Which is right size resistor

Discussion in 'The Projects Forum' started by mrel, Oct 27, 2010.

  1. mrel

    Thread Starter Active Member

    Jan 20, 2009
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    Hello
    Have 6 volt battery , device call for 3volts at 350ma .
    Could it be a 10ohms resistor at 25 watt or am I wrong.?
    mrel
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
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    You don't mention what the device is.

    If its current varies, there is no way for you to use a fixed resistor to get exactly 3V.

    Can you consider the use of a 3.3V low dropout voltage regulator instead?
     
  3. t06afre

    AAC Fanatic!

    May 11, 2009
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    Perhaps a LM317 could do the job here.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    A regulator is definitely better, however...

    3V ÷ .35A = 8.57Ω , R = E/I

    3V X .35A = 1.05W , P = E I
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The 350mA is the Maximum current the device will draw, usually the average is less than that.

    Trying to use a resistor voltage divider for power wouldn't work out too well.

    As others have already stated, an LM317 or other regulator would be the way to go.
     
  6. mrel

    Thread Starter Active Member

    Jan 20, 2009
    97
    1
    thatone guy
    Do you have that circuit plan for using the LM317
    mrel
     
  7. mrel

    Thread Starter Active Member

    Jan 20, 2009
    97
    1
    I am using 6 volts battery on a dc to dc conventer that need 3 volts at 350 MA so am I doing right thing using resister to drop the voltage from 6 volt to 3 volts and 350 MA
    Is there better way to drop voltage from 6 volts dc to 3 dc volts at 350 MA?
    mrel
     
  8. Wendy

    Moderator

    Mar 24, 2008
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  9. wayneh

    Expert

    Sep 9, 2010
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    I agree with Bill's picture, worth a thousand words.

    You could alternatively drop the 6v thru 4 diodes to get ~3.2v (exact value would depend on the diode choice). Not very elegant, but I suppose it would work.
     
  10. mrel

    Thread Starter Active Member

    Jan 20, 2009
    97
    1
    Bill Mareden
    What is valve of R2 looks like adjustable resistor.
    mrel
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    It is meant to be an adjustable power supply, however, put a fixed resistor in there and it would work very well for your application. You could also build it as is and adjust it for the voltage you need.

    For 3V a 180Ω in parallel with a 2.7KΩ (both being R2, 168Ω) would create 3.008 volts, ± the tolerance of the chip and the resistors.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    It's kind of a shame to use a linear regulator to step 6v down to 3v for use as an input to a DC-DC converter. The efficiency will be less than 50% due to the power dissipation in the regulator and R1/R2 divider.

    There are some very interesting all-in-one IC packages available nowadays that are switching buck regulators; they just need caps on their inputs and outputs - and they're tiny. However, QFN and similar SMD/SMT packages are somewhat difficult for hobbyists to use; you almost have to make a PCB.

    Just a couple of examples:
    http://www.mouser.com/ProductDetail...r/FAN5362UC29X/?qs=l8TZ72NkpXmSWLFFmL%2bWeg==
    http://www.mouser.com/ProductDetail/Enpirion/EP5352QI-T/?qs=AG1tZYOK7s50PQLZ9ZDk7A==

    Happened to run across the VIPER16 family during a search; pretty interesting switching regulator in a 7-pin DIP package.
    http://www.mouser.com/ProductDetail...IPER16HN/?qs=sGAEpiMZZMthiYuEY6QoeRtlBkWtHgST
    Kind of complex for a beginner to attempt using though.
     
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