HowTo: Electronics – The Functions of a Multimeter (Use for Troubleshooting)

Voltmeter, amperemeter, ohmmeter or as a “compact device” = multimeter are the ultimate life or at least a good mood savior if the self-made circuit does not work.

Sometimes it just happens that although you have kept to a detailed construction plan, the built circuit still does not work. At least then you sit very frustrated after the initially motivated started work in front of the built circuit and would like to throw everything back down.

In such situations, good advice is often expensive. Thats why i think an article like this can be helpfull. It should act like a little guide as you may at least be able to track down the problem.

With the help of these small compact devices, different electrical variables can be measured and errors can be located quickly.

Hints for our lovely english readers: Basically, many of the articles on Nerdiy.de are translations from the original german articles. Therefore, it may happen here and there that some illustrations are not available in english and that some translations are weird/strange/full of mistakes or generally totaly wrong. So if you find some obvious (or also not obvious) mistakes don't hesitate to leave us a hint about that in the comment section. 
Also please don't get confused, that instead of a "dot" often a "comma" is used as decimal separator. 🙂

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Safety instructions

I know the following hints are always a bit annoying and seem unnecessary. But unfortunately, many people who knew it "better" from carelessness lost their eyes, fingers or other things or hurt themselves. In comparison, a loss of data is almost not worth mentioning, but even these can be really annoying. Therefore, please take five minutes to read the safety instructions. Even the coolest project is worth no injury or other annoyance. https://www.nerdiy.de/en/sicherheitshinweise/

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Requirements

Helpful Articles:
Not really a requirement, but an example of a practical application area can be found in the article Electronics – Commissioning a new circuit

Required material:
-none-

Required tools:

In the following list you will find all the tools you need to implement this article.

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The functions of a multimeter

Important for later troubleshooting is knowing and using the basic functions of a multimeter. This is not always easy, since modern multimeters are often equipped with all sorts of functions. To give you an overview of the individual functions, these are described in the following sections.

Measuring range selector switch

The range selector is the main control of your meter. This determines roughly what size you want to measure. For some measuring ranges, such as “Hz/Duty”, you can then use the buttons to switch between the options. In order to set the measuring range selector switch to the desired option, it is turned so that it points like a pointer to the desired measuring range.

Buttons

The available buttons on a meter are very often different from model to model. Mostly only a “range selector switch” is available with which you can select the electrical size to be measured.
On some models, however, additional functions can be selected via the buttons. As an example, therefore, the functions of the buttons of the imaged meter are described.

With the “HOLD/B.L” -button you can “hold” the measured value on the screen at the time of pressing the button. The displayed reading will therefore remain on the display and, for example, you have enough time to note down the corresponding value. A long press on this button also activates or deactivates the backlight illumination of the LC-display.

The Range-button allows you to manually adjust the measuring range. For many measuring instruments, this measuring range is now determined automatically. Nevertheless, it may sometimes be necessary to be able to set this measuring range manually. That is why it is always important, especially in the measurement of current and voltage, that you are “grossly aware” of the magnitude of the electrical variable to be measured before the measurement. For example, if you only want to measure 12V, you have to take very different safety precautions than when measuring 500V or more. Please note the safety instructions!

The “°C/°F-button” allows you to change the unit between Celsius and Fahrenheit when measuring temperatures. Fahrenheit is the standard unit in the US. Celsius, however, in most other countries.

With the “Hz/Duty”-button, you can switch between these when measuring the frequency or duty cycle. Since these two parameters share a setting position on the measuring range selector, it is possible to switch between them.

The “REL”-button allows you to set a new “zero point” for subsequent measurements. For example, you can compare two voltages. For example, if you measure a voltage of 5V during the first measurement and then press the “REL” key, all subsequent voltages are measured in relation to the first one. If, for example, you measure a voltage of 8V after pressing the “REL” button, only 3V will be shown on the display of your meter because  (the former measured) 5V will be used as a reference for this measurement.

With the help of the “Select”-button you can choose if you want to measure an alternating size (=AC) or DC size (=DC). For example, if you want to measure the DC voltage of a battery or a rechargeable battery, it is important that you set both the range selector to “V” (=voltage measurement) and the type of size to DC. On the other hand, if you want to measure the voltage of a power outlet, you need to set the type of size to AC.

Connectors:

In the lower area there are four connection sockets in which the test leads can be inserted. The “most important” is the “COM”-socket. The black wire is always plugged into this. Only the red test lead is changed depending on the electrical size to be measured.

For example, if you want to measure a current whose magnitude is in the ampere range, you must plug the red test lead into the “10A”-socket. It is also important that the designation “10A” here indicates that this connection is protected with “only” 10A. Higher currents can not be measured directly. If you do not measure any current when using this socket, most likely this fuse is defective. How to exchange them is described in the instruction manual of your multimeter.

Right next to the “10A” socket you will find the “TEMP/μA/mA” socket. In this you have to plug in the red test lead, if you want to measure currents that are in the mA and μA – ie less than 1A – range. This socket is also secured with a fuse. In many, as in this case, but with a self-resetting fuse. An external temperature sensor can also be connected to this socket with this meter. That’s why the “TEMP…” is in the socket name.

The last jack is located on the right side of the meter shown. This jack, labeled with a capacitor, diode, voltage, resistance symbol and “Hz”, is required for all other measurements. So if you want to measure capacitance, resistance, voltage or frequencies, you have to use this socket. Also for the diode test – to determine the forward direction and forward voltage of a diode – you must use this socket.

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Voltage measurement

One of the important electrical parameters is the voltage. It is relatively easy to measure with a multimeter.

Nevertheless, one should first think about the expected voltage. That’s because the precautionary measures must be adapted during the measurement. It is not only important to know what voltage value you should expect, but also what type of voltage. So whether alternating voltage (AC=Alternating current) or DC voltage (DC=Direct current).

Here is a little quote from the very detailed (german) Wikipedia article (https://de.wikipedia.org/wiki/Ber%C3%BChrungsspannung):

In healthy adult humans one goes with the exceeding of the low voltage starting from 50 V alternating voltage (AC) or 120 V direct voltage (DC) from a life-threatening situation. Among other things, in children and larger farm animals, the contact voltage is set only to a maximum of 25V AC or 60V DC, in wet room installations in some cases even to 12V.

This means that you can normally measure at least voltages up to 25V AC and 60V DC without much danger to your life. If you are not “off the shelf” you should generally avoid working with greater voltages.

To measure the voltage with a multimeter, you first have to set it to the voltage measurement range. This is usually marked with a “V” for volts (ie the unit of electrical voltage). Unfortunately, the correct setting is not standardized here and varies with many measuring devices. If you want to be sure, take a look at the manual of your meter.

If you have set the voltage measurement, it is still important that you set the voltage type. For the shown meter, the voltage is set using the “Select” button. However, as I said, this is not uniform in all measuring devices.

For the actual voltage measurement, the measuring tips are then always connected in parallel to the consumer or producer whose voltage you want to measure.

It is important to know that a measuring device has a very high but measurable internal resistance in the voltage measurement. During the voltage measurement, therefore, you will inevitably switch a resistor in parallel with your load to be measured, as a result of which a parallel connection of two resistors is created and thus a (slightly) higher current flows through the entire circuit. This can also cause the voltage to be measured to be slightly lower than it would be without a measuring device. This is negligible in most cases but still good if you keep this in mind. 🙂

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Current measurement

The current is another important electrical quantity. It is also easy to measure with a multimeter. In contrast to the voltage measurement the circuit whose current should be measured, needs to be valued/it’s current needs to be estimated.

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Resistance-measuring

The resistance measurement is actually a combination of a voltage and current measurement. But since the resistance is also one of the important electrical quantities, its measurement is directly possible with many measuring devices.

Actually, the resistance of a circuit (in simplified terms) is the proportionality constant of voltage to the current. That is, the resistance indicates how much current flows through a circuit at a certain applied voltage. This means, if you know what voltage is applied to a circuit and then also know what current flows through the circuit at this voltage, you can easily calculate the resistance.

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Continuity test

The continuity test is actually a luxury version of the resistance measurement. With it can be determined very easily whether two contacts are electrically connected.

For example, if you want to check that you’ve made the right circuit from the schematic to the real world, you can use the continuity test to measure whether the contacts that should be electrically connected according to the wiring diagram are really connected. If two contacts are electrically connected you can find out by touching the first contact with the black and the second contact with the red tip. If both contacts are connected, the meter signals this usually with a beep and the displayed resistance value in the display. Whether two contacts are connected is determined here by the resistance measurement. If the resistance from one to the other contact is very low there is also a connection between them.

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Capacitance measurement

The capacitance measurement allows you to determine the capacitance of a capacitor. This is especially helpful if you find a capacitor in your component drawer whose value can no longer be clearly read.

To measure the capacitance, the measuring tips of the meter are simply held against the two contacts of the capacitor.

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Diode test

With the help of the diode test you can determine the forward direction and the forward voltage of a diode. This is especially helpful with SMD diodes, because they are not always printed on the housing.

You can recognize the passage direction by holding the measuring tips to the two contacts and trying out in what combination a voltage is shown on the display. For example, if you hold the red tip to the first contact of the diode and the black tip to the second contact and you then see a voltage on the display, you know that the contact to which you just held the red tip is the anode of the diode is.

The indicated voltage is then the forward voltage of the diode. On the other hand, if you do not see any voltage, you will need to flip the probe tip map. In this case, you have only found the reverse direction of the diode.

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Frequency measurement/Duty

The frequency or duty cycle measurement is not integrated in all measuring devices. With this you can measure frequencies of an AC voltage or the duty cycle of a switched DC voltage.

For example, it is also relatively easy to check whether a pulse width modulation you have programmed works.

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Possible additional functions

temperature

Some meters also have the ability to measure temperatures using an externally connected thermocouple. To do this, you must connect this and set the meter accordingly.

dB

Using dB measurement, you can measure the sound pressure of a sound.

lux

By measuring the illuminance, you can measure the illuminance in a room.

x10Lux

In order to be able to measure very high illuminance levels – for example in strong sunshine – you can switch the sensitivity of the illumination measurement. As a result, you can measure higher illuminances or display them on the screen.

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Troubleshooting

Now for the actual troubleshooting. It is difficult to present a precise, universal, always functioning “Roadmap”. Unfortunately, mistakes often hide where you do not expect them to.

But many mistakes are simply careless mistakes. These can usually be found very quickly with the following plan.

1. Continuity check of all connections in the de-energized – ie switched off – state.
   • Check that all connections that are made according to the wiring diagram are really connected. The continuity test of your multimeter is perfect for this.
   • You can also check that adjacent contacts on the board have not been short-circuited.
   • Above all, the energy supply lines should not have a short circuit. You can also test this with the continuity test. The meter should then show no continuity.
2. Check installed resistors for correct value.
   • Measure the installed resistors in a de-energized state.
   • In the case of deviating values, make sure that the respective resistor could also be connected in parallel with other resistors, which would change its value. If in doubt, you have to once again disconnect a contact of the resistor from the circuit and thus measure the resistance separately.
3. Check power supply and other voltages
   • Often it is helpful to check the individual supply voltages. If you have multiple supply voltages (for example 3.3V and 5V) in your circuit, you should ensure that both are available and that they do not break down under load.
4. Check input current.
   • In which you check the input current, you can easily determine if there is a short circuit or a missing connection.
   • If the current is very high you will most likely have built a short circuit in. Then look for a connection between the positive and negative contact of the power supply of your circuit.
   • If the current is very low, it is possible that you forgot to connect the power supply. It could also be that a voltage regulator is defective. In circuits where microcontrollers are used it could happen that this is held in reset mode by a faulty circuit.
5. Check the voltage of the logic levels.
   • Especially with digital communication between a microcontroller and connected sensors, it is important that they use the same logic levels. For example, a microcontroller that works at 1.8V logic levels will find it very difficult to work with a sensor that expects the 5V logic level. This can be solved by measuring the voltages and/or looking into the data sheets of the devices involved.

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Step by step construction to avoid/detect errors

Granted, this is a bad tip for troubleshooting after the complete circuit is already set up. But probably it helps you to build your next circuit:

For very complex or circuits with multiple sensors, actuators, etc., it is advisable to take this step by step “piece by piece” in operation. This means that you first check that the microcontroller works without connected sensors and is programmable.

If this works, the next sensor will be connected and checked again if it works. If everything runs, the next sensor/actuator is connected. This continues until all circuit parts are built up.

If a newly added part of this or even the complete circuit stops working you already have a troubleshooting approach. Because then you can pretty much assume that the newly added component has or is the cause of the error.

In this case, you can as a kind of “counter sample” remove the added component again. If the error no longer occurs in this case, you have the confirmation that the error is related to the corresponding component.

In this case, it is then to find out why this component causes the error?

• Is it maybe broken? If present, try to install and test another copy of the corresponding component.
• How does the current consumption of your circuit change when the component is installed? Is she maybe so high that not enough power reserves are available to power the entire circuit? Here you should check by a voltage measurement if the supply voltages “breaks down”/sag.
• For sensors/actuators that communicate via the I2C/IIC or other buses, it may be that the other bus device interferes with the communication. Check for any terminators, pull-ups and shorts present. Keep in mind that especially with the I2C bus, another bus user must increase the bus capacity and consequently the pull-ups must also be adapted.

The advantage of this “procedure” is that you have a direct approach that could have caused the error. It will make troubleshooting easier and hopefully fix the bug faster.

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Additional information

https://de.wikipedia.org/wiki/Ber%C3%BChrungsspannung

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I hope everything worked as described. If not or you have any other questions or suggestions, please let me know in the comments. Also, ideas for new projects are always welcome. 🙂

Fab

P.S. Many of these projects - especially the hardware projects - cost a lot of time and money. Of course I do this because I enjoy it, but if you appreciate it that I share these information with you, I would be happy about a small donation to the coffee box. 🙂