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[ Technology sharing ] Washing machine controller static electricity rectification case

 

 The product description


The product is a clothes-drying machine controller, the power supply voltage is AC 220V, the product is plastic shell (as shown in Figure 1), and the PCB weak current line is unpaved (as shown in Figure 2).


The product has two groups of signals, one is the control signal, the other is the LED lamp signal, the control signal is added with pull resistance and capacitance to the ground, the LED lamp signal line is connected with resistance, no pull resistance and capacitance to the ground.





Electrostatic test conditions


Contact 4KV, air 8KV. Test the control signal line and LED signal line.



The test phenomenon


1. Test the air discharge on the control signal line, and the product will reset;

2.Test the air discharge on the LED signal line, the product will reset.



Problem analysis


1. Static electricity may be transmitted to MCU through signal leading to reset;

2. The pull-up resistance of the signal cable may be transmitted to the power supply, resulting in a reset.




Improvement ideas



1. Parallel ESD (TESD05CB) on the control signal line and LED signal line, test the air discharge, product reset;

2.String magnetic beads on the ground wire, the signal port on the ground and power supply, MCU on the ground isolated, test air discharge, product reset;

3. Replace the ESD on the control signal line with the patch varistor (VESD5R6DG), the product test is normal.




The solution


1. The pressure sensitive resistor (VESD5R6DG) is connected in parallel with the control signal line and the LED signal line.

2.In the later stage, the ground of signal line is separated from that of MCU, and the floor area of PCB working ground is increased.



Conclusion



1. Weak current line floor area should be as large as possible;

2. The protective device with certain capacitance has better absorption effect on static electricity.



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