1. Diode reverse polarity protection circuit

Schottky diodes are commonly used to protect circuits, such as reverse polarity circuits, because their forward voltage decreases. The following figure shows a common reverse polarity circuit.

When VCC is connected to ground with the correct polarity, the diode conducts forward and the load receives power. Compared with the rectifier diode's 0.7V, the forward voltage drop on the Schottky diode is very low at around 0.04V, so the power loss on the diode is not too large. Moreover, Schottky diodes can allow more current to pass through them and have faster switching speeds, making them suitable for high-frequency circuits.

2. Diode reverse current protection circuit

The diode placed in series with the positive pole of the power supply is called a reverse protection diode, which ensures that current can only flow in the forward direction and that the power supply only applies positive voltage to your circuit.

This type of diode is useful when the power connector is not polarized. The disadvantage of reverse protection diodes is that they can cause some voltage loss due to forward voltage drop.

A voltage regulator is used to reduce the input voltage to the desired level and maintain it constant in the event of power supply fluctuations, and can also be used to regulate the output voltage.

Zener diodes are commonly used as voltage regulators because they are designed to operate under reverse bias conditions. When in forward bias, it behaves like a normal signal diode. On the other hand, when a reverse voltage is applied, the voltage remains constant over a wide range of currents.

In the following circuit, the input voltage can vary between 0V and 12V, but the output voltage will never exceed 5.1V because the reverse breakdown voltage (Zener voltage) of the Zener diode is 5.1V. When the input voltage is below 5.1V, the output voltage will be equal to the input voltage, but when it exceeds 5.1V, the output voltage will be adjusted to 5.1V.

A freewheeling diode is essentially a diode connected to an inductive load terminal to prevent high voltage from being generated across the switch.

When the inductor circuit is turned off, the freewheeling diode provides a short-circuit path for the flow of attenuated current in the inductor, thereby consuming the energy stored in the inductor.

The main purpose of a freewheeling or flyback diode is to release the energy stored in the inductor by providing a short-circuit path, otherwise sudden attenuation of circuit current will generate high voltage at the switch contacts and diode.

When switch S is closed, the steady-state current I through the circuit is (V/R), so the energy stored in the inductor is (LI 2)/2. When this switch S is turned on, the current will suddenly decay from the stable value I=(V/R) to zero.

Due to the sudden decay of the current, a high reverse voltage equal to L (di/dt) (according to Lenz's law) will appear on the inductor terminals, and therefore on the diodes and switches, which will cause sparks at the switch contacts.

If the reverse voltage exceeds the peak reverse voltage of the diode, it may be damaged. To avoid this situation, a diode called a freewheeling or flyback diode is connected to the inductive load RL