Resistors Part 2- Color Code and How Resistance is Measured

Let's continue of what we have discussed yesterday before we go on the full discussion of a Series Circuit which will be our next topic that we will going to study here in Learn Electrical Engineering for Beginners.

Today, we will be dealing on how the resistor color coding is being used and how do we going to interpret it to obtain the reading. Then, afterward s we will touch a little bit on how the resistance is being measured. That's all will be discussed within this new post.

Let's begin now... timer start now!

The Resistor Color Code

We all know that we can find the resistance value of any resistor by using an ohmmeter. But what if we don't have an ohmmeter to use? Most of the case we can find the resistance value easier by interpreting its marking. Some resistors like wire-wound resistor have its printed value in ohms in their body. If they don't have the mark, you would require to use an ohmmeter. An example of a resistor which usually have all of the data printed directly on the resistor body with the information such as tolerance, temperature characteristics, and exact resistance value is the precision wire wound resistor. Other resistor like the carbon resistors usually do not have the data of characteristics directly marked on them, instead they have a so called color code by which they can be identified. You will wonder why it is being done this way for carbon resistors. The reason of using a color code for a carbon resistor is that they are small which is difficult to read the printed values especially when they are mounted.

Before we forgot something, there are two types of carbon resistors. The radial and an axial. They are only differ in the the way the leads are connected to the body of the resistor. Both employ the same color code but they are printed in the different manner. Radial lead resistors are not found in modern equipment. They are widely used in the past. I can't see any example of this now. Below is an example of an axial resistor.

In the picture above this axial lead resistors have its leads molded into the ends of the carbon rod of the resistor body. If you will see, the leads extends straight out in line from the body of the resistor. The carbon rod is coated with a good insulator.

Moving on...

Color coding system for resistors consists of three colors to indicate the resistance value in ohms of a certain resistor, sometimes the fourth color indicate the tolerance value of the resistor. By reading the color coded in correct order and substituting the correct value of each corresponding color coded as shown in the table below, you can immediately tell all you need to know about the resistor. The only thing that you will practice on how to use it and familiar yourselves for those value so that you can easily determine the value of the resistor color coded at a glance.

This is how you will do it.

The color of the first color band indicates the first digit of the resistance value or the first significant digit. Let's have an example below. Supposed that you have a given resistor below, the first color is yellow. If you would look at the table above it is equivalent to 4.

The second color coded of the resistor given below is violet, so this is now your second digit which is equivalent to 7 as shown in the table above.

The third color would served as your multiplier. In the case below since it is color red which is equivalent to 100 multiplier, or just simply add 2 zeros so this would look like this now:

47 ohm x 100 = 4, 700 ohms or 4.7 kilohms

Ooooppps! it seems that we are not done yet. The last color band or the fourth color band is gold which have 5% tolerance according to our table above. Therefore our final answer would be:

4.7 kilohms +/- 5% - answer

How to Measure the Resistance

We all know that voltmeter and ammeter are used for measuring the voltage and the current respectively. For the resistance, the meters that use to measure it is the ohmmeter. When using an ohmmeter, there should be no voltage present across the resistors except for the ohmmeter battery, otherwise your ohmmeter would be damaged. I can see two types of ohmmeter nowadays, the analog and the digital. Among the two ohmmeters, digital is widely used nowadays.

The above ohmmeter usually used to measure the resistance of the resistors. Ohmmeter ranges usually vary from 0-1,000 ohms to 0 -10 megohms. There are some special ohmmeters called the MEGGERS. This ohmmeter was used to measure high resistance values which are over 10 megohms. Some meggers use high voltage batteries and other use special type of hand generator to obtain the necessary voltage. These megohmeters is used to measure and test the resistance of insulation. Picture below is the example of a megger.

Ohmmeter is very easy to use by following two steps. First, the voltage must be set to the proper value. This is done with the zero adjustment by shorting outor by connecting together the two leads from the ohmmeter and setting to zero ohms on the meter with the zero adjustment control. This should be always done whenever you changed the meter range selector switch to a different scale. Now, the meter is now calibrated for the given range, you will notice that when the leads shorted out, the meter reads zero ohm, but when it opens, the meter reads infinity which indicates an open circuit. Therefore, when these leads touches the resistors subject for measurement, it will directly read the resistance in the meter multiplying it with the range selector switch. The range selector switch is serves as the multiplier or the multiplying factor whenever you are measuring the resistance using ohmmeter. The range selector switch usually marked as R, RX 10, RX 100, RX1,000, etc...
For example if the ohmmeter is switch on to R X 1,000 meaning the value of the meter will be multiplied to 1,000 to get the actual value of the resistance being measured.

That's it for today.

Tomorrow we'll continue dealing with circuits here in Electrical Engineering.

Cheers!

Resistors Part 1 - Use and Properties

Now that you have learned the basic concept of Ohm's Law, we can now proceed in discussing the use, properties, and construction of resistors. All you can learn it here in Electrical Engineering.

Before we continue our study of circuits, we need to know more about resistance and resistors though we have touched it a little on my previous post. But its just a review. Today, it would be a bit deeper.

We know that there is a certain amount of resistance in all electrical equipment which we use. Sometimes, this resistance is not enough to control the flow of current to the extent desired. If you did not get my point here, let's have a few example of this. I will going to give an example by illustration as what had shown below. The circuit shown below, a switch and a current limiting resistor are used to control the flow of current through the motor. When starting a motor, the switch is kept open and the resistance is thereby added into the circuit to control the flow of current. After the motor has started, the switch is then closed in order to bypass the current limiting resistor.

There are wide variety of resistors, some of them have fixed values and some are variables.

What resistor is made up of?

Resistors are made up of special resistance wire, graphite (carbon) composition, or of metal film. Wire wound resistors are usually used to control large currents, while carbon resistors controls current which are relatively small.

Vitreous enameled wire-would resistors are constructed by winding resistance wire on a porcelain base, attaching the wire ends to metal terminals, and coating the wire and base with powdered glass and bake enamel to protect the wire and conduct heat away from it.

Fixed wire wound resistors with a coating other than vitreous enamel are also used. The example below is the example of this one.

Wire-wound resistors may have fixed taps, which can be also used to change the resistance value in steps, or sliders, which can be adjusted to change the resistance of any fraction of the total resistance. The picture below is the example of this one.

Precision wound resistors of manganin wire (a special wire that does not change resistance very much with high temperature) are used where the resistance value must be very accurate, such as in test instruments. The picture below is the example of this one.

Carbon resistors are used for low current applications. They are made from a rod of compensated graphite (carbon) that is mixed with clay and binders. By varying the amount of each component, it is possible to vary the resistance values obtained over a very wide range. Two lead wires are called pigtails are attached to the end of the resistance rod, and the rod is embedded in a ceramic or plastic covering, leaving the pigtails protruding from the ends. Take a look for a sample below.

You will find other type of resistor called a deposited film resistor used for special applications. These resistors are made by depositing a thin film of resistance metal or carbon on a ceramic core and then coating the resistor with either a ceramic or enamel protective coating. In many cases you will find that these resistors have radial leads meaning the leads come off at right angles to the body of the resistor. In some cases the deposited film is laid down on the core as spiral, similar to winding a wire around the tube, in order to increase the length of the resistance element without making the resistor too long. The example of this one was shown below.

Resistor Tolerance and Values

Let's consider this topic before we go on the color code for resistors. You need to find out the something about resistor tolerances and something about the preferred values of resistance that you will find in the circuits. Special resistors may have tolerances of as little as 1% , 0.1% or even 0.01% but most resistors that you will see have much greater tolerances. Large wire-wound resistors usually have tolerances of 10% or 5%. Carbon resistors are available in 20%, 10% and 5% tolerances.

So what are those tolerance mentioned above means? Let's take an example...

If you had a 10 kilohm resistor with a tolerance of 20%, the actual value of the resistor could anywhere from 10 kilohm - 10 kilohm (0.20) = 8 kilohm to 10 kilohm + 10(0.20) = 12 kilohm. That is how you will going to use the tolerance given for a specific resistor.

You will wonder how many different resistance values you can get for a resistor. It depends on the tolerance. Considerations such as this have led to the establishment of a set of preferred values of resistance in each tolerance where the highest tolerance of one value is about equal to the lowest tolerance of the next highest value. The table of preferred resistance was shown below. Later, you will find that resistors are available in different power ratings as well.

The numbers on the chart above show only the first two digits. Thus, it means for example, 33 means that 3.3, 330, 3.3 kilohm, 330 kilohm and 3.3 megohm resistors are available.

On my next post I will show you how the resistor color code is being used for carbon resistors and how the resistance is being measured.

I will tee off shortly. Stay more here in Learn Electrical Engineering for Beginners.

Cheers!

Solutions To Brain Teasers Number 2

As I promised last time, I will going to show you the solutions to teasers on our previous topic about the Factors Affecting Resistance here in Electrical Engineering. By the way, this topic is still covered on our review of basic Physics for this is very important one when we reached our major topics in this Electrical Engineering course.

Last time, I leaved you four worded problems in order for you to analyze and understand the principles fully. But if you still running out of time to solve it, I will show to you it now.

The first problem given was:

Problem 1 : The resistance of a copper wire 2, 500 cm long and a 0.090 cm in diameter is 0.67 ohm at 20 degree celcius. What is the resistivity of copper at this temperature?

The solution here is quiet simple. We just have to substitute the given values from the formula since we have uniform units:

p = RA/ l = 0.67 ohm x [ TT ( 0.090 cm) ^2/ 4 ] / 2, 500 cm
    = 1.7 x 10 ^-6 ohm.cm - answer

The unit of resistivity in the British engineering system of units differs from that just given in that different units of length and area are employed. The unit of area is the circular mil., the area of the circle 1 mil (0.001 in) in diameter, and the unit of length is the foot. Since the areas of two circles are proportional to the squares of their diameters, the area of a circle in circular mils is equal to the square of its diameter in mils. In this system of units the resistivity of a substance is numerically equal to the resistance of a sample of that substance 1 ft long and 1 circular mil in area, and is expressed in ohm-circular mils per foot.

The abbreviation CM is often used for circular mils. This should not be confused with the abbreviation used for centimeters (cm). We will use the more standard cmil.

Let's solve another problem using the principles above.

Problem 2 : Find the resistance of 100 ft of copper wire whose diameter is 0.024 in and whose resistivity is 10.3 ohm.cmils/ft.

Convert first the given diameter in mils. Since, 1 mil = 0.001 in as mentioned above.

Therefore, d= 0.024 in = 24 mils.

Then getting the area we have, A = d^2 = 24 ^2 cmils. Substituting the values:

R = pl / A = (10.3 ohm.cmils/ft)(100 ft) / 24 ^ 2 cmils.
R = 1.8 ohm - answer

Problem 3 : A silver wire has a resistance of 1.25 ohm at 0 degree celcius and the temperature coefficient of resistance of 0.00375 per degree celcius. To what temperature must the wire be raised to double the resistance?

Since we are asking for the temperature, just derive the formula of EQ . 1 in our previous post here in Electrical Engineering topics. It will be:

 

t = Rt - R0 / Ro oo = ( 2.50 -1.25 ) ohm / 1.25 ohm x 0.00375 /C
t = 266 degree celcius - answer

It should be clearly understood that R0 in the above equation ordinarily refers to the resistance at 0 degree celcius and not to the resistance of any other temperature. A value of oo based upon the resistance at room temperature, for example, is appreciably different from the value based upon 0 degree celcius. This may be made clearer by the graphic analysis of the variation of resistance with temperature. ( Click the image to enlarge)

In the above illustration, the resistance Rt of a conductor at any temperature t is plotted. For a pure metal, this curves gives a linear relation ( approximately). Note the fact that the curve does not pass through the origin; i.e. at 0 degree celcius the resistance is not zero. Hence, we cannot say that R oo t. The slope of the curve delta R/delta t is constant . Since,

oo = delta R / delta t /Ro = slope / Ro

it is clear that the value of oo depends upon the base temperature chosen for Ro. In computations involving temperature variation of resistance, the value of Ro must be obtained by using the equation below:

Rt = Ro + Ro oo t = Ro( 1 + oo t)

Understanding the principles of effect of temperature in resistance.

Problem 4 : A tungsten filament has a resistance of 133 ohm at 150 degree celcius. If oo = 0.0045/C., what is the resistance of the filament at 500 degree celcius?

From the EQ 1: ( of our previous post)

Ro = Rt / 1 + oo t = 133 ohm / 1 + (0.00450/C ) x 150 degree celcius

Ro = 79.4 ohms

Getting the resistance at 500 degree celcius,

R500 = R0( 1 + oo t500)
R500 = 79.4 ohms [ 1 + (0.00450/C) x 500 degree celcius]

R500 = 258 ohms - answer

Since it is the resistivity factor that changes with temperature. The equations of EQ 1 and EQ 2 from our previous lecture may be written with p in place of R.

pt = p0( 1 + oot) ------------------------------------- EQ. 3

I will not give the table for resistivities and temperature coefficients of resistivity of materials for it is always given in the problem. You don't have to memorize it.

I think this is now over for the basic review....

On my next post, we will now begin to discuss the real scope of Electrical Engineering.

Cheers!

Factors Upon Which The Resistance Of Conductor Depends

Let's continue our discussion with our basic concepts here in Electrical Engineering. I guess most of you are familiar with this topic but this should not be neglected because it has a big role during board exam of Electrical Engineering.

What is the topic for today?

Let's begin....

We all know that Georg Simon Ohm who formulated the law that the resistance of a conductor varies directly with its length, inversely with its cross-sectional area, and depends upon the material of which it is made.

From the study of resistors in series, one would expect that the resistance of a piece of uniform wire is directly proportional to its length, since it can be thought of as a series of small pieces of wire whose total resistance is the sum of the resistances of the individual pieces.

Let's have an example. Consider a wire 1 ft in length and having a cross- sectional area of 0.3 in^2. By thinking of this as equivalent to three wires ( 1 ft in length) each having a cross-sectional area of 0.1 in^2 connected in parallel, we may infer that

1/R = 1/R1 + 1/R2 + 1/R3

or since R1 = R2 = R3,

1/R = 3/R1 and R1 = 3R

showing that the resistance of one of the small wires is three times as great as that of the large wire. This suggests that the resistance of a wire is inversely proportional to the cross-section, a fact that was verified experimentally by Ohm.

Using R varies as l and R varies as 1/A, as mentioned above, we can write R varies as l/A where l is the length and A the cross sectional area of a uniform conductor. This relation can be written in the form of equation

R = p l/A

where p is a quantity, characteristics of the material of the conductor, called the resistivity of the substance. The term specific resistance is sometimes used instead of resistivity.

From the equation above:

p = RA/l

If A and l are given values of unity, it is seen that p is numerically equal to the resistance of a conductor having unit cross section and unit length.

If R is in ohms, A in square centimeters, and l in centimeters, then p is in ohm-centimeters. This unit is somewhat more convenient than the mks unit the ohm-meter.

Conductance and Conductivity

Since the reciprocal of the resistance, 1/R occurs often in parallel circuits, it is frequently convenient to designate this concept as the conductance of the resistor. The symbol used for conductance is G, and the unit is mho. In a parallel circuit the total conductance is given by G = G1 + G2 + G3. Less often the reciprocal of resistivity 1/p is used, and this concept is called the conductivity of the material. The symbol for conductivity is o (not exactly the symbol) and the unit is mho/cm.

Change of Resistance With Temperature

The electric resistance of all substances is found to change more or less with the changes of temperature. Three types of changes are observed. The resistance may increase with increasing temperature. This is true of all pure metals and most alloys. The resistance may decrease with increase of temperature. This is true of a semiconductor like carbon and of glass and many electrolytes. The resistance may be independent of temperature. This is approximately true of many special alloys, such as manganin ( Cu 0.84, Ni 0.12, Mn 0.04).

Experiments have shown that, for moderate temperature range, the change of resistance with temperature of metallic conductors can be represented by the equation.

Rt = Ro + Ro oo t = Ro( 1 + oo t) -----------------EQ. (1)

where Rt is the resistance at temperature t, Ro is the resistance at 0 degree celcius, and oo is a quantity characteristic of the substance and known as the temperature coefficient of resistance. The defining equation for oo is obtained by solving Eq 1, giving,

oo = Rt - Ro/ Rot ----------------------------------EQ.(2)

The temperature coefficient of resistance is defined as the change in resistance per unit resistance per degree rise in temperature, based upon the resistance at 0 degree celcius.

Although Eq 1 is only approximate, it can be used over medium ranges of temperature for all but very precise work.

Since Rt - Ro and Ro have the same units, their units will cancel in the fraction in Eq 2. Hence, the unit of oo depends only upon the unit of t. For instance, for copper oo = 0.004/C, but only 5/9 x 0.004/F.

For clear understanding of the principles above. I will show you on my next post some illustrative problem solving. For you to review in advance I will show it now.

A Teaser

Problem 1 : The resistance of a copper wire 2, 500 cm long and a 0.090 cm in diameter is 0.67 ohm at 20 degree celcius. What is the resistivity of copper at this temperature?

Problem 2 : Find the resistance of 100 ft of copper wire whose diameter is 0.024 in and whose resistivity is 10.3 ohm.cmils/ft

Problem 3 : A silver wire has a resistance of 1.25 ohm at 0 degree celcius and the temperature coefficient of resistance of 0.00375 per degree celcius. To what temperature must the wire be raised to double the resistance?

Problem 4 : A tungsten filament has a resistance of 133 ohm at 150 degree celcius. If oo = 0.0045/C., what is the resistance of the filament at 500 degree celcius?

I will going to reveal the solutions on my next post with additional discussions again for this topic here in Learn Electrical Engineering for Beginners.

Cheers!