Hardware

How to use a multimeter to test resistors

How do you know if a resistor is correctly restricting the flow of electricity? Using a multimeter is an obvious answer. However, there is more to it than meets the eye. This article will explain what you need to know.

When a PC stops working, it’s often cheaper and easier to replace it than to repair it. After all, why repair a computer that your company bought two years ago when you can buy a new one that’s twice as powerful for half the cost of your original machine? However, for many IT support technicians today, the time and energy spent repairing electronic equipment is still necessary because of budget constraints or because of the sensitive nature of data kept on many desktops. Fortunately, there are quite a few tools at the technician’s disposal. And when it comes to repairing electronics, few tools are as handy as a multimeter. In this article, I’ll show you how to use a multimeter to troubleshoot some basic electronic components, such as resistors.

Before we begin
Every multimeter is different, so the instructions that I give you may not exactly match up with your multimeter. Therefore, make sure you understand how to use your specific model of multimeter before you try any of these techniques. Failure to do so could result in injury or damage to the components that you are testing.

Resistor ratings
Resistors are probably the easiest component to test with a multimeter. Resistors are designed to decrease electrical current. For example, if a circuit required the use of a transistor, but the amount of electricity being used was sufficiently high enough to damage the transistor, then one way of being able to use the transistor is to place a resistor in front of it.

Color band
Before you can test a resistor, you need to know its strength and tolerance. Resistors are color-coded. If you look at a resistor, one end should have a gold, silver, or white band. Turn the resistor so that this band is to your right. That band represents the resistor’s tolerance. Before I discuss tolerances, you need to know how to read a resistor’s values. You begin by translating the colored bands into numbers and recording those numbers. For the first and second colored bands, the values are as follows:
  • Black = 0
  • Brown = 1
  • Red = 2
  • Orange = 3
  • Yellow = 4
  • Green = 5
  • Blue = 6
  • Violet = 7
  • Grey = 8
  • White = 9

Multiplier band
Once you find the values for the first two bands, write them down. For example, if you have a red band and a black band, then the values will be 2 and 0. Put these two numbers together and you’ll get the number 20. The third band is the multiplier band. This is the number you'll multiply the first two bands by to get the resistor’s value. The color scheme for the third band is as follows:
  • Black = 1
  • Brown = 10
  • Red = 100
  • Orange = 1000 (or 1 K)
  • Yellow = 10,000 (or 10 K)
  • Green = 100,000 (or 100 K)
  • Blue = 1,000,000 (or 1 M)

Pretend that a resistor had red, black, yellow, and silver bands. I already explained that the red and black bands in the first two positions would translate into 2 and 0, which are joined to read as 20. The yellow band in the third position is a multiplier. The multiplication value is 10,000 (or 10 K). Now, multiply 20 by 10,000 and you’ll get 200,000. This means that the resistor is rated at 200,000 ohms, more commonly expressed as 200 K ohms.

Tolerance band
Let’s take a look at the tolerance band. The reason for having a tolerance band is that no resistor performs at exactly its rated value. The tolerance band is there to let you know how much the resistor could potentially be off by. A gold resistor means that the rated value is within plus or minus 5 percent of being accurate. A silver band means that the resistor’s actual value may be within plus or minus 10 percent of the rated value. If there is no tolerance band, it means that the resistor has an actual value within plus or minus 20 percent of the rated value.

Now we'll go back to our 200,000 ohm resistor. This resistor had a silver tolerance band, meaning that it is accurate within plus or minus 10 percent of the rated value, with 10 percent of 200,000 equaling 20,000. If we add 20,000 to 200,000, we determine that the resistor’s actual measurement could be as high as 220,000 ohms. Likewise, if we subtract 20,000 from 200,000, the resistor could have a resistance of as low as 180,000 ohms.

Testing resistors
Now that you know how to read a resistor’s estimated values and potential values, let's take a look at how to check for a bad resistor. Generally, resistors are pretty durable, but they can be cooked by excessive amounts of electricity. Back in my college electronics class, I remember more than one classmate cooking resistors with too much juice. Usually, the resistor gets hot, starts smoking, and makes a strange high-pitched squeal.

Once a resistor has been blown, often no electricity can pass through it. Such resistors are said to have infinite resistance. At the same time, if the resistor was damaged by excessive voltage but not destroyed, the resistor may allow some electricity to pass but have an incorrect level of resistance. This is why it is so important to know about tolerances. For example, if you knew that a resistor was supposed to have a value of 200,000 ohms but tested the resistor at 180,000, you might assume that the resistor was bad.

When testing a resistor, the multimeter is passing a known amount of electrical current through the resistor and then measuring the amount of current that actually makes it through. Since the multimeter is passing current through the resistor, you want to ensure that the device containing the resistor you are testing is unplugged and turned off. If a normal amount of current were flowing through the resistor and you tried to test the resistor, not only will your reading be inaccurate, but you could damage the resistor and other components. You could also damage your multimeter or receive a nasty electrical shock.

With that said, multimeters are designed to use scales. These scales determine how much current the multimeter will use during the test. For example, my multimeter has scales for 200 ohms, 2 K ohms, 200 K ohms, 2 M ohms, and 20 M ohms. If I were to test our fictitious 200 K ohm resistor with this particular meter, I would set the scale at 200 K ohms. However, it’s purely a coincidence that my meter has a setting for 200 K ohms. Normally, there won’t be a scale setting that matches the value of the resistor. In such situations, you’ll want to go to the nearest scale value above the resistor's rating. For example, if you had a 100 K ohm resistor, you would use the 200 K ohm scale. If you had a 300 K ohm resistor, you’d use the 2 M ohm scale. The available scales will differ among brands and models of multimeters, but the concept remains the same.

Once you've verified that the device is unplugged and powered off and that your meter is set to the correct scale, it’s time to take a measurement. Resistors aren’t polarized, so it doesn’t matter which side of the resistor you place the meter’s red or black probes on. Once you place the probes against the resistor’s leads, you should receive a value for the resistor.

For demonstration purposes, I decided to use my meter to actually test a 200 K ohm resistor. The resistor tested at 197.6 ohms. This was well within the 180 K to 220 K range allowed by the resistor’s 10 percent tolerance. Had the resistor tested outside of this range, the resistor would have been bad and would have needed to be replaced.

More information on multimeters
Multimetersare versatile tools that all PC support technicians should be familiar with for troubleshooting electronic equipment. If you want more information on multimeters, give these other TechProGuild articles a try:
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