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Electronics Reduce USB 5v to 3v (2xAAA Batteries)

Discussion in 'Modding' started by CaseyBlackburn, 22 May 2007.

  1. CaseyBlackburn

    CaseyBlackburn Network Techie

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    I need to convert the power from a USB port that is 5v, down to be equivalent to 2 AAA batteries. I do not know much about resistors and such, so what would I need to do this?
     
  2. SteveyG

    SteveyG Electromodder

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    A 3V LDO regulator such as TI's TPS76830?
     
  3. cpemma

    cpemma Ecky thump

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    An LDO regulator would be certain (there's also the fairly common LM1117T-3.3, a 3.3V LDO that's probably near enough) but with about 2V headroom available you might just get away with a dead common 317T adjustable.

    Or with three 1N4001 diodes in series you'd get around 2.6V (assuming your 5V rail is spot on 5.0V) which is more than two rechargeable cells. That's the easy solution I'd go for. Two diodes would leave you with 3.4V.

    What's being powered?
     
  4. SteveyG

    SteveyG Electromodder

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    The diodes would yield an output voltage that would vary between 3.2 and 2.6V depending on load, so I guess that is acceptable for something that was battery powered.
     
  5. CaseyBlackburn

    CaseyBlackburn Network Techie

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    What is being powered is a Belkin TuneCast that I want to setup to run off USB power instead of tons of batteries.

    Which method do you believe will be easiest and cheapest? Just buying the LM1117T-3.3? Where could I pick up the TPS76830?
     
  6. cpemma

    cpemma Ecky thump

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    Three 1N4001 diodes in series; if it will run off either rechargeables or dry cells (2.4V or 3V) then it's not that fussy. KISS. :thumb:

    Code:
    
    USB 5V-------->|-->|-->|------2.9V
    
    USB 0V------------------------0V
    
    
     
  7. scimmy

    scimmy Member

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    Hi

    Would it be possible to use the orange wire from the PSU in any way, l believe that it's 3.3v. But then again l might be completely wrong
     
  8. CaseyBlackburn

    CaseyBlackburn Network Techie

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    This is for in my car, I have a "hub" that provides power through a few USB ports for my phone, GPS, and some assorted other things. I am going to convert it to use this now.
     
  9. ComputerKing

    ComputerKing <img src="http://forums.bit-tech.net/images/smilie

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    sorry but here we go I will try the 3 X 1N4001 diodes in series :)

    thanks guys
     
  10. Rum&Coke

    Rum&Coke New Member

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    Erm, just to remind everyone that 3 diode in series from a 5v DC source will blow since the Amperage will jump exponentially. Also I hate to see people use USB connections for powering menial stuff since its not really meant to be used in that way. Just remember 100mA safely without having to request more.
     
  11. cpemma

    cpemma Ecky thump

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    Only if you accidentally short out the supply on the end. The load limits the current, as always. Try shorting out the original 5V supply, it will blow just as loud, maybe more so. :eeek:
     
  12. ComputerKing

    ComputerKing <img src="http://forums.bit-tech.net/images/smilie

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    WTF I don't Whant to Blow my 1K watts PSU .. :sigh:

    I was going to use the 5V from the PSU not USB so it will blow too ?

    WTf I do now .. :wallbash: :wallbash:
     
  13. cpemma

    cpemma Ecky thump

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    Put a 1A (or appropriate) fuse in the line. It always makes sense. Any car spares shop will sell inline fuses.
     
  14. ComputerKing

    ComputerKing <img src="http://forums.bit-tech.net/images/smilie

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    you mean I get 1A Fuse no less or no more. OK put the Fuse before the 1N4001 Diode or after them..

    also they will blow up every time then I change them? not stable or what

    thanks
     
  15. CaseyBlackburn

    CaseyBlackburn Network Techie

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    Well, I bought some inline fuses, a LM1117T-3.3, and three 1N4001 Diodes.
     
  16. ComputerKing

    ComputerKing <img src="http://forums.bit-tech.net/images/smilie

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    that LM1117T-3.3 here Link Says that it will drop to 3.3 :( I want less that 3.0 like 2.8 ~ 2.9

    Also If this will work can you draw a diagram to me simple one to show were the things will be ? the orlder and this things

    thanks very much
     
  17. ComputerKing

    ComputerKing <img src="http://forums.bit-tech.net/images/smilie

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    I bought 1N4001 Diodes and Fuse holde still the LM1117T-3.3 and 1AMP Fuse :(

    This deal cost me 1 $ :wallbash:

    [​IMG]

    Any one will help me soon ? I will buy it today or tomrrow need help soon ..
     
  18. ComputerKing

    ComputerKing <img src="http://forums.bit-tech.net/images/smilie

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    Bump ..

    mean no need for that LM1117T-3.3 :worried:

    just

    5V-------->|-->|-->|------Fuse --- 2.9V

    or what ?
     
  19. cpemma

    cpemma Ecky thump

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    Just that. The LM1117T-3.3 would make a 3.3V supply in a different circuit, get the datasheet for example circuits. There is a 2.85V version but I don't know who sells it.

    You haven't bought the easiest fuseholder to use - that looks like a bolt-down fitting, not an in-line. :duh: You'll need to insulate the terminals so they can't touch any bare metal. The plastic-bodied in-line ones (or even the panel-mount ones) are much easier to use.
     
  20. Bas van der Werff

    Bas van der Werff New Member

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    By in line fuse holder do you mean the plastic tube ones where put the fuse in just like a battery in a flashlight?
     

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