Three Common Types of Power Adapter Failures
May 09, 2025
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Generally, power adapter failures can be categorized into three types:
High-voltage failure.
Low-voltage failure.
Both high-voltage and low-voltage failures.
High-voltage failure is primarily characterized by the indicator light not turning on. Common symptoms include a blown fuse, a short-circuited rectifier diode (D1), a bulging or cracked capacitor (C11), a damaged Q1, an open R25, a short between pin 7 of U1 and ground, or an open R5 preventing U1 from receiving startup voltage. Replacing these components usually fixes the issue. If pin 7 of U1 has a voltage above 11V and pin 8 has 5V, U1 is likely functioning normally. In this case, check for poor soldering on Q1 and T1 pins. If Q1 repeatedly fails without overheating, the issue is typically due to a faulty D2 or C4. If Q1 burns out and overheats, the cause is usually a leak or short in the low-voltage section, excessive current, or an abnormal pulse waveform from pin 6 of UC3842, leading to excessive switching loss and heat buildup in Q1.
Other high-voltage failure symptoms include flickering indicator lights and unstable, low output voltage, often caused by poor soldering on T1 pins or open circuits in D3 or R12, cutting off the power supply to TL3842 and its peripheral circuits.
A rare high-voltage failure involves the output voltage exceeding 120V, usually due to U2 failure, an open R13, or a shorted U3 pulling pin 2 of U1 low, causing pin 6 to send excessively wide pulses. In such cases, prolonged power-on should be avoided to prevent severe damage to the low-voltage circuit.
Low-voltage failure is mostly caused by reverse connection of the charger to the battery terminals, burning R27 and damaging LM358. Symptoms include a constant red light, no green light, low output voltage, or near-zero output. Replacing these components typically resolves the issue.
Additionally, if W2 is unstable, the output voltage may drift. Excessively high output voltage can overcharge the battery, causing severe water loss, overheating, and eventual thermal runaway (potentially rupturing the battery). Low output voltage, on the other hand, leads to undercharging.
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For both high and low-voltage failures, a thorough inspection of all diodes, transistors, optocouplers (e.g., 4N35), MOSFETs, electrolytic capacitors, ICs, and resistors (especially R25, R5, R12, R27, D4 (16A60V fast-recovery diode), and C10 (63V, 470μF)) should be performed before powering on to avoid expanding the fault range.
Some chargers feature anti-reverse-connection and anti-short-circuit protection by adding a relay at the output. In cases of reverse polarity or short circuits, the relay disengages, cutting off output. Others use a different approach, where the low-voltage circuit's startup power is supplied by the battery itself, with a diode preventing reverse connection. Once the adapter starts normally, it takes over the low-voltage power supply.