Beginners Guide to Using a Multimeter
If you're just starting out with multimeters, some of the symbols and functions may look very strange to you. And perhaps you don't fully understand how to operate the meter, or exactly what it is you are measuring.
The good news is - once you understand a few basic things, it's really not that complicated! So, let's break it all down... we'll begin with a little bit of theory, which will help you understand what, how and why you test a particular way.
The theory -
Voltage, Current and Resistance. These are three electrical terms that you would have heard before. But do you know exactly what they are? Sometimes it’s difficult to get your head around, particularly with the first two. I’ll do my best to explain in basic terms...
Voltage is the electrical pressure that exists between two points. In a nutshell, there is a positive charge at one point, and a negative charge at another. These charges desperately want to get together to equal things out. The more positively charged and negatively charged each point is, the more they are desperate to get together and create equal balance. In other words, there is an electrical pressure between the two points that is wanting to merge. This electrical pressure is known as voltage. Its unit is known as Volts and is denoted by the capital V. The more Volts (V), the greater the electrical pressure is. There are two main types of voltage. AC voltage and DC voltage.
With AC voltage the electrical pressure is actually changing from zero to a particular value in the positive direction and back to zero, then to a particular value in the negative direction and back to zero again. In Australia on the domestic supply, it does this 50 times every second.
A DC voltage is a constant voltage in one direction and doesn’t change. By convention, as discussed above DC voltages measure from positive to negative.
Current is the movement of electrons in a conductive substance. In an electric wire, it is the movement of electrons within the wire from one point to another. What causes this movement in the wire? Well, remember our paragraph above when we said that voltage is electrical pressure between two oppositely charged points that are desperately trying to equal out? Well, current, or the movement of electrons, is how these two points equal out. In order for them to equal out, there must be a way for the electrons to travel from the negatively charged point to the positively charged point. What we do is put an electrically conductive wire between the two, which gives the electrons a super highway to travel along. The movement of electrons is called current, and it’s unit is amperes, or amps for short. Current is denoted by the capital A. The more current (A), the more electrons that are flowing.
At this point you may have noticed that I stated that the electrons (negative) are going to the positively charged point. But doesn’t electricity go from positive to negative? The answer is…..’’No’’…...When electricity was first being investigated, it was conventionally thought that the positive charge moved to the negative. However, it was later discovered that only the free electrons move, meaning the movement is from negative to positive. But, it was too late by the time industry learnt this, so we left it how it was. The fact that it is actually going the other way to what is generally thought has little consequence on the majority of electrical installations. The building of electronic components and semiconductors are a little different, however, as the electron direction flow can be critical to the function of the device. But this is a whole different topic!
Like voltage, current can be either AC or DC. Remembering that current is the result of electrical pressure, if the electrical pressure is AC, it follows that the resultant current will mimic the pressure that is causing it. That is, go from zero to a particular value in the positive direction and back to zero, then to a particular value in the negative direction and back to zero. If the electrical pressure is DC, then the current will travel in one direction only, and by convention as discussed above, this is said to be from positive to negative.
Resistance is like a reduced speed zone to the electrons on their conductive highway. It slows the electrons down so that they can’t just race to their positive charge. Not only do we need resistance to slow the current down and prevent a short circuit, but we can design resistance to do work for us. This is the case for an electric heater or a traditional incandescent light. Their elements are resistors, but their side effects are heat and light. In electronic circuits resistance is used to give circuits particular characteristics.
The unit for resistance is Ohms. It is represented by the omega symbol, Ω. The greater the Ohms, the more the resistance.
At this point I will mention the word impedance. Unlike resistance, which is using a physical barrier to electrons to slow current down, impedance refers to electrical/magnetic reactions in an AC system that, by default, can slow current down. It is a bit beyond the scope of this article, but I mention it because you may come across the word from time to time.
Distinguishing voltage, current, and resistance with an example...
Let’s put voltage, current and resistance together in a common safety question to give a mental picture of what’s going on...
"How much voltage will kill you?"
The answer is, voltage itself doesn’t kill you. It’s the current that kills you. However, it's the voltage that gives the electrical pressure required to push that current through you, and the resistance of your body is the only thing that will slow the current down.
The more voltage you have, the less effective the resistance of your body is, and the more current will be pushed through. That's why all the danger signs give a voltage warning. They are telling you that there is heaps of electrical pressure here and it will overcome your body's resistance to it. The amount of damage the resultant current causes depends on how much current is passing through, and for how long this current exists.
A safety switch in a house in Australia is designed to switch off on earth fault before 0.03A (amps) and within 400 milliseconds (as per Section B4 and Table B1 of the Australian Standards 3000) because any more current than this, for any longer, is dangerous.
Here’s another example...
You can put your hands on the terminals of a 6 Volt battery and nothing will happen, but if you join a wire straight between those two terminals, there will be sparks, and if you leave it there, the wire will get hot. At six volts electrical pressure, your body has enough resistance to limit the current to a very small amount, but when you short circuit the battery with a wire, there is very little resistance to slow the current and arcs, sparks and heat result.
You can actually weld with the 12V electrical pressure of a car battery because there is very little resistance in the welding rod and metal you are welding. The rush of current is enough to heat the metal to melting point and weld.
Using your multimeter -
OK, now we have a handle on voltage, current and resistance, we can learn how to measure them with a multimeter, and know exactly what it is we are measuring.
Before we do, a quick word on ranges. Your meter has several different circuits that are designed to handle different values of measurements. Each of the circuits have a minimum and maximum value that they are designed to measure. These values are called the range. If you try and obtain a value that is outside a range, the meter will not measure it correctly. If the value is too high for the range, it will show OL, meaning over limit. If the value is too low, then your reading will not be as accurate as it could be. You should find the correct range in order to get the most out of your multimeter.
Most multimeters these days are auto ranging, but some do still require manual input, and some have both auto and manual ranging. For more information on range see our blog on Understanding Terms - Count, Range and Resolution.
When you are measuring voltage, you are trying to determine how much electrical pressure exists between two points.
A word on safety...
- If you are expecting to be measuring dangerous voltages, then you should be wearing insulated gloves and eye protection at a minimum. In Australia, these voltages are considered to be 120VDC and above, and 50VAC and above. Bear in mind that lower voltages can still cause damage if the circumstances are right, such as wet conditions or short circuits. So if in any doubt, gloves and eye protection should be worn regardless.
- If you are using the voltage function to test that a circuit is off, always test your meter on a source that you know is on, then conduct your test, then test your meter again to prove it is still working.
Setting up your probes
This is important. If you put your probes in the current ports on your multimeter to measure voltage, the usual result is blowing the fuse in the meter…… which means no more measurements until you replace the fuse!
So, make sure you put the red lead in the port that is labelled V for volts, and the black lead in the Common. The V for volts port may have other symbols as well, as this port is also used for other measurements. (Some meters, for example some clamp meters, may use the same port for voltage and current, but this is by special design. The majority of multimeters will have separate ports for the red lead for current and voltage).
AC or DC Voltage
Some meters will automatically determine whether you are measuring DC or AC voltage. However, if your meter is not that clever, you will have to choose AC or DC Volts, either on the selector or with the mode button. Most meters will have a capital V for the volts function followed by the letters AC or DC. Some will have the capital V, but have a wavy line above it for AC and a straight line above it for DC.
Once you've selected Voltage, confirm that AC or DC (or the wavy line or the straight line) is displayed on the screen.
If your meter is auto ranging it will just have symbols for volts AC and volts DC to switch to, however, if it is not auto-ranging it will have a series of range choices within an appropriate AC or DC voltage section. This means you are required to place the meter on the appropriate range for the value you are expecting. For example, if you are expecting 12V, you would put your meter on the 20V range, or similar, as 20V is bigger than 12V so it will be able to measure it. If you put it on the 2V range, it won’t measure as the range gets maxed out well before it reaches 12V. If you are not sure of what the expected result is, start with the maximum range and work your way down to the appropriate range.
Reading the meter
Once you have put your meter on the voltage function with either auto range or the correct range for the expected measurement, place the red probe tip on one of the points to be measured, usually the one you expect to be positive, and the black probe on the other point, then read the result off the screen. Adjust the range if necessary - that is, if the meter shows OL (then you’d need a higher range), or if the value is less than a lower range option on your meter (then you’d choose a lower range option).
** You could try practising voltage measurements on your car battery, or even any AA's you've got lying around.
When you are measuring current, you are trying to determine how many electrons are flowing through a circuit.
Before continuing, check that you are not expecting more than 10A as most multimeters will only handle 10A of current before blowing a fuse. If you are expecting more than 10A, and your multimeter is only rated for up to 10A, you may have to consider purchasing a dedicated ammeter, or a clamp meter to conduct the test.
Setting up your probes
Place the red probe in the port labelled A for current, and the black probe in the common. This time the danger of blowing a fuse by having the leads in the incorrect ports, is not an issue. If you put the probes in the wrong the ports to measure current, the result is nothing. No damage, and no measurement result.
AC or DC Current
The selection of the current function is pretty much the same as selecting voltage, only this time you are looking for capital A’s.
You will have to choose AC or DC current, either on the selector or with the mode button. Once you've selected Current, confirm that AC or DC (or the wavy line or the straight line) is displayed on the screen.
If you do not have auto ranging you will also have to choose the appropriate range. It is best to start off on the highest range, and work your way down.
Setting up for current measurement
Now things get a little bit different. Current is the amount of electrons travelling through a conductive material. So, we need to direct these electrons through the multimeter to measure them. To do this, we need to put the multimeter IN the circuit.
** Make sure you turn the circuit off before proceeding. If you leave the circuit on whilst setting up the meter, you are at risk of possible electric shock for higher voltages, and/or short circuiting to another part of the circuit or to ground. **
To put the meter into circuit you need to disconnect one end of the wire you wish to measure.
Attach one probe to the circuit where you disconnected the wire. Attach the other probe to the end of the wire you disconnected.
With DC, it helps to have an idea which way the current flows in order to display polarity right, but most meters will read positive and negative direction for current anyway. If you know which end of the wire is positive, make your probe attachment so that red is on the most positive end of the circuit.
Reading the meter
With both probes attached you can now turn your circuit back on. Whatever current is directed to go through that wire you are testing, has to go through the multimeter as well. Read the result off the screen. Adjust the range if necessary - that is, if the meter shows OL (then you’d need a higher range), or if the value is less than a lower range option on your meter then choose a lower range option for better resolution in your result.
** You could try practising current measurements on your Arduino board circuits, or by placing your meter in circuit with the battery supply for a remote control car, or battery powered radio, etc.
When you are measuring resistance, you are measuring the ability to resist current flow.
Resistance measurement works almost opposite to current measurement. Instead of directing current from a circuit through the multimeter as when measuring current, the meter instead puts current into the circuit using a small voltage. The amount of current that gets through is directly proportional to the amount of resistance in the circuit.
Setting up the probes.
To measure the resistance, place the red probe in the port that has the Omega (Ω) symbol, which is usually the same as the voltage port, and the black probe in the common. Select the resistance mode on the meter. This will be a selections with the Omega (Ω) symbol. On modern meters the range is usually automatic, but like current and voltage you may have to select an appropriate range. Because resistance can be so varied, it is usual to start on the highest range, then work your way down.
Test the meter.
Take note that when the probes are apart the meter will read "OL''. This is because the resistance of the air between the two probes, is much greater than the largest range that the meter can measure.
Place the probes together. Your meter should read somewhere very close to zero.
** Make sure you turn the circuit off before proceeding. If you leave the circuit on whilst setting up for testing, you are at risk of possible electric shock for higher voltages, and/or short circuiting to another part of the circuit or to ground. **
Setting up for the resistance test
If you are measuring the resistance in a component which is in circuit, you may be measuring in a big circle around the circuit, and not actually through the component you wish to measure. It is for this reason you should try and isolate the component by disconnecting it at one end, before measuring.
Reading the meter
Place the red probe at one end of the component you're testing (doesn’t matter whether it’s the disconnected end or not) and the black probe at the other end. Read the measurement.
If you only get an ''OL''result, check that you are on the highest range. Adjust if necessary and test your meter again by touching the two probes together. If the range is not the problem, switch the probes to opposite ends, and measure again, just in case there is some sort of diode circuit where you are testing. If the meter still shows ''OL'', then you have an open circuit, meaning there is no currrent path between the probes.
** You could try practising resistance measurements on a length of wire, or on a halogen or incandescent light bulb.
There are many other measurements that your multimeter may be capable of. Some common ones including diode testing, frequency and capacitance. And further again, some meters have special functions such as insulation resistance, duty cycle, 4-20mA%, DC offset functions, and many more. You're best off getting started with the basics though, before moving on to these!
For more information on multimeters, measurements, and technical jargon see our 'Understanding Terms' articles here.