Converting a laptop power supply into an adjustable power supply: from theory to practice

Introduction: Why a Laptop Power Supply is the Ideal Base for Modification

Old laptop power supplies often gather dust on shelves, although they can become the basis for a useful device. Their compact size, high power (usually 65-120 W) and stabilized output voltage make them ideal candidates for conversion into regulated power supplies. Unlike homemade circuits on LM317, such blocks already contain high-quality filters, protection against short circuits and overheating - all that remains is to add adjustment.

The main advantage of this approach is price/quality ratio. A new laboratory power supply with similar characteristics will cost 3-5 thousand rubles, while modification of an old power supply will require an investment of only 200-500 rubles for radio components. In addition, many laptop units (for example, from Dell PA-10, HP 65W or Lenovo 90W) have unified schemes, which simplifies the revision process.

In this article we will look at a unique conversion method using pulse width modulation (PWM) based on the TL494 chip, which allows you to smoothly regulate the voltage from 0 V up to maximum (usually 19-20 V) without power loss. This approach compares favorably with traditional linear regulators, where excess energy is dissipated as heat.

Which power supplies are suitable for conversion: selection criteria

Not every laptop power supply can be successfully modified. Here are the key parameters to pay attention to:

• 🔌 Circuit type: pulse units based on TL494, KA7500 or similar PWM controllers. Linear units (a rarity for laptops) are not suitable.
• 📊 Maximum power: blocks on 65-90 W give more room for experimentation than low-power ones (30-40 W).
• 🔄 Availability of feedback: there must be an optocoupler on the board (usually marked PC817, EL817) - it is responsible for stabilizing the voltage.
• 🔧 Constructive: blocks in a collapsible metal case are preferred (for example, Delta ADP-65JH or Lite-On PA-1900-02).

Top candidates among popular models:

Power supply model	Power	Max. current	Features
Dell PA-10	90 W	4.62 A	On TL494, easy to modify
HP 65W (HSTNN-LA02)	65 W	3.33 A	Compact, but requires feedback improvements
Lenovo 90W (ADP-90RH)	90 W	4.74 A	Good short circuit protection, but difficult to disassemble
Acer ADA090NE	90 W	4.74 A	Suitable for beginners, little interference

⚠️ Attention: power supplies from Apple (for example, series MagSafe) are not suitable for such modification due to proprietary communication protocols between the unit and the laptop. Their schemes are very different from the standard ones.

Before purchasing or disassembling, check the unit for functionality: connect it to a laptop or measure the output voltage with a multimeter. If the unit “hums” or heats up without load, most likely it is already partially faulty and is not worth the effort.

Required components and tools for modification

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

• 🔧 Soldering iron (power 40-60 W) with a thin tip and solder.
• 🔍 Multimeter for measuring voltage and testing circuits.
• ⚡ Potentiometer on 10-20 kOhm (preferably multi-turn for fine tuning).
• 📉 Resistors: 1-2 kOhm (for voltage divider) and 220 Ohm (for indication LED).
• 💡 LED (any color) with a current-limiting resistor.
• 🔄 Variable resistor (if you want smooth current control).
• 🛠️ Electrical tape/heat shrink for contact insulation.
• 📏 Wires cross-section not less 0.75 mm² for output terminals.

Additionally you may find it useful:

• 🔨 Drill or needle file for making holes for the potentiometer in the housing.
• 🧲 Magnetic screwdriver for working with small screws inside the block.
• 📈 Oscilloscope (optional) to check the quality of stabilization.

The cost of all components (except for the soldering iron and multimeter) rarely exceeds 300-400 rubles. If you already have a basic ham radio kit, the cost will only come down to purchasing a potentiometer and resistors.

⚠️ Attention: Never use for modification power supplies with a damaged case or traces of melting inside. Even if they are still working, the risk of short circuit or electric shock is much higher.

Step-by-step instructions: modifying the power supply to TL494

Let's look at the process using the example of a popular block Dell PA-10 (90 W) with chip TL494. Blocks from HP, Lenovo and other manufacturers on the same chip.

Step 1: Disassembly and Finding Key Items

Carefully disassemble the unit body; it is usually secured with latches or Phillips screws. Find inside:

• 🔹 Microcircuit TL494 (eight-legged, marked 494 or KA7500).
• 🔹 Feedback optocoupler (usually next to the TL494, in a black case with 4 legs).
• 🔹 Output diode Schottky (large, on the radiator) - no need to replace it.
• 🔹 Resistors connected to the 1st leg of the TL494 (this is a feedback divider).

Take photographs of the board from all sides - this will help with reassembly. Pay attention to the tracks leading from the 1st leg of the TL494 to the optocoupler - they will have to be modified.

Step 2: Feedback Circuit Modification

The main idea is to replace the fixed voltage divider with adjustable. To do this:

1. Unsolder the resistor connecting the 1st leg of the TL494 to the optocoupler (usually a resistor on 10-20 kOhm).
2. Solder in its place potentiometer 10-20 kOhm, the middle pin of which is connected to the 1st leg of the TL494, and the outermost pins to the feedback circuit (one to the optocoupler, the other to ground).
3. Add a constant resistor in parallel to the potentiometer 1-2 kOhm to limit the minimum voltage (otherwise, when the potentiometer is turned to its extreme position, the voltage will drop to zero and the unit may not start).

Connection diagram:

TL494 (ножка 1)
|
✠ (потенциометр 20 кОм)
|
┌─┴─┐
│ 1k│
└───┘
|
Оптрон → обратная связь

Step 3: Refinishing the body and installing controls

Drill a hole in the block body for the potentiometer handle. To do this:

• Mark a spot on the top or side cover.
• Use a drill with a diameter 6-8 mm (depending on potentiometer size).
• Clean the edges of the hole with a file to ensure there are no burrs.

Attach the potentiometer to the board so that its axis extends into the drilled hole. For reliability, you can use hot melt glue or epoxy resin.

Step 4: Testing and Settings

Before turning on for the first time:

• Check all solder connections for short circuits with a multimeter.
• Connect an incandescent lamp to the output of the unit 12 V (as a load).
• Set the potentiometer to the middle position.

Connect the unit to the network. If everything is done correctly, the output voltage will be regulated from ~2 V up to maximum (usually 19-20 V). If the unit does not start or makes a squeaking sound, check the feedback circuit for breaks.

Additional improvements: protection and functionality

The basic modification allows you to regulate the voltage, but for a full-fledged laboratory power supply it is worth adding a few improvements.

1. Voltage and current indication

The easiest way is to connect voltmeter and ammeter based on Chinese modules (for example, DSN-VC288). They can be purchased for 150-200 rubles on AliExpress. Connection:

• 🔴 Voltmeter: parallel to the block output terminals.
• ⚡ Ammeter: in series with the load (via a shunt or directly if the module supports current measurement up to 5 A).

For accurate measurements, calibrate the modules using a reference multimeter.

2. Current limit (CC/CV function)

So that the unit can operate in mode current stabilization (useful for charging batteries), add a second potentiometer to the current feedback loop. To do this:

1. Locate the current limiting resistor on the board (usually located next to the output diode Schottky).
2. Replace it with a potentiometer 0.5-1 kOhm or add in parallel to the existing resistor.
3. Set the maximum current so that it does not exceed 80% from the nominal (for example, for a block on 4.74 A set a limit in 3.5-4 A).

3. Reversal protection

Add a diode to the output of the block Schottky (For example, SB540) in reverse switching. This will protect the circuit if the polarity is accidentally reversed when connecting a load. The diode must withstand a current of at least 5 A.

4. Cooler for cooling

If the unit will operate at maximum power for a long time, install a small fan (40×40 mm) on 12 V. It can be powered from a separate stabilizer on 7812 or directly from the output of the unit through a resistor to reduce the speed.

Finalization	Difficulty	Cost (RUB)	Necessity
Voltage/current indication	Low	150-200	Recommended
Current limit	Average	50-100	Preferably
Reverse polarity protection	Low	30-50	Required
Cooler	Low	100-150	Optional

Testing and calibration: how to check the operation of the unit

After assembly, you need to check the unit in different modes. Here's a step-by-step test plan:

1. Checking idle speed: Connect the unit to the network without load. The voltage should be adjusted smoothly, without surges. If the unit turns off, check the feedback circuit.
2. Load test: Connect the resistor with power 10 W on 10 ohm (current ~1.5 A at 15 V). The voltage should not "sag" by more than 0.5 V.
3. Checking short circuit protection: Briefly short-circuit the output terminals. The unit should turn off or limit the current (if the CC circuit has been modified).
4. Heat test: Run the unit under load for 30 minutes 2-3 A. The case should not heat up above 60°C.

To calibrate the voltmeter and ammeter:

• Connect a reference multimeter in parallel with the voltmeter and calibrate it with a trimmer resistor (if available).
• For an ammeter, use a load with a known current (for example, a lamp 12 V / 2 A) and adjust the readings.

If the unit behaves unstable (for example, the voltage “floats”), check:

• 🔹 Quality of solder connections (especially around TL494).
• 🔹 Presence of parasitic connections (wires should not intersect with high-voltage circuits).
• 🔹 Capacity of the output capacitors (if they are swollen, replace them).

⚠️ Attention: when testing blocks with a power of more than 60 W Use a forced-cooling load (for example, a fan directed at a resistor). Overheating may cause a fire!

What to do if the unit does not start?

1. Check the input fuse (it often blows during a short circuit).

2. Make sure that the potentiometer is not set to the minimum voltage position (turn it to the middle position).

3. Check the power circuit of the TL494 - on the 12th leg there should be ~15 V relative to ground.

4. If the unit “squeaks”, the overvoltage protection may have tripped. Check the divider on the 1st leg of the TL494.

Common mistakes and how to avoid them

Even experienced radio amateurs sometimes make mistakes when modifying power supplies. Here are the most common ones and ways to prevent them:

• 🔥 Overheating of output diodes: Occurs when operating at maximum current without cooling. Solution: Install diodes on the radiator or add a cooler.
• 📉 Unstable voltage: Usually caused by poor filtering or long output wires. Solution: add capacitors 1000 µF to the output and shorten the wires.
• ⚡ Spontaneous shutdown: May be due to incorrect feedback setting or overload. Solution: Check the circuit around the 4th leg of the TL494 (overvoltage protection).
• 🔌 Poor potentiometer contact: leads to voltage “jerking”. Solution: Use a multi-turn potentiometer or clean the contacts with alcohol.

Another common problem is noise in the output voltage. They appear as ripples on an oscilloscope or flickering LEDs. Causes and solutions:

Cause of interference	How it manifests itself	Solution
Weak filtration	High frequency noise (~50 kHz)	Add an LC filter to the output
Unstable feedback	Low frequency vibrations (1-10 Hz)	Increase the capacitor capacity in the feedback circuit
Bad wiring	Pulse surges when load changes	Shorten the feedback wires

If you encounter a problem not described above, try searching for a solution in datasheet for TL494 (section Troubleshooting). Often malfunctions are associated with improper operation of the internal comparator of the chip.

Practical application of a modified power supply

An adjustable power supply from a laptop will find use in a variety of tasks:

• 🔋 Charging batteries: from Li-ion 18650 to lead acid 12 V. Mode CC/CV (if modified) is ideal for this.
• 💡 Power supply for LED strips: You can smoothly adjust the brightness by connecting the tapes to 12 V or 24 V.
• 🎮 Powering homemade devices: from Arduino projects to audio amplifiers. Stabilized voltage will protect sensitive electronics.
• 🔧 Equipment repair: checking boards, testing motors and other components without the risk of damaging them due to unstable voltage.
• 🚗 Automotive electronics: powering the on-board computer or testing relays when repairing a car.

Examples of specific applications:

• Power supply Raspberry Pi or Orange Pi with the ability to adjust voltage for overclocking.
• Food 3D printer (if the block is powerful enough - from 120 W).
• Creation adjustable soldering iron based on a nichrome spiral.

To power sensitive electronics (such as audio amplifiers), it is recommended to add a additional linear stabilizer (For example, LM317) to reduce interference.

FAQ: answers to frequently asked questions

Is it possible to modify a power supply without a soldering iron?

Theoretically, yes, but it is extremely inconvenient and unreliable. The minimum required soldering iron is:

• Feedback resistor taps.
• Soldering the potentiometer.
• Voltmeter/ammeter connections.

An alternative is to use alligator clips, but they can cause vibration to break contact.

How to increase the maximum block current?

The maximum current of the unit is limited by the output diodes and transformer. You can increase it:

1. Replacing the output diodes Schottky to more powerful ones (for example, SBR10U45SP5 on 10 A).
2. By increasing cooling (adding radiators or a cooler).
3. By connecting a second similar block in parallel (but this requires PWM synchronization).

However, exceed the rated current by more than 20% not recommended - it shortens the life of the components.

Why does the voltage change abruptly during adjustment?

This is a common problem when using a single turn potentiometer. Solutions:

• Replace the potentiometer with multi-turn (For example, 10 kOhm / 10 turns).
• Add a capacitor in parallel with the potentiometer 10-100 nF for smoothing.
• Check the quality of the contacts - sometimes surges are caused by oxidation of the tracks.

Is it possible to make a charger for a car battery from a laptop power supply?

Yes, but with reservations:

• The block must produce at least 14.4 V (for Pb- batteries).
• It is necessary to modify the current limiting circuit (maximum 5-10% of battery capacity, for example, for 60 Ah - no more 6 A).
• Add reverse polarity protection (diode Schottky at the exit).

For a full-fledged charger, it is better to use specialized circuits (for example, on LM358), but for emergency recharging a modified unit is suitable.

How to protect the unit from moisture and dust after modification?

After modification, the block becomes more vulnerable to external influences. Tips:

• Cover the board with a thin layer varnish for electronics (For example, Urushiol).
• Seal the vents dust net with small cells.
• If the unit will be used in wet conditions, place it in sealed housing with cable entry.
• For additional protection, install silica gel pack inside the case.