Modify 6v to 5v

Discussion in 'The Projects Forum' started by sgalle, Nov 27, 2010.

  1. sgalle

    Thread Starter New Member

    Nov 27, 2010
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    I pulled this board from my portable speaker box. It originally powered an XM radio receiver. I get 6.1V from a 3pin plug on the right of the picture. I would like to use this plug to power a 5V plug to charge my iphone. I understand how to wire the plug after I have 5V through this plug. I should be able to change something on this board to get the 5V, instead of just adding another regulator or diode.

    Can anyone help?
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    The LM2576 is an adjustable switching regulator.

    Here is the datasheet

    6V isn't a standard device, and since yours has "ADJ" on it, I would guess it is the adjustable version.

    Need photo of board from top, and from bottom, medium high resolution with sharp focus.

    It could be a matter of changing out a single component such as a resistor to change the output voltage of that regulator to a 5V output.

    Unless you still need to use it for a 6V regulator, what is that regulator board from? Looks like it would make a handy 3A Adjustable Voltage SMPS!
     
  3. sgalle

    Thread Starter New Member

    Nov 27, 2010
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    Thanks for you quick response. I have no need for the 6V anymore. I want to permanently change it to 5V.

    Thanks!
     
    Last edited: Nov 27, 2010
  4. jatinah

    New Member

    Jul 26, 2010
    10
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    Hi....I saw your post, and yep, the pic of the underside of the board shows the feedback resistor path, the resistor in front of the 2576 regulator. If you decrease the value of this resistor about 15% , should dial the output down a bit ;o) A pot would be handy to dial it in...good luck....:eek:
     
  5. sgalle

    Thread Starter New Member

    Nov 27, 2010
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    0
    It sounds like you are saying I should change the resistor on the left. I am a little green at this, so I want to be sure. That is the blue one on the left side of the board.

    Next I don't know how to read these. I think I see brown, brown, black,black, brown.

    I am one step closer, thanks for your patience.
     
    Last edited: Nov 28, 2010
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    R2 and R3 make a voltage divider to determine output voltage.

    In your circuit, R3 is missing.

    The layout for a 5V regulator is on page 6 of the datasheet linked above. It would seem that you only need to add in a resistor of the same value as the board's R2 to get 5V out, the actual formula for replacing both R2 and R3 on your board is on page 6 of the datasheet.
     
  7. sgalle

    Thread Starter New Member

    Nov 27, 2010
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    0
    I can't tell what R2 is. All of the resistor calculator show it isn't a correct value. I have looked many times. Brown, brown, red, white, gold.

    Any suggestions?
     
  8. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
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    It appears that there are only 2 resistors and not 3. They more than likely are the voltage divider mentioned above that determine the output voltage. You could try to measure the resistors without power attached. It does really seem to be a matter of determining which resistor to replace to obtain a lower voltage. I am guessing, but I think the USB's have a tolerance of +/-.5V. Either a diode or a change of one resistor will solve your problem.

    If, If R1/R2 correspond to R1/R2 on the datasheet It would be R2 you would change to a value of ~7.15k, provided R1 is 1K.
    I have had similar issues figuring out resistor values on metal film resistors as the color code seems different, maybe someone else can shed some light on this issue.
     
    Last edited: Nov 28, 2010
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