Changing a laptop power supply voltage

[vinicius.jlantunes]

All, need some help if you can.

I am trying to re-purpose a laptop power brick and need to change its output voltage from 19V to 12V. I reckon I can do so by changing the resistors in the feedback loop, am I right?

It's a LiteOn model PA-1650-32HN. I tried to find its schematic but not sure if what I found is right (see page 7 of this PDF, and also attached: https://ac-dc.power.com/sites/default/files/PDFFiles/der197.pdf). There are definitely some differences between the schematic and the board.

I also tried tweaking the potentiometer on the board but doesn't seem to work, seems to only change the voltage slightly (it was set with some sort of glue that I tried to remove, I guess factory calibration?).

There is a chip on the board close to what I believe are the feedback resistors, marked "1002 PGUY". I can't find what chip that is to try and understand how the feedback loop works. Or maybe that has nothing to do with the feedback...

The only other chip on the board is what I believe to be the main switching controller. I'm having a hard time reading it without proper magnification but I think it's something like SC1L704, or maybe SC1170. The "S" could be a "5" maybe.
Here is a picture of the board but I didn't manage to make it visible...
https://1drv.ms/u/s!AgoNznOUIIVFi9sfcqddvS3mk_Mwtg

Any further advice on how to approach this? I am going to use it for powering a relatively high current 12 V device (~2-3 A).

[orbanp]

Key to the output voltage control is keeping reference voltage on the LM431 (U2) at 2.5V, as divided  from the output voltage by R17 and R18. Keep R18 the same, calculate R17 for the new output voltage for 2.5V at the LM431 control pin.

You also might want to keep the current through the optocoupler diode part (U3A) the same. So you want to recalculate R16 for the new output voltage. Calculate the current for the existing output voltage ((Vout - Vdiode) / R16). Vdiode is the forward bias voltage of the optocoupler diode. Measure it or look it up in the datasheet. Based on this calculate new value for R16 for the changed output voltage.

Good luck, Peter

[oldmannewstuff]

Hello!

You might find this post useful: https://www.eevblog.com/forum/projects/modifying-old-laptop-charger-for-ham-radio-use/msg1410365/#msg1410365.

I attempted to do the same with my brick, but the voltage became unstable. Hope you have better luck with yours 

[Neilm]

I attempted to do the same with my brick, but the voltage became unstable. Hope you have better luck with yours 

Some supplies use the transformer turns ratio to optimise the feedback loop. If you move the output voltage too far this will result in loop oscillation as the transformer characteristics are not correct.

[Zero999]

I attempted to do the same with my brick, but the voltage became unstable. Hope you have better luck with yours 

Some supplies use the transformer turns ratio to optimise the feedback loop. If you move the output voltage too far this will result in loop oscillation as the transformer characteristics are not correct.

Yes, 19V to 12V is quite a big jump. It might be better to compromise, by choosing a higher voltage, which is still safe to run the 12V device on, such as 14V.

[vinicius.jlantunes]

Thank you all for your suggestions! I will give it a try and maybe aim at 14 V since the device I want to power doesn't need exact 12 V. It's one of these portable car tire pumps so it's used to a not so precise supply.

Will let you know how it goes when I make some progress.

Thanks again!

[NiHaoMike]

Just get a cheap PWM motor speed controller. You'll most likely need to in order to overcome inrush.

[westfw]

It's one of these portable car tire pumps

You'll most likely need to in order to overcome inrush.

Indeed.  I had a 5 or 10A "car battery eliminator" (AC-in, lighter-plug out) and tried to run a 12V tire pump using it, and it blew the fuse pretty much immediately.  Don't underestimate the startup and/or highly loaded current consumption of cheap motors!

[vinicius.jlantunes]

You guys mean something like this one?
https://www.ebay.com/itm/Mini-DC-5A-Motor-PWM-Speed-Controller-3V-35V-Speed-Control-Switch-LED-Dimmer-ST/172255836155?hash=item281b3f77fb:g:HwUAAOSw61haWaOV

Well I can get one but it's not as fun as I don't get to spend a few hours at the bench 

Perhaps I should pivot my solution here to building a PWM controller?
I guess these ebay ones are basically a 555 + MOSFET?