Converting a laptop power supply into an adjustable voltage source: a complete guide

The power supply from an old laptop is not garbage, but a potentially useful device for a radio amateur. With minimal modifications, it can be turned into an adjustable voltage source with a current of up to 4-6 A, which covers the needs of most homemade circuits, charging batteries or powering LED strips. The main advantage of such a modification is cheap and accessible: blocks from laptops 19V/3.42A or 19.5V/4.62A often sell for pennies on the secondary market.

However, not all blocks are suitable for modification. In this article we will look at how to choose a suitable donor, which circuits to use to regulate the voltage (from 1.2V to 20V), and how to avoid mistakes that lead to device failure. We will pay special attention security: Working with high voltage requires knowledge of nuances that are rarely written about in superficial guides.

Which laptop power supplies are suitable for conversion?

Not every power supply (BP) can be adapted to a controlled source. Main selection criteria:

• 🔌 Power: optimal 60W–90W (For example, Dell PA-10 or HP 65W). Blocks are more powerful 120W often have complex protection schemes that are difficult to bypass.
• 📏 Output voltage: 18.5V–20V. Blocks on 12V (for example, from netbooks) are less versatile.
• 🔧 Connector type: cylindrical connectors with a diameter of 5.5×2.5 mm or 7.4×5.0 mm — they are easier to adapt to crocodiles or bananas.
• 🛠️ Availability of a scheme: blocks with markings Delta, Lite-On or Chicony often have documentation publicly available.

Avoid blocks with active PFC (power factor correction) - they are more difficult to modify due to additional feedback circuits. Ultra-compact units (for example, from MacBook) where the components are too tightly packed to be soldered.

⚠️ Attention: Laptop power supplies Lenovo series ThinkPad (For example, 20V/4.5A) often have non-standard inclusion logic. Without proper signal emulation PS_ON# they will not start even after modification.

Required Components and Tools

To convert the power supply into a regulated source you will need:

• 🔧 Soldering iron (power 40–60W) with a thin tip and solder Sn60Pb40.
• 🔍 Multimeter (required with current measurement mode up to 10A).
• 📉 Buck DC-DC converter: module on XL4015 or LM2596 (for currents up to 5A).
• 🔌 Connectors: alligator clips, banana clips or terminal blocks for voltage output.
• 🛡️ Insulation materials: heat shrink tube, electrical tape, plastic case for installation.

Additionally you may find it useful:

• 🔄 Potentiometer on 10k–50kΩ for smooth voltage regulation.
• 💡 LED with resistor 1kΩ to indicate power on.
• 📊 Voltmeter-ammeter (for example, module DSN-VC288) for visual inspection.

If you plan to use the unit to charge batteries, add to the circuit current stabilizer (for example, on LM317). This will prevent overcharging and extend the life of the batteries.

Remodeling schemes: from simple to advanced

There are three main approaches to modifying the power supply:

1. Direct connection with DC-DC regulation - the simplest and safest option. The block produces a fixed voltage (for example, 19V), which is reduced to the required level by an external converter.
2. Feedback Modification - changing resistors in the circuit FB (feedback) to adjust the output voltage. Requires knowledge of circuit design.
3. Replacing the controller - complete rework of the circuit with installation of a new PWM controller (for example, TL494). Suitable for experienced radio amateurs.

For most tasks, the first option is sufficient. Below is the module connection diagram XL4015 to the power supply Dell PA-10:

Блок питания (+19.5V) → [Вход XL4015]
→ [Выход XL4015] → Нагрузка
→ [GND] → Общий

Important: module XL4015 must be designed for an input voltage of at least 24V (although our block outputs 19.5V, the reserve is needed to protect against surges). Also don't forget about heat sink: at currents higher 3A the converter will heat up.

⚠️ Attention: If you modify the feedback circuit, never disable optocoupler (optocoupler) completely. This will result in loss of galvanic isolation and risk of electric shock.

Step-by-step instructions for assembling an adjustable power supply

Let's look at the rework process using a block as an example Dell PA-10 (19.5V/3.33A) using the module XL4015.

Step 1: Turn on the unit without a laptop

Most laptop power supplies require a power-on signal (PS_ON#). B Dell PA-10 This is the green wire in the loop. To start the unit without a laptop, you need to:

1. Find the green wire and connect it to GND (black wire).
2. Connect a load (for example, a lamp to 12V) for the first test.
3. Measure the output voltage - it should be 19.5V ± 0.5V.

Step 2: Connecting the DC-DC converter

Install the module XL4015 to a heatsink (for example, from an old processor). Connect:

• VIN+ → +19.5V from the block.
• VIN- → GND block.
• VOUT+ → terminal "+" load.
• VOUT- → terminal "-" load.

Step 3: Setting the Output Voltage

Using a potentiometer on the module XL4015 set the desired voltage (for example, 12V). Check stability under load:

1. Connect resistor 10Ω/10W (current ~1A).
2. Measure the voltage after 5 minutes - it should not “float” by more than ±0.2V.

Testing and calibrating a homemade power supply

After assembly you need to check:

1. Voltage stability under different loads (from 0.1A to 3A).
2. Ripple level (should be no more 50mV when measured with an oscilloscope).
3. Short circuit protection: short-circuit the output - the unit should turn off, and after eliminating the short circuit, restore operation.

For calibration use reference voltmeter (For example, UNI-T UT61E). Compare its readings with a cheap Chinese module DSN-VC288 - the difference should not exceed ±0.1V. If the error is greater, calibrate the ammeter/voltmeter using trimmers on the board.

⚠️ Attention: If under load > 2A The power supply starts to overheat, add active cooling (cooler on 12V). Connect it in parallel with the output through a resistor 1kΩso that it only works when the power supply is turned on.

Common mistakes and how to avoid them

Even experienced radio amateurs sometimes make mistakes when converting power supplies. Here are the most common:

• ⚡ Ignoring capacitor discharge. The capacitors in the block can store a charge for days. Always discharge them with a resistor 10kΩ/5W before work.
• 🔥 DC-DC converter overheating. Modules LM2596 without a radiator they fail at currents > 2A. Use XL4015 with heat sink.
• 📉 Unstable voltage. The reason is that the wires from the unit to the converter are too long. Use twisted pairs of length < 15 cm.
• 🔌 No reverse polarity protection. Add a diode 1N5822 to the output if you plan to connect batteries.

Another common problem is output noise. If your power supply is making noise in your speakers or interfering with sensitive circuits, add LC filter:

• Inductor 10–100 µH.
• Capacitors 1000 µF/25V And 0.1 µF (ceramics).

Practical application of the converted power supply

An adjustable laptop power supply can be used for:

• 🔋 Charging batteries: Li-ion (via module TP4056), Pb (with current limitation).
• 💡 LED strip power supply: for example, 12V ribbons up to 5 m.
• 📻 Radio equipment testing: receivers, amplifiers, Arduino projects.
• 🔧 Equipment repair: checking boards, searching for short circuits.

For charging Li-ion It is recommended to add batteries to the circuit:

• Balancer (For example, BMS-fee for 3S or 4S assemblies).
• Timer on NE555 for automatic shutdown by time.

When powering LED strips, do not forget about current limiting resistor (if the tape does not have a built-in driver). For tape on 12V with current 1A a resistor will do 0.5Ω/5W.

FAQ: Frequently asked questions about power supply conversion

Кнопка → [База 2N2222] → [Реле] → [Цепь PS_ON#]