What happens if the positive and negative poles are reversed? What is the connection between these four reverse connection protection circuits?

Once the positive and negative poles are reversed, it can lead to the destruction of many electronic components.

To prevent the positive and negative poles from being reversed, we use reverse connection protection circuits. How do they protect against this?

There are four common reverse connection protection circuits: diode series, bridge rectifier, NMOS, and PMOS.

Diode series: Utilizing the unidirectional conductivity of diodes (forward conduction, reverse cutoff), a diode is serially connected at the output end of the positive power supply for normal conduction. If the power supply is reversed, a circuit loop cannot be formed, and the diode will be cutoff.

Now, is there a circuit that can conduct in both directions?

Certainly, that would be the bridge rectifier circuit.

Regardless of whether it's forward or reverse connected, the bridge rectifier always maintains a fixed output, and the circuit can operate normally.

However, whether it's one diode or multiple diodes, it will produce a voltage drop across the diode and consume power.

At this point, the reverse connection protection circuit design of MOSFETs can be utilized.

Reverse connection protection circuit design for NMOS and PMOS:

When the input is positive on top and negative on the bottom, the current passes through R1, R2, and the parasitic diode of the MOSFET to ground. After voltage division by R1 and R2, the GS voltage exceeds the threshold voltage Vgs of the MOSFET, and then the MOSFET conducts.

When the polarity is reversed, i.e., negative on top and positive on the bottom, the current path is reversed cutoff by the parasitic diode of the MOSFET, and since GS is cut off due to the absence of voltage, the circuit loop is cut off.

The principle of reverse connection protection for PMOS is the same as NMOS, just in a different position, usually placed at the top.