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5V power adapter without voltage output repair tips

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5V power adapter without voltage output repair tips

Symptom: No power is output from a 5V power adapter, and the indicator light is off. Analysis and maintenance: According to the analysis of the fault phenomenon, it indicates that the power supply is abnormal. After disassembly, check that the skin of the current-limiting resistor R1 (1Ω / 0.5W) at the mains input terminal has cracked, and confirm that it is open after testing, indicating that there is an overcurrent phenomenon. In order to facilitate maintenance, a schematic diagram is drawn according to the actual circuit board (labeled KS-518 on the circuit board), as shown in the accompanying drawings.

It can be seen from the drawings that this is a parallel switching power supply composed of discrete components. On the road, check that the switching tube Q1 (13003F) has broken down, the overcurrent detection resistor R5 (5.6Ω / 0.5W) is open, and the junction of the control tube Q2 (C945) is short-circuited. Then, it is found that the photocoupler ICl (317I ) ③, ④ The forward and reverse resistance between the feet is about 1.6kΩ (normally should be ∞). What causes the above components, especially ICl, to be damaged?
It is found in the analysis that the C pole of Q2 is connected to the primary coil ② of the switching transformer T, and the C pole of Q2 is not connected to the anti-peak suppression circuit. This part of the circuit board has a position number and a mounting hole, as shown in the virtual box in the figure. , But no component is installed, it is estimated that the fault is related to this.
C4, R6, and D6 in the virtual frame circuit are completed with 4700P / 1kV, 220kΩ, and FRl07, and the test is performed after the damaged components are replaced. The 5V voltage across C5 is stable. Repeated power failure and power on. It is damaged, and it is not hot to touch Q1 and Q2, indicating that the fault of the power supply has been eradicated. So far, the failure has not been reproduced.

[Tips] When charging, you should first connect the load and then power on. After charging, you should unplug the charger and then disconnect the load. Try to avoid no-load power or disconnect the load when the utility power is too high.

A power adapter emits a "squeak" sound when working, affecting the user's work mood.

Maintenance process: Generally speaking, it is normal for the power adapter to have a weak working noise, but if the noise is disturbing, it is a malfunction. Because the power adapter only causes a squeaking when there is a large gap between the switching transformer or the magnetic ring of the inductor and the coil. After disassembling the adapter, gently turn the coils on the two inductors by hand without power. If there is no loose feeling, you can be sure that the working noise source of the adapter comes from the switching transformer.
The methods to eliminate the “squeaking” sound of the switching transformer are:
(1) Re-weld the connection pins of the switching transformer with the printed circuit board with an electric soldering iron. When welding, press the switching transformer against the circuit board by hand so that the bottom of the switching transformer is in close contact with the circuit board.
(2) Insert a suitable plastic sheet between the magnetic core and the coil of the switching transformer or seal it with epoxy resin.
(3) Insert a hard paper or plastic sheet between the switching transformer and the circuit board.
In this example, the first method was not effective, and the switching transformer was removed from the circuit board. The second method was used to try and the "squeak" sound was eliminated.


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| Release time: 2019.12.25 Source:
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Dongguan Qiqi Industrial Co., Ltd. specializes in producing: power adapters , chargers, LED drive power supplies, car chargers, switching power supplies, etc ...