The problem of "neutral line" and "live line" in switching power supply

bigbat

The problem of "neutral line" and "live line" in switching power supply [Copy link]

The mains electricity standard in my country is 220V effective value and 50HZ AC frequency. It is divided into "neutral line" and "live line", the live line is connected to the phase line of the transformer, and the neutral line is connected to the neutral line of the transformer. Most of the household electrical appliances we commonly use switching power supplies for AC/DC conversion. The wiring terminals of the switching power supply are often marked with N and L signs. However, my country's two-phase plugs usually do not distinguish between N and L, but I have seen that American standard plugs do distinguish between them. So is it necessary to distinguish between N and L in the absence of PE wire?

However, it is still necessary to distinguish in daily use. For example, the two ends of a fluorescent lamp sometimes glow slightly when it is turned off. This may be caused by connecting the neutral and the fire in reverse. I have always wondered why the light is on when the circuit is clearly disconnected.

cruelfox

Light is emitted when a weak electric current flows. It is not necessary to connect a wire for the current to flow. A capacitor formed by a conductor and surrounding conductors (such as metal pipes in the wall) can also pass AC current.

maychang

The two ends of a fluorescent lamp sometimes glow slightly when it is turned off. This is because the switch is connected to the neutral line (for normal connection, the switch should be connected to the live line). There is a distributed capacitance between the electrode and the ground in the fluorescent tube, which causes the gas in the fluorescent lamp to be slightly ionized and glow.

bigbat

cruelfox posted on 2020-7-3 15:24 The light is emitted when a weak current flows. It is not necessary to connect a wire for the current to flow. The conductor and the surrounding conductors (such as the metal pipes in the wall) form an electric...

There must be current when the light is on, but how is the circuit formed?

maychang

bigbat posted on 2020-7-3 15:34 There must be current when the light is on, but how is the circuit formed?

From the live wire through the lamp, and then through the distributed capacitor to the ground. The neutral wire is connected to the ground.

maychang

bigbat posted on 2020-7-3 15:34 There must be current when the light is on, but how is the circuit formed?

If the fluorescent lamp is connected to the neutral line, it will often glow slightly after the switch is turned off. If you hold the lamp with your hand, the glow will become more obvious. This is because the human body is grounded (at least the distributed capacitance between the human body and the earth is greater than the distributed capacitance between the wire and the earth), and the distributed capacitance between the human hand and the lamp electrode is greater than when the lamp is not held.

maychang

bigbat posted on 2020-7-3 15:34 There must be current when the light is on, but how is the circuit formed?

If you rub a fluorescent tube that is not connected to anything with a plastic film or fur, you can see the tube glow in the dark. In a completely dark environment, you can even see the tube glow by shaking the plastic film next to the tube.

DIAG

That cheap LED light switch placed on the neutral wire will also glow, which is more obvious at night and flickers slightly, commonly known as ghost lights!

bigbat

maychang posted on 2020-7-3 15:46 If a fluorescent tube that is not connected to anything is rubbed with plastic film or fur, the tube can be seen glowing in the dark. When the environment is completely dark, even...

I have seen this phenomenon, but fluorescent lamps need very high voltage to work and emit light. Of course, the voltage of static electricity can sometimes be very high. But I have never found that fluorescent lamps have ever emitted strong light under the action of static electricity, even for a flash. Could it be that the current is not enough?

maychang

bigbat posted on 2020-7-3 15:56 I have seen this phenomenon, but fluorescent lamps need very high voltage to work and emit light. Of course, the voltage of static electricity can sometimes be very high. But I didn't find...

The voltage of static electricity is generally higher than that of AC mains electricity, and it is common to reach thousands of volts or even tens of thousands of volts. However, the current cannot be sustained, so the energy of a single discharge is not large enough.

Fluorescent lamps (and even all gas discharge tubes) cannot produce strong flashes visible to the human eye under the action of static electricity.