The essence of diode protection in DC circuits is quite simple - a diode is connected in parallel to the inductive load in the direction opposite to the current that passes through the coil. The 1N4004 diode is a regular rectifier diode with a maximum current of 1 A and a reverse voltage of 400 V. In my experiments, I will use a 1N4007 diode with a current of 1 A and a reverse voltage of 1000 V. If you buy this diode in China, it will cost 34 kopecks. The easiest way to demonstrate protection against back EMF resulting from the phenomenon of self-induction when switching powerful inductive loads is to assemble a simple device called an electromechanical buzzer. A buzzer is basically a relay with a normally closed contact. There is no current in the coil - the contact is closed. When current appears, the contact is open. The relay coil is connected to the power source through this contact and thus, when a magnetic field occurs, the contact opens and the current in the coil disappears. The contact returns to its place and supplies power to the circuit again. In Soviet times, such a device was widely used in electric bells and was even installed in electromechanical alarm clocks. Of course, this device works perfectly well without a protective diode - but if you look closely at the contacts, you can see that strong sparking occurs. If you add a regular rectifier diode to this simple device, the sparking practically disappears. Sparking has two problems. The first is that the contacts burn out and, therefore, their service life is reduced. And the second is that the spark is a strong source of radio interference. By adding a regular rectifier diode to the coil circuit, we solved both problems. The buzzer was assembled purely for clarity - all this applies to any electromechanical contacts, be they relay contacts or a regular button that switch inductive loads - installing a reverse diode will significantly extend their service life. Of course, a diode is not needed to switch LEDs, incandescent lamps, and heaters. But what about electronic keys? Let's assemble another circuit. An Arduino board powered by a separate power source supplies +5V pulses to the input of a transistor key. A conventional electromechanical relay is connected to the transistor collector. To understand what is happening, an LED is connected in parallel to the relay coil in the opposite direction to the current passing through the coil. As we can see, when the current in the relay coil disappears, the LED flashes. How does this flash threaten the transistor key? Since the LED flashed, it means that + appeared on one of its terminals and minus appeared on the other terminal. Our inductance coil turned into an additional power source at the moment of opening. And what happens when we connect two power sources in series. It was 9V and now it is 18. Here, exactly the same thing happens. The supply voltage appeared on the transistor collector + the voltage created by the coil. Of course, such a small relay will not be able to break through a transistor key - but if you include a powerful starter or an electric motor in the circuit, the sum of the voltages can break through the transistor with all the ensuing consequences. Therefore, in my opinion, it is better to add one penny part to the circuit and sleep peacefully than to think whether the key will break through or not during the next load disconnection.