Converting the power supply from a laptop into regulated laboratory source - one of the most budget-friendly ways to get stable voltage with smooth tuning for amateur radio projects. Unlike factory LBPs (laboratory power supplies) costing from 5,000 ₽, this homemade option will cost 200–500 ₽ if you already have a donor power supply from Dell, HP, Lenovo or Acer. The main advantage is the opportunity to get voltage regulation from 1.5V to 24V with current up to 5–10A (depending on the power supply model), which is enough to power Arduino, amplifiers, LED strips and even small soldering stations.

However, not all laptop power supplies are suitable for conversion. For example, compact adapters at 19V/3.42A (typical for ultrabooks) will not withstand the load due to the weak element base, but full-size power supplies from gaming laptops (eg. Dell PA-12 at 19.5V/11.8A) are ideal candidates. In this article we will analyze 3 working schemes for refurbishment (on TL431, LM317 and a digital PWM controller), we will show how to avoid common soldering errors, and test the finished unit under a real load. If you have never soldered SMD components or worked with high voltage, it is better to practice on less demanding projects.

Which laptop power supplies are suitable for conversion?

Not every power supply can be adapted to a regulated source. Main selection criteria:

  • 🔌 Power from 90W. Weaker units (for example, 45W or 65W) will not be able to produce sufficient current after modification.
  • 🔧 Availability of feedback (FB). Without it, it is impossible to stabilize the output voltage. Looking for a microcircuit TL431 or similar on the board.
  • 📏 Form factor. “Bricks” with a metal body are preferable - they are better cooled and have a current reserve.
  • 🔄 Availability of protection. Blocks with OVP (from overvoltage) and OCP (from overload) safer for experiments.

The best donors for alteration:

PSU model Power Max. current Notes
Dell PA-12 19.5V / 230W 11.8A Easily modified, there is a circuit with replacement resistors
HP 19.5V 9.23A 180W 9.2A Cooling needs improvement
Lenovo 20V 9A 180W 9A Suitable for LM317 circuit
Acer 19V 6.32A 120W 6.3A Budget option, but rather weak for powerful loads

⚠️ Attention: Do not use power supplies from netbooks (such as Asus Eee PC) - their circuits are often simplified, and the maximum current does not exceed 2–3A. Also avoid BP with active PFC (power factor correction) - their modification requires deep knowledge in power electronics.

📊 What power supply do you already have?
  • Dell
  • HP
  • Lenovo
  • Acer
  • Other brand
  • No suitable one

Modification schemes: 3 proven options

There are several ways to make a power supply adjustable. We will consider three most reliable, from simple to complex:

  1. Replacing the feedback resistor (the simplest method, but with a limited range of adjustment).
  2. Adding LM317 (allows you to smoothly adjust the voltage from 1.5V to 30V, but requires soldering skills).
  3. Digital PWM controller (for advanced users, gives precise control and protection).

Let's consider each option in detail.

1. Replacing the feedback resistor ("two resistor" method)

This method is suitable for blocks with TL431 on the board. The bottom line is to replace one of the resistors in the feedback circuit with variable resistor (potentiometer), which will allow you to change the output voltage within ±20% of the nominal value.

Steps:

  1. Find the chip on the board TL431 (usually 3 legs, marked "TL431" or "KA431").
  2. Identify the resistors connected to the pin REF (usually the middle leg). One of them (usually 1–10 kOhm) needs to be replaced with a 10–20 kOhm potentiometer.
  3. Solder the potentiometer in parallel or instead of the standard resistor (depending on the circuit).
  4. Add a voltmeter to the output to monitor the voltage.

⚠️ Attention: This method does not provide a wide range of adjustment. For example, if the initial voltage is 19V, you can only change it within 15–22V. For a larger range, other circuits are needed.

Soldering iron with thin tip (25–30W)|

Solder and flux (preferably no-clean) |

Multimeter for checking voltages|

Potentiometer 10-20 kOhm (e.g. 3296W)|

Electrical tape or heat shrink for insulation -->

2. Circuit with LM317 (range 1.5–30V)

A more universal method is to use stabilizer LM317, which allows you to smoothly regulate the voltage from 1.5V to 30V. The circuit is connected after the main power supply converter and works as a step-down stabilizer.

Procedure:

  1. Remove the output voltage from the power supply (usually 19V) and apply it to the input of LM317.
  2. Assemble the circuit according to the standard LM317 documentation: 
    

    +19V ---[LM317]--- +OUT (регулируемое)
    

    |
    

    240Ω
    

    |
    

    ADJ ---[Потенциометр 5кОм]--- GND
  3. Add a heatsink to the LM317 (at currents above 1A it will heat up).
  4. Install a voltmeter and ammeter at the output.

Advantages of the method:

  • 🔹 Wide adjustment range (from 1.5V to 30V).
  • 🔹 Stable voltage even when the load changes.
  • 🔹 Possibility to limit the current (by adding a resistor to the circuit ADJ).

Disadvantages:

  • 🔸 Voltage drop on LM317 (minimum output voltage ~1.5V).
  • 🔸 Current limitation (maximum 1.5–2A without additional amplifiers).
💡

If you need more than 2A, use LM338 (up to 5A) or LT1083 (up to 7.5A) instead of LM317. The connection diagram is similar, but a more powerful radiator will be required.

3. Digital PWM controller (for experienced ones)

The most advanced option is to use PWM controller (For example, XL4015 or MP2307) to reduce voltage with high efficiency. This method allows you to:

  • 📉 Adjust the voltage from 0.8V to 36V (depending on the controller model).
  • 🔋 Limit the current programmatically (short circuit protection).
  • 📊 Control parameters via the display (if you add Arduino + OLED screen).

Connection diagram:

  1. The power supply output (19V) is fed to the input VIN controller.
  2. Exit VOUT goes to load.
  3. Resistors R1 And R2 set the output voltage (calculation using the formula Vout = 0.8 * (1 + R2/R1)).
  4. A potentiometer is added for smooth adjustment.

⚠️ Attention: When using PWM controllers, be sure to check maximum input current module. For example, XL4015 can withstand up to 8A, but only with good cooling. Also avoid cheap Chinese modules without protection - they can burn out during a short circuit.

Step-by-step assembly instructions (using the example of LM317)

Let's consider the most balanced option - modification of the power supply using LM317. To work you will need:

  • 🔧 Power supply from a laptop (for example, Dell PA-12).
  • 🔧 Stabilizer LM317T (in TO-220 housing).
  • 🔧 Potentiometer 5 kOhm (linear).
  • 🔧 Resistor 240Ω (0.5W).
  • 🔧 Radiator for LM317.
  • 🔧 Voltmeter and ammeter (for example, DSN-VC288).
  • 🔧 Housing for installation (you can use a plastic box).

Step 1: Disassemble the power supply

Carefully open the PSU case. It is usually glued or snapped together. Be careful - there may be some inside high voltage capacitors (even after disconnecting from the network!). Wait 10-15 minutes before working.

Step 2: Find Output Pins

Find the points on the board where the output wires are soldered (usually thick traces marked +V And GND). Solder wires to them to connect to LM317.

Step 3: Assembling the LM317 Circuit

Assemble the circuit on a breadboard or mounting panel:



+19V (от БП) ---[LM317]--- +OUT (на нагрузку)


|


240Ω


|


ADJ ---[Потенциометр 5кОм]--- GND (от БП)

Step 4: Mounting the Voltmeter and Ammeter

Connect DSN-VC288 or similar module to the output of the circuit. Observe polarity strictly!

Step 5: Testing

Connect a load (for example, a 12V/21W car lamp) and smoothly rotate the potentiometer, observing the voltage changes. If LM317 gets hot, add a heatsink or reduce the current.

💡

Always start testing with the minimum voltage (1.5V) and gradually increase it. Never connect a load of more than 50% of the maximum current of the power supply unit when starting up for the first time!

Common mistakes and how to avoid them

Even experienced radio amateurs make mistakes when modifying the power supply. Here are the most common:

  • ⚡ Output short circuit. Always test the circuit with a multimeter in test mode before turning it on for the first time.
  • 🔥 Overheating LM317. If the stabilizer heats up above 60°C, add a heat sink or reduce the current.
  • 📉 Unstable voltage. The reason is poor contact in the feedback circuit or incorrectly selected resistors.
  • 💥 PSU failure. Often occurs when connecting a load above the maximum current. Always use safety!

⚠️ Attention: If, after modification, the power supply begins to “squeak” or emit a high-frequency whistle, this is a sign unstable feedback. Unplug it immediately and check the circuit around TL431 or LM317. Often the problem is solved by adding a 100nF ceramic capacitor in parallel with the potentiometer.

What to do if the power supply does not turn on after modification?

If the power supply shows no signs of life, check:

1. Integrity of the fuse (usually located at the 220V input).

2. There is a short circuit at the output (disconnect all connected devices).

3. Correct soldering (especially around TL431 and LM317).

4. Voltage at the stabilizer input (should be ~19V).

If the problem is not solved, the key transistor or diode in the primary circuit may have burned out. In this case, it is easier to find another donor BP.

Testing and calibration of the finished power supply

After assembly, it is necessary to check the operation of the unit in different modes. Here is the minimum set of tests:

  1. Idling: Measure the output voltage without load. It must be stable (±0.1V fluctuations are acceptable).
  2. Minimum load: Connect a 10Ω/10W resistor. The voltage should not drop by more than 0.2V.
  3. Maximum load: Connect a load that consumes 70–80% of the maximum current of the power supply. For example, for Dell PA-12 (11.8A) is ~8–9A. Watch the LM317 heat up!
  4. Short circuit test: Briefly (no more than 1 second) close the output. The unit must either turn off or limit the current (if there is protection).

For calibration:

  1. Connect an accurate voltmeter (eg UNI-T UT61E).
  2. Set the desired voltage with the potentiometer.
  3. Fix the position of the handle with a marker or add a scale to the body.

⚠️ Attention: If during testing you notice that the voltage "floats" when the load changes, add a 1000uF/35V electrolytic capacitor to the output of LM317. This will smooth out the pulsations.

Improvements for professional use

If you plan to use the power supply regularly, there are a few upgrades worth adding:

  • 📊 Digital indicators. Replace the analog voltmeter/ammeter with DSN-VC288 or YB27VA for precise measurements.
  • 🔄 Current adjustment. Add a second potentiometer and circuit to LT3080 to limit current.
  • 🔌 Universal connectors. Set to output banana nests, USB Type-C (for powering gadgets) and crocodiles.
  • 🛡️ Additional protection. Add a fuse to the output and a relay to automatically shut down if it overheats.

For those who want full automation, you can integrate Arduino Nano with OLED display and an encoder. This will allow:

  • Save voltage/current presets.
  • Display consumption graphs.
  • Control the unit via Bluetooth (via the module HC-05).
💡

To power the Arduino from the power supply itself, use a step-down DC-DC converter AMS1117-5.0to get stable 5V from 19V.

FAQ: Frequently asked questions about converting a laptop power supply

Is it possible to make an adjustable power supply from a 65W PSU?

Technically yes, but the maximum current will be limited to ~3–4A, which is only suitable for low-power loads (for example, Arduino or LED strips). For a soldering iron or power amplifier, it is better to use a power supply from 120W.

How to check if my power supply has feedback (FB)?

Look for the chip on the board TL431 or similar (for example, KA431, TLV431). Also pay attention to the optical isolation (optocoupler, for example, PC817) - it is often used in the FB chain.

What should I do if, after modification, the power supply produces a voltage higher than the rated voltage?

This is a sign of a faulty feedback circuit. Check:

  1. Correct soldering of resistors around TL431.
  2. The integrity of the tracks (perhaps there is a break somewhere).
  3. There is a short circuit at the output.

If the problem is not solved, try replacing TL431 to a new one.

Can this power supply be used to charge a car battery?

No, it's dangerous! To charge the battery you need specialized charger with current and voltage control (usually 14.4V for lead batteries). A homemade power supply based on LM317 does not have reverse polarity protection and may fail.

How to increase the maximum output current?

There are several ways:

  1. Use a more powerful stabilizer (for example, LT1083 instead of LM317).
  2. Add an external transistor amplifier (circuit at BD139/BD140).
  3. Connect two LM317s in parallel (but this requires precise selection of resistors).

Remember that the maximum current is still limited by the capabilities of the original power supply!