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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

-soldering tools

-Dell PA-10 power supply for laptop

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! :)

<p>Nice workout!</p>
<p>Thank you for your feedback!</p>
<p>nice job of re-using electronics. i got a box old laptop power bricks somewhere i might have to give this a whirl. if a guy wanted to he could mount the variable resistor in the case and have a variable power supply. also for those that like to mess with the math the formula is given in the data sheet to figure this out without the variable resistor. just gotta scratch your head a bit more :) so i did the formula given in the datasheet for practice, and replacing the bottom two resistors with one 12K resistor would get you to around 16.5V output.</p><p>who needs to buy cheap wall wort's when you can convert high quality power bricks to your desired voltages! :)</p>
<p>By the way, for certain values of the variable resistor, this Dell power supply will shut down it's output and will not turn it on again unless you disconnected it from the mains (in advance you need to rotate the variable resistor to a suitable value). For sure it was designed to provide a fixed voltage with a good efficiency and maybe a modification of + / - 4 Volts can be ok, but to have a nice variable power supply, not really. But for such mods, BeachsideHank made a useful comment about using a linear regulator :) (I think there are as well Instructables on this)</p>
<p>Thank you for your feedback! I wanted to check for myself the formula (on this Dell power supply it is Vout = 2.5 x (R41 + R42||R42A) / (R42||R42A) ). I used values of the resistors on board (see my schematic from the Instructable) and the output voltage (19,5 Volt), I expected to calculate 2.5 Volt from the voltage divider at the VSENSE pin, since this will be compared with the internal 2.5 Volt reference. (R42 || R42A = 1996 Ohm). Voltage divider output (R41 and R42||R42A) for 19.5 Volt input is 0.556 Volt. For a certain frequency, R41 is parallel with R41A, in that situation divider output is 17,17 Volt. Not sure why. </p>
<p>A heroic effort to be sure, but why not use something like an LM317 and regulate the voltage outside of the power supply? This question is not meant to diminish your useful Instructable either. ☺</p>
<p>Thanks for your feedback! This effort was mostly on mechanical side, on the electronics was less effort. I think this was the reason: somewhere on the circuit I have only to replace a single resistor without considering the mechanical part of the project. This modified power supply will serve my Lithium charger only and can supply around 4 Amps max, while linear regulators can supply less current, have voltage drop on them, need extra heat sink, need a case too. I used a linear regulator for a bench power supply.</p>
<p>I understand the objective now, thanks. These devices are notoriously compact and well seled, so this was a major acomplishment on your part.</p>

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Bio: I like making things, trying to utilize my hands and my brain. There is no limit!
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