They cross a line marked Primary / Secondary, so I assume they are a discharge path, to divert a lighting or static overvoltage so it doesn't burn through the insulation in the transformer. They're even labelled "SP1"
A discharge across the PCB may not necessarily be a disaster, while a discharge across the transformer will destroy it.
I think the lack of solder mask, and the round tips, are to try to make the breakdown voltage a bit more predictable. It will still depend on the air pressure and humidity.
Normally this would be done with a gas discharge tube, which has a much lower and more reliable striking voltage, but this might be too expensive in a phone charger.
Earthing or grounding is designed to protect the users of electrical equipment by preventing dangerous voltages to appear on metal casings, etc.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Building supply neutral is earthed / grounded at the supply transformer.
In most countries the electrical utility grounds one of the wires from the local step-down transformer. This "neutralises" the wire as there is now no potential relative to earth. The live wire, on the other hand, has full voltage between it and earth. This explains why your transformer will work with the neutral wire connected to earth. The fact that it works also suggests that you don't have "earth leakage" or "ground fault" protection in your house. (You should have.)
My question is, What is wrong with this design, and why did it sort of work?
It sort of worked for the reasons explained above.
There is plenty wrong with your circuit:
- You are using ground as a return for your neutral current. Ground resistance can vary and can be high. This can give problems with your circuit.
- Safety: if the ground wire becomes disconnected from your ground connection the wire will be live. This provides a potentially lethal shock hazard.
Additional information:
simulate this circuit
Figure 2. Supply transformer supply and earthed appliance.
Yes Live is the supply, Neutral is the return and the Earth is protection. Normally there is no current in the earth wire but if, for example, a live wire fell off inside the appliance and contacted the metal there is a risk of electrocution. Earthing the appliance prevents the case rising to dangerous voltages. In the event of a severe fault a high current may flow but the fuse will then blow, making the system safe.
Double insulated devices don't need to be earthed so a two-pin plug is adequate.
See also:
In any electrical equipment having single ph 230 v ac supply ,if neutral and earth wire exchanged then what will be the consequence.
Finally, what you are doing is dangerous given your poor understanding. I recommend that you work on low-voltage circuits until your knowledge of electrical theory is improved.
Best Answer
The coupling capacitance in the isolation transformer causes common mode noise voltage from the primary side switcher not the low voltage side. So this Common Mode DC pair extends the CM noise to the laptop. Some have a torridal ferrite to absorb some of the CM signal but when the laptop is floating there is not much attenuation unless RF caps were used to the primary earth ground. This current is limited by the Earth ground leakage test limits often 250uA or 500uA depending on country. So the winding coupling capacitance often radiates with line frequency modulated 50kHz or whatever switching rate. Then external unbalanced mic’s often induce some of this E field into the high CM input impedance and causes nonlinear conversion to differential mode noise.
Earthing gnd shunts and attenuates the CM high impedance Signal to low levels. That is simply an impedance divider.
This also works with any VGA video cable as this cable has earth ground from the 3 pin power plug.
The only risk is a ground fault to the grid could energize the metal edges of the laptop case momentarily or an open ground elsewhere but to a machine with high ground current may pass to a user touching a good ground.
A better solution would be to have a perfectly balanced high inductive CM choke with RF caps to earth ground to both shunt voltage but at lower CM noise current levels But this adds great expense it seems so it is not done for the few that might benefit from it.