# HowTo: Electronics – Commissioning a new circuit

After the last component of a kit or the first prototype of your own circuit is done, 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 comes the long-awaited reward in the form of a perfectly functioning circuit.

This is not always the case, probably. even rarely. Often a prototype does not work after the first power up. Sometimes it’s just small mistakes that can be fixed quickly. But sometimes a mistake can lead to the destruction of one or more components.

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

Before the initial or re-commissioning of a circuit, you should always take a few measurements. These cost you in the worst case about five minutes and can save you a lot of money, frustration and time at the same time.

How to carry out such a “commissioning measurement” and what to pay attention to, is described in the following article.

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## Safety instructions

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## Requirements

Of course, as you will need a measuring device for the following measurements, it is very helpful to know the basic functions of a measuring device. Information on this can be found in the following article.
Electronics – Functions of a multimeter

Required material:
-none-

Required tools:

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

## Warning of high voltages and resulting currents

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

These voltages are the maximum allowable contact voltages for adults. These values assume that the current potentially flowing through the human body is not life threatening. Nevertheless, you should always avoid becoming a “part” of any kind of circuit. Information on this can be found again in the article Electronics – The functions of a multimeter (use for troubleshooting).

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

## Set the multimeter correctly

As already mentioned in the prerequisites, you will need some functions of your multimeter during commissioning measurement. The individual functions are described in the article Electronics – Functions of a Multimeter. In this you will find information on how to set the multimeter for the respective measurements and what else should be considered.

## Check for short circuit of supply lines and appropriate power

The first test you can do before your circuit is connected to a supply voltage is the short circuit test. It tests that the supply lines are clearly isolated from each other. They are clearly isolated from each other when they have a very high electrical resistance.

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

You must now be able to classify the measured value to be able to assess whether it is “okay” or “not okay”. In many cases, you will see an “OL” (Open Load), which means that the resistance is so high that it can not be determined by the meter (so that’s “Okay”), but in some cases you will get it lower values. In order to check whether this “lower” resistance value points to an error, you must 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 – let’s say 1ohm – it’s most likely an indication of a short circuit.

It may also be that this value is absolutely appropriate. Let’s say you have designed a DC motor to take 25W at 5V supply voltage. In this case, the resistance of 1Ohm is absolutely fine.

You must always try to classify the measured value correctly.

The Ohm’s law is a great tool. Thanks to this you can determine from the measured resistance and the applied voltage which current your circuit would take up. Where:

$Voltage=Resistance*Current$

or in formula symbols

$U=R*I$

Practically, the active power of a circuit is also determined by the current and the voltage. Here is true:

$Power=Voltage*Current$

or again in formula symbols:

$P=U*I$

By mutual use of the respective formulas, you can also determine the power directly from the measured resistance and the corresponding current or voltage. Then:

$P=\frac{U^2}{R}$

or

$P=I^2*R$

So, and what does that bring us now? You can now practically calculate what power your circuit would pull. If you still have in mind what performance you expect, you can very well assess whether there is already an error or the measured values ​​are realistic for a functioning circuit.

The theoretical performance that your circuit should take can be determined by adding up the recorded services of the installed individual components. If you use in your circuit, for example, a microcontroller at 5V about 50mA, 10 LED’s at 5V record about 60mA and two sensors at 5V each about 10mA record so you can easily calculate the expected total recorded power.

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

$P=U*I$

to calculate.

The individual powers are then as follows:

$Mikrocontroller:&space;5V*0,05A=0,25W$

$10*LEDs=&space;10*5V*0,06A=3W$

$2*Sensoren=2*5V*0,01A=0,1W$

Together, this results in an expected drawn power of:

$P_{Gesamt}=P_{Mikrocontroller}+P_{LEDs}+P_{Sensoren}$

$P_{Gesamt}=0,25W+3W+0,1W=3,35W$

So now we have a clue in which area the recorded power of the circuit would be approximately. If the previously measured resistance of the circuit and the resulting power calculated from it is equal to, below, or just above it, you can assume that this should not cause any problems.

Attention: This procedure is only approximate for circuits that work with DC voltage. For devices that are also supplied with AC voltage (ie all devices that hang, for example, without power supply directly to the supply network), it is a bit more complicated, because these still a possibly existing inductive resistance must be considered.

## Continuity check of connections

A popular mistake that has often happened (at least for me) is that some connections are forgotten. Just one forgotten soldering point at the wrong position is enough to make the whole circuit stop working.

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

To do this, you must set the multimeter for continuity testing or, if not available, for resistance measurement. The continuity test is a bit more comfortable as the multimeter confirms a correct connection in this case with a “beeping”. Therefore, the phrase “beep through connections”.

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

## Checking short circuits to signal lines

Even with concentrated work, mistakes can sometimes creep into the structure of one’s own circuit. Most of these result in unwanted short circuits. Especially with signal lines which are used, for example, for communication of a microcontroller with connected sensors, this often leads to the fact that the sensor no longer works or even can be destroyed.

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

Put your meter on continuity and check that there is really no connection between signal lines and adjacent lines and supply voltage and ground. Orient yourself to the wiring diagram and check again if only the contacts are electrically connected that should be electrically connected. Information on the continuity test can be found again in the article Electronics – The functions of a multimeter (use for troubleshooting) in the section Continuity test

## Voltage test of the (secondary) supply voltage

Now you have already taken a few measurements and if you have come here without errors that looks very good. The following measurement is actually no measurement that is performed “before” the commissioning, because you have to connect your circuit for the first time to the supply voltage.

The goal is to ensure that your circuit is supplied with the desired voltage. If you’re using a power supply to power your circuit, it’s a good idea to turn on the power supply first and measure the voltage on the low-voltage plug on the power supply.

Set your multimeter to voltage measurement. Be sure to set the correct voltage type. In many cases, although low-voltage power supply units use direct current (DC), it makes sense to play it safe by checking the imprint on the power supply. You will also find tips on the information printed on the power supply units in the article Power Supplies – Correctly Dimensioning and Understanding the Data Sheet.

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

## Start up the circuit step by step

In the paragraph “Step by step structure to avoid/detect errors” of the article Electronics – The functions of a multimeter (use for troubleshooting) is described why it makes sense to take a circuit “piece by piece” in operation. This procedure can already avoid many mistakes or makes it easier to fix them.