Want to know how to measure resistance with a multimeter? This guide will teach you all about the basics.
Resistance is one of the most important values to be measured in electronics. For this reason, every multimeter comes equipped with an ohmmeter. With an ohmmeter, tinkerers and engineers alike can design and troubleshoot various electric and electronic circuitry.
Although resistance values of components are freely available online, due to various factors such as the quality of manufacturing, weather, corrosion, and general wear and tear, actual resistances may vary significantly. This is why everyone working with electronics will need to learn how to measure the resistance on the fly using a multimeter. Continue reading below to know how!
Electrical resistance is a kind of force that resists or impedes the flow of electrical current. Resistance is measured in values of ohms represented by an omega symbol, Ω. It is one of the values calculated using Ohm's Law, alongside voltage and current.
With proper resistance values, people can control and direct electrical current. Resistance has many possible functions inside a circuit. Some of the most popular uses include voltage dividers, setting frequency and timers, controlling circuit functions, and producing heat.
Before performing resistance measurement, you must understand what a resistor is since it will most likely be the component you’ll be measuring for resistance.
There are several 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 non-linear resistors.
Linear resistors can further be classified into two types: fixed value resistors (e.g. regular through-hole-resistors) and variable resistors (e.g. potentiometers).
On the other hand, non-linear resistors will change their resistive values according to different circumstances like temperature, voltage, and light (e.g. thermistor, diode).
Since impurities can cause resistance, every component in a circuit will have some levels of resistive values. Even the copper wires that are supposed to transfer electricity as efficiently as possible will have small amounts of resistance. A good thing about electronics is that values don’t have to be perfect for circuits to work. We just have to ensure our values are within tolerance or margin of error.
As for resistors, manufacturers are required to state the tolerance of their resistors. A resistor’s tolerance can be identified by looking at its specification sheet online or by identifying the metallic color of the last band marked on the component. These bands will be colored bronze (±1 % tolerance), gold (± 5% tolerance), or silver (± 10% tolerance). For everyday DIY projects, a ± 10% tolerance will often be fine, but for precision work, having tolerances of ± 5% or even ± 1% might be required.
So, when measuring resistance, expect that the values won’t be exact: a 270-ohm resistor may read 268 ohms or 272 ohms. As long as it doesn’t exceed the tolerance indicated by the last band of the resistor, you should be fine.
Measuring resistances in components or nodes will greatly benefit your troubleshooting skills in electronic circuits. And to know whether a resistor or a specific node has gone bad (not working), you’ll need a reference of the correct values.
As stated earlier, you can find the resistive values of components if you search for its component datasheet online. For regular THT fixed-value resistors, a more convenient way of knowing their resistive value is to familiarize yourself with the resistor color coding illustration below:
To read a resistor’s color code, you’ll first need to orientate the resistor properly. Remember that when reading a resistor, you always read from left to right. Metallic colors such as bronze, silver, and gold should be oriented to the right-most part of the resistor.
There will be four to five bands on a resistor. On a five-band resistor, the first three bands will indicate the first three digits of the resistor’s value; the fourth band is a decimal multiplier, indicating how many zeros you append to the first three digits. On a four-band resistor, only the first two bands represent digits, while the third is a decimal multiplier. For both types, the last band will always be metallic, corresponding to the resistor’s tolerance.
If you memorize this color coding scheme, you'll have a way to measure a circuit's resistance without using a multimeter.
Before measuring resistance, you'll first need to get acquainted 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:
Now that you know the basics of resistance and why we measure it, it is time to show you how to check resistance with a multimeter.
Step 1: Insert the black probe’s jack in the multimeter’s COM or common port. Insert the red probe in the ohm input port.
Step 2: Select the ohmmeter function on your multimeter and select the resistance range. Use your function switch to select the ohmmeter function. The function will usually be indicated by an omega symbol (Ω).
If you are using an auto-ranging multimeter, your ohmmeter will automatically set the correct resistance range (so there's no need to set it). As for manual multimeters, you’ll need to use your function switch to select the range or resistances you expect to measure.
If you are measuring THT resistors, use the resistor color coding scheme to estimate the resistance range you’ll need to set your multimeter. If it’s an SMD (surface-mount device) type of resistor, the value will likely be 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 its resistance through its specification sheet. If you really cannot estimate its value, just set the range at the lowest value. You can then keep adjusting the range if the ohmmeter don’t show any value.
Step 3: Take red and black probes and let each probe touch the metallic ends of the component or node you’re trying to measure.
Step 4: Look at the display for the resistance value. If you are using an auto-ranging multimeter, make sure to check for the symbol on display. An “MΩ” symbol means megohms (1 MΩ = 1,000 kΩ), a "kΩ" means kiloohms (1 kΩ = 1,000 Ω) a “Ω” symbol means ohms (1Ω = 1,000 mΩ). If the result is a decimal value with the “Ω” symbol, it’s on milliohms (mΩ).
Handling electronic and electrical circuits has its own hazards. To ensure you don’t damage the circuit, and for your personal safety, you’ll have to keep the following in mind.
When measuring resistance with an ohmmeter, please ensure the circuit is not powered (unless you need to). Scan the circuit. If you see an inductor, a capacitor, or a battery, make sure to remove the battery and then discharge the circuit by connecting a high-value resistor on both ends of the node or components.
And that’s about 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 (make sure to discharge capacitors and coils) in and out of a circuit. Familiarizing yourself with the 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 current as they will greatly enhance your troubleshooting capabilities.
Craving to learn how things worked, Jayric Maning started tinkering with all kinds of electronic and analog devices during his earlier teens. He took up forensic science at the University of Baguio to where he got acquainted with computer forensics and cyber security. He is currently doing lots of self-study and tinkering with tech figuring out how they work and how we can use them to make life easier (or at least cooler!).
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