Resistance is one of the most important values to measure in electronics. For this reason, each multimeter is equipped with an ohmmeter. With an ohmmeter, both technicians and engineers can design and troubleshoot various electrical and electronic circuits.
Although component strength values are freely available online, due to various factors including build quality, weather, corrosion, and general wear and tear, actual strengths can vary significantly. This is why everyone who works with electronics will need to learn how to measure resistance on the fly with a multimeter. Keep reading below to find out how!
What is resistance measured in?
Electrical resistance is a type of force that resists or prevents the flow of electrical current. Resistance is measured in ohm values represented by an omega symbol, Ω. It is one of the values calculated using Ohm’s Law, along with voltage and current.
With proper resistance values, people can control and direct electrical current. The resistor has many possible functions within a circuit. Some of the more popular uses include voltage dividers, frequency adjustment and timers, control of circuit functions, and heat production.
Before making a resistance measurement, you need to understand what a resistor is, as it will most likely be the component whose resistance you will be measuring.
What is a resistor?
There are various electronic components specifically designed to provide resistance in a circuit. These components are known as resistors. Resistors can be classified into two basic types: linear and nonlinear resistors.
Linear resistors can be further classified into two types: fixed value resistors (eg regular through hole resistors) and variable resistors (eg potentiometers).
On the other hand, non-linear resistors will change their resistive values based on different circumstances like temperature, voltage, and light (eg thermistor, diode).
Understanding Resistor Tolerance
Since impurities can cause resistance, every component in a circuit will have some level of resistive value. Even copper wires that are supposed to transfer electricity as efficiently as possible will have small amounts of resistance. The nice thing about electronics is that the values don’t have to be perfect for the circuits to work. We just have to make sure that our values are within the tolerance or margin of error.
Regarding resistors, manufacturers must indicate the tolerance of their resistors. A resistor’s tolerance can be identified by looking at its spec sheet online or by identifying the metallic color of the last marked band on the component. These bands will be bronze (±1% tolerance), gold (±5% tolerance) or silver (±10% tolerance). For everyday DIY projects, a tolerance of ±10% will often be fine, but for precision work, tolerances of ±5% or even ±1% may be required.
So when you measure resistance expect the values to be inaccurate – a 270 ohm resistor may read 268 ohms or 272 ohms. As long as you don’t exceed the tolerance indicated by the last band on the resistor, you should be fine.
Where to Base Resistor Values
Measuring resistances in components or nodes will greatly benefit your troubleshooting skills in electronic circuits. And to know if a specific resistor or node has gone bad (not working), you’ll need a reference to the correct values.
As stated above, you can find the resistive values of components by looking up their component data sheet online. For regular THT fixed value resistors, a more convenient way to know their resistive value is to familiarize yourself with the resistor color code illustration below:
To read the color code of a resistor, you will first need to orient the resistor correctly. Remember that when you read a resistor, you always read from left to right. Metallic colors such as bronze, silver, and gold should be oriented towards the far right part of the resistor.
There will be four to five bands in a resistor. On a five-band resistor, the first three bands will indicate the first three digits of the resistor value; the fourth band is a decimal multiplier, indicating how many zeros you add to the first three digits. In a four-band resistor, only the first two bands represent digits, while the third band is a decimal multiplier. For both types, the last band will always be metallic, corresponding to the resistance tolerance.
If you memorize this color coding scheme, you will have a way to measure the resistance of a circuit without using a multimeter.
Essential parts of a multimeter
Before measuring resistance, you will first need to familiarize yourself with a multimeter. In general, there are two types of multimeter: analog and digital. Although they have differences in interface, both can measure voltage, current, and resistance. Here is an illustration of both types of multimeter and the essential parts you need to know to measure resistance:
How to measure resistance with a multimeter
Now that you know the basics of resistance and why we measure it, it’s time to show you how to check resistance with a multimeter.
Step 1: Insert the black probe connector into the common or COM port of the multimeter. Insert the red probe into the ohm input port.
Step 2: Select the ohmmeter function on your multimeter and select the resistance range. Use its function switch to select the ohmmeter function. The function will usually be indicated by an omega symbol (Ω).
If you’re using an auto-ranging multimeter, your ohmmeter will automatically set the correct resistance range (so you don’t need to set it). As for manual multimeters, you’ll need to use their function switch to select the range or resistances you expect to measure.
If you’re measuring THT resistors, use the resistor color coding scheme to estimate the resistance range you’ll need to set up your multimeter. If it’s an SMD (surface mount device) type of resistor, the value is likely written on the resistor itself.
If for some reason you can’t find it or the value is too small to see, you can find your resistor via its spec sheet. If you really can’t estimate its value, just set the range to the lowest value. Then you can continue to adjust the range if the ohmmeter does not show any value.
Step 3: Take red and black probes and let each probe touch the metal ends of the component or node you are trying to measure.
Step 4: Look at the screen to see the value of the resistor. If you’re using an auto-ranging multimeter, be sure to check the symbol on the display. An “MΩ” symbol means megohms (1 MΩ = 1000 kΩ), a “kΩ” means kiloohms (1 kΩ = 1000 Ω) and a “Ω” symbol means ohms (1Ω = 1000 mΩ). If the result is a decimal value with the symbol “Ω”, it is in milliohms (mΩ).
Be safe when testing circuits and components
Handling electronic and electrical circuits has its own dangers. To make sure you don’t damage the circuit and for your personal safety, you should keep the following in mind.
When you measure resistance with an ohmmeter, make sure the circuit is not powered (unless necessary). Scan the circuit. If you see an inductor, capacitor, or battery, be sure to remove the battery, and then discharge the circuit by connecting a high value resistor across both ends of the node or components.
Reading resistance values
And that’s all you need to know about the basics of resistance and reading resistance values. To hone your skills, try measuring the resistance of various electronic components (be sure to discharge capacitors and coils) inside and outside of a circuit. Familiarizing yourself with common resistor values and the resistor color coding scheme will also make you more proficient in using an ohmmeter. You may also want to learn how to measure voltages and currents as these will greatly improve your troubleshooting abilities.