Warning: following project is dealing with voltages which are dangerous for human life (110 - 240 Vac)! Please be very careful and start this project only if you have working experience with such dangerous high voltages!
I have a HobbyKing Turnigy Accucel Lithium charger (link) and I needed to feed it with maximum 17 Volt DC. I was missing such power supply. Fortunately I found a genuine Dell power supply for laptops, it was in good condition and was very cheap. Unfortunately it was delivering a fixed 19.5 Volt, but a good amount of current (4.62 Amps).
I searched the Internet for advises on how to modify such power supply, I found nothing about this particular type (PA-10 family from Dell).
So here is a little tutorial of reverse engineering combined with a short guide on how to tackle almost any electronic product you want to modify in case you have no schematic on hand (or if you want to fix it if it is defective).
You will need:
-electronic meter able to measure continuity of printed circuit board (PCB) traces
-internet connection for electronic components data sheet
-paper and pencil
-Dell PA-10 power supply for laptop
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Step 1: Open the Plastic Case of the Power Supply
First you need to pry open the plastic case of the power supply.
For laptops, these are built as bricks, fully enclosed, sometimes the plastic is welded. You can find various opening methods on the Internet, my choice was to use a red hot knife blade and to cut the contour. This helped to open the case (no pictures, sorry).
Disadvantage: you will make it look bad and the 2 parts will not fit anymore as a tight box.
Step 2: Identify the Main Functional Electronic Blocks Inside the Device
Now that we have the power supply opened (consider any electronic device), we have to identify the main functional blocks and the type of electronic components.
Given the fact that we know that we are dealing with a switching mode power supply (SMPS), we will go from input side to the output side:
-input side is 110-240 Vac from the AC mains connector
-then are some coils, transistors, capacitors etc dealing with lowering the voltage and changing it from ac to dc (details are on the Internet, SMPS it is an interesting thing to learn about, search flyback SMPS, buck SMPS, boost SMPS)
-feedback block (this one will keep the output voltage fixed and will control the SMPS)
-output side (where the cable is connected and where the identification circuit is located)
As a matter of fact, a SMPS 110-240 Vac to any DC output will have an optical isolation between controlling and drivers and as well slot cut on the PCB - these are easy to be found on our PCB. Therefore between the output side (place where the DC cable is soldered) and the slot cut on the PCB it should be located our feedback block as well.
On that area are 2 small black bugs and we should read the markings on these 2 electronics components: TEA1761 (2 x 4 pins, SO8 SMD) and DS2xzy (2x3 pins, TSOC SMD)
Step 3: Check the Datasheet of the Electronic Components
We have to find the data sheet of the 2 components found on the previous step: DS2xzy (2x3 pins, TSOC SMD) and TEA1761 (2 x 4 pins, SO8 SMD).
DS2xyz is a one wire memory from Maxim Semi (link). This is not the guy we need, it is a memory used by the laptop to identify the power supply (in case you wonder why the laptop is not working with an HP power supply with the same connector, same parameters etc).
TEA1761 is an integrated circuit "member of the new generation of Synchronous Rectifier (SR)
controller ICs for switched mode power supplies" according to this data sheet.
So TEA1761 is our guy and we need to focus on the electronic parts around this integrated circuit.
In the data sheet / application note is it specified that the circuit is controlling the output voltage of a SMPS by feedback through a voltage divider on pin 5:
Step 4: Reverse Engineering the Schematic From the PCB
Now that we know so many things about the integrated circuit TEA1761, we have to find the electronic parts on the board itself and to draw the schematic on paper.
We need to use the continuity tester from the multimeter and to draw the schematic on paper. Since the feedback voltage is through a voltage divider and the voltage must be precise over time, I assumed that the voltage divider contains the 2 resistors marked with 4 digits (that means improved tolerance).
So eventually I checked the continuity only on the right side of the TEA1761 and I draw the schematic.
Step 5: Tweak the Voltage Divider
Now we know which resistors are part of the voltage divider, so we can replace such fixed resistor with a variable one.
I decided to remove from the board R41 (68k Ohm) and to put instead a variable 100k Ohm resistor connected by 2 wires to the board.
Here comes a dangerous part: need to connect the power supply to the mains (110-240 Vac), this time the power supply is opened and live circuit is exposed! Please pay maximum attention during this operation and avoid touching the board of the power supply!
Rotate the variable resistor, this will modify its value, see where the power supply will stop working
(minimum voltage output) and see where the voltage output is 17 Vdc.
Remove the mains from the power supply and continue.
We should desolder carefully the variable resistor and measure its value for an output voltage of 17 Vdc - I measured around 58k Ohm.
I found a resistor of 56k Ohm around and I soldered on the board. The output voltage is 16.5 Volt, good enough.
Step 6: Close the Plastic Case and Use the Power Supply
I think that the title is self-explanatory :)
I decided to complicate the things: I removed the wire which was going to the laptop and I replace it by a female 5.5mm connector. I had to make an opening on the case, but it is ok.
The case was not willing to close again like it was used to, so I used some cable ties.
Make sure no interior of the power supply is accessible from exterior, this is risky due to the mains voltage inside.
As a conclusion for this Instructable: Internet is giving us a tremendous power, only combined with some neuron 's effort - if you have almost any electronic device, you can try to repurpose it or even fix it if it is broken just by starting from the integrated circuits used inside. Check their functionality, check the small parts around (resistors, capacitors, diodes, inductors etc) and draw a schematic, compare it with the application recommendation from the IC manufacturer and see what can you do from that point on.
Enjoy the electronics! :)