How to Use a Multimeter
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Multimeters are devices which allow measurement of electrical current, voltage, and resistance.
Contents
Overview of the multimeter
A multimeter is exactly what it sounds like… a bunch of meters bundled into a single instrument. It’s like buying a multipack of chips or soda from Costco. You only have to get one item and you get everything… least that’s the idea. Most multimeters come equipped with a core set of basic features outlined in the next section, Basic Features. More advanced multimeters come with additional instrumentation options which are outlined under the Advanced Features section. Multimeters come in many different shapes, sizes, and forms with varying levels of sophistication so just be aware that the multimeters that you encounter may operate differently but the core measurement instrumentation is approximately all the same. The multimeter that we will be using is a Fluke 115 so each section will also outline how to perform each measurement on that model. Every multimeter comes with a set of two probes. One probe is red which is usually used to probe either the high voltage or positive terminal of a component. The other probe is black which is used to probe the lower voltage relative to the high voltage or the negative terminal of a component. Depending on the measurement, the probes will plug in to different terminals of the multimeter itself. On the Fluke 115, the black probe will usually plug into the black terminal of the multimeter which is labeled COM or common ground. The red probe will plug into one of the other two terminals depending on which measurement you want to perform. Top of Page
Basic features
All multimeters are equipped with the following meters:
- Voltmeter – Voltage
- Ohmmeter – Resistance
- Ammeter – Current
Voltmeter
In circuit applications, there are two types of voltages: AC and DC. DC stands for Direct Current where the voltage is characterized by a constant. AC stands for alternating current where the voltage is characterized by a periodic sinusoidal, or pseudo-sinusoidal oscillation. All multimeters come equipped with a DC voltage source meter but cheaper ones may not have the AC voltage source meter. All measurements are in Volts (V) which are the SI units for voltage.
DC voltage
To operate the DC Voltmeter, you will have to connect your red probe to the terminal indicated for voltage measurements. This is usually indicated by a ‘V’ above the terminal on the multimeter. On the Fluke 115, this will be the red terminal on the right of the COM terminal. For other multimeters, the indicator for the high voltage probe should be fairly obvious. If you can’t identify this terminal, then you should take a quick peek at the operating manual found under the External resources section. Once you have your probes connected to the correct terminals, make sure your multimeter is switched to DC voltmeter mode. On the Fluke 115, this should be the second option right of OFF. Take the other end of your two probes and connect the red probe to the positive terminal of whatever you want to measure, and connect the black probe to the negative terminal or ground. After letting your multimeter think for a few seconds, it should automatically adjust and display the DC voltage across the two terminals you are connected to. To make more measurements, simply reconnect the probes and repeat as many times as necessary.
AC voltage
AC voltage is a little trickier. First make sure that you have switch to the AC voltmeter mode on your multimeter. This is usually marked by a ‘V~’ or ‘AC V’ marking on your multimeter. On the Fluke 115, it’s the first option next to OFF. Also make sure the black probe is connected to COM and the red probe is connected to the plug to the right of COM. The reason we have an AC voltmeter is that since the DC voltage meter measures constant voltage, it ends up outputting the average voltage across the terminals. For DC applications it works great but for AC applications, the DC voltmeter is pretty useless since the average value of a sinusoidal waveform is zero but the voltage is clearly not constant at zero voltages. The AC voltmeter solves this problem and measures the root mean square (RMS) AC voltage. Note that the RMS voltage is NOT the same as the peak to peak voltage. AC voltages are usually expressed as an RMS quantity so the multimeter also outputs an RMS quantity.
Ohmmeter
The Ohmmeter is an instrument that measures resistance. Make sure that you are switched to Ohmmeter mode which is indicated by a resistor symbol. This is the 4th option on the Fluke 115. Most multimeters will also have a capital Omega over the connector for one of the probes for the Ohmmeter. The other probe goes into the ground terminal of the Multimeter. On the Fluke1 15, connect the probes the same way as the voltmeter; black goes into COM and red goes into the terminal to the right of COM. To measure, simply apply the two probes across the resistor in question, let the multimeter think, and read off the stable value. Honestly though, it’s probably faster to just memorize the resistor code mnemonic.
Ammeter
This instrument is used to perform current measurements. As with DC voltage and AC voltage, there is also the option of having DC current and AC current.
DC current
As with the other measurements, change the setting so that the multimeter is in ammeter mode. This is usually marked by ‘DC I’, ‘Amp’, or ‘A’ on the terminals. On the Fluke 115, this is the last option on the wheel. Note that for current measurements, the probes will connect into different terminals as for the ammeter. On the Fluke 115, the black probe will still be connected to the ground terminal but the red probe will be connected to the pin LEFT of COM. Once you have plugged into the proper terminals, identify which component of your circuit you want to measure the current flow through. You will then have to insert the ammeter in series with that component. Take note that this is not the same as the voltage measurement where we connected the probe in parallel with components.
AC current
To make an AC current measurement, switch the settings so that multimeter is in AC ammeter mode. This is usually indicated by a tilde-like symbol ~ in front of the symbols for the DC ammeter (‘A’, ‘Amp’, ‘AC I’). On the Fluke 115, this is the second to last option on the multimeter. Once again, use the same probe configuration as when measuring DC current and connect the probes in series with the component that you are trying to measure the current through. The measurement on the multimeter will again be an RMS measurement, NOT a peak to peak measurement.
Advanced features
For more advanced instrumentation applications, multimeters have more sophisticated features. Some of the more advanced features that MOST multimeters come with are:
- Capacitance Meter - Capacitance
- Inductometer – Inductance
- Frequency Counter - Frequency
Capacitance meter
As the name implies, this mode measures the capacitance across the terminals of a capacitor in Farads (F). Again, make sure that the meter is switched into the correct mode. The capacitance meter is usually marked by the circuit symbol for a capacitor. To access this on the Fluke 115, you will first have to toggle to the diode measurement mode and then hit the yellow button. The yellow button is like a shift key so make sure to shift in and out of modes accordingly. Again, connect the black probe to COM and the red probe to the right terminal on the multimeter. When you measure the capacitance, for polarized capacitors such as electrolytics, you will have to apply the red probe or high input terminal to the positive lead and the black probe or ground input terminal to the negative lead of the capacitor. Otherwise your multimeter will not give you correct readings. The positive lead of a capacitor is usually indicated by a longer lead.
Inductometer
This instrument measures the inductance across an inductor in Henrys (H). If you don’t know what an inductor is, don’t worry too much about it since they are usually rare in basic circuit applications. In a nutshell, the inductor is the current counter-part of the capacitor. For more on how inductors work, you can check out the wiki. To measure inductance, first configure the multimeter to the mode marked by an inductor symbol or coil. Then make sure the probes are plugged into the same terminals as you used when measuring capacitance. Again, apply the probes across the terminals of the inductor and the meter should read the inductance of the inductor.
Frequency counter
The frequency counter is an instrument used to measure the frequency of the waveform in Hertz. To access the frequency counter mode on the Fluke 115, you will have to toggle to the AC voltage mode (first setting) and hit the yellow “shift” key. Again, plug the black probe into COM and the red probe into the right terminal. Then apply the probes across the terminals of waveform you want to measure the frequency of. Make note that the waveform should be periodic otherwise you won’t get appropriate readings.
Common applications
Measuring component values
Every once in a while you will encounter a cryptically labeled circuit component which has an obscure sequence of alphanumeric markings which the manufacturer just assumes you know how to read. Suppose we have a capacitor marked “105”… so it must be a 1uF capacitor. Got that? Yeah we didn’t either… If you ever end up stumbling across a component that wasn’t labeled explicitly or you’re just too lazy to spend the CPU cycles to decode the resistor, just throw it across the multimeter and set it to the quantity you want to measure. Note: make sure for polarized components like capacitors that you apply the probes across the correct leads, otherwise you will probably get an unintelligible reading.
Checking power supplies and node voltages
When you first build your circuit, often times you are going to realize that it doesn’t quite work the way you wanted it the first time. Sure you popped in all of your components the right way and you thinking you connected all of the wires correctly but they’re all the same color so how can you be sure? Furthermore, the mistake that you made could be anywhere so randomly rebuilding modules to fix the problem is the equivalent of trying to hit a target with a slingshot while blindfolded and riding a horse backwards (well maybe not THAT hard…). In order to prevent you from entering rage quit mode and throwing fragile objects across the room, circuit gurus liberally use the multimeter to performs some basic checks on your circuit when trying to debug misbehaving circuits. There are often a few simple checks that you can perform in order to isolate the problem to a particular part of your circuit:
- Check power supplies to all modules of your circuit. If active components in your circuit don’t receive power, they don’t work… period. If you are using integrated circuits and CPUs, you will want to make sure that power supplies are connected and adequate power is being delivered. You can do this by making a simple DC voltage measure at the supply pins. If you readings you get are not correct, you probably want to see if there is a problem and trace it to the source of your problem.
- Check node voltages on your circuit. Although this is best done with an oscilloscope, you can also use the multimeter to make sure that the node voltages of you circuit are what you expect. If your readings are artificially low or not what you expect, the problem may be traced to components around that node. Remember that the multimeter does not allow you see the waveform if your circuit is oscillating. To see a waveform, you will want to use an oscilloscope instead.
- Check component values and verify that the component you are using is the same as the component you have in your design. This is just an inevitable human error thing when you work really fast. For example yYou might have chosen a 100 Ohm resistor instead of a 1000 Ohm resistor.
Common mistakes
Inevitably people make mistakes because we are all human. Since you’re already reading this section, you probably are looking for some solutions to your problem before crawling the interwebs.
No measurement reading
Description or symptoms of the problem:
- Multimeter not responding to attempted measurement
- Display shows measurement but the measurement is too far off to be a problem with your circuit
Possible or known solutions:
- Make sure probe terminals are correctly connected. If you are measuring current, remember that probes go in series not parallel with the component.
- Check input source. It may be that the power is not connected or the node that you probe is not connected.
- Make sure you are in the correct measurement mode. Trying to measure current while you are in voltmeter mode is usually problematic.
- Check if you are measuring AC or DC. If you are measuring AC but in DC mode, you will get the average value which is not even close to the AC value you’re looking for. If your measuring AC but in DC mode, the multimeter will oscillate
Ammeter does not work
Description or symptoms of the problem:
- Ammeter does not work
- Multimeter beeps when attempting measurement
Possible or known solutions:
- Fuse is blown. Check the fuse and replace the fuse if necessary.
- Ammeter is connected in parallel not series with component value you wish to measure. When the multimeter beeps in ammeter mode, it usually signifies a short circuit connection across the terminals of the probes.
