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Easy circuit and resistor calculator for different type LEDs?

Discussion in 'LEDs and Optoelectronics' started by sailboat, Jul 28, 2011.

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  1. sailboat


    Jul 28, 2011
    I have some tiny surface mount LEDs that I wish to light up in my project. [The information below in square brackets is from datasheets I found online; the rest is directly from the packaging].

    NTE30005 - Red, V(F) 1.85V Typ, I(F) 30mA, V(R) 5V, P(D) 100mW. [2.5V max]
    NTE30006 - Grn, V(F) 2.2V Typ, I(F) 25mA, V(R) 5V, P(D) 105mW. [2.5V max]
    NTE30027 - Blu, V(F) 4.2V Typ, I(F) 20mA, V(R) 4V, P(D) 120mW. [4.2V max, 3.5V Typical -- I guess we go with what is on the packaging?]

    My original thought for this project was to use 2 of each LED, but that might change to less. I have found tons of information on LED wiring, but all seems to focus on calculating resistor values when using LEDs of the same type. How does one calculate resistor values for LEDs when different LED types are used in the same circuit?

    I am also planning to power the circuit using a LiPo battery, 1S 3.7V 138mAh. I think this might eliminate using the Blue LEDs, unless I can add a second LiPo. But even with 1 LiPo, the voltage is higher than the requirements for the Red and Green LEDs. Is there an easy way to still hook this up?

    Also, given some formula for calculating the resistor value for each LED, what voltage value would be used in the formula, since the battery voltage will change over time? In this case, fully charged (no load) is 4.2V, and somewhere around 3.7V is when it is ready for a recharge.

    Right now, I would be happy if I can wire a single LiPo to power one of the Red and one of the Green LEDs.

    Your thoughts, suggestions, and knowledge sharing are greatly appreciated.

    Additional info: The resistor(s) will also have to be tiny surface mount, except for parts next to the battery. Only two leads run to the section where the surface mount components will be used.
  2. TBennettcc


    Dec 4, 2010
    First of all, see this:

    After you've read through that, here is some more information:

    The V(f) of your LEDs means the forward voltage drop, not the voltage it runs on.

    LEDs run off of constant current, not constant voltage.

    Let's say you have three red LEDs in series. That gives a total forward voltage drop of (3 x 1.85 = ) 5.55 volts. So you would need at least a 6V power supply in order to power those three LEDs in series. Then, to calculate the resistor, you take the voltage of the power supply, subtract the forward voltage drop of the LEDs, and then divide the result by the current you want to flow. With the LEDs in series, the same amount of current will flow through each LED.

    So, to continue our example, a 6V power supply gives a difference of 0.45 volts. Ohm's Law gives V = I x R. Rearranging gives V / R =I. 0.45 V / 0.030 A = 15 ohm resistor.

    It's going to be tricky to incorporate all three LEDs if you want to make them all in series. You might have to put them in parallel; that way, you can control the amount of current through each LED.

    In parallel, using one green and one red LED, you would need (3.7V - 1.85V) / 0.030 A = 61 ohm resistor. Looking at E12 values, you'll choose either a 56 or 68 ohm resistor for the red LED.

    Doing the same calculations for the green LED, (3.7V - 2.2V) / 0.025 A = 60 ohm resistor. So you would also choose either a 56 or 68 ohm resistor for the green LED.

    You would wire each LED in series with one resistor, then wire the LED/resistor assemblies in parallel.

    Hope this helps. Good luck.
  3. sailboat


    Jul 28, 2011
    Thank you very much for taking the time to help me out, and for the link to that very well written post. I feel I have a much better understanding now. (It is a lot to take in, but these explanations have helped tremendously!)

    As I understand, this would be the current through each LED for each of the following resistors over the battery's voltage range (understanding that Vf may vary depending on brand and temperature, thus possibly changing the current somewhat, but I think probably okay to ignore). (Also the lower battery voltages are not as much of a concern since the battery must be recharged before it gets to 3.0V under load).

    ...Vf .....R ..If | 3.0V 3.2V 3.4V 3.6V 3.8V 4.0V 4.2V

    1.80V 56-Ohm 30mA | 21mA 25mA 29mA 32mA 36mA 39mA 43mA
    1.80V 62-Ohm 30mA | 19mA 23mA 26mA 29mA 32mA 35mA 39mA
    1.80V 68-Ohm 30mA | 18mA 21mA 24mA 26mA 29mA 32mA 35mA

    2.20V 56-Ohm 25mA | 14mA 18mA 21mA 25mA 29mA 32mA 36mA
    2.20V 62-Ohm 25mA | 13mA 16mA 19mA 23mA 26mA 29mA 32mA
    2.20V 68-Ohm 25mA | 12mA 15mA 18mA 21mA 24mA 26mA 29mA

    Would these resistors still be okay, given the higher than rated current at the more fully charged battery voltage? (Or should I really be looking at higher resistance values or voltage regulation to help?)

    I should also clarify (also given the temperature aspect), that the LEDs (and likely resistors) will be mounted on a model helicopter blade, and with my current plan at least one of the two LEDs will have a large amount of airflow over it (since I will mount one LED on the top and the other on the bottom of the blade, one or the other will get more or most of the airflow depending on the pitch of the blade). Of course, the final mounting approach might require me to add a piece of clear tape or something over the LEDs to ensure they are secure.

    So getting the LEDs at their 'optimal' brightness is a goal, and having the LiPo last a decent amount of time with them lit as well.

    I believe if I add the approximate current through each LED together, I can determine approximately how long the battery will last:
    138mAh / (30mA + 25mA) = 2.5 hours
    But I think this is the time to completely drain the battery (which is very bad), so maybe it can do 80% of that, so maybe 2 hours (plenty!). (This is under my current understanding; I could be wrong here).

    Thank you again!
  4. TBennettcc


    Dec 4, 2010
    If your voltage is going to vary that much, you will probably want to look at building a 'constant-current' source. That way, your LEDs are always receiving exactly how much current they need to keep them bright. If you're going to use the red and green LEDs, you might try building a CC source for 25mA, and see how the red LEDs look at 25mA versus 30mA. If that's satisfactory, then you're all set; otherwise, you'll be building one CC source for 25mA and one for 30mA.

    Here's one example I found:

    You'll have to look around for one you feel you can construct. This particular CC regulator only has a V(f) of 0.5 volts, which is good, considering a similar CC circuit using an LM317 has about a 3V forward-voltage drop. (I'm sure you can find other circuits; this was just one of the first I happened to come across. Feel free to experiment!)
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