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Friday, August 28, 2009

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.



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