Resistance is a measurement of the difficulty level of electrons to flow through a particular object. Resistance is similar to the friction an object experiences when it moves or is moved over a surface. Resistance is measured in ohms; 1 ohm is equal to 1 volt of voltage divided by 1 ampere of current. Resistance can be measured with an analog or digital multimeter or ohmmeter.
Step
Method 1 of 3: Measuring Resistance with a Digital Multimeter
Step 1. Select the object whose resistance you want to measure
For the most accurate measurement, test the resistance of a component separately. Remove components from the circuit or test components before you install them. Testing components in a circuit may result in inaccurate readings due to the presence of other components.
If you are testing a circuit or even if you are simply removing components, make sure that all power flowing to the circuit is turned off before proceeding
Step 2. Insert the probe cable into the correct probe cable hole
In most multimeters, one of the probe wires is black and the other is red. Multimeters often have multiple apertures, depending on how they are used to test resistance, voltage, or current. Usually, the right hole used to test resistance has the label "COM" (from the word common) and the hole is labeled with the Greek letter omega,, which is the symbol for "ohm".
Insert the black probe lead into the hole labeled "COM" and the red probe lead into the hole labeled "ohms"
Step 3. Turn on the multimeter and select the best test range
The resistance of a component ranges from ohms (1 ohm) to megaohms (1,000,000 ohms). To get an accurate resistance reading, you must set the multimeter to the correct range for your component. Some digital multimeters will automatically set the range, but others must be set manually. If you have a general estimate of the resistance range, adjust the range according to your estimate. If you're not sure, you can determine the range through a trial run.
- If you don't know the range, start with a midrange setting, usually 20 kiloohms (kΩ).
- Touch one of the probe wires to one end of your component and the other probe to the other end of your component.
- The number on the screen will display 0.00 OL, or the actual resistance value.
- If the value is zero, the range is too high and should be lowered.
- If OL (overloaded) appears on the screen, the range is too low and should be increased to the next highest range. Retest the component with the new range setting.
- If a certain number appears on the screen such as 58, that is the resistance value. Remember to take into account the range used. On a digital multimeter, the top right corner is your range setting. If k appears in the corner, the real resistance is 58 k.
- Once you get the range right, try lowering the range one more time to see if you can get a more accurate reading. Use the lowest range setting for the most accurate resistance reading.
Step 4. Touch the multimeter probe wire to the ends of the component you are testing
Just as when you adjust the range, touch one probe lead to one end of the component and the other probe to the other end of the component. Wait for the numbers to stop rising or falling and record them. This is the bottleneck of your component.
For example, if your reading is 0.6 and MΩ appears in the upper right corner, your component resistance is 0.6 megaohms
Step 5. Turn off the multimeter
When you have finished measuring all your components, turn off the multimeter and remove the probe wire for storage.
Method 2 of 3: Measuring Resistance with an Analog Multimeter
Step 1. Select the object whose resistance you want to measure
For the most accurate measurement, test the resistance of a component separately. Remove components from the circuit or test components before you install them. Testing components in a circuit may result in inaccurate readings due to the presence of other components.
If you are testing a circuit or even if you are simply removing components, make sure that all power flowing to the circuit is turned off before proceeding
Step 2. Insert the probe cable into the correct probe cable hole
In most multimeters, one of the probe wires is black and the other is red. Multimeters often have multiple apertures, depending on how they are used to test resistance, voltage, or current. Typically, the right hole used to test resistance has the label "COM" (from the word common) and the hole is labeled with the Greek letter omega,, which is the symbol for "ohm".
Insert the black probe lead into the hole labeled "COM" and the red probe lead into the hole labeled "ohms"
Step 3. Turn on the multimeter and select the best test range
The resistance of a component ranges from ohms (1 ohm) to megaohms (1,000,000 ohms). To get an accurate resistance reading, you must set the multimeter to the correct range for your component. If you have a general estimate of the resistance range, adjust the range according to your estimate. If you're not sure, you can determine the range through a trial run.
- If you don't know the range, start with a midrange setting, usually 20 kiloohms (kΩ).
- Touch one of the probe wires to one end of your component and the other probe to the other end of your component.
- The multimeter needle will move across the screen and stop at a certain point indicating the resistance of your component.
- If the needle moves to a high range value (left side), you must increase the range setting, turn the multimeter down, and try again.
- If the needle moves to a low range value (right side), you should lower the range setting, turn the multimeter off, and try again.
- Analog multimeters must be zeroed each time the setting is changed and before testing components. Touch the probe wires to each other to create a short circuit. Make sure the needle is pointing to zero using the Ohm Control or Zero Adjustment after the probe wires touch each other.
Step 4. Touch the multimeter probe wire to the ends of the component you are testing
Just as when you adjust the range, touch one probe lead to one end of the component and the other probe to the other end of the component. The resistance range in the multimeter is read from right to left. The right is zero and the left is about 2k (2,000). There are several scales in an analog multimeter. So, make sure to look at the scale labeled from right to left.
As the scale goes up, the higher values get clustered and close together. Setting the right range is critical to getting accurate readings for your components
Step 5. Read the barriers
Once you touch the probe wires to the component, the needle will stop somewhere between the highest and lowest scales. Check to make sure you see the ohm scale and note the value indicated by the needle. This is the bottleneck of your component.
For example, if you set the range to 10 and the needle stops at 9, the resistance of your component is 9 ohms
Step 6. Set the voltage to a high range
After you are done using the multimeter, you want to make sure that the multimeter is stored properly. Setting the voltage to a high range before turning off the multimeter ensures that the multimeter will not be damaged if one time is used by someone who forgets to set the range first. Turn off the multimeter and unplug the probe for storage.
Method 3 of 3: Ensuring Good Testing
Step 1. Test the resistance in components, not circuits
Measuring resistance in components in the circuit will lead to inaccurate readings because the multimeter also measures the resistance of the component under test as well as other components in the circuit. However, sometimes, it is necessary to test the resistance of the components in the circuit.
Step 2. Test only powered components
Current flow through the circuit will cause inaccurate readings as increased current will cause greater resistance. In addition, the additional voltage can also damage the multimeter. (Therefore, resistance testing of the battery is not recommended.)
Any capacitor in the circuit being tested for resistance must be recharged before being tested. A discharged capacitor may absorb charge from the multimeter current causing a momentary fluctuation in the reading
Step 3. Check the diodes in the circuit
Diodes conduct electricity in only one direction; thus, reversing the position of the multimeter probe wire in a circuit containing a diode will cause a different reading.
Step 4. Watch your fingers
Several resistors or components must be held to maintain contact with the multimeter probe wires. Holding the resistor or probe wire with your finger will cause an inaccurate reading as your body absorbs current from the circuit. This is not a big problem when using a low-voltage multimeter, but it can be a problem when testing resistance with a high-voltage multimeter.
One way to keep your hands off components is to stick them in a test board or breadboard when testing resistance. You can also attach an alligator clip to the multimeter probe wire to keep the resistor terminals stationary while testing
Tips
- The accuracy of a multimeter depends on the model. Inexpensive multimeters are usually accurate to 1 percent of the correct value. You will of course have to pay more for a more accurate multimeter than this.
- You can identify the resistance of a given resistor based on the number and color code of the lines on the resistor. Some resistors use a 4-line system, while others use a 5-line system. One of the lines is used to represent the level of accuracy.
