1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Maximum resistance and power for LED

  1. Jan 18, 2015 #1
    1. The problem statement, all variables and given/known data
    You wish to use an LED (D1) which provides essentially zero resistance and is rated to accept either a maximum of 75 mW of power or 30 mA of current. In the following two circuits, what range values for resistor R1 will be acceptable in each case if the voltage provided by V1 is 12 V?


    2. Relevant equations
    R = V / I
    V = P / I

    3. The attempt at a solution
    Rmax = V / I = 12 V / 30 mA = 400 Ω.

    V2 = P / I = 75 mW / 30 mA = 2.5 mV

    So I need a resistor that will reduce the voltage from 12 V to 2.5 mV, requiring a voltage drop of 11.9975 V:

    Rmin = V / I = 11.9975 V / 30 mA = 399.916667 Ω

    Rounding to one or two significant figures (as mentioned in the instructions) would remove any sort of "range" from my answer. Furthermore, a 400V resistor would leave no energy for the LED. Is the problem in my answer or the large voltage supply?
    Last edited: Jan 18, 2015
  2. jcsd
  3. Jan 18, 2015 #2


    User Avatar

    Staff: Mentor

    Check the units for your V2 calculation. You have mW over mA, so the "milli's" should cancel.

    What does V2 represent? What's the potential drop required across the resistor taking V2 into account?

    I'm not sure what criteria they want you to apply to find a "range of values" for the resistor. Perhaps they want you to select suitable values from the set of standard resistor values? See, for example, Resistors - Standard Values.
  4. Jan 19, 2015 #3
    I see now that what I was doing was flawed. So 400 Ω is the minimum resistance without allowing more than 30 mA of current. I don't know what the maximum resistance would be. No matter the resistance, the voltage drop over the resistor is 12 V. The LED will never get to its maximum of 75 mW of power.
    Last edited: Jan 19, 2015
  5. Jan 19, 2015 #4


    User Avatar

    Staff: Mentor

    Actually, the drop across the resistance will be less than 12 V. Remember V2 that you calculated before? The LED will exhibit an almost constant voltage drop due to its junction potential. LED's typically have a drop in the neighborhood of 2 or 3 V (depends upon the details of the construction and materials of the LED). So you'll want to work that into your figuring.

    I'm still unsure how to select a maximum resistance without knowing something else about the LED (like a minimum current that will produce visible illumination for the application at hand).
  6. Jan 19, 2015 #5


    User Avatar
    Gold Member

    most LEDs are speced with a lumen/watt curve. You can either use that.

    If you are not given that data and you do not when the minimum current through the LED is, then you have two options.

    1. make an assumption, do some research, pick a LED and min current
    2. assume the problem is simply talking about the ranges of R1 that would protect the diode, in which case R1 could be infinate
  7. Jan 23, 2015 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Have you considered that the question might be expecting you to work out the forward voltage from the max power and max current? (Not always a legitimate method).

    PS: What does the second circuit look like? Perhaps the bit about a "range of values" makes more sense for the other circuit?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted