Table of Contents
- Multimeter Basics
- Multimeter Overview
- Measuring Voltage
- Measuring Current
- Measuring Resistance
- Measuring Continuity
- Leads and Probes
- Important Features
1. Multimeter Basics
In this tutorial I will show you how to use digital multimeters to measure voltage, current, resistance and continuity, how to use probes and and leads and how to exchange the fuse of a multimeter.
In the first section we will have a closer look on multimeters and see which parts they contain and what you can do with them.
Every multimeter consists of three main parts. The display, the settings where you can change measurements ranges and choose between measuring voltage (V), current (A) or resistance (Ω) and the ports where the probes are plugged in. Usually probes are black or red, the black one is for the COM port (COM for common) and is connected to ground or (-), the red probe (+) can be either connected to the VΩ port when measuring voltage or resistance or to the current port (A) when measuring current. More advanced multimeters often have more than one port for current one for measuring lower current (mA) and one for higher currents usually up to 10A. Some multimeters include the port for lower current in the VΩ port.
The difference between the ports is the fuse they are protected with. Using a low fused port with high current can overload the fuse and the circuit. Trying to measure very low currents with the 10A port will mostly fail because of the resistance of the fuse. Most fuses used in multimeters can be overloaded for a short amount of time (slow blow fuses), for example the Fluke 87V can sustain up to 20A for up to 30 seconds. But be careful with cheap multimeters, they often can’t sustain 10A for longer than a few seconds.
The probes are so called banana type connectors and can use different probes, every probe with a banana type plug will work with multimeters.
2. Multimeter Overview
Multimeters can be used for many different things, for example test batteries, circuits and power cables or measuring the resistance of resistors. It’s the basic instruments for troubleshooting an electrical device or circuit. Everyone who is working with electronics can make use of them. In section one we already had a look at the basics and parts of a multimeter. Basic multimeters have a manual range selection, more advanced models include automatic range detection. Better models come with higher accuracy and some high-end models have displays that show a graphic representation of the measurement.
One important feature when measuring AC is a True RMS converter. The true RMS values are not just the average of the absolute value of the curve, this only works with sinusoidal waveforms but not for more complex nonlinear waveforms. The value is proportional to the root mean square (RMS) which can be quite a complex calculation, usually only meters in the price range above $100 are coming with that feature. Some meters will lock the ports according to the knob settings, so it’s not possible to plug the probes in the wrong ports.
3. Measuring Voltage
When measuring voltage or current you have to choose between direct current (DC) like found in batteries and alternating current (AC) which you can find at your standard electrical socket. First we plug the black banana probe to the COM port and the red probe to the VΩ port. At the other end connect the black probe to the battery’s ground or „-“ and the red probe to power or „+“. Because it’s a battery we will choose the setting for DC Voltage. Press the probes with little pressure against the ends of the battery, if it’s a new one you should measure around 1.5V with your multimeter.
If you switch the black and red probe the reading of the multimeter will be negative. It’s just a definiton thing and if we hold the COM Probe at the „+“ side of the battery we will define that point as zero. If we measure the voltage in relation to that point the reading will be about -1.5V.
As next step, let’s measure voltage on a breadboard. Again, we will set the knob to DC Voltage. If you don’t have a meter with autorange function you have to set the range manually. For example if you want to measure that 1.5V battery you will need to set your meter to 2V. For measuring a 12V powered circuit you will need to set it to 20V. Otherwise your meter will read „1“ (some meters will show something like „1.999“) if set too low or „0“ if set too high.
Measuring voltage on cuircuits or devices will quickly show you whether or not it’s powered correctly. If it should be 12V but you measure 10V or even more than 12V you will know that probably something is wrong and you have to check the connections and wires of your circuit. When measuring voltage you can measure between any two points of your circuit as long as they are connected.
There is a video series which explaines very detailed (electricty in general is also explained) how to use a multimeter to measure voltage, current and resistance.
Here is the first part of the video series:
4. Measuring Current
Measuring current is a bit more complicated than measuring voltage. If you want to measure current you have to do it in series, that means you will need to place your multimeter between the flow of current. To be able to do that you have to interrupt the power connection. In the picture above the white wires are disconnected from each other so you can place you meter in between. The current will then flow through the meter and can be measured. Connect the red probe to the side where the power source is located and the black probe to the other side. Same as when measuring voltage if you connect the wrong probe to the ends of the wire, you will see a negative measurement aswell. On the multimeter side you plug the black one in the COM port as always, the red probe will be connected to the mA or A port (depends on your meter). For a normal breadboard you’ll have less than 200 mA. The multimeter knob should go to DC Current.
When measuring current the amount measured will fluctuate a bit over time. So it’s good to watch what it says during a time of a couple of seconds or minutes. For that reason it’s great to have probes with alligator clips which you can attach to the wires without the need to hold them. The better your meter the better the accuracy, cheaper meter will slower adapt to changes in current flow.
After measuring current you always should switch your meter off or to Voltage before measuring something different. If you try to measure Voltage while having your meter still on current settings you will connect through your meter to ground. This can blow your fuse within seconds!
5. Measuring Resistance
As always we will start with setting the meter to the right settings and connecting the right probes in the right ports. For measuring resistance put your knob to the Ω-symbol. Resistance is measured in Ohms. Black probe is plugged to COM as always. Red probe should be connected to the VΩ port. Resistors normally have color codes on them, indicating the resistance and tolerance value. Resistors usually have a 5% or sometimes 10% manufacturing tolerance. But sometimes you will find resistors without color code, or you just want to measure the exact value.
For measuring the resistance of a resistor you have to connect the probes at both sides of the resistor. On which side you connect the red and black probe makes no difference now, the resistance between the two points will be always the same. Measuring resistance is influenced by the temperature and surrounding magnetic fields, if you have a lot of wires and/or power sources near where you take the measurement the results can vary greatly.
The third part of the video series will explain measuring resistance and continuity:
6. Measuring Continuity
When measuring continuity we are testing the resistance between two points. With continuity test you can detect whether two points are connected or not. It allows to test the conductivity of materials and trace where connections have been made. If troubleshooting some broken hardware it’s the most important test to find out what went wrong. First set the multimeter to contiunity mode (see image in 1. Multimeter Basics). As for measuring resistance the red probe connects to the VΩ port and the black probe to the common (COM) port. After setting your meter in continuity mode (some cheaper meters may don’t have one) the meter will indicate connections by tone. If there is a low resistance (usually less than a few Ω) a tone will be emitted, this means the two points are connected. If there is a very high resistance, there is no connection (or a large resistor is in the way) no tone will be emitted. If you touch just your two probes together you should here a tone.
On a device or breadboard that is not powered you can use the probes to check the ground pins. You should hear a tone when they’re connected as expected. With continuity testing you go along wires and can find breaks in the line (this is very importatant as an electrician).
When a system is not working, continuity is one thing that can help you to troubleshoot the system:
- If the system is on, carefully check VCC and GND to make sure the voltage is the correct level. If a 5V system is running at 4.2V check your regulator carefully, it could be very hot indicating the system is pulling too much current.
- After power the system down check continuity between VCC and GND. If there is continuity then you’ve got a short somewhere.
- With power off check that VCC and GND are correctly wired to the pins on the microcontroller and other devices. The system may be powering up, but the individual ICs may be wired wrong.
7. Leads and Probes
High-quality multimeters come already with a set of probes included. Standard probes are alligator clips, hook clips, pincer clips and test probes. Many high-quality meters already include alligator clips. Having different probes at hand gives a great flexibility. Alligator clips are great for connecting to wires or to pins on a breadboard, there good when you have no ports for plugging in a probe but don’t want to hold them the whole time like with standard test probes. Pincer Clips are great for hard to reach locations like small gaps. Hook clips work wel with smaller integrated circuits (IC).
8. Important Features
Now we’ve covered, what multimeters can do and how to actually use them. But what makes the difference between good and bad multimeters? What are the features and differences of let’s say a DMM like the Fluke 87V compared to a cheap $20 meter. In my „Multimeter Buyers Guide“ I have a closer look at this questions. So if you’re interested to buy a meter I recommend you to check it out here.