In Part 1 of the Electronics for Kids series, we looked at LEDs; in Part 2, we explored capacitors. Now in Part 3, we will take an in-depth look at electrical measurements.
It is impossible to discuss electronics without including electrical measurements. In order to choose the correct values for the electronic components and calculate some working conditions, the behavior of an electrical circuit must be predicted using some simple arithmetic calculations.
It’s similar to a cooking recipe in which all of the ingredients must be precisely measured in order to produce a wonderful meal.
The main issues of electrical measurement will be addressed with some simple examples, with particular reference to electrical voltage and the calculation of ohmic resistance.
There isn’t a practical field where measurements aren’t used, such as length, weight, school grades, video game points, and body temperature.
Any physical, chemical, or electrical quantity must be measured to be well controlled and managed. If you don’t take some measurements, you won’t be able to predict how the circuit will behave. We’re obviously not talking about measuring length with a tape measure or a ruler.
Instead, we’re talking about electrical measurements taken with an instrument known as a tester or multimeter, as shown in Figure 1.
To take the first electrical measurements, you don’t need to buy a high-end model that costs hundreds of dollars. A simple analog or digital model, which can be purchased for a few euros, will suffice. Analog meters are becoming increasingly rare, and most devices are now digital.
The tester, also known as a multimeter, is an essential measuring tool. It can have a variety of functions, but the following are common to all models:
- A voltmeter for alternating and direct voltage measurement
- An ammeter for current measurement
- An ohmmeter for resistor measurement
Before moving on to the electrical measurement examples, it’s a good idea to review the concepts of voltage, current, and resistance, as shown in Figure 2. These are the three electronic pillars that no design can be completed without.
In everyday language, the terms “voltage” and “current” are used interchangeably, but they actually refer to two different concepts. Knowing the difference between these two terms is crucial for avoiding dangerous situations and getting the most out of electronic devices.
Because electricity has a behavior that is very similar to that of water, the electrical voltage could be represented by water pressure, while the electric current could be represented by water itself.
Looking at the figure, we can find the following three items:
- Electrical voltage is the pressure which, from the power source, pushes electrons through the circuit. It is measured in volts.
- The electric current is the set of electrons flowing in the circuit. It is therefore an ordered flow determined by the electrical voltage. It is measured in amperes.
- Resistance is a measure that expresses the opposition to the flow of current in an electrical circuit and is measured in ohms.
All materials are characterized by a certain degree of resistance to current and are divided into the following categories:
- Conductors: materials that offer very little resistance and electrons can move easily (for example, silver, gold, copper, and aluminum)
- Semiconductors: materials that have characteristics that include those of conductors and insulators
- Insulators: materials that offer high resistance and limit the flow of electrons as much as possible (for example, paper, glass, rubber, and plastic)
Measuring the voltage of a battery or cell
It is possible to read the electric voltage at the ends of a battery using this method, and it is useful to determine whether it is charged or should be discarded. Connect the red test lead to the voltage socket (V) and the black test lead to the common socket to put the tester in voltmeter mode (COM or earth), as shown in Figure 3.
It is always advisable to set the maximum flow rate (full scale) to a higher position and then, if necessary, lower it. In our example, because the batteries to be measured have a voltage of 1.5 V, 4.5 V, and 9 V, it is advisable to use the maximum range of 20 V, which can be selected via the relative rotary switch. It is now possible to directly measure the battery voltage. Do not expect a voltage exactly the same as that indicated on the label of the battery. For a 4.5-V battery, for example, you could read the 4.7-V value if it is new and charged, the 4.1-V value if it is a little discharged, or the 3.4-V value if it is already sufficiently discharged.
The same rule is true for all other voltage generators. When measuring the electrical voltage, it is important to respect the correct polarity of the test leads. The red lead must be connected to the positive pole of the battery, while the black lead must be connected to the negative pole. However, any polarity reversal does not destroy the tester; the voltage will be indicated with a negative value. Remember, once again, not to put the two poles of the battery into direct electrical contact. It could generate a lot of heat and even a flame.
Measuring the voltages of a circuit made up of various components
Figure 4 shows the wiring diagram that allows the lighting of two LED diodes connected in series. The calculation of the resistance value will be studied in one of the next installments. Let’s take some tension measurements now, to improve our general vision. These measurements are not intended for further purposes; they are only carried out to become more familiar with the tester. Recall that in this type of connection, the electrons passing through the battery, the resistance, and the two LED diodes are all the same, as is the water flowing through a single tube. Set the tester to measure the voltage with a voltage of 20 V full scale. Now let’s examine the following measurements, considering that the various points of the circuit are called nodes:
- Red tip on node N1 and black tip on node G: The voltage value read is about 4.5 V. In practice, we are measuring the voltage present at the battery terminals.
- Red lead on node N1 and black lead on node N2: The voltage value read is about 0.7 V. We are measuring the voltage across the resistor.
- Red lead on node N1 and black lead on node N3: The voltage value read is about 2.6 V. We are measuring the voltage present between the positive pole of the battery and the cathode of the first LED diode.
- Red tip on node N2 and black tip on node N3: The voltage value read is about 1.89 V. We are measuring the voltage present at the ends of the first LED diode.
- Red tip on node N2 and black tip on node G: The voltage value read is about 3.78 V. We are measuring the voltage present at the ends of the two LED diodes.
- Red tip on node N3 and black tip on node G: The voltage value read is about 1.89 V. We are measuring the voltage present at the ends of the LED diode.
The most important measurements concern the first (N1-G), which has the purpose of measuring the voltage of the battery (4.5 V); the fourth and sixth (N2-N3 and N3-G), which have the purpose of measuring the voltage at the ends of the first and second LED diodes (1.89 V); and the fifth (N2-G), which has the purpose of measuring the voltage across the two LED diodes (3.78 V). The measurements taken may differ from those reported here, as batteries and electronic components are always slightly different from each other. As also seen from the measurements, the black lead does not always have to be connected to the negative pole of the battery but can connect to any node, as well as the red lead. You need to become familiar with this fact. By connecting the tester to the various nodes of the circuit, we obtain the reading of a certain electrical voltage on the tester. It is not generated by the individual electronic components but by the battery that is spread over the various components. In fact, in this type of circuit, we will never find a voltage higher than 4.5 V.
The final experiment aims to determine the value of the resistors, which, as shown in Figure 5, can be compared to water pipes of various diameters. The following is the general rule for resistors:
- The larger the resistor value, the less current passes through it, like a narrow tube (for example, 330,000 Ω, equivalent to 330 kΩ);
- The smaller the resistor value, the more current passes through it, like a large tube (for example, 15 Ω).
To measure a resistor, it is not necessary to respect the polarity. In fact, these types of components are not polarized, and the test leads can be oriented at will, as well as their direction in an electrical circuit. The body of the resistor is usually provided with some colored rings that indicate the ohmic value. The international color code is as follows:
- Black: 0
- Brown: 1
- Red: 2
- Orange: 3
- Yellow: 4
- Green: 5
- Blue: 6
- Purple: 7
- Gray: 8
- White: 9
For example, for a resistor with the colors of the brown, black, and brown bands, we will have the following value:
- First brown band: 1
- Second black band: 0
- Third brown band: 1 zero to add
Therefore, the resistor value is 100 Ω.
Another example, for a resistor with the colors of the yellow, purple, and orange bands, we will have the following value:
- First yellow band: 4
- Second purple band: 7
- Third orange band: 3 zeros to add
Therefore, the resistor value is 47,000 Ω, equivalent to 47 kΩ. Depending on the value of the resistor to be measured, it is necessary to position the rotary switch of the tester in the correct position, as can also be seen in the example. The human body also has its own resistance. If you try to touch the test leads in the ohm position with both hands, you will be able to read the following approximate values:
- Approximately 10,000,000 Ω (10 MΩ) when your hands are dry
- Approximately 700,000 Ω (700 kΩ), when your hands are wet or sweaty
- Approximately 50,000 Ω (50 kΩ) when your hands are very wet
Furthermore, salt water is much more conductive than fresh water.
To design simple electronic circuits, kids should perform several experiments. Even if some concepts are still not clear, it does not matter. With time and a lot of practice, they will be understood perfectly. The tests should only be carried out using batteries, as they are not dangerous. It is also advisable to use the help of an adult while performing experiments.
Electronics for Kids: Part 1 — LEDs
Electronics for Kids: Part 2 — The Capacitor