S NAGA MALLESWARA REDDY

Varistor Introduction

The varistor is a passive two-terminal solid state semiconductor device that is used to provide protection to electrical and electronic circuits. Unlike the fuse or circuit breaker which offers over-current protection, the varistor provides over-voltage protection by means of voltage-clamping in a similar way to the zener diode.

The word "Varistor" is an acronym, and is a combination of the words VARI-able resi-STOR used to describe their mode of operation way back in their early days of development which is a bit misleading since a varistor can not be manually varied like a potentiometer or rheostat.

A Varistor

But unlike a variable resistor whose resistance value can be manually varied between its minimum and maximum values, the varistor changes its resistance value automatically with the change in voltage across it making it a voltage-dependant, nonlinear resistor or VDR for short.

Nowadays the resistive body of a varistor is made from Semiconductor Material making it a type of semiconductor resistor with a non-ohmic symmetrical voltage and current characteristics suitable for both AC and DC voltage applications.

In many ways the varistor looks similar in size and design to a capacitor and is often confused as being one. However, a capacitor cannot suppress voltage surges in the same way a varistor can. When a high voltage surge is applied to a circuit, the outcome is usually catastrophic to the circuit, therefore the varistor plays an important role in the protection of delicate electronic circuits from switching spikes and over voltage transients.

Transient surges originate from a variety of electrical circuits and sources regardless of whether they operate from an AC or DC supply as they are often generated within the circuit itself or transmitted into the circuit from external sources. Transients within a circuit can rise rapidly increasing the voltage to several thousand volts, and it is these voltage spikes which must be prevented from appearing across delicate electronic circuits and components.

One of the most common sources of voltage transients is the L(di/dt) effect caused by the switching of inductive coils and transformer magnetizing currents, DC motor switching applications and surges from the switching-on of fluorescent lighting circuits or other supply surges.

AC Waveform Transients

Varistors are connected in circuits across a mains supply either phase-to-neutral, phase-to-phase for AC operation, or positive-to-negative for DC operation and have a voltage rating to suit their application. A varistor can also be used for DC voltage stabilization and especially for electronic circuit protection against over voltage pulses.

Varistor Static Resistance

Under normal operation the varistor has a very high resistance, hence part of its name, operating in a similar way to the zener diode by allowing lower threshold voltages to pass unaffected.

However, when the voltage across the varistor (either polarity) exceeds the varistors rated value, its effective resistance decreases strongly with an increasing voltage as shown.

We know from Ohms Law that the current-voltage (I-V) characteristics of a fixed resistor is a straight line provided that R is kept constant. Then the current is directly proportional to the potential difference across the ends of the resistor.

But the I-V curves of a varistor is not a straight line as a small change of voltage causes a significant change of current. A typical normalised voltage versus current characteristics curve for a standard varistor is given below.

Varistor Characteristics Curve

We can see from above, that the varistor has symmetrical bi-directional characteristics, that is the varistor operates in both directions (quadrant Ι and ΙΙΙ) of a sinusoidal waveform behaving in a similar way to two zener diodes connected back-to-back. When not conducting, the I-V curve shows a linear relationship as the current flowing through the varistor remains constant and low at only a few micro-amperes of "leakage" current. This is due to its high resistance acting as an open circuit and remains constant until the voltage across the varistor (either polarity) reaches a particular "rated voltage".

This rated or clamping voltage is the voltage across the varistor measured with the specified DC current of 1mA. That is, the DC voltage level applied across its terminals that allows a current of 1mA to flow through the varistors resistive body which itself is dependant upon the materials used in its construction. At this voltage level, the varistor begins to change from its insulating state into its conducting state.

When the transient voltage across the varistor is equal to or greater than the rated value, the resistance of the device suddenly becomes very small turning the varistor into a conductor due to the avalanche effect of its semiconductor material. The small leakage current flowing through the varistor rapidly rises but the voltage across it is limited to a level just above the varistor voltage. In other words, the varistor self-regulates the transient voltage across it by allowing more current to flow through it and because of its steep nonlinear I-V curve it can pass widely varying currents over a narrow voltage range clipping-off any voltage spikes.

Varistor Capacitance

Since the main conducting region of a varistor between its two terminals behaves like a dielectric, below its clamping voltage the varistor acts like a capacitor rather than resistor. Every semiconductor varistor has a capacitance value that depends directly on its area and varies inversely with its thickness.

When used in DC circuits, the capacitance of the varistor remains more or less constant provided that the applied voltage does not increase above the clamping voltage level, and drops off abruptly near towards its maximum rated continuous DC voltage.

However, in AC circuits, this capacitance can affect the body resistance of the device in the non-conducting leakage region of its I-V characteristics. As they are normally connected in parallel with an electric device to protect it against over voltages, the varistors leakage resistance drops rapidly with an increase in frequency.

This relationship is approximately linear with the frequency and the resulting parallel resistance, its AC reactance, Xc can be calculated using the usual 1/2piƒC as for a normal capacitor. Then as the frequency increases so to does its leakage current.

But as well as the silicon semiconductor based varistor, metal oxide varistors have been developed to overcome some of the limitations associated with their silicon carbide cousins.

Metal Oxide Varistor

The Metal Oxide Varistor or MOV for short, is a voltage dependant resistor in which the resistance material is a metallic oxide, primarily zinc oxide (ZnO) pressed into a ceramic like material. Metal oxide varistors consist of approximately 90% zinc oxide as a ceramic base material plus other filler materials for the formation of junctions between the zinc oxide grains.

Metal oxide varistors are now the most common type of voltage clamping device and are available for use at a wide range of voltages and currents. The use of a metallic oxide within their construction means that MOV's are extremely effective in absorbing short term voltage transients and have higher energy handling capabilities.

As with the normal varistor, the metal oxide varistor starts conduction at a specific voltage and stops conduction when the voltage falls below a threshold voltage. The main differences between a standard silicon carbide (SiC) varistor and a MOV type varistor is that the leakage current through the MOV's zinc oxide material is very small current at normal operating conditions and its speed of operation in clamping transients is much faster.

MOV's generally have radial leads and a hard outer blue or black epoxy coating which closely resembles disc ceramic capacitors and can be physically mounted on circuit boards and PCB's in a similar manner. The construction of a typical metal oxide varistor is given as:

Metal Oxide Varistor Construction

To select the correct MOV for a particular application, it is desirable to have some knowledge of the source impedance and the possible pulse power of the transients. For incoming line or phase borne transients, the selection of the correct MOV is a little more difficult as generally the characteristics of the power supply are unknown. In general, MOV selection for the electrical protection of circuits from power supply transients and spikes is often little more than an educated guess.

However, metal oxide varistors are available in a wide range of varistor voltages, from about 10 volts to over 1,000 volts AC or DC, so selection can be helped by knowing the supply voltage. For example, selecting a MOV or silicon varistor for that matter, for voltage, its maximum continuous rms voltage rating should be just above the highest expected supply voltage, say 130 volts rms for a 120 volt supply, and 260 volts rms for a 230 volt supply.

The maximum surge current value that a varistor will take depends on the transient pulse width and the number of pulse repetitions. Assumptions can be made upon the the width of a transient pulse which are typically 20 to 50 microseconds (uS) long. If the peak pulse current rating is insufficient, then the varistor may overheat and become damaged. So for a varistor to operate without any failure or degradation, it must be able to quickly dissipate the absorbed energy of the transient pulse and return safely to its pre-pulse condition.

Varistor Applications

Varistors have many advantages and can be used in many different types of applications for the suppression of mains borne transients from domestic appliances and lighting to industrial equipment on both AC or DC power lines. Varistors can be connected directly across mains supplies and across semiconductor switches for protection of transistors, MOSFET's and thyristor bridges.

Varistor Applications

Varistor Summary

In this tutorial we have seen that the basic function of a Voltage Dependant Resistor, or VDR, is to protect electronic devices and electrical circuits against voltage surges and spikes, such as those generated by inductive switching transients.

As such varistors are used in sensitive electronic circuits to ensure that if the voltage does suddenly exceeds a predetermined value, the varistor will effectively become a short circuit to protect the circuit that it shunts from excessive voltage as they are able to withstand peak currents of hundreds of amperes.

Varistors are a type of resistor with a non-linear, non-ohmic current voltage characteristic and are a reliable and economical means of providing protection against over voltage transients and surges.

They achieve this by acting as a high resistance blocking device at lower voltages and as a good low resistance conducting device at higher voltages. The effectiveness of a varistor in protecting an electrical or electronic circuit depends on the proper selection of the varistor with regards to voltage, current and energy dissipation.

Metal Oxide Varistors, or MOV's are typically made from a small disk-shaped metal zinc oxide material. They are available in many values for specific voltage ranges. An MOV's voltage rating, called the "varistor voltage" is the voltage across a varistor when a current of 1mA is passed through the device. This varistor voltage level is essentially the point on the I-V characteristic curve when the device starts to conduct. Metal oxide varistors can also be connected in series to increase the clamping voltage rating.

While metal oxide varistors are widely used in many AC power electronics circuits to protect against transient over voltages, there are also other types of solid state voltage suppression devices such as diodes, zener diodes and suppressors which all can be used in some AC or DC voltage suppression applications along with Varistors.

Resistor Colour Code Wheel

Resistor colour codes can sometimes be a little confusing until you understand how they work. But once you get the hang of them it becomes easier to read the values of those simple colour coded bands. There is a lot of information both online and on this Electronics Tutorials website too, to help you read and understand how resistor colour codes work and this free, simple to use and practical resistor colour code wheel will hopefully help you on your way.

Generally, resistors are too small in size for manufacturers to print numbers and letters on them to indicate their resistive value and tolerance. Luckily for us, some bright spark somewhere invented a resistor colour coding system to make our lives easier and help us to work it out with our free resistor colour code wheel. Fixed resistors have different coloured rings or bands around them to indicate their resistive value with each coloured band having a decimal value associated with it.

Our Resistor Colour Code Wheel

There are many clear advantages to using a colour coding system on electrical and electronic components. The main advantage of using coloured rings or bands around a resistors body, is that they can be very easily seen and read no matter what the position or orientation of the resistor on a board. These coloured bands can also be read even if the resistors body is a little dirty or badly burnt.

We saw in our Ohm's Law tutorial that resistors are used to limit the amount of current flowing in a circuit so it is important to know their resistive value and depending upon the type, size and tolerance of the resistor, there can be three, four or five coloured bands used to do this.

Although these coloured bands represent nominal or ideal values, they are a good approximation of the actual resistance value. This is because the actual resistive value can have a percentage variation of resistance either side of the nominal value. This variation is called the "tolerance".

All fixed resistors have a tolerance ranging from less than a tenth of a percent, (0.1%) up to 20% for large carbon types. So a tolerance of 5, 10 and 20 percent, means that the actual value of the resistor can vary from the expected nominal value by as much as ±5, ±10, and ±20%. For example, a 100Ω resistor with a tolerance of ±10% can have a value from 90Ω's (-10%) to 110Ω's; (+10%). That's a variation of 20Ω's and still be in tolerance.

Resistors with just three coloured bands generally have no (none) tolerance band as they have a fixed tolerance of ±20%. The first two coloured bands are the digit or number bands and the third is known as the multiplier. When there are four bands, the first two coloured bands are the digit bands, the third is the "multiplier", with the fourth band being the "tolerance" value. On resistors with five coloured bands, the first three colours are always digit bands followed by the multiplier and tolerance bands.

Step 1 – Download and Print

Using our simple and free resistor colour code wheel we can now make sense of all these different coloured bands and what they mean. But first things first. We need to download and print out the Resistor Colour Code Wheel template using the download button below. Its completely free!

Please click the button to download the related PDF file.  

Note that this is a PDF (Acrobat) Document File. Please make sure that you have an application to open this file type before downloading the resistor colour code wheel template.

After you have download and printed out the resistor colour code wheel template, you should have an A4 sized paper (or whatever size you want to print out) colour wheel template looking like this:

The Resistor Colour Wheel Template

Note that you can print this resistor colour code wheel template onto any size or density (weight) of paper that you want. It all depends upon your printer and what you have. The standard A4 sized printer or office copier paper has a paper density or weight of 80 g/m2. This standard printer paper is fine and can be used to make the resistor colour wheel.

However, being thin printing paper it can easily be torn or damaged with use, so a paper density of 150g/m2 or more such as thin card would be a better choice and more durable. It would also be possible to laminate the template sheet after printing to both strengthen and prolong its life, the choice is yours. Anyway once you have the resistor colour code wheel template downloaded to your hard drive, you can print it out as many times as you want.

Step 2 – Cut Out the Disks

Having printed out our resistor colour code wheel template from the link above on to suitable paper, we now need to carefully cut out the five coloured disks as shown using scissors or a sharp knife.

Once we have our five coloured disks cut out we can now start to assemble them together to form the finished colour wheel design.

Step 3 – Assemble the Disks

In order to assemble our five individual coloured disks together to create the finished resistor colour code wheel, we need to use a brass paper fastener like the ones shown or something similar that you may have easily available.

Carefully poke or make a hole through the center of each wheel using a pin, needle, compass point, knife or any other sharp object you have, being careful not to cut yourself in the process. The hole you make needs to be the same diameter as the brass paper fasteners. Make sure that the hole you make is big enough to both insert and rotate the paper fastener, or whatever you decide to use. In my case the hole was about 4mm (5/32″) in diameter.

Insert the paper fastener through each hole in turn, starting with the smallest disk and working upwards making sure that each disk is free to rotate as you assemble. Once complete bend the fasteners tabs over at the back of the larger disk. You should now have an assembled resistor colour code wheel that looks something like this.

Assembled Resistor Colour Code Wheel

Step 4 – Using the Colour Wheel

Now that we have assembled our resistor colour code wheel its time to start using it. In the following examples we are going to use it to find the values of a 4-band and a 5-band resistor. But first we will define what each coloured disk is in relationship to the meaning of the coloured bands on a resistors body.

• Disk One (1st Digit) – This is for the first coloured band closest to the end of the resistor on the left hand side and represents the first digit of the resistors value.
• Disk Two (2nd Digit) – This is for the second coloured band along representing the second digit of the resistors value.
• Disk Three (3rd Digit) – This is used for metal film resistors which use a five and six-band colour code for more precise values. In this case the first three coloured bands indicate the first three numerical digits. For a three or four band colour coded resistor this 3rd digit can be ignored.
• Disk Four (the multiplier) – The next coloured band is the mathematical multiplier which represents the number of zeros to be added to the first two (or three) digits. If the third colour, for a 4-band resistor or the fourth colour, for a 5-band resistor, is either gold or silver, this represents a fractional decimal multiplier as the resistive value is less than 10Ω's. If the coloured band is Gold, multiply the first two or three digits by 0.1 (divide-by 10) and if the coloured band is Silver multiply by 0.01 (divide-by 100).
• Disk Five (the tolerance) – The final coloured band represents the tolerance of the resistor. A Gold band indicates a tolerance of ±5% while a Silver band indicates a tolerance of ±10%. If there is no coloured band as in a three band resistor, then the tolerance is ±20%.

Note that a resistor can have 3, 4 or 5 coloured bands to indicate its resistive value. The coloured bands that are grouped, or closer together on one side of the resistors body indicate the resistive value of the resistor and you should start here reading from left to right. A single coloured band separate from the group and on its own will be the tolerance value.

In this first example, we are going to use the resistor colour code wheel we have just made to find the resistive value of the following 4-band resistor which is used for most resistors.

4-band Resistor Colour Code

The coloured bands are shown as:  YELLOW , VIOLET, ORANGE and GOLD. Then the resistance using the colour wheel is found as:

As this is a 4-band resistor and the resistor colour code wheel can be used to find the resistive values of 5-band resistors, then the 3rd digit wheel is not used in this case. Then the colour code wheel shows:

The first colour band (yellow) gives the first digit value of 4. The second colour band (violet) gives the second digit value of 7. This gives a two digit value of 47. Multiply this by the value of the third band. In this case, orange which has a value of 1000 or 1k, so the resistor has a resistive value of 47,000 ohms (47 × 1000 = 47000) or 47kΩ's. The last band gives the resistors tolerance value andgold equals a tolerance range of ±5%.

Then using the resistor colour wheel, the resistor has the following resistance:

Yellow Violet Orange = 4 7 3 = 4 7 x 103 = 47000Ω or 47kΩ ±5%.

In this second example, we will use it to find the value of the following 5-band resistor. Five band colour codes are used to provide more precise values in precision metal-film resistors with lower tolerances.

5-band Resistor Colour Code

The coloured bands are shown as: BLUE, RED, BLACK, BROWN and BROWN. Then the resistance using the colour wheel is found as:

As this is a 5-band resistor, all the disks of the resistor colour code wheel can be used to find the resistive value. Then the colour code wheel shows:

The first colour band (blue) gives the first digit value of 6. The second colour band (red) gives the second digit value of 2. The third colour band (black) gives the third digit value of 0. This gives a three digit value of 620. We now multiply this by the value of the fourth band, brown which has a value of 10. So the resistor has a resistive value of 6200 ohms (620 × 10 = 6200) or 6k2Ω's. The last band gives the resistors tolerance value and brown equals a tolerance range of ±1%.

Then using the resistor colour wheel, the resistor has the following resistance:

Blue Red Black Brown = 6 2 0 0 = 6 2 0 x 10 = 6200Ω or 6k2Ω ±1%.

So there you have it, a fun little project to do at home for use at school or the science lab, just download, print and cut out to give you a very useful reference tool for finding the resistive values of 4 or 5-band resistors using this free and simple Resistor Colour Code Wheel.

This resistor colour code wheel is simple to use, just position the colours of the disks and read off the number its that easy and simple, and for checking the value of more resistors, just rotate the coloured disks and you will get another colour scheme. But remember, if you are still unsure of a resistors value, you can always find its resistance using a multimeter or check out our Resistor Colour Codes tutorial.

If you have still not done so you can download the related PDF file here:  

Have fun making it, using it and sharing it, and let me know what you think. Enjoy 

Resistor Tutorial and Summary

We can summarise this resistor tutorial section and what we have learnt as follows:

• The job of a Resistor is to limit the current flowing through an electrical circuit.
• Resistance is measured in Ohm's and is given the symbolΩ
• Carbon, Film and Wirewound are all types of resistors.
• Resistor colour codes are used to identify the resistance and tolerance rating of small resistors.
• The BS1852 Standard uses letters and is used to identify large size resistors.
• Tolerance is the percentage measure of the accuracy of a resistor from its preferred value with the E6 (20%), E12 (10%), E24 (5%) and E96 (1%) series of tolerance values available.

Series Resistor Tutorial

• Resistors that are daisy chained together in a single line are said to be connected in SERIES.
• Series connected resistors have a common Current flowing through them.

• Itotal = I1 = I2 = I3 …. etc

• The total circuit resistance of series resistors is equal to:
• Rtotal = R1 + R2 + R3 + ….. Rn etc.
• Total circuit voltage is equal to the sum of all the individual voltage drops.

• Vtotal = V1 + V2 + V3 …. etc

• The total resistance of a series connected circuit will always be greater than the highest value resistor.

Parallel Resistor Tutorial

• Resistors that have both of their respective terminals connected to each terminal of another resistor or resistors are said to be connected in PARALLEL.
• Parallel resistors have a common Voltage across them.

• VS = V1 = V2 = V3 …. etc

• Total resistance of a parallel circuit is equal to:
• 
• Total circuit current flow is equal to the sum of all the individual branch currents added together.

• Itotal = I1 + I2 + I3 …. etc

• The total resistance of a parallel circuit will always be less than the value of the smallest resistor.

Resistor Power Rating

• The larger the power rating, the greater the physical size of the resistor to dissipate the heat.
• All resistors have a maximum power rating and if exceeded will result in the resistor overheating and becoming damaged.
• Standard resistor power rating sizes are 1/8 W, 1/4 W, 1/2 W, 1 W, and 2 W.
• Low ohmic value power resistors are generally used for current sensing or power supply applications.
• The power rating of resistors can be calculated using the formula:
• 
• In AC Circuits the voltage and current flowing in a pure resistor are always "in-phase" producing 0o phase shift..
• When used in AC Circuits the AC impedance of a resistor is equal to its DC Resistance.
• The AC circuit impedance for resistors is given the symbol Z.

Resistors in AC Circuits

We have looked at resistors, their connections and used Ohm's Law to calculate the voltage, current and power associated with them. In all cases both the voltage and current has been assumed to be of a constant polarity, flow and direction, in other words Direct Current or DC.

But there is another type of supply known as Alternating Current or AC whose voltage switches polarity from positive to negative and back again over time and also whose current with respect to the voltage oscillates back and forth. The oscillating shape of an AC supply follows that of the mathematical form of a "sine wave" which is commonly called a Sinusoidal Waveform. Therefore, a sinusoidal voltage can be defined as V(t) = Vmax sin ωt.

When using pure resistors in AC circuits that have negligible values of inductance or capacitance, the same principals of Ohm's Law, circuit rules for voltage, current and power (and even Kirchoff's Laws) apply as they do for DC resistive circuits the only difference this time is in the use of the instantaneous "peak-to-peak" or "rms" quantities.

When working with AC alternating voltages and currents it is usual to use only "rms" values to avoid confusion. Also the schematic symbol used for defining an AC voltage source is that of a "wavy" line as opposed to a battery symbol for DC and this is shown below.

Symbol Representation of DC and AC Supplies

Resistors are "passive" devices, that is they do not produce or consume any electrical energy, but convert electrical energy into heat. In DC circuits the linear ratio of voltage to current in a resistor is called its resistance. However, in AC circuits this ratio of voltage to current depends upon the frequency and phase difference or phase angle ( φ ) of the supply. So when using resistors in AC circuits the term Impedance, symbol Z is the generally used and we can say that DC resistance = AC impedance, R = Z.

It is important to note, that when used in AC circuits, a resistor will always have the same resistive value no matter what the supply frequency from DC to very high frequencies, unlike capacitor and inductors.

For resistors in AC circuits the direction of the current flowing through them has no effect on the behaviour of the resistor so will rise and fall as the voltage rises and falls. The current and voltage reach maximum, fall through zero and reach minimum at exactly the same time. i.e, they rise and fall simultaneously and are said to be "in-phase" as shown below.

V-I Phase Relationship and Vector Diagram

We can see that at any point along the horizontal axis that the instantaneous voltage and current are in-phase because the current and the voltage reach their maximum values at the same time, that is their phase angle θ is 0o.  Then these instantaneous values of voltage and current can be compared to give the ohmic value of the resistance simply by using ohms law. Consider below the circuit consisting of an AC source and a resistor.

The instantaneous voltage across the resistor, VR is equal to the supply voltage, Vt and is given as:

The instantaneous current flowing in the resistor will therefore be:

As the voltage across a resistor is given as VR = I.R, the instantaneous voltage across the resistor above can also be given as:

In purely resistive series AC circuits, all the voltage drops across the resistors can be added together to find the total circuit voltage as all the voltages are in-phase with each other. Likewise, in a purely resistive parallel AC circuit, all the individual branch currents can be added together to find the total circuit current because all the branch currents are in-phase with each other.

Since for resistors in AC circuits the phase angle φ between the voltage and the current is zero, then the power factor of the circuit is given as cos 0o = 1.0. The power in the circuit at any instant in time can be found by multiplying the voltage and current at that instant.

Then the power (P), consumed by the circuit is given as P = Vrms Ι cos Φ in watt's. But since cos Φ = 1 in a purely resistive circuit, the power consumed is simply given as, P = Vrms Ι the same as forOhm's Law.

This then gives us the "Power" waveform and which is shown below as a series of positive pulses because when the voltage and current are both in their positive half of the cycle the resultant power is positive. When the voltage and current are both negative, the product of the two negative values gives a positive power pulse.

Power Waveform in a Pure Resistance

Then the power dissipated in a purely resistive load fed from an AC rms supply is the same as that for a resistor connected to a DC supply and is given as:

• Where:
• P  is the average power in Watts
• Vrms  is the rms supply voltage in Volts
•  Irms   is the rms supply current in Amps
• R  is the resistance of the resistor in Ohm's (Ω) – should really be Z to indicate impedance

The heating effect produced by an alternating current with a maximum value of Imax is not the same as that of a DC current of the same value. To compare the AC heating effect to an equivalent DC the rms values must be used. Any resistive heating element such as Electric Fires, Toasters, Kettles, Irons, Water Heaters etc can be classed as a resistive AC circuit and we use resistors in AC circuits to heat our homes and water.

Resistors in AC Circuits Example No1

A 1000W heating element is connected to a 250v AC supply voltage. Calculate the impedance (AC resistance) of the element when it is hot and the amount of current taken from the supply.

Resistors in AC Circuits Example No2

Calculate the power being consumed by a 100Ω resistive element connected across a 240v supply.

As there is only one component connected to the supply, the resistor, then VR = VS

Then to summarise, in a pure ohmic AC Resistance, the current and voltage are both said to be "in-phase" as there is no phase difference between them. The current flowing through the resistor is directly proportional to the voltage across it with this linear relationship in an AC circuit being called Impedance. As with DC circuits, Ohm's Law can be used when working with resistors in AC circuits to calculate the resistors voltages, currents and power.

Resistor Power Rating

When an electrical current passes through a resistor due to the presence of a voltage across it, electrical energy is lost by the resistor in the form of heat and the greater this current flow the hotter the resistor will get. This is known as the Resistor Power Rating.

Resistors are rated by the value of their resistance and the Electrical Power given in watts, (W) that they can safely dissipate based mainly upon their size. Every resistor has a maximum power rating which is determined by its physical size as generally, the greater its surface area the more power it can dissipate safely into the ambient air or into a heatsink.

A resistor can be used at any combination of voltage (within reason) and current so long as its "Dissipating Power Rating" is not exceeded with the resistor power rating indicating how much power the resistor can convert into heat or absorb without any damage to itself. The Resistor Power Rating is sometimes called the Resistors Wattage Rating and is defined as the amount of heat that a resistive element can dissipate for an indefinite period of time without degrading its performance.

The power rating of resistors can vary a lot from less than one tenth of a watt to many hundreds of watts depending upon its size, construction and ambient operating temperature. Most resistors have their maximum resistive power rating given for an ambient temperature of +70oC or below.

Electrical power is the rate in time at which energy is used or consumed (converted into heat). The standard unit of electrical power is the Watt, symbol W and a resistors power rating is also given in Watts. As with other electrical quantities, prefixes are attached to the word "Watt" when expressing very large or very small amounts of resistor power. Some of the more common of these are:

Electrical Power Units

Resistor Power (P)

We know from Ohm's Law that when a current flows through a resistance, a voltage is dropped across it producing a product which relates to power.

In other words, if a resistance is subjected to a voltage, or if it conducts a current, then it will always consume electrical power and we can superimpose these three quantities of power, voltage and current into a triangle called a Power Triangle with the power, which would be dissipated as heat in the resistor at the top, with the current consumed and the voltage across it at the bottom as shown.

The Resistor Power Triangle

The above power triangle is great for calculating the power dissipated in a resistor if we know the values of the voltage across it and the current flowing through it. But we can also calculate the power dissipated by a resistance by using Ohm's Law.

Ohms law allows us to calculate the power dissipation given the resistance value of the resistor. By using Ohms Law it is possible to obtain two alternative variations of the above expression for the resistor power if we know the values of only two, the voltage, the current or the resistance as follows:

[ P = V x I ]      Power = Volts  x  Amps

[ P = I2 x R ]      Power = Current2  x  Ohms

[ P = V2 ÷ R ]      Power = Volts2  ÷  Ohms

The electrical power dissipation of any resistor in a DC circuit can be calculated using one of the following three standard formulas:

• Where:
•       V  is the voltage across the resistor in Volts
•       I  is in current flowing through the resistor in Amperes
•       R  is the resistance of the resistor in Ohm's (Ω)

As the dissipated resistor power rating is linked to their physical size, a 1/4 (0.250)W resistor is physically smaller than a 1W resistor, and resistors that are of the same ohmic value are also available in different power or wattage ratings. Carbon resistors, for example, are commonly made in wattage ratings of 1/8 (0.125)W, 1/4 (0.250)W, 1/2 (0.5)W, 1W, and 2 Watts.

Generally speaking the larger their physical size the higher its wattage rating. However, it is always better to select a particular size resistor that is capable of dissipating two or more times the calculated power. When resistors with higher wattage ratings are required, wirewound resistors are generally used to dissipate the excessive heat.

Power Resistors

Wirewound power resistors come in a variety of designs and types, from the standard smaller heatsink mounted aluminium body 25W types as we have seen previously, to the larger tubular 1000W ceramic or porcelain power resistors used for heating elements.

Typical Power Resistor

The resistance value of wirewound resistors is very low (low ohmic values) compared to the carbon or metal film types. The resistive range of a power resistor ranges from less than 1Ω (R005) up to only 100kΩ's as larger resistance values would require fine gauge wire that would easily fail.

Low ohmic, low power value resistors are generally used for current sensing applications were, using ohm's law the current flowing through the resistance gives rise to a voltage drop across it.

This voltage can be measured to determine the value of the current flowing in the circuit. This type of resistor is used in test measuring equipment and controlled power supplies.

The larger wirewound power resistors are made of corrosion resistant wire wound onto a porcelain or ceramic core type former and are generally used to dissipate high inrush currents such as those generated in motor control, electromagnet or elevator/crane control and motor braking circuits.

Generally these types of resistors have standard power ratings up to 500W and are connected together to form resistance banks.

Another useful feature of wirewound power resistors is in the use of heating elements like the ones used for electric fires, toaster, irons etc. In this type of application the wattage value of the resistance is used to produce heat and the type of alloy resistance wire used is generally made of Nickel-Chrome (Nichrome) allowing temperatures up to 1200oC.

All resistors whether carbon, metal film or wirewound obey Ohm´s Law when calculating their maximum power (wattage) value. It is also worth noting that when two resistors are connected in parallel then their overall power rating is increased. If both resistors are of the same value and of the same power rating, then the total power rating is doubled.

Resistor Power Rating Example No1

What is the maximum power rating in watts of a fixed resistor which has a voltage of 12 volts across its terminals and a current of 50 milliamperes flowing through it.

Given that we know the voltage and current, we can substitute these values into the following equation: P = V x I.

Resistor Power Rating Example No2

Calculate the maximum safe current that can pass through a 1.8KΩ resistor rated at 0.5W.

Given that we know the resistor power rating and resistance, we can now substitute the values into the power equation of: P = I2R.

All resistors have a Maximum Dissipated Power Rating, which is the maximum amount of power it can safely dissipate without damage to itself. Resistors which exceed their maximum power rating tend to go up in smoke, usually quite quickly, and damage the circuit they are connected to. If a resistor is to be used near to its maximum power rating then some form of heatsink or cooling is required.

Resistor power rating is an important parameter to consider when choosing a resistor for a particular application. The job of a resistor is to resist current flow through a circuit and it does this by dissipating the unwanted power as heat. Selecting a small wattage value resistor when high power dissipation is expected will cause the resistor to over heat, destroying both the resistor and the circuit.

Thus far we have considered resistors connected to a steady DC supply, but in the next tutorial about Resistors, we will look at the behaviour of resistors that are connected to a sinusoidal AC supply, and show that the voltage, current and therefore the power consumed by a resistor used in an AC circuit are all in-phase with each other.