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Power adapter arc characteristics

Date:2019-03-23 Hits:607
Power adapter arc characteristics

This article will provide a deeper discussion of the arc characteristics in power adapter gas. While this may not be necessary, some of the decisions made by an engineer designing a power adapter ballast at some point are of great value. The concentrated study of the conductive properties of gases began in the 19th century. This has also led to a deep understanding of the properties and nature of electrons, as well as a preliminary understanding of atomic structures and the use of X-rays in medical diagnostics.

 

In 1889, Friedrich Paschen studied the relationship between DC breakdown voltage and gas pressure between a pair of electrodes in air. The spherical electrode he uses is much larger than the distance between the two electrodes, which avoids the creation of very high voltage gradients near the tip or edge. His conclusion is the famous Paschen curve, as shown in Figure 16.6. When the two electrodes are separated by 0.3 to 0.5 cm, the breakdown voltage is close to 1000 V at one atmosphere. As the air pressure drops, the breakdown voltage will continue to drop to the lowest point around 300V, and then rise rapidly. Other gases also exhibit the same characteristics, except that the critical gas pressure at the lowest breakdown voltage point is different.

Paschen's law provides an experimental explanation for the phenomena mentioned above, namely impact ionization. At high pressures, the average distance between atoms (ie, the mean free path) is small. Thus, when an electron or positive ion is accelerated by an electric field to a sufficiently large rate to ionize a neutral atom, it collides with other atoms. As the gas pressure drops, the mean free path will increase, and the electrons or positive ions will accelerate over a longer distance before the collision, thus accumulating enough speed. In the event of a final collision, they have enough energy to ionize the atoms, which creates an avalanche effect of the carriers, which triggers the arc.

Arc characteristics of power adapter under DC voltage


As early as the end of the 19th century, physicists studied the visible light characteristics of electrode arc discharge under DC voltage. In their early experiments, cold-emitting solid electrodes were placed at the ends of the glass tube, and a high voltage of several hundred volts was applied to both ends of the electrode through a current limiting resistor.

 

When the gas pressure inside the tube drops low enough, they observe that the bright and dark areas extend from the cathode to the anode as shown. Starting from the vicinity of the cathode, you can first see a small illuminating area CG, followed by a long dark quarantine CDS, followed by a longer illuminating area NG, followed by an equal length dark area FDS. After that, close to the anode is a light and dark band of light. These regions are sequentially named as Cathode Zone (CG), Crookes Dark Zone (CDS), Negative Light Zone (NG), Faraday Dark Zone (FDS), and Positive Arc Column Zone (PC).


The above phenomenon can be explained as follows.

As the pressure drops to near the lowest point of the Paschen curve, the stray free electrons (which are produced by the high voltage gradient of the ray or cathode) are accelerated to obtain sufficient energy to ionize the neutral gas atoms. . The positive ions thus ionized, because of their large mass, do not move very quickly and do not move away from the cathode, so a positive charge region is established here and a high voltage gradient is formed near the cathode. This voltage gradient, now referred to as the cathode voltage drop, accelerates the impact of positive ions on the cathode, causing the cathode to emit some electrons.


When neutral or ionized mercury atoms near the cathode are bombarded with electrons of sufficient energy, some of the electrons in their atoms absorb these energies and transition to higher-energy orbits. When these electrons again transition back to their original orbit, they emit visible light in the CG region.

At the outer edge of the CG region, all of the power adapter electrons deplete their energy on the way to the anode, causing the velocity to drop so that they do not have enough energy to excite those neutral atoms to a higher energy state. After passing through the CDs region, these electrons are re-accelerated. At the edge of the NG region, they regain the energy that can excite the neutral atom to reach a higher energy state, and the electrons in the neutral atom that is excited when passing through the NG region. It returns to the original track and emits visible light in the NG area. When free electrons pass through the FDS region, they are no longer able to excite those neutral atoms to reach a higher energy state because they are depleted of energy, thus forming dark regions.

At the beginning of the PC area, there is another bright area. Then in the entire P area, dark and bright alternately appear. The dark zone is the electron acceleration zone, while the bright zone is the region where the electrons have enough energy to excite the atoms to emit visible light. Most of the voltage applied to the electrodes falls mainly in this section and its length accounts for 80% to 90% of the total length of the lamp.


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