I am trying these days to learn how to repair my laptops, PC, cellphones radio and antenna and other electronics devices. I wrote few notes already about how to check electronics components with multimeter, how to check SMD capacitor, how to find short circuit on PCB quickly. Today I wanted to write note on my malfunctioning Dell inspiron laptop. I have download its schematic diagram and wanted to write note on those.
To begin with below is picture of the PCB of my Dell Inspiron laptop main motherboard.
The following is the circuit schematic of the power supply from 19V adapter.
This Page 38 of the Dell Inspiron 14z (5423) schematic focuses on the DC power input section—essentially how the laptop receives and manages external power from a 19V AC adapter. This is a crucial part of the power delivery system, as it conditions the incoming voltage and protects the system. This dell laptop circuit diagram show the 19V DC input path from the power adapter (DC jack) to the charger IC and initial power rail filtering and switching. Here's a breakdown of what's on the page:
🧩 Key Sections and Components
1. DC Power Input (DCIN1)
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Connector: DC-JACK255-GP
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This is the input jack for the 19V power adapter.
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AD+ is the 19V rail coming from the adapter.
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2. Resistors and Capacitors for Signal Conditioning and Protection
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PR3814 (100kΩ) – Pull-down or sense resistor for the AD+ line.
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PR3815, PR3816, PR3817 – More resistors forming voltage dividers or pull-ups for adapter detection.
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PC3815, PC3816, PC3817 – Ceramic capacitors for filtering high-frequency noise on the AD+ line.
3. MOSFETs / Power Switches
The component PQ3810 labeled 2N7002K-2-GP on page 38 of the schematic is a MOSFET, specifically:
🔍 PQ3810: N-Channel MOSFET
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Part Number: 2N7002K
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Type: N-channel enhancement-mode MOSFET
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Package: Likely SOT-23 (small surface-mount)
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Use Case in the Circuit: Most likely used as a switch or logic-level control transistor.
In the above DC input/power path section, PQ3810 is:
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Likely controlling or enabling/disabling a path related to the 19V input (AD+)
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It may be used in combination with other components (e.g., resistors like PR3814, and perhaps a P-MOSFET) to form a load switch or adapter detection logic
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The gate is often controlled by a pull-up resistor and logic signal
🧩 Why Use 2N7002K Here?
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It can switch relatively high voltages (up to ~60V)
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It has low gate threshold, suitable for logic-level control
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It's reliable for enabling/disabling signals or power lines
🧠 Example Role in Adapter Detection:
A common setup could be:
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Gate pulled high or low through resistors
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Drain/source used to pull down a signal (like ACOK) when adapter is present or absent
4. Fuse or Protection Devices
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Usually labeled PF** (e.g., PF3801) — providing overcurrent protection.
5. BQ24727 Charger IC (or nearby interface)
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This chip handles battery charging and power switching.
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Inputs from DCIN1 (AD+) and outputs regulated voltages for downstream power rails.
🔄 Signals and Rails
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AD+: Adapter Positive (19V input)
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ACOK, ACDET, or similar: Signals for detecting if the adapter is present and valid
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DCIN1: General name for the adapter voltage net
🧭 Interpretation of the Design
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The circuit detects and filters the 19V input from the adapter.
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It protects against overvoltage or incorrect polarity.
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It then passes the voltage to the charger IC, which decides whether to charge the battery or power the system.
🔍 Summary
The power supply circuit is the entry point for external power into the laptop. It includes:
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The DC input jack
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Protective resistors and capacitors
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MOSFETs for switching and protection
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Connection to the BQ24727 charger IC
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Voltage detection and conditioning circuitry
If you want a trace of how the power moves from this point to the battery or power rails, there are pages on charging system and battery management. Let me know if you'd like help with those too.
