Power Supply
Introduction : Why is the power supply considered the eye of the technician in the world of phone maintenance?
In the fast-paced world of mobile phone maintenance, a power supply (or laboratory power supply) is an indispensable tool, serving as the "eye of the technician," revealing the hidden secrets of the board and its faults. Whether you're a beginner or a professional maintenance technician seeking to advance your diagnostics, understanding how to use this device effectively is key to providing quick and accurate solutions.
A power supply acts as a smart replacement for a battery, supplying the phone's board with the necessary power and, more importantly, monitoring and recording the current draw (amps) consumed by the various circuits. These readings, displayed on the device's screen, serve as the board's language, telling us about its status: Is there a short circuit ? Is the device trying to boot but failing? Is there a power leak?
In this comprehensive guide, we'll delve into the ins and outs of using a power supply to diagnose mobile phone faults, starting with the basics of wiring, analyzing various amperage readings, and finally gaining a deeper understanding of common faults and how to accurately identify them.
Chapter One: Basics of Using a Power Supply
Before diving into the troubleshooting, let's make sure you're familiar with how to properly connect and operate the device:
1. Voltage adjustment:
The power supply voltage is typically set to a value very close to the phone's normal battery voltage. Common values are 3.7 volts (for incompletely charged batteries or some older phones) or 4.2 volts (for fully charged batteries).
Important tip : Always start with a lower voltage and gradually increase it if necessary, to avoid damaging delicate components.
2. Connecting to the board:
The power supply has two main wires : a positive wire (usually red) and a negative wire (usually black).
The positive wire is connected to the positive terminal of the battery input on the phone board (positive battery pin).
The negative wire is connected to the negative terminal of the battery input on the phone board (negative battery pin).
Ensure the connection is tight and secure to avoid any sudden power failure that may affect the diagnosis.
3. Monitor the device screen:
After connecting, monitor the power supply's display. You'll see an initial reading of the current drawn (amps). This initial reading is the starting point for your diagnosis.
Chapter Two: Understanding Ampere Readings: Board Language You Should Know
Accurate diagnosis depends on understanding what the numbers you see on the power supply screen mean. These readings change depending on the phone's condition and operating stage. We'll divide them into three main categories:
Category 1: Automatic withdrawal (before pressing the power button) - an indication of immediate problems
Here, current is drawn as soon as the power supply is connected to the board, even before the power button is pressed. This often indicates a serious problem that requires immediate attention.
Condition: Very High Draw (Full Short)
Amperage reading : Continuous, very high current draw, often exceeding 1000 milliamps (1 amp) or more. The device may emit a warning beep.
Diagnosis : This means there is a "hard short" (direct short) in one of the main paths that feeds directly from the battery (such as the V_BAT line or other main feed lines).
Possible components : The culprit is often a small capacitor connected to the main power supply, the PMIC itself , or even the processor ( CPU ) in rare cases.
Action : Disconnect the power supply immediately to avoid damaging components. Start by inspecting components near the power inlet, using a microscope to identify swollen or charred components.
Condition: Simple and continuous withdrawal ( Leakage )
Amperage Reading : Low, constant current draw, typically between 20 and 100 milliamps.
Diagnosis : This indicates a leak in the charge. This means that a component is constantly drawing power even when the phone is idle, causing the battery to drain quickly.
Possible components : It could be a leaky capacitor, a certain IC that isn't going into sleep mode properly, or even a problem with the lighting or audio circuit.
Procedure : This type of fault requires a careful examination of the components, and you may need to use techniques such as "freezing" (using a coolant) to determine which component is leaking power.
Category 2: Pulling after pressing the power button (No Power)
In these cases, there is no significant current draw before the power button is pressed, indicating that the primary circuits are intact. The problem appears after attempting to power on the device.
Status: No draw at all (0 mA)
Ampere reading : 0 milliamps exactly after pressing the power button.
Diagnosis : This indicates a problem with the power button path itself or with the primary boot circuit.
1. The power button is broken: The button itself may be damaged, or its path on the board may be broken.
2. Power IC failure : The power IC (PMIC) is responsible for starting the processor and other components. If it is faulty, the circuits will not be triggered for output and no current will be drawn.
Procedure : Check the power button and its paths, then check the power IC and surrounding components.
Condition: Very light and stable draw (10-30mA)
Ampere reading : Very low and constant draw that does not change after pressing the button.
Diagnostics : This means that the power IC may be partially working, but is not outputting the voltages needed to power the main components such as the processor (CPU) or memory ( eMMC/UFS ).
Possible components : A faulty primary power circuit within the power IC, or a problem with the processor or memory power lines.
Procedure : Careful inspection of the power IC and the power lines coming out of it is required.
Condition: Simple, fluctuating draw (10-20 mA then returns to zero )
Ampere Reading : The pressure rises slightly, then drops rapidly, returns to zero, and repeats this cycle.
Diagnosis : This pattern often indicates a problem with the memory IC (eMMC or UFS) or the paths connecting it to the processor. The processor attempts to read instructions from memory, but fails and then tries again.
Possible components : The memory IC itself, or problems with the data or clock lines between the memory and the processor.
Action : This may require re-soldering the memory IC, or in rare cases replacing it.
Condition: Very high and stable draw (300-800mAh)
Amperage reading : Steady high draw after button press, but not a complete short.
Diagnosis : This indicates a "short after boot." This means that a component or circuit began to power up after the button was pressed, but it continually draws too much current, preventing the device from booting normally.
Possible components : It may be a fault in the power IC itself, the processor (CPU), or one of the secondary power lines that the power IC outputs to operate other components (such as the audio circuit, Wi-Fi, etc.).
Procedure : Requires careful inspection of the components being fed after takeoff.
Category 3: Partial Boot
Here, the phone starts to boot up, but stops at a certain stage, or restarts constantly.
Status : Stop at first logo (e.g. manufacturer logo)
Amperage reading : The draw reaches a certain value (e.g. 100-300mA) and then stabilizes or drops slightly without completing take-off.
Diagnosis : This often indicates a problem with the internal memory (eMMC/UFS) or the boot files stored on it. There may be a corruption in a portion of the memory or in the operating system.
Action : This may require reprogramming the device (software) or in more complex cases, there may be a problem with the memory itself.
Status : Continuous Restarts
Amperage Reading : Shows a normal draw for startup, then suddenly drops to zero or the device starts restarting repeatedly.
Diagnosis : This condition can be caused by several factors:
- Software problem : The operating system is corrupted or faulty.
- Supply voltage problem : One of the important supply lines (such as lighting voltage, data voltage) has a problem or is out of order.
- Thermal Failure : A major component overheats too quickly, causing it to shut down as a precaution.
Action : Start by checking the software. If the problem persists, check the power lines and the components they supply.
Status: Normal draw then rises at the second logo
Amperage reading : Normal draw at start-up, and when the second logo appears (like the OS logo), the draw increases significantly.
Diagnosis : This often indicates a faulty power supply IC (PMIC) or the power supply circuits that control it. Power consumption increases significantly when attempting to load the system interface.
Procedure : Check the power IC and its associated components that are activated at this stage.
Very important note for professional technician:
Readings are indicators, not strict rules!
It is very important to understand that the exact amperage values you see on the power supply screen vary greatly from phone to phone (between Android devices, iPhones, and even between different models of the same company).
Experience is key : A professional technician relies on his accumulated experience and on comparing the current draw of a faulty phone with that of a working phone of the same make and model.
Use a microscope : Always use a microscope to closely examine suspected components.
Don't be afraid to experiment (carefully) : In some cases, you may need to try disconnecting certain components (such as small capacitors) to see if the draw returns to normal.
Chapter Three: Types of Power Supplies Available
When choosing a power supply, you will find different types, each with its own advantages:
Basic Models: Provide basic voltage and amperage readings, great for beginners and common faults.
Advanced Models: May contain additional features such as:
- High-resolution digital displays: for more accurate readings.
- Built-in overcurrent protection : to protect the board from damage.
- Additional USB ports : For charging or testing devices.
- Additional functions : such as resistance measurement or diode testing.
- Programming Interfaces : For some specialized devices.
Tip : Start with a device that meets your current needs, and you can upgrade later as your skills develop.
Chapter Four: Additional Tips to Increase Diagnostic Accuracy
- Board cleanliness : Make sure that the phone's board is clean and free of any solder residue or dirt that may affect the current readings.
- Check the battery : Sometimes, the battery itself may be the cause of abnormal current draws. Try using another healthy battery if possible.
- Using a Multimeter : The power supply tells you the current draw, but a multimeter (like the Fluke 17B+ we'll mention later) is necessary to measure voltages and resistances in specific paths.
- Documentation : Keep a record of the amperage readings for the various faults you encounter. This will help you in the future.
Chapter Five: Multimeter - Power Supply Partner
While the power supply reveals what's happening in terms of power consumption, the multimeter tells you "why" in terms of voltages and resistances.