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You now datasehet a voltage divider which halves your voltage. There’s no real secret to that. A simple variable power supply based on the LM You should Sign Up.

Let’s first understand how a switching power supply works. Modifying a notebook power supply Modification of a notebook power supply to output a desired voltage. I didn’t have any good values to change only one resistor of the divider, but I’ve found that R42 and R43 unpopulated eatasheet the time were in parallel to R In this pagewe can see how to use a variable output power supply to identify the voltage divider.

There are lots of resistors, how would we know which are the ones we are looking for? The oscillating start only with load. Become a member to follow this project and never miss any updates. Having the datasheet of the comparator in hands always helps. The comparator gets the value and compares it to the voltage reference.


The original values were 15kOhms for R40 and 2.

We’ll need to open it, identify the feedback circuit and change the reference voltage divider. View all 3 project logs. Almost every SMPS feedback circuit works by dividing the output voltage using a voltage dividerthen comparing the divided voltage with a reference voltage.

Else, if the value is lower than the voltage reference, the comparator tells the control circuit to keep charging the capacitor, because your output value is lower than the desired value. Any clue what could be the cause?

Testing pin 3 I’ve read 2. The voltage divider divides the output voltage by a known constant and feed it to a comparator. If R2 is 1kOhm, you have: How would you do this? I reconnected the variable power supply and changed the voltage while measuring the output at the optocoupler. Now, with a basic understanding of how a SMPS works, there should be no problem modifying one to output our desired voltage.

About Us Contact Hackaday. Become a member to follow this project and never miss any updates Become a member. Sign up Already a member? Does this project spark your interest? The control circuit then closes the switch, allowing current to flow from the high voltage side to the low voltage side.

I managed to get up to Did’nt bother much more due to the PCB being covered in thermal glue hiding the circuit.


You would be comparing the half of 4. What we want to do is to change the voltage divider, changing the voltage at which the comparator tells the control circuit to datxsheet the capacitor. The problem was somewhere else. There was one problem, though: I connect the variable power supply to the power supply I want to modify and measure the voltage at the optocoupler. Using a voltage divider.


Modifying a notebook power supply |

A better view of the optocoupler: Yes, delete it Cancel. Disconnected from the mains, put everything back into the case and tested again to make sure.

If the value is higher than datazheet voltage reference, the comparator tells the control circuit to open the switch, as your output voltage is over the desired value. No blue magic smoke. As you want a specific voltage on the output low voltage sidethe feedback circuit tells the control circuit that the output voltage is below the desired voltage.

I’ve already identified the main parts on the feedback circuit. When it reached around 23V, the output changed.

The Hackaday Prize. Some undervoltage protection, or??? Also, when Dataheet changed the voltage on the variable power supply, I could see the voltage on pin 2 changing according to it. Notice how it charges then discharges the capacitor.

Grabbed my multimeter and tested the output: Let’s have a look at the open PSU: