Problems with a potential divider

Discussion in 'The Projects Forum' started by MMH, Mar 7, 2013.

  1. MMH

    Thread Starter Member

    Feb 8, 2013
    143
    4
    Hello there everybody!!

    I have some tiny problems with a potential divider. I needed 1.8v and 120mA out of 5v 500mA supply for my project. I constructed one using a 1k resistor on one side and 1.75k resistor on another side. I got 1.85v but alas! The current dropped down to 2.8mA!!

    Can you tell me any other ways to get this 1.8v 70-120mA from 5v 500mA??
    Please try to make it not THAT complicated. Thanks in advance for your help!!
     
  2. tindel

    Active Member

    Sep 16, 2012
    568
    193
    Hmm - I'm a bit confused about where you want your current and how you arrived at 2.8mA. 5V/2.75k=1.8mA Maybe a schematic would help

    If you want 100mA out of your 5V supply then you need a total load of 50 ohms off of your 5V. I.E. Both of your resistors should sum to 50 ohms. Try 18ohm and 33ohm. That should give you ~1.8V and ~100mA out of your 5V supply. BTW - You'll need at least 1/2W resistors for this.

    Note when you add a load to your 1.8V that your voltage will go down and your current will go up!
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    You could use an LM317 regulator.
     
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  4. MMH

    Thread Starter Member

    Feb 8, 2013
    143
    4
    Thanks you very much!!

    What about using a 6.8ohm(1/4W) and 10 ohm(1/2W) resistors. i cant find 1/2W 6.8ohm resistor anywhere in the home.:p:p
     
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    You can get a certain voltage drop through a resisitor only if the current stays exactly the same, all the time.

    To calculate it use Ohms Law.
    5v - 1.8v = 3.2v through the resistor.
    3.2/120ma = 26.7 ohms ~ 27 ohms
    Then wattage = 3.2v x 120ma = 0.38 watts.

    The trouble with using a resistor is that as soon as the current changes the voltage out changes.
    The same thing happens with a resistor voltage divider.

    A voltage regulator like the LM317 keeps the voltage constant when the current changes.
     
  6. DickCappels

    Moderator

    Aug 21, 2008
    2,647
    632
    You can only get away with using a resistive divider at that current if your load is well known and absolutely stable. Otherwise the LM317 is probably a better solution for your application.

    You did not mention the kind of load you expect and how much variation in load you expect, and to what tolerance you want to keep the 1.8 volts. Knowing so can help determine whether merely using a resistive divider is a good solution.
     
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  7. MMH

    Thread Starter Member

    Feb 8, 2013
    143
    4
    Thank you very much for answering!!

    I will now use the LM317 for my projest:):)
     
  8. Munir Ahmad

    New Member

    Mar 6, 2011
    2
    0
    Assalam-o-Alaikum, Mohtarim I saw a tiny bulb light haveing 4 LED's of 1.5 volt DC current, one resistance, one diod and one capacitor working at 220 volt AC, can any body supply me circuit for the same
     
  9. MMH

    Thread Starter Member

    Feb 8, 2013
    143
    4
    O alai kum As Salaam! Good to see That ther is one more Muslim like me in this forum!
     
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