Lower laptop power supply output voltage from 20v to 12v

[Mr_Max]

Hi everyone

I have an old laptop power supply (Fujitsu Siemens adp-65hb, DELTA b/pec dvo-3a PCB) that I wish to use with IMAX B6 Balance Charger
For that purpose the output must be lower than 18 volts for the IMAX to accept it without warning beeps
I can't find schematic for it unfortunately, it has a TL431 connected to an Optocoupler, but I don't know what resistors values to change
in order to lower the output voltage to 12v or < 18v kindly I need your help

 

[AnalogKid]

R8-R11 are interesting, a very high resistance bridging the isolation barrier. Since it is a single-sided board, you should (with effort) be able to work up a partial schematic.

ak

 

[Nigel Goodwin]

You probably only need to draw out the circuit around the TL341, that presumably is the regulator? - and the output voltage is set by the resistors connected to the adjustment pin - one of them should go to ground, the other to the output voltage rail. Adding a resistor (of a suitable value) in parallel with the resistor from the output rail will lower the voltage.

You just need to apply a little ohms law to the resistor values.

 

[Mr_Max]

Thanks a lot for your replies



R135 and R136 as I found somewhere on the web, are the ones connected to the Ref pin
Their values are :
R135 4.32 Kohm
R136 5.49 Kohm

Other resistors near by:
R134 17.40 Kohm
R132 470 ohm

Unfortunately I don't have the knowledge to fully understand the theory of operation of TL431 in that circuit
What resistors should I change and values ?! I hope if anyone can help

 

[MikeMl]

When used as a voltage comparitor, the TL431's output V(k) switches from high to low as V(ref) exceeds 2.495V. We would need to know if R136 is on the high side of V(ref) or on the low side, but the following shows one possibility:

V2 and RL is just there to show what happens at V(k).

 

[Mr_Max]

Thanks to you all for your help

I managed to do it, a variable voltage PSU, I have removed R134 and replaced it with a 20 KΩ pot in series with a 220 Ω SMD resistor according to this topic
thanks for the author and thanks Nigel Goodwin for putting me on the right track.
I have connected a 12v mabuchi 540 DC motor as a test load, at 16v it runs ok, but gets warm and I noticed some sparks inside after running it for a minute.

 

[Mr_Max]

I have been using the PSU for a while, but today I made some tests, I found out that the max amps it gives at 16v is 2A,
when I connect a 12v 21w car bulb, I see and hear arcing, and voltage drops to zero for a second then goes up to 16v is that normal ?!