DC-DC 5v converter usb resistor selection

Discussion in 'The Projects Forum' started by Dsmroots, Aug 7, 2014.

  1. Dsmroots

    Thread Starter New Member

    Aug 7, 2014
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    I'm new to the forum and a newb when it comes to electronics. I have built a few hobby boards from Ramsey electronics and have decent soldering abilities but as far as component selection I'm a little stuck at the moment.

    I am basically trying to build a circuit identical to this
    http://forums.vwvortex.com/showthread.php?6983041-DIY-Built-in-USB-Charging-Ports

    Now i will be running the 5v converter off of a 12v DC source and the converter is rated at 5v DC @ 3 amps. For anyone using the search function, most smartphones use the d+ and d- contacts on a USB port to communicate to the device which amount of current to draw. In the previously posted link they were trying to charge at 2000 mah. To get 2000 mah an iphone needs to see 2.75v on d+ and 2v on d- . I am only trying to pull 900 mah by using 2v at d+ and 2.75v on d-. I know that this voltage orientation will work for 900 mah based on this sites listed information
    https://www.voltaicsystems.com/blog/choosing-usb-pin-voltages-for-iphones-and-ipads/

    So to sum all of this up, my intentions are to use the listed 47k ohm, 51k ohm and 75k ohm resistors but wire them in opposite to the way the original link listed to get the voltages on the pins I need. This should work with most smart phones.

    So the problem I'm having is resistor selection. I've tried to use the online resistor calculators and formulas both to find what wattage resistors to buy so that they can handle the heat. Also the type of resistors needed. Also looking at mousers site they list heat ranges in their selection. I'm just trying to find common parts but if it needs to be anything special I have no problem with going that route. Any help or direction would be greatly appreciated.

    Btw I'm going with USB ports rather than the common charger for a specific reason, so please don't refer me to the thousands of typical chargers. I simply need direction on resistor selection

    Thanks for your time
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The maximum power dissipated would be with 5V directly across the 47k resistor (which is not the actual situation). That power would be 25/47k = ~0.5mW, which is negligible. The smallest resistors you can find (perhaps 0.1W) or can conveniently solder will do nicely and won't get noticeably warm.
     
  3. Dsmroots

    Thread Starter New Member

    Aug 7, 2014
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    So look for 1/10W resistors for all three? I've read somewhere that when trying to size the power of a resistor, that it's a good rule of thumb to double the needed rating. Some online calculators tell you to multiply your numbers by 1.33 to give a 33% added safety margin. Just wondering if .5 mW or .1w is allowing for that safety margin? I just want it to work the first time with no magical smoke. Really it was the math that was confusing me. Thanks for you input
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    0.5mW is half of one-thousandth of a Watt !! So even 0.1W gives a MASSIVE safety margin of 20000%. Yes, 0.1W will do for all three resistors.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    If you are starting out in electronics as a hobby and are planning on acquiring a collection of components, choosing 1/4W (0.25W) 5% axial thru-hole resistors is a good all round choice.
     
  6. Dsmroots

    Thread Starter New Member

    Aug 7, 2014
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  7. Dsmroots

    Thread Starter New Member

    Aug 7, 2014
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    Ok so you squared the voltage and divided by 47 to get that correct? Sorry for the stupid questions. I'd just rather figure it out then to have to ever ask again
     
  8. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Divided by 47,000. Keep track of the units. The answer is in W, and then converted to mW
     
  9. Dsmroots

    Thread Starter New Member

    Aug 7, 2014
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    Ahh. Gotcha. Thanks again everyone.
     
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