HowTo: Electronics - Commissioning a new circuit

After the last component of a kit or the first prototype of your own circuit is finished, you are often excited and impatient. Sometimes at this point you have two, three or more hours of often fiddly and highly concentrated work behind you. So now the long-awaited reward comes in the form of a perfectly functioning circuit.

Unfortunately, it doesn't always work that way, maybe. even rarely. Often a prototype does not work after it is switched on for the first time. Sometimes it's just small errors that can be quickly corrected. Sometimes an error can also lead to the destruction of one or more components.

In this case the frustration is often huge. You have invested time and money and now you have none of it. To save you this frustration, there are one or a few important rules to follow.

Before putting a circuit into operation for the first time or again, you should always take a few measurements. In the worst case scenario, these will cost you around five minutes of time, but at the same time they can save you a lot of money, frustration and time.

How you carry out such a “commissioning measurement” and what needs to be taken into account is described in the following article.

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Safety instructions

I know the following notes are always kind of annoying and seem unnecessary. Unfortunately, many people who knew "better" have lost eyes, fingers or other things due to carelessness or injured themselves. Data loss is almost negligible in comparison, but even these can be really annoying. Therefore, please take five minutes to read the safety instructions. Because even the coolest project is not worth injury or other trouble.
https://www.nerdiy.de/sicherheitshinweise/

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Requirements

Since you will of course need a measuring device for the following measurements, it is very helpful to know the basic functions of a measuring device. You can find information about this in the following article.
Electronics – functions of a multimeter

Required material:
-no-

Required tool:

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

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Warning of high voltages and the resulting currents

At this point the important note again: High voltages can be very dangerous. If you are unsure, you should never work with voltages higher than 50V AC (alternating voltage) or 120V DC (direct voltage).

These voltages are the maximum permissible touch voltages for adults. With these values it is assumed that the current potentially flowing through the human body is not life-threatening. Nevertheless, you should of course always avoid becoming “part” of any kind of circuit. You can also find information about this in the article Electronics – The functions of a multimeter (use for troubleshooting).

If you are unsure about anything, it is always better to ask trained specialists for help. Because even the most annoying and stupid questions cause less work and suffering than a funeral.

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Set the multimeter correctly

As already mentioned in the requirements, you will need some functions of your multimeter for the commissioning measurement. The individual functions are in the article Electronics – functions of a multimeter described. In this you will find information on how to set the multimeter for the respective measurements and what else you need to take into account.

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Check for short circuit in the supply lines and appropriate power

The first test you can carry out before your circuit is connected to a supply voltage is the short circuit test. This tests whether the supply lines are clearly insulated from each other. They are clearly isolated from each other if they have a very high electrical resistance.

To do this, set the multimeter to measure resistance and check the resistance from the positive to the negative supply line.

You now have to be able to classify the measured value in order to be able to assess whether it is “okay” or “not okay”. In many cases you will get an “OL” (Open Load), which means that the resistance is so great that the meter cannot determine it (so that is “Okay”), but in some cases you will also get lower values. In order to check whether this “lower” resistance value indicates an error, you have to check whether it can be plausible. You compare the measured value with the expected resistance value.

For example, if you measure a very low resistance value - let's say 1 ohm - this is most likely an indication of a short circuit.

However, it may also be the case that this value is absolutely appropriate. Let's assume you have designed a DC motor that should draw 25W with a 5V supply voltage. In this case the resistance of 1Ohm is absolutely fine.

So you always have to try to classify the measured value correctly.

Ohm's law is a great tool for this. Thanks to this, you can determine what current your circuit would draw from the measured resistance and the applied voltage. The following applies:

or in formula symbols

Practically, the active power of a circuit can also be determined using the current and voltage. The following applies here:

or again in formula symbols:

By using the respective formulas together, you can also determine the power directly from the measured resistance and the corresponding current or voltage. Then the following applies:

or

So, what does this mean for us? You can now practically calculate what power your circuit would consume. If you then have in mind what power consumption you expect, you can very well estimate whether there is already an error or whether the measured values are realistic for a functioning circuit.

You can determine the theoretical power that your circuit should absorb by adding up the power consumed by the individual components installed. For example, if you use a microcontroller in your circuit that draws approx. 50mA at 5V, 10 LEDs that draw approx. 60mA at 5V and two sensors that each take approx. 10mA at 5V, you can easily calculate the expected total power recorded.

To do this, you first have to calculate the individual services. Since we know the currents consumed by the individual components at the respective voltages, the power consumed can be calculated using the formula

calculate.

The individual services are then as follows:

Added together, this results in an expected power consumption of:

So we now have an indication of the range in which the power consumed by the circuit would lie. If the previously measured resistance of the circuit and the calculated power consumption corresponds to this value, is below or only slightly above it, you can assume that this should not cause any problems.

Attention: This procedure only applies approximately to circuits that work with direct voltage. For devices that are also supplied with alternating voltage (i.e. all devices that are directly connected to the power supply without a power supply, for example) it is a little more complicated because any inductive resistance that may be present must be taken into account. 

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Continuity testing of connections

A common mistake that often happens (at least to me) is that some connections are forgotten. All it takes is a forgotten soldering point and a connected sensor, for example, no longer works or no longer works correctly.

To avoid this, it is recommended that all connections “beep” once after assembly. This means that it is checked whether all the connections specified in the circuit diagram also exist in the circuit constructed.

To do this, you must set the multimeter to test continuity or, if not available, to measure resistance. The continuity test is a little more convenient because the multimeter confirms a correct connection with a “beep” in this case. Hence the phrase “beeping through connections”.

Now go through all the connections shown in the circuit diagram and check whether they also exist in the circuit you have built. You can also find information about this in the article Electronics – The functions of a multimeter (use for troubleshooting) in the section Continuity test

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Checking for short circuits to signal lines

Even when you're working hard, errors can still creep into the structure of your own circuit. These usually result in unwanted short circuits. Especially with signal lines that are used, for example, to communicate between a microcontroller and connected sensors, this often means that the sensor no longer works or can even be destroyed.

In this case, a few measurements per signal line are enough to rule out this error.

Set your measuring device to the continuity test and check that there really is no connection between the signal lines and neighboring lines as well as to the supply voltage and ground. To do this, refer to the circuit diagram and check again whether only the contacts that should be electrically connected are actually electrically connected. You can find information about the continuity test in the article Electronics – The functions of a multimeter (use for troubleshooting) in the section Continuity test

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Voltage test of the (secondary) supply voltage

Now you have already carried out a few measurements and if you have gotten this far without errors, it looks very good. The measurement that follows is actually not a measurement that is carried out “before” commissioning, because you have to connect your circuit to the supply voltage for the first time.

The goal is to ensure that your circuit is supplied with the desired voltage. If you use a power supply to supply your circuit with energy, it is recommended to first switch on the power supply and measure the voltage at the low-voltage plug of the power supply.

Set your multimeter to measure voltage. Also make sure to set the correct type of tension. In many cases, low-voltage power supplies use direct voltage (DC), but it makes sense to be on the safe side by checking the printing on the power supply. You can also find tips on the information printed on the power supplies in the article Power supplies – size correctly and understand the data sheet.

Now measure the voltage of the low-voltage connection. This should be in the range of the voltage value also printed on it.

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Put the circuit into operation step by step

In the paragraph “Step-by-step structure to avoid/recognize errors directly” of the article “Electronics – The functions of a multimeter (use for troubleshooting)” describes why it makes sense to put a circuit into operation “piece by piece”. This approach allows many errors to be avoided or easily corrected.

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Further information

https://de.wikipedia.org/wiki/Pr%C3%BCfen_(VDE)

https://de.wikipedia.org/wiki/Kleinspannung

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

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Have fun with the project

I hope everything worked as described for you. If not or you have questions or suggestions please let me know in the comments. I will then add this to the article if necessary.
Ideas for new projects are always welcome. 🙂

PS 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 think it's cool that I share the information with you, I would be happy about a small donation to the coffee fund. 🙂