A basic guide for new and seasoned makers alike, with some of the most common mistakes and frequently asked questions.
The vast majority of makers own and use a multimeter. These are one of the most basic tools to begin the electronics journey, and very few makers can get away without one. They vary between five dollar ultra-basic units that may or may not accurately tell you a plugpack voltage or resistor value, up to bench units costing thousands of dollars and claiming phenomenal accuracy. However, most multimeters found on maker benches are in between those extremes.
RATIONALE AND SCOPE
The challenge is that many makers come from non-electronic backgrounds. What would traditionally have been considered ‘assumed knowledge’ or ‘general topic knowledge’ can no longer fairly be assumed as such, in the same way, it is not fair to assume that everyone who checks their own fluid levels in their car knows anything about the engine.
We live in an age where information is accessible in many ways from many sources, and this sometimes results in an incomplete picture through no fault of anyone.
With this in mind, and recalling common customer questions from years of electronics retailing experience by some of the DIYODE staff in their pre-DIYODE careers, we have to put together a basic primer on how to use a multimeter, but with a bigger picture included. Often, many people are told to ‘do this’ and ‘do not do that’, without being told why. Knowing why helps you remember what to do and what not to do, and it helps to apply the knowledge to other situations or make decisions about circumstances that have not been specifically explained.
This article will cover all of the common test parameters like voltage and current measurement, resistance and continuity, and on through the middle-ground things like temperature, capacitance, and frequency that only some meters have. We will explain the difference between auto-ranging and manual multimeters, and take a look inside one as well. What we will not do is cover anything that involves mains electricity. While many multimeters are capable of reasonably high-voltage work, any work on mains electricity and onwards to street and main lines is all covered by licensed restriction in Australia, New Zealand and other countries.
MYTH AND REALITY: MAINS WORK AND THINGS THAT PLUG IN
You can only perform any work on mains electricity, even changing a light switch or fan controller, if you are qualified and licensed.
If you are qualified and licensed and do not know how to use a multimeter, we’re seriously worried! The definitions from NSW Fair Trading state that this applies at 50V Alternating Current and 120V Ripple Free Direct Current. Other states have similar rules.
There is a long-standing belief that anything plugging into a wall socket was exempt from these rules. In particular, New South Wales’ legislation carries the phrase ‘any electrical article connected to, and extending or situated beyond, any electrical outlet socket’ in the section about what is exempt from the Act.
The problem is, different legislation states that connected appliances (‘articles’ in the legislation) must be compliant to Australian Standards, and these laws carry separate rules about repair, modification, or construction by qualified people. We spent the better part of four hours trawling through the legislation from different states and while the picture is often not crystal clear and the language can be vague, there is definitely a consensus that the myth about the wall socket being the limit of the law is invalid.
Even making your own extension leads is illegal. If it’s above 120V DC or 50V AC, it’s licence-only, no exceptions, even if it’s something you’re making or repairing that plugs into a wall socket for its mains connection. These are still covered by law, just not the same law that governs building and fixed wiring, which was the origin of the myth.
In addition to that, the laws covering appliances were traditionally less well-publicised and enforced, and have also been updated in recent years, which may have helped the myth build. The upshot of this is that makers, unless licensed, have no reason to be testing for 240V AC.
MULTIMETER TYPES AND RANGING
Multimeters are usually either hand-held, or bench-top styles. The benchtop varieties are usually far more expensive than most of the hand-held types, have a raft of additional features as well as higher accuracy, resolution (number of digits), and often software functions.
Often all but the budget end of the range will read to a higher resolution than a hand-held meter. They’re also mains-powered and many have storage or data logging functions. In a similar vein, some multimeters are USB-plug-in add-ons for computers, tablets, and even phones. These send signals to the device where the rest of the interpreting and display occurs.
Most multimeters used by makers are hand-held. This is the most familiar format and the most common digital formats use an LCD screen to display the data, with a dial underneath to select ranges and functions. There may also be buttons for additional selections.
Of these, some are auto-ranging and some are manually ranged. Auto-ranging means that, once a given function like voltage measurement is selected, the multimeter chooses whether it needs the 200mV, 2V, 20V, or 200V range, or whatever that meter’s range settings are.
Ranges are just the brackets of upper and lower limits to a measurement. These are used for resolution, as measuring and displaying a 15mV signal on a scale meant for 200V will not produce much of a result. In fact, on many multimeters, there will be no reading.
Manually ranged meters just use a switch for this function, and the user has to choose which range is relevant. Choosing the right range gives you a good resolution to read from, so the 15mV signal above on the 200mV setting will show as mV with a decimal point.
That’s much better than being 0.01V on the 2V scale, if the decimal point moves at all on your meter. Some meters only have a fixed decimal point between the right-hand and second-to-right digits.
By the way, most auto-ranging meters still have manual override, for times when you want to choose yourself.
There are also analog multimeters. These are less common today but once were the standard. We didn’t find any current analog benchtop units but there are a few hand-helds around, in different quality scales. These use a moving needle coil over a multiple-graduation scale display, and are only manually ranged. This is because all the measurement circuitry is discrete, not digital or microcontroller-based.
There are two critical points to note when choosing ranges on a digital DMM. When manually ranging, start at the highest, and work down to the lowest unless you absolutely know what range to expect. For example, the 20V range would be fine for a 12V car battery under load. The next range down is usually 2V. If the 20V range gives no reading, then try the 2V range to see if your battery is just very, very flat. If you have no idea what to expect, start high and work down.
The other critical point is that auto-ranging does not mean automatic mode selection. If you have your meter set to DC volts and measure AC, you will see a zero reading. The reverse is true as well, where a meter set to DC Volts will not display a reading for AC volts. The multimeter uses completely different circuitry to measure these two functions, even though the dial position can be the same. The fact that they’re often combined is that only some of the circuit is on the PCB of the multimeter: The rest is contained within an IC.
That last point sounds simple, but is actually a very common mistake. It accounts for a great many of the customer returns in retail stores, closely behind blown fuses. More on that soon. If you do make this easy mistake and do not realise that it is the source of your nil reading, you can spend quite a lot of time looking for power supply problems that do not exist.
The only saviour you may have is that because multimeters are designed to be sensitive, a meter at idle will often display a few millivolts from induction or static, and this number dances all over the place. If you see an unexpectedly low but stable reading, then you may have your meter set to the wrong voltage function.
INSIDE AND OUTSIDE A MULTIMETER
While there are differences between multimeters, there are themes throughout. For example, some multimeters have a screen that backlights and a control button to suit, but almost all multimeters have a function dial.
Auto-ranging meters will also have a function or ‘mode’ button, and these are used in combination. In the previously mentioned AC/DC situation, the function dial selects voltage measurement, while the ‘mode’ button selects AC or DC. At the bottom (usually) of the multimeter are three sockets for 4mm banana plugs.
The majority of modern meters use shrouded plugs and sockets, and while it is possible to use regular banana plugs in these, the connection is not great. The contact section comes very close to the transition to the plastic shroud section and may result in a small contact area.
One socket will always be the ‘COM’ socket, for ‘common’, meaning ‘common to all functions’. It is, in effect, the ground connection of the multimeter. When colour coded, it is generally black, but on meters with dark plastic housings, it is often not coded at all. The reason it isn't labeled Ground/GND is because the multimeter is always isolated from the circuit - unlike an oscilloscope which has a common ground/earth with the mains - and so, the COM can connect to any part of the circuit, not just ground, without causing issues.
The symbols on this socket reflect the features of the meter but may not show all of them. Importantly, many modern meters also have a ‘mA’ label at this socket. That’s relevant later. We’re going to call this the ‘multi-use ‘V’ socket’ from here onwards, because ‘V’ is the one symbol this socket will always have on every meter. The final socket will be labelled ‘A’ and is for current measuring. As discussed later, current measurement requires a completely different circuit and is separate from the other functions.
The LCD display is the heart of your interaction with a multimeter. This is where both the results of measurements, as well as additional information, are displayed. Displays are often quoted in specifications as ‘count’ or ‘digits’. These two terms mean a similar thing, in different ways.
Digits are almost always quoted as ‘3.5 digits’, ‘4.5 digits’ or some other number ending in ‘.5’. That is because the first digit at the left of the display is only half a digit - it can only display a value of ‘1’ or ‘0’, and it displays ‘0’ by not being present at all rather than forming the numeral. The rest of the segments of a regular seven-segment display are not fitted. So, a 3.5 digit display can display a value of 1999. This is why many older multimeter ranges were in increments of factors of two tens: 200mV, 2V, 20V, 200V.
More recently, with advancements in microcontrollers, fully-functioning displays are the preference. The range values are still limited by other circuit considerations, but the displays themselves are generally made up of full digits. In this case, the value of the maximum display is expressed as a ‘count’. So, a 4000-count meter can display a value of 3999 for a given range. Because of this, the count value also tells you the range increment. A 6000-count multimeter will have ranges incremented in factors of six tens: 600mV, 6V, 60V, or whatever value is being measured, not just volts.
Also on the display are an array of small symbols which activate when needed. The critical AC and DC indicators are there to tell you what voltage setting you have selected. Also present are things like a lowercase ‘m’ that appears when any unit is being measured in its ‘milli’ range, and extra symbols such as Hz,
Note: If at any time, you see ‘OL’ displayed on the screen, this means that the signal being measured is either above the range set, or above the limits of the multimeter. For example, a 5V power rail measured on the 2V setting will show ‘OL’.
Knowing what a multimeter looks like inside can help you understand what’s going on with the function dial, and, therefore, shape your expectations of what a multimeter does and how. We don’t recommend pulling your own multimeter apart, as quite often, there are breakable clips, warranty stickers, spring contacts and usually a spring-loaded ball detent that indexes the mode wheel, which goes flying across the room when released. Because these are tiny, you’ll need a forensic team to find them. Instead, we pulled one apart for you!
Inside the multimeter, you can see that the function dial connects the sockets to different circuits in the multimeter. It’s getting harder and harder to see this, as modern PCBs are often well-coated and use multi layer boards, as well as extensive use of multi-function integrated circuits under a resin blob. However, the current shunt with its thicker paths, and some different routing from the multi-use ‘V’ input through the function dial should still be visible.
FUSES
There are usually two fuses inside a multimeter. One protects the main section of the circuit from overcurrent, and is often between 100mA and 500mA. This entire section of the meter is low-current, which is why the separate current socket exists.
Some modern meters can cope with up to 400mA measurement on the multi-use socket, but any more than this, or all current on other meters, must be input through the current socket labelled ‘A’. This has a 10A fuse, generally, and runs to the current shunt. Fuses are there for absolute protection, and they do not ‘just go’. A blown fuse is an indication that something has been done incorrectly, yet they are a constant reason for returns of multimeters to retail stores.
Many retail store staff members can tell you that a ‘faulty’ multimeter often has a blown fuse, and the customer quite often thinks this is a warranty issue. A blown fuse is no more a warranty issue than damage to a car from having an accident after running a red light.
The car’s brakes didn’t fail, the driver used the car outside procedures. Often, the blown fuse is caused by having the leads in the wrong socket. Trying to measure with the mode dial on voltage but the probes in the current socket is one thing, but measuring current with the probes in the voltage sockets is another story entirely.
With that covered, let’s get into some specifics for measuring things with your multimeter.
VOLTAGE
When measuring voltage in a circuit, there is a need for the meter not to be a load on the circuit. Any change in load can affect the voltage being read. For this reason, multimeters are designed with a voltage reading circuit that has the highest impedance practical. Impedance is the opposition to the flow of electricity, but encompasses much more than just resistance. That fact sets this part of the multimeter circuit apart from some others, and is why this function has its own position on the mode dial.
Voltage is always measured across the source. You can measure a supplied voltage by measuring between power terminals or rails, or you can measure voltage drop across a component. This last point is another reason that the impedance needs to be high: Any appreciable current flowing through the meter when testing for voltage drop can affect other parts of the circuit.
Connect your multimeter with its black test lead in the ‘COM’ socket, and the red lead on the socket with ‘V’ marked next to it. This ‘V’ will likely be with a bunch of other symbols, as ours is, because this socket goes to several parts of the multimeter’s circuit. Make sure you have the selector dial on
Now, you can touch the probes to the circuit points to be measured. This might be battery terminals, or it might be power rails. It could also be the voltage drop across a component, as in our demonstration.
Make sure that the probe tips do not touch anything else. At best, this would result in a false reading. At worst, it could result in a destructive short circuit, if the probe touches two places at once. Many multimeter probes have covers to make only the very ends of the probes exposed, and we have used these where practical throughout this article.
If you don’t know the polarity of the current you are measuring, your multimeter will tell you. If the black lead is connected to ground when measuring rails or a lower potential when measuring voltage drop, and the red lead is to the positive rail or a higher potential, then the display shows just the reading.
However, if the polarity is reversed so that the red lead is to ground and the black to positive, or black to a higher potential than red, a small ‘-’ sign appears at the left of the display. This is something to watch closely for, as it may not always be obvious when the numbers themselves have your attention.
CONSIDERATIONS
When measuring voltage, be aware that only loaded circuits display a true voltage. For example, a battery that will not start your car may well show above 12V on the multimeter. Only when a suitable load is applied will the internal resistance of the battery not cope with the current demands, and the voltage falls.
Hold the multimeter on the battery while the engine tries to start, and all of a sudden you see a much lower voltage. This is also true for testing AA or other disposable batteries: A cell may show 1.5V but drop to 1.1V or lower when loaded. The same can be said for power supplies. A plugpack with certain faults may show the correct or slightly higher voltage when measured on its own. However, as soon as the load is applied, the voltage can fall or disappear completely. The only use for measuring a plugpack at the plug with no load is determining the voltage if it is unlabelled.
Also, remember the AC/DC function issue. If you do not get a reading where you are expecting one, this may be the cause. However, if you see, say, 1V when you were expecting 12V, this will not be the cause. Other things to check if you have no reading are that the meter is not damaged in some way. A known good source is always useful to have at hand, but many engineers and makers have at least two multimeters, to check one against the other. Of course, if you make the same mistakes over and over and kill both the same way, that won't help. We may or may not be speaking from experience here.
Although a battery will not show its true voltage unloaded, it’s still a good way to verify if a meter is reading at all. This works even better if the battery is in a device that can demonstrate that the battery is not dead, but rarely will a battery show absolutely 0V. If your meter shows no reading across more than one source, chances are a fuse has been blown or other damage has occurred to either the meter or the probes. Of course, check that you have the right mode and that the cables are in the right ports!
One last thing to be aware of only applies to measuring AC voltages. Cheaper multimeters display an average value for AC. Mathematically, the average is the total of all data point values, divided by the number of values. The number of data points taken from a given wave will vary by meter but is generally defined by time: a certain number of samples per second will be taken, rather than a number of points on each wave.
However, AC voltages are generally stated as RMS values. This is Root Mean Square, a more complex mathematical operation that involves taking the square root of the mean (average) of the squares of the data points. In mathematics, it’s often called the Quadratic Mean, but it is always RMS in electrical terms.
Without getting into a maths lesson, the average value is lower than the RMS value for the same set of data. The RMS value happens to be the value of AC voltage that causes the same power dissipation in a purely resistive load as the same DC voltage. The average value, however, is lower. Keep this in mind and be aware of your meter’s measuring system: The manual will tell you whether it measures an average or RMS value: If it is average, values may not be what you are expecting from calculations or the specifications of a product such as an AC plugpack.
CURRENT
Measuring current incorrectly is the source of many a multimeter returned to retailers, and of much frustration for makers. While obvious to those having come to making through the traditional path to electronics, there are plenty of people who are justifiably unaware of this issue. Getting it wrong can kill your multimeter.
The way current is measured is based on Ohm’s Law. Current is always measured with the meter in series with the circuit under test, as the current to be measured is passed through a ‘shunt’. That is a very small-value, precise resistor over which a tiny but predictable voltage drop occurs.
This voltage drop is what is measured to calculate the current. Since Ohm’s Law states that the voltage drop across a constant resistance is equal to the current in Amperes multiplied by the resistance in Ohms, the tiny voltage drop corresponds to the amount of current in a linear fashion.
Voltage drop is used because analog to digital converters, the IC doing the work, are voltage-driven components. In addition, most sensitive measuring devices cannot handle such current. Even moving-coil analog meters use a shunt.
Of course, there is no such thing as an absolutely constant resistance, as even small temperature changes alter the resistance, but in practical terms with a resistor constructed in the right way, we can consider the shunt to be constant.
To measure current, connect the black lead to the ‘COM’ socket, and the red lead to the ‘A’. Note that the socket may be labelled ‘10A’ or whatever maximum current the multimeter is designed for. Turn the dial to the highest current range if it is manual, or to the ‘bulk’ current setting if it is auto-ranging. Now connect the red lead to the power supply and the black lead to the load. You can also connect the red lead to the load and the black lead to ground, but this may not result in an accurate reading. More on that shortly.
With the meter connected in series with the load or circuit under test, and the dial in the right position, turn on the power to the circuit. If you have a manual ranging meter, you can turn down the dial until the reading is a reasonable resolution. If it’s auto-ranging, the multimeter should do this for you.
The reason for a separate circuit is that the large current-handling paths needed to carry the measured current through the shunt are not suitable for other measurements, and, of course, having other measurements in series just doesn’t work: Voltage, for example. Other measurements use high-impedance inputs as discussed for voltage testing. Drawing multiple amps through these circuits with the meter in series can be very destructive.
On more expensive meters, there is often advanced protection built in. On cheaper meters with similar features to those costing ten times as much, corners have to be cut somewhere. Drawing 5A through a multimeter with the red lead plugged into the multi-use ‘V’ socket will probably destroy something if the fuse does not provide enough protection. And depending on dial settings, it may not.
However, some modern meters have trended towards measuring smaller currents with the multi-use ‘V’ socket. The cut-off here is usually around 200mA to 400mA. Always use the ‘A’ socket first until you know that the current involved is below your multimeter’s low-current limit. At best, you’ll blow a fuse if this is exceeded. At worst, damage can still occur despite the fuse. Having moved the red lead to the ‘V’ socket, the measurement is still performed in series. Also keep in mind that some circuits, particularly ones with inductive loads like coils or motors, draw bigger start-up currents than their constant operating current value.
The other point to note is that all of this is that some multimeters only measure Direct Current (DC). The symbols on the meter we have photographed have the ‘~’ symbol, and the manual clearly states that it can measure Alternating Current (AC). If only the DC symbol
If you have a manually-ranged meter, you will probably have DC and AC current positions on the dial. Auto-ranging meters require you to push the ‘mode’ or ‘function’ button to select AC, as DC will be the default.
As for voltage, the first thing to do if you find a zero reading when you’re expecting to see a current value displayed, is to check your AC/DC settings. After this, check lead positions and dial selections. If there is still no joy, check the load you were testing without the multimeter connected. If it works, but it does not work with the meter connected, then there is likely something wrong with the meter.
Check for blown fuses first, then try to read across the voltage supply of the load with the voltage setting on the meter, remembering to move the red lead to the correct socket. If you get no voltage reading either, chances are there has been a mistake and damage has occurred to the multimeter.
One last note on current: Many cheaper multimeters, (any meter costing less that $100 could be a candidate) use 0.1Ω or even 1Ω resistors as their shunts. This can cause an appreciable voltage drop and may actually affect the circuit being measured. This may render the reading less accurate, but it also may cause unexpected behaviour if the circuit is operating close to a voltage threshold.
RESISTANCE
Resistance testing is probably more common than current checking, but is less complex. You’re also unlikely to kill the multimeter, which is nice. To test resistance, the multimeter emits a small, known current, and measures the voltage drop.
We tested our multimeter with an oscilloscope and found that the voltage was only 200mV across the oscilloscope’s 1MΩ input impedance, and 20mV across the 10MΩ input when the probe was switched to 10x. That means a current, according to Ohm’s Law, of 200 nanoamperes! Yes, 200nA, nowhere near enough to cause issues in a circuit or component.
However, measuring resistance is not as simple as just holding the probes across any resistor or conductor, or other component. Current will always move through any path available to it, and the amount going each way is related to the resistance of those paths. If there are two resistors in parallel, and one is double the value of the other, then a third of the current goes through the bigger resistor and two thirds through the smaller one.
A resistor, wire, or other component undergoing a resistance test must be tested in isolation, disconnected from the circuit. In many circuits, there are multiple current paths. Take the example here. It has a resistor in parallel with a capacitor, with another resistor in series with an LED. Current will travel through the other paths, including the capacitor, which will charge but also has leakage current across it. All of these things will alter the voltage drop across the resistor under test.
While it might be tempting to be able to test a resistor connected in series with an LED if those are the only two components in the entire circuit and there is no power applied, the reality is that even then there is no guarantee of unseen or unnoticed current paths. In addition, any stray voltages across the component can alter the reading as well. It’s best to just test with the probes across the resistor with at least one end disconnected from the circuit.
To test resistance, connect the black lead to the ‘COM’ socket and the red lead to the multi-use ‘V’ socket. Move the mode selector dial to the relevant position. This is a series of dedicated positions on manually ranged meters, but on auto-ranging meters, the resistance function will be grouped with the diode, continuity, and possibly capacitance functions.
By default, the function at this dial position will be resistance, and you should see Omega Ω to the edge of the screen. If it isn’t, push the ‘mode’ or ‘function’ button until you see that resistance is selected. Now, you can touch the probes to either side of the test subject, making sure you have good contact.
Do not hold the test subject with your fingers! Though the current is small and the voltage low, the human body will still conduct enough to give an inaccurate reading, particularly above 10kΩ. For values under 10kΩ, the difference by adding the human body is negligible, but it's best to develop the habit. Wait until the reading stabilises, and you have your resistance measurement.
CONTINUITY
Continuity is one of the simpler tests you can perform with a multimeter. All but the cheapest handheld multimeters include a continuity buzzer, which works in a similar fashion to the resistance measurement, but with stronger signal. It’s still not a regulated, fixed voltage and so will change with resistance. On our oscilloscope, we calculated a 420mV signal into a 1MΩ load. The voltage drop across the probes is fed to an internal comparator, and when the voltage drop is below a certain threshold, indicating a low-resistance current path, a buzzer sounds.
The actual value of resistance where the buzzer stops varies wildly, even among meters from the same brand we use. Do not assume continuity means less than 1Ω! A good idea is to use a potentiometer to check where your meter's cut-off is.
You will still see numbers on the display, because resistance is still being measured. There’s just a sound so that you don’t have to keep looking at the screen while working with the probes and trying to find solder bridges, shorted wires, good connections, and so on.
To test continuity, there will be a dial position that has a symbol much like the ‘WiFi’ symbol.
This will likely be on its own for manual ranging meters or with the resistance function for auto-ranging multimeters. In this case, the ‘mode’ or ‘function’ button needs to be pressed until the symbol appears on the screen. The probes need to be set up in the standard form, with black to ‘COM’ and red to the multi-use ‘V’ socket. After that, you can concentrate on where you’re placing the probes during fault-finding.
DIODE CHECK
Diode checking is available on many meters and is similar in principle to resistance and continuity checks. A voltage is applied to the probes and the drop across a diode is measured. The setting is usually solitary on manually ranged meters and with the resistance and continuity position on auto-ranging meters.
As before, the ‘mode’ or ‘function’ button needs to be pressed until the diode symbol is shown on the display. The voltage is higher: We measured our meter outputting 1.5V. Set-up is the standard, with the red lead in the multi-use ‘V’ socket and the black lead in ‘COM’. Again, diodes should be tested in isolation, as other current paths can provide false readings.
A word of warning: The voltage drop of a diode shown by a multimeter is at a low test current. Actual currents in use will cause a higher voltage drop. That's especially true of LEDs, which are not designed as current-carriers like a rectifier diode is. The diode function on a mater will give you a guide, but the best picture comes from measuring the voltage drop in-circuit under operating conditions.
CAPACITANCE
Capacitance testing is not a basic feature, and some multimeters are capable of it while others are not. After browsing retail listings, we found that the feature is available on a little under half the range. That makes it worth covering.
Capacitance measurements involve the multimeter feeding out a square wave to the probes and measuring voltage over time in both the charge and discharge cycles. Because of this, you must start with a discharged capacitor and the capacitor needs to be disconnected from the circuit so that other current paths are not present.
It is also important to make sure that your fingers do not touch the metal ends of the probes or the capacitor contacts. The current and voltage involved won’t hurt you, but the human body has capacitance and will affect the reading.
Capacitance may be on a dial position on its own, or grouped with something else. This is usually, but not always the resistance function. Press the ‘function’ or ‘mode’ button until the correct symbol is displayed on the screen of the dial setting as a grouped function. The symbol varies.
Some meters use ‘μ’, others
The setup is the same as most other functions: Black lead to ‘COM’ and red to multi-use ‘V’. Either use clips or hold the capacitor against a surface with the probes so you don’t have to touch it. Keep the leads separated as much as possible, although stray capacitances are unlikely to be big enough to give a reading.
The capacitance measurement on most multimeters is not hyper-accurate, but it’s good enough to identify unlabeled components or determine faulty ones. If you require precisely tuned circuits, you’re better off with an LCR meter.
TEMPERATURE
Multimeters that measure temperature depend on thermocouples. The thermocouple exploits the fact that two dissimilar (different) metals joined together at a spot will exhibit a potential difference (voltage) at the junction. This voltage varies with temperature in a consistent and known way. To measure temperature, the multimeter measures this voltage and scales it to read in known temperature scales.
The thermocouple generally plugs into an adaptor into the ‘COM’ and multi-use ‘V’ sockets. The adaptor usually has ‘+’ and ‘-’ marks on it, because the thermocouple does too. Most thermocouples are of the ‘K’ type, and the connector used on these has a slightly thicker tab on one side. Some multimeters have a socket for the thermocouple directly.
On the function dial, some meters, manual- or auto-ranging, have a dedicated position. Other meters group temperature with other features. On our multimeter, it is on its own with the word ‘TEMP’, but often °C/°F is used. If the feature is grouped with others, the ‘mode’ or ‘function’ button will need to be pressed to make the icon display on the screen.
Many meters with the function on its own dial position use the ‘mode/function’ button to select between degrees Celsius, degrees Fahrenheit, or even Kelvin on some meters.
When testing temperature, handle the thermocouple with care. Many are exposed at the tip, and can easily be damaged. They are also susceptible to moisture causing current paths between the conductors, falsifying the readings.
The tip where the two metals (usually wires of different metal rather than copper wire joined to other metals) join is very brittle, and hairline fractures not noticeable to the eye can also cause headaches. Thermocouples on multimeters often become inaccurate over time because of these and other manual handling factors. If you can, get hold of a thermocouple encased in an aluminium probe: these take much longer to equalise to the temperature to be measured, but are sealed against moisture and much more rugged to handle.
Always wait until the thermocouple has equalised with the test subject before taking the reading.
FREQUENCY
The range of multimeters that can measure the frequency of a signal largely, in our research, mirrors the pattern for capacitance. The frequency can be measured but no information can be gained about the waveform involved. Some multimeters will even measure duty cycle (the amount of time that a signal is high as a percentage of the total high/low time), but they assume a square wave to do so. Reading a sine wave will still give a duty cycle value, but it will not likely be 50%.
Frequency is usually on a dial function all on its own, with Hz/% being the most common symbol: Hz for Hertz, the unit of frequency measurement, and % for the duty cycle expressed as a percentage. The ‘mode’ or ‘function’ button should select between the two when they are present. Connection is the black to ‘COM’ and red to multi-use ‘V’. Keep your fingers away from the metal of the probes, because the human body has a lot of AC signals induced in it and can contaminate the signal being measured.
GENERAL POINTS AND LIMITATIONS
Always be familiar with your multimeter’s specifications. For example, not realising that your multimeter is limited to 100μF while trying to measure a 330μF capacitor could lead to lots of wasted time, frustration, or incorrect conclusions. Frequency is often limited too, and neither of these functions will ever have the range on a multimeter as they do on dedicated meters. Costs have to be cut somewhere to make them affordable, and dedicated meters usually cost more each than a multimeter.
It is also helpful to keep in mind things like whether or not your multimeter uses the multi-use ‘V’ socket for lower currents. Measuring current incorrectly, and we cannot stress this enough, can damage your meter and cause many headaches and incorrect conclusions.
Besides that, just remember that current is measured differently from almost anything else, and most functions require an isolated test subject. Make sure anything like a capacitor is discharged before testing. Also, remember your legal limitations! No mains work!
NOT WHAT YOU EXPECTED?
If you don’t get the reading you expect from a test situation, first check the function dial and the selections made by the ‘mode/function’ button: is the meter set up to measure what you want, and in the correct range? Also, make sure the leads are in the correct sockets.
Next, check connections. Make sure the banana plugs are firmly seated and that the probes are making good contact. Double check that your subject is isolated if that is relevant, or that you have the right connection set-up: In parallel for most things, and in series for current.
A good way to check if the probes, and their connection, are good is a quick change into continuity mode, and tap the probe tips together. This will confirm if there is an issue with them or not.
Failing that, check the meter against a known good source. This will help eliminate (or suggest) damage to the meter. If you get expected readings on a known reference subject but unexpected results from your actual test subject, then the test subject itself is likely to be the source of the issue and you can begin the fault-finding process.
If the problem still persists, particularly zero readings even when connected to a known source, check the fuses.
Finally, if your readings are erratic and inconsistent, check the battery of the multimeter. Most meters have a battery symbol of some form on the display. It’s usually not visible and only appears when the battery is getting low. Some flash, some are constant. Other multimeters lack this feature, so you cannot assume the battery is not the source of erratic readings. A dimming screen is also a sign of a failing battery.
If you keep all of that in mind, you should have much success with your multi-metering pursuits, and your multimeter itself should enjoy a long and happy, life without letting the smoke out.