This instruction will help you convert the power source into an adjustable 3-25 V. If you have a 19 V power supply from a laptop or a 12 V LED garland unit, then all such sources can be converted into adjustable ones, and it is easy to set any output voltage by rotating the variable resistor.
Will need
- Two capacitors 470 uF 25 V.
- Variable resistor 10 kOhm.
- Resistor 2.2 kOhm.
Converting a fixed voltage power supply to an adjustable voltage source
The entire switching power supply board appears before us.
We will not touch everything to the left of the blue transformer. This is the high voltage part and we are not interested in it. On the right, the low-voltage part consists of several elements, and that’s what we’ll be finalizing.
Schemes and theory of revision
The block is stabilized by feedback through an optocoupler. This optocoupler is controlled by a TL431 stabilizer chip. It has 3 terminals and looks like a transistor.
One resistor in the optocoupler circuit is limiting, the other two are dividers at the output of the microcircuit. These resistors are clearly visible on the back of the board.
That is, if you change the division factor at the input of the microcircuit, then the output voltage at the output of the power supply will change accordingly.
To do this, you need to replace one resistor and connect a variable instead of the other. Something like this:
Solder the divider resistors.
It is imperative to replace the output capacitors with others with a higher operating voltage.
We also solder them.
We solder new ones.
We solder a 2.2 kOhm resistor, according to the modification diagram.
We take a variable resistor and solder wires to it.
We solder the wires to the board instead of the resistor chip.
Now, very carefully, turn on the unit to the network and check the operation. Let's connect to the output multimeter.
If everything works properly, then we assemble the body. Since there is no additional space in the case, we will move the resistor outside by gluing it on the side with glue.
We check under load. The source is well regulated and produces a voltage in the range of 3.4-21.5 V.
Everything works fine.
A few words about safety precautions
- Before disassembling the unit, if you have just unplugged it, be sure to wait a couple of minutes until all internal capacities are discharged.
- The output voltage, at the maximum position of the variable resistor, should not exceed 25 V, since the output capacitors may fail.To decrease the regulated voltage, increase the resistance of the 2.2 kΩ resistor.
