USB power 5V convert to 4.6V or 5.2V?

Discussion in 'The Projects Forum' started by Lavrenti, Feb 2, 2014.

  1. Lavrenti

    Thread Starter New Member

    Feb 2, 2014
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    I'm sure this is probably asked a million times but is it possible to modify a USB cable using resistors or whatnot to output 4.6V? Would the current stay relatively the same? I have an external battery that currently outputs either 2.1A or 1A (pun intended).

    Also, is it possible to step up to 5.2V? Would this be the solution?
    http://www.prodctodc.com/dc-to-dc-15v-to-5152v-stepup-module-power-converter-for-mp3-mp4-phone-p-66.html#.Uu6x_XkyxAQ

    Reason being, I want to be able to charge my various Nintendo handhelds safely via usb power source.

    Thank you in advance!
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    It depends on the rules of the game; what is allowed or not allowed. If you check the USB specifications you will find out what the range of the 5V supply is. To go lower you build a buck converter that lowers the voltage. To go higher you build a boost converter that raises the voltage.

    The first rule of thumb that you apply is that power in is always strictly greater than power out. I generally use 80% as a first approximation. I would not bother futsing around with resistors unless you want to risk toasting your expensive equipment.

    Keep in mind that you only get 100 mA to play with. You can get more if your device has the capability to negotiate with the USB hub for it. You don't really want to go to all that trouble -- do you?
     
  3. #12

    Expert

    Nov 30, 2010
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    1) The most recent USB ports can only provide 1/2 amp.
    2) most devices aren't all that picky about a voltage that is less than 10% off the optimum.
    3) Most people that post here think their battery will be charged by exactly the voltage on its label, which is not true.
    More likely that you would need more than 5 volts to start with and regulate that down to what the battery needs.

    These things seem discouraging from my point of view.
     
  4. TANDBERGEREN

    Member

    Jan 20, 2014
    74
    4
    Use a schotky diode or more to have the voltage drop wanted.
    From 5V down to 4,6V would usually be good with two diodes in series.

    You would have to spend some time to study datasheets.

    To get higher, You would need a buck-converter as already stated.
     
  5. takao21203

    Distinguished Member

    Apr 28, 2012
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    USB is normally 4 volts something
     
  6. paulktreg

    Distinguished Member

    Jun 2, 2008
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    The ATX power supply specification states the 5VUSB rail can be between +4.75V and +5.25V. I suppose any designs must presume +4.75V.
     
  7. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Does it really matter when your so close to the voltage? ??
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Mine measure between about 5.1 and 5.25v, and they change depending on system load (what task the PC is doing).
     
  9. Lavrenti

    Thread Starter New Member

    Feb 2, 2014
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    My usb source outputs says it outputs 5v at 1A effectively. How would the current be affected if I were to use Schottky diodes?

    Question: is it true that if my device only needs 320mA, but my source offers 1000mA, is that safe? i.e. will the device just draw what it needs?

    Also, does anyone know/own any good cheap meters used to test output voltage and current? like a digital tester or something?
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    This is a common misunderstanding that keeps cropping up. I believe there is a sticky for this. You need to understand basic electricity, specifically Ohm's Law:

    I = V/R

    You cannot arbitrarily choose V and I at the same time for a fixed R. If V = 5V and I = 1A that must mean that R = 5Ω.

    The USB source can provide 500mA if you need it. If you ask for 320mA that's what you get.
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    That's a boost converter mate. A buck converter regulates to a lower voltage.
     
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  12. Lavrenti

    Thread Starter New Member

    Feb 2, 2014
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    yes thank you that is what I thought just wanted to confirm <3
     
  13. #12

    Expert

    Nov 30, 2010
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    MrChips likes this.
  14. TANDBERGEREN

    Member

    Jan 20, 2014
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    Typo. :D

    And agree.
     
  15. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    USB defaults to a lower current mode. To get a port to output the max current you need to set it up in software, either on the PC side, or by data into the USB port.
     
    #12 likes this.
  16. burger2227

    Member

    Feb 3, 2014
    190
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    Do you really want to overdrive a PC USB port? I'd use a cheap USB power adapter for a wall socket for experimenting first. A PC is a terrible thing to waste unless it is a Vista...
     
  17. TANDBERGEREN

    Member

    Jan 20, 2014
    74
    4
    To get out of any confucions here, I must put his out.

    1. The USB-port defaults to none spesific current output.
    2. The maximum crrent output from a USB2.0 port is typically 0.5A to 0.9A
    3. For USB Pre 3.0 the Voltage output is 5,00V +/- 0.25V USB 3.0 has 5.00V, +0.25/-0.55.
    From chargers You may see a current capability of up to 5A in a USB-port.

    To explain why the USB doesn't default to ant spesific current output, one must know how theese are regulated.
    Any power wich is to be fixed at any spesific current output will need to NOT be fixed to any spesific Voltage, unless we are to see a constant power into the load.
    For USB-ports we have a fixed Voltage, where the load determines what current there will be drawn from the port.
     
  18. Lavrenti

    Thread Starter New Member

    Feb 2, 2014
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    Now since I am absolutely an amateur with resistors and diodes, does anyone know exactly what Schottky diode(s) I would need to implant into my cable to effectively lower my output from 5v to 4.6v?

    Sorry that this is dragging out so convolutedly, but this is pretty much all I need to know.

    And just to be clear, my power source is not a computer of any kind. It is a back up rechargeable battery with two USB ports both outputting 5v . One port at 1A, and one at 2.1A.

    Thank you for all you responses thus far!
     
  19. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Try two 1N5819 diodes in series, if the voltage drops too much (it depends on current) you could reduce it to one diode.

    And I assume you are NOT trying to charge a battery directly from this? So you do have a battery charge controller that runs from 4.6v?
     
  20. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Why exactly do you need a 4.6 volt DC output?

    Anything designed to be charged by a connection to a USB port will work with most any DC input between 4.5 and 5.5 volts.
     
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