modifying dc regulator

Discussion in 'The Projects Forum' started by yonubear, Feb 11, 2014.

  1. yonubear

    Thread Starter Member

    Jan 10, 2013
    73
    1
    ok all I am sure this is simple but I am trying to alter a 4.5vdc car power adapter to output 3 or 3.3v however I am unable to make out the righting on the IC chip so I am hoping that someone here maybe able to help me out based on the fact that there are 3 resistors I would assume one of them controld the regulator chip output.

    Thanks,
    Yonu
     
  2. burger2227

    Member

    Feb 3, 2014
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    Run the 4.5 volts output through two diodes in series and it should be about right. .6 volts drop across a 1N4004 with a load. Cathodes toward new circuit from plus side of power..
     
  3. yonubear

    Thread Starter Member

    Jan 10, 2013
    73
    1
    hey thanks

    I will give it a shot.
    In case it matters I managed to read the ic it is a
    CHN 063AC K01225

    thanks,
    yonu
     
  4. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    The chip is most likely a MC34063A. This is a old but very common buck / boost regulator.



    Just look at the datasheet for the MC34063A. Formulas are not very complicated.

    There are also online calculators.
    http://www.nomad.ee/micros/mc34063a/
     
  5. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    The right way would be to change out the sensing resistor as per the datasheet.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Good call, it is absolutely a 34063. :)

    I have seen them often in car 12v stepdown adapters. And not just old ones, it is still probably the most popular IC for small cheap china gizmos that plug in a 12v cigarette lighter socket.
     
  7. yonubear

    Thread Starter Member

    Jan 10, 2013
    73
    1
    ok I had a look at the data sheet and if I understand right the sensing resistor is on pin 7
    which based on that it is the circled resistor but I am confused on how to use the calculator you linked to get the new value
     
  8. yonubear

    Thread Starter Member

    Jan 10, 2013
    73
    1
    would someone mind using the pics I posted and tell me what the resistor values are I have set and worked them up from the color bands and am unable to match the values I come up with to identify each resistor to change them.

    Thanks,
    yonu
     
  9. burger2227

    Member

    Feb 3, 2014
    190
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    What's V in?

    With just 12 volts in and 3.2 out in the calculator I get: R1=3.6k R2=5.6k (3.19V)
    Nothing for Rsc but it would need a current value to calculate.

    The resistors are numbered R1 and R2 on the board aren't they? Check the pins.
     
  10. yonubear

    Thread Starter Member

    Jan 10, 2013
    73
    1
    yeah on the board there is an r1, r2, and r3 that is what was confusing me and and maybe that was my issue I was assuming 13.6 as the input

    Thanks,
    yonu
     
  11. yonubear

    Thread Starter Member

    Jan 10, 2013
    73
    1
    ok I just started looking at the board again there is R1-R5
     
  12. yonubear

    Thread Starter Member

    Jan 10, 2013
    73
    1
    I think I have this figured out r2 and are r4 are in series totaling 3.7kohm and r3 is 1kohm

    R1= 0.39ohm 5%

    based on the calculator that spinnaker was kind enough to link for 4.5v I should have sense being .5ohm R1 should be 1.5kohm R2 should be 3.9kohm

    so based on that should I be able to assume that R3 on the board is R1 in the datasheet and R2 and r4 on the board are R2 in the datasheet diagrams or am I way off.

    Thanks,
    Yonu
     
  13. yonubear

    Thread Starter Member

    Jan 10, 2013
    73
    1
    ok I am a bit confused I decided to try removing the 2.7k resister and replace it with a 760ohm to get me the close to the 1.8k that the calculator says I need however whe I connected it the output drops to 1v so I am sort of confused any advice would be great.

    Thanks,
    yonu
     
  14. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    You are confusing us. What value do you have for R1? What value do you have for R2? And try using a capital letter at the beginning of your sentences and a period at the end. You need to put some effort into making it easier for others to help you.
     
  15. yonubear

    Thread Starter Member

    Jan 10, 2013
    73
    1
    Ok about the grammar sorry I am posting from my cell phone. The current sense resistor is 0.39ohm, R1 is 1k, and R2 is 3.7k but it is a 1k and 2.7k in series.

    yonu
     
  16. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    I am posting with my tablet right now and I use caps.

    Did you check the data sheet? That combination would get you 5.8 V.

    If you had the resistors reversed then you would get 1.6v close to what you are measuring.
     
  17. yonubear

    Thread Starter Member

    Jan 10, 2013
    73
    1
    Yes I noticed the datasheet but that is what is currently on the board. I based what R1 and R2 are in the diagrams by the values of the original resisters.

    Yonu
     
  18. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    R2=1.4k. R1=1k. would get 3v.
     
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