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

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Mr_Max, Aug 16, 2017.

  1. Mr_Max

    Mr_Max New Member

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    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

    1021172300_1502114159.JPG 2222595400_1502114157.JPG 3376682100_1502114165.JPG 4405718000_1502114157.JPG
     
  2. AnalogKid

    AnalogKid Well-Known Member

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    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
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    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.
     
  4. dave

    Dave New Member

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  5. Mr_Max

    Mr_Max New Member

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    Thanks a lot for your replies

    Delta_SMPS_Shc.jpg

    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
     
    Last edited: Aug 17, 2017
  6. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    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:
    431.png
    V2 and RL is just there to show what happens at V(k).
     
  7. Mr_Max

    Mr_Max New Member

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    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.
     
    Last edited: Aug 17, 2017
  8. Mr_Max

    Mr_Max New Member

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    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 ?!
     

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