in Volts (V).

Potential Difference is denoted by ∆V and is defined as the difference in potential or voltage between two points.

If V_A is the potential at A and V_B is the potential at B, then from the definition of potential difference,

∆V_BA = V_B – V_A

Current flows in an electrical circuit in the form of charge whereas potential doesn’t flow or move. Potential difference is applied between two points.

The unit of potential difference between two points is Volt. Volt is defined as the potential drop across a 1 Ohm (Ω) resistor with 1 Ampere of current flowing through it.

Hence

1 Volt = 1 Ampere×1 Ohm

V = I × R

According to Ohm’s law, the current flowing in a linear circuit is directly proportional to the potential difference across the circuit. Hence if the potential difference applied across the circuit is greater, then the current flowing in the circuit is larger.

Generally in electrical circuits the lower potential is earth or ground. This value is usually considered 0 V. Hence the potential difference is equal to the applied voltage. Earth is considered as the common point in a circuit. This reference of earth or ground as common point in electrical circuits is useful in easy understanding of the circuit. Potential difference is also called voltage.

Voltages connected in series are added to give total voltage in a circuit. This can be observed in resistors in series connection. If V1, V2 and V3 are connected in series then total voltage VT is given by

VT = V1 + V2 +V3.

In case of elements connected in parallel, the voltage across them is equal. This can be observed in resistors in parallel tutorial.

VT = V1 = V2 = V3.

Voltage Divider Circuit

Resistors in series connection are used to produce a voltage divider circuit. Voltage divider is a linear circuit whose output voltage is a fraction of input voltage.

Applications of Voltage Divider Circuits

Resistors in series will form Voltage Divider Circuits. Voltage Divider principle is the basis in the construction of Potentiometer which acts as a simple voltage regulator.

Voltage Divider Circuits are used in Sensing circuits. The most frequently used sensors in the form of Voltage Divider circuits are thermistors and Light Dependent Resistors.

Basic Electronics on the Go - Resistor Power Rating

From  http://www.electronicshub.org/resistor-power-rating/

Resistor Power Rating

Introduction

Resistors can be rated based on two values. First they can be  based on the resistance of the resistor. Second they can be  based on the power in Watts that a resistor can dissipate safely.

The power rating of a resistor can be defined as the loss of electrical energy in the form of heat in a resistor when a current flows through it in the presence of a voltage.

Resistors can be used in any circuit based on the requirement in any combination of current and voltage. These different combinations of currents and voltages are selected such that the dissipating power rating of a resistor does not exceed the resistor power rating, which indicates the amount of power a resistor can convert into heat without causing any damage to it. The resistor power rating can also indicate the amount of power a resistor can absorb without causing any damages.

The Resistor Power Rating is also called as Resistor Wattage Rating. The Resistor Power Rating is defined as “the amount of heat a resistor can dissipate for an indefinite time period without affecting or degrading its performance.”

The resistor power rating is measured in Watts which are the units of power.

Since the power dissipation is dependent on the size of the object, the resistor power rating can vary from a value as small as one tenth of a Watt to a large value as hundreds of Watts based on the size, temperature and construction procedure of resistor.

Generally the temperature used to define the power rating is ambient temperature or room temperature. Generally, most of the resistors have their maximum power rating at an ambient temperature of  70^oC  or less.

Power Dissipation

Resistors are basic electrical components which obey Ohm’s Law. When a voltage V is applied between the leads of the resistor of resistance R, a current I flows through it. This current I is given by

I = V / R

The movement of electrons is the cause for this current and they are accelerated by the electric field due to the applied potential. These accelerated electrons which possess kinetic energy, try to move towards the positive side of the material and in this process they collide with atoms and lose their energy. And the result of this collision is conversion of electrical energy to heat.

The rate of loss of energy or power dissipation can be calculated from the formula P = I × V.

From Ohm’s Law, we have V = I × R

Therefore P = I2 × R

And P = V2 / R

Power Resistors

Some resistors are designed for larger power dissipation. These are called Power Resistors. Resistors with power rating of at least 5W come under power resistors. The material used for construction of power resistors must be of high thermal conductive in nature. Power resistors often come with heat sink which helps them in dissipation of heat.

Wire wound power resistors are common, but they can also be found in other types. If Nichrome alloy based wire wound resistors are used with proper non-conductive enamel paint, they can withstand temperatures up to 4500C.

Another type of resistors used to withstand large amounts of currents is Grid Resistors. Grid Resistors can withstand a current up to 500 Amps and can have a resistance values as low as 0.04 Ω. The construction of Grid Resistors includes two electrodes with large metal stripes connected between them in the form of a matrix. Grid resistors are used as grounding resistors, brake resistors and harmonic filters for electric substations.

Another type of power resistors are Water Resistors. The construction includes tubes which carry saline solution with electrodes connected at both ends of the tube. The concentration of saline solution or salt water will determine the resistance. Because of the presence of water in the tubes, water resistors provide large heat capacity, which in turn result in high power dissipation.

Power resistors can also be made in the form of Surface Mounted Devices. Because of their small size, the power dissipation capacity of SMD resistors is less than grid type resistors and water resistors. Usually the power dissipated by SMD resistors is in the order of few Watts.

The range of power dissipated by different types of power resistors are as follows

• SMD Resistors 5 W or less
• Helical Wound 50 W or less
• Edge Wound 3.5 KW or less
• Grid Resistors 100 KW or less
• Water Resistors 500 KW or less

Power rating Examples

1. For example, to choose an appropriate power rated resistor of resistance 800 Ω and a supply voltage of 12 V. The available power ratings are 0.25 W, 0.5W and 1 W.

Power P in the resistor is equal to P = V2 / R

Therefore P = (12)2  / 800 = 0.18 W.

Hence a resistor of Power rating 0.25 should be used.

Applications of Power Resistors

Every resistor is rated with its maximum power rating. This power rating indicates the maximum power a resistor can dissipate without causing any damages to itself or the circuit. Some applications require less power dissipation and others require large power dissipation. Power resistors are used in applications where we need to dissipate large power. Some of the applications of Power resistors are

1. Engine brakes in heavy locomotives and trams use power resistors. Locomotives move at high speed and possess high kinetic energy. While stopping these high speed locomotives, their kinetic energy is converted to heat.

Depending on the velocity of the locomotives, the amount of heat generated can be in the order of few Kilo Watts. Classic disc brakes can’t be used as they wear out easily. Hence regenerative brakes or high power resistors in the form of Grid resistors are used in locomotives.

2. Power resistors are used as grounding resistors to limit fault currents, high voltages and act as protective relays. These resistors can be rated up to 8 Kilo Amps.

3. Power resistors are used as load resistors in turbines and Uninterruptable Power Supplies. They can be designed to provide adjustable resistance and can dissipate a power of up to 6 Mega Watts. Because of this high power dissipation, load resistors are equipped with an efficient cooling system to control the temperature and prevent the devices from burning out.

Basic Electronics on the Go - Resistor Color Codes

From http://www.electronicshub.org/resistor-color-code/

Resistor Color Codes

There are many different types of resistors available. In order to identify or calculate the resistance value of a resistor, it is important to have a marking system. Resistor Color Code is one way to represent the value of the resistance along with the tolerance.

The electronic color code is a way of indicating the ratings or values of electronic components like resistors, capacitors, inductors, and others. The electronic color code was developed in the early 1920s by the Radio Manufacturers Association. Color code is used as they are cheap and can be printed on small components as well.

The standards for color coding registers are defined in international standards IEC 60062. This standard describes color coding for axially leaded resistors and numeric code for SMD resistors.

There are several bands to specify the value of resistance. They even specify tolerance, reliability and failure rate. The number of bands vary from three to six. In case of 3 band code, the first two indicate the value of resistance and the third band acts as multiplier.

Three Band Resistor Color Code

The three band color code is very rarely used. The first band from the left indicates the first significant figure of the resistance. The second band indicates the second significant number.

The third band indicates the multiplier. The tolerance for three band resistors is generally 20%. 

For example if the colors on the resistor are in the order of Yellow, Violet and Red from left, then the resistance can be calculated as

47× 10^2± 20 %. This is 4.7 KΩ± 20%.

Four Band ResistorColor Code

Four band color code is the most common representation in resistors. The first two bands from the left are used to indicate the first and second significant digits of resistance. The third band is used to indicate the multiplier.

The fourth band is used to indicate tolerance. There is a significant gap between third and fourth bands. This gap helps in resolving the reading direction. 

For example, if the colors on a four band resistor are in the order Green, Black, Red and Yellow then the value of resistance is calculated as 50 * 10^4± 2 % = 500KΩ± 2%

Five Band ResistorColor Code

High precision resistors have an extra band which is used to indicate the third significant value of the resistance. The rest of the bands indicate the same things as a four band color code.

Therefore the first three bands are used to indicate the first three significant values of resistance. Fourth and fifth bands are used to indicate multiplier and tolerance respectively.

There is an exception when the fourth band is either Gold or Silver. In this case, the first two bands indicate the two significant digits of resistance.

Third band is used to indicate multiplier, fourth band is used for tolerance and fifth band is used to indicate the temperature coefficient with units of ppm/K. 

For example, if colors on a five band resistor are in the order Red, Blue, Black, Orange and Gray then the value of resistance is calculated as 260×10^3± 0.05 = 260 KΩ ± 0.05%.

Tolerance Letter Coding for Resistors

The letter code for tolerance is shown below

• B = 0.1%
• C = 0.25 %
• D = 0.5 %
• F = 1 %
• G = 2 %
• J = 5 %
• K = 10 %
• M = 20 %

K and M should not be confused with kilo and mega Ohms.

SMD Resistor Code

There are three types of coding systems used to mark SMD Resistors. They are

• Three digit coding
• Four digit coding
• E96 coding

Three Digit Code

In three digit coding, the first two numbers indicate the significant value of the resistance and the third number indicates the multiplier like 10 in case the digit is 1, 100 in case the digit is 2 or 1000 in case the digit is 3 and so on.

Some examples of three digit codes are

450 = 45 * 10^0 = 45 Ω

221 = 22 * 10^1 = 220 Ω

105 = 10 * 10^5 = 1 MΩ

If the resistance is less than 10Ω then the letter R is used to indicate the position of the decimal point. For example

3R3 = 3.3Ω

47R = 47 Ω

Four Digit Code

For more precision resistors, a four digit code is marked on them. The calculation is similar to three digit code. The first three numbers indicate the significant value of the resistance and the fourth number indicates the multiplier.

Some examples under this system are

4700 = 470 * 10^0 = 470 Ω

1001 = 100 * 10^1 = 1 KΩ

7992 = 799 * 10^2 = 79.9 KΩ

For resistors less than 100 Ω, R is used to indicate the position of the decimal point.

For example,

15R0 = 15.0 Ω

E series

Electronic Industries Association (EIA) specified a standard preferred value system for resistors and is named as E series. IEC 60063 is an international standard which defines the preferred number series in resistors (and also for capacitors, inductors and Zener Diodes). The coding is based on the tolerance values and different E series available are

• E3 50% tolerance
• E6 20% tolerance
• E12 10% tolerance
• E24 5% tolerance
• E48 2% tolerance
• E96 1% tolerance
• E192 0.5, 0.25, 0.1% and higher tolerances
• E3 coding is no longer in use and E6 coding is very rarely used.
• The E96 coding system is used for high precision resistors with a tolerance of 1%.

There is a separate coding system in EIA E96 marking system. This system uses three figures for marking. The first two are numerals which indicate the three significant digits of the value of resistance. The third figure is a letter used to indicate the multiplier.

The EIA 96 code scheme for multipliers is shown below

CODE	MULTIPLIER
Z	0.001
Y or R	0.01
X or S	0.1
A	1
B or H	10
C	100
D	1000
E	10000
F	100000

Some examples of EIA 96 coding system are

92Z = 887 × 0.001 = 0.887 Ω

38C = 243 × 100 = 24.3 KΩ