Beginner question about resistance

Discussion in 'General Electronics Chat' started by phase ghost, Mar 5, 2010.

  1. phase ghost

    Thread Starter New Member

    Mar 3, 2010
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    I'm trying to understand how to calculate resistance for a basic circuit, but I'm confused how I get from one voltage/amp to another. For instance:

    I have a (a) 9v/650ma source and I want to get it to (b) 2v/20ma before an led.

    Using ohm's law I get 13.846 ohms from (a) and 100 ohms from (b). I'm not sure what size resistor to use to get the input voltage and amps to the desired state though. Would a 100 ohms resistor do it? I've done a bunch of exercises to find out ohms from voltage and current and vice versa, but getting from point a to point b is baffling. thanks!
     
  2. phase ghost

    Thread Starter New Member

    Mar 3, 2010
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    Pardon my extreme beginner question, but I think I may have be getting somewhere.

    If I want 20ma to be the result do I use ohm's law and divide 9v by 20ma to get the proper resistance needed? I got 450Ω as my answer.
     
  3. aurorathor

    New Member

    Feb 18, 2010
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    Why not put in a voltage limiter circuit instead of using resistors?
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,646
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    Hello,

    The values you calculated are the minimum load resistance for the powersupply
    and the internal resistance for the led at 20 mA.

    To calculate the resistor needed to connect the led to the powersupply with the following calculations:
    Seriesresistor = (powersupplyvoltage - ledvoltage) / ledcurrent.
    I your case it would be (9 - 2) / 0.02 = 7 / 0.02 = 350 Ohms.
    The nearest standard values will be 360 or 390 Ohms.

    Here is a drawing how things are connected:

    [​IMG]

    Greetings,
    Bertus
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Here is a link to the Ebook chapter that covers LED's - http://www.allaboutcircuits.com/vol_3/chpt_3/12.html

    Briefly, you use the source voltage and the LED conduction drop. 9 volts (source) minus 2 volts (conduction drop of LED) leaves you 7 volts to consider. Limiting current to 20 ma needs a resistance, given by R = E/I. 7 volts divided by .02 amps is 350 ohms. The closest safe standard value is 360 ohms, so that is the resistor to use.
     
  6. phase ghost

    Thread Starter New Member

    Mar 3, 2010
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    Excellent answers, thank you! Just what I was hoping to learn.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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