Forward Voltage

Discussion in 'General Electronics Chat' started by Sidewinder, Jun 1, 2008.

  1. Sidewinder

    Thread Starter New Member

    Jun 1, 2008
    Hello, I'm pretty new to electronics (and my knowledge is limited to a one semester class in electromagnetism). I have 3 IR LED's with a forward voltage(?) of 1.2V, forward current(?) of 100mA and a radiant power output of 16mW min(?). I also have a battery pack with one 9V battery and would like to connect the LED's in parallel. How do I determine the amount of resistance I need?

  2. thingmaker3

    Retired Moderator

    May 16, 2005
  3. mik3

    Senior Member

    Feb 4, 2008
    It is not good to connect leds directly in parallel because if one led has a slightly smaller forward voltage than the others, the other leds wont light and the one led which will light will burn due to passing excessive current. Use a resistor for each leds independently and then connect them in parallel. You can use the first website thingmaker3 told you to calculate the value of the resistor for each diode.
  4. SgtWookie


    Jul 17, 2007
    If you purchased those IR LEDs from Radio Shack, beware!

    I have very good reason to believe that the 100mA is a typographical error on the Radio Shack package; if you attempt to put anywhere near that much current through the LED's, you will burn them up! :eek:

    I had a few of those in my junkbox, so I decided to experiment with one of them using a laboratory-quality 0-54vdc 3.5a variable voltage power supply, current limiting resistor, and a reasonably accurate digital mA meter in series, while monitoring LED light output using a digital camera.

    I don't recall the exact current at which it occurred, but suddenly at around 60mA, I(led) dropped to zero and IR light output ceased. The LED had burned open.

    These are not the only LED's that Radio Shack has mismarked for current ratings; they carry some super-bright red LEDs that the package states are rated for 200mA. It is not likely they would survive for very long at current exceeding 30mA.

    Unless you don't care about the possibility of destroying your IR LED's, I suggest that you limit the current to around 25mA as an absolute maximum.
  5. Sidewinder

    Thread Starter New Member

    Jun 1, 2008
    Thank you, those links are very helpful!

    Would it be better do use a series circuit or do as you suggest? 1 resistor per led?

    I did indeed buy those from Radio Shack and will heed your warning. Thanks! Ive read numerous posts where people criticize Radio Shack for problems and their prices, is there a place online you would recommend for buying leds/resistors/ect...?
  6. SgtWookie


    Jul 17, 2007
    Major vendors like Mouser, Allied Electric, Digi-Key and Newark are first rate, and stand behind what they carry. Some of them have minimum dollar amounts for ordering.

    Discounters like Jameco, Electronic Goldmine, Marlin P. Jones & Associates ( and the like I've had decent luck with; they seem to ship very quickly.

    I've had mixed results on Ebay; no outright "horror stories", but it helps a great deal to ensure that the seller and yourself can at least communicate before the purchase. Beware of shipping & handling costs regardless of the seller, E-bay or elsewhere. Many on E-bay are downright deceptive; you have to dig through the "fine print" in the auction to discover they have a "flat rate" $25 or other ludicrous shipping/handling fee for a $0.99 item.

    Some make outrageous claims and outright misrepresentation of what their product is capable of. If it seems too good to be true, it most likely is.

    The Radio Shack of the 60's and 70's was an experimenter's paradise compared to their stores of today. In the "good old days" they carried at least a reasonable assortment of CMOS and TTL IC's. Nowadays, they seem to concentrate on cell phone sales. :rolleyes: However, they seem to actually be carrying a few new electronic items lately. Prices there are generally quite high, but you have to expect that in a retail store. Also, you can get it today rather than waiting (and paying extra) for expedited shipping from halfway across the planet.

    If you want a taste of those "good old days", pick up their "Electronics Learning Lab", it's around $70 but occasionally goes on sale for around $50. The lab was designed by Forrest M. Mims III, a noted citizen-scientist, who has a real knack for explaining things in a way regular folks can understand without wandering into techno-geek speak. It's worth the price of admission just for the project board alone, but it also comes with components and a couple of very nicely written project notebooks by Forrest M Mims.
  7. SgtWookie


    Jul 17, 2007
    In your particular circumstance, you're using a 9v source with three LEDs that I've already suggested should be subjected to no more than 25mA current.

    It will be far more efficient to run them in a series string, with a single current-limiting resistor.

    9v batteries (if that's what you're using) vary a good bit as to output voltage under load, even when brand new. Some are as low as 8.4v. Some industrial 9v batteries are closer to 10v. Rechargeables might be lower than 8.4v. Alkaline typically have the highest voltage and highest mAh rating, but they are not rechargeable.

    But let's assume for now that you will be using a 9v source.

    Your typical Vf for your LEDs is 1.2v, and my suggested I(LED) is 25mA maximum.
    You need to subtract the total Vf from your supply voltage, and then using Ohm's Law, calculate what resistance (Rlimit) is needed to allow 25mA or less across the remaining voltage.
    Ohm's Law states:
    R = E/I (Resistance = Voltage / Current)
    Our adaptation:
    Rlimit = (Vsupply - VfLED(total)) / I(LED)
    Rlimit = (9v - (3x1.2)) / 25mA
    Rlimit = (9 - 3.6) / 0.025 Amperes
    Rlimit = 5.4 / 0.025
    Rlimit = 216 Ohms
    The nearest standard value of resistance is 220 Ohms. So, let's see what the actual current will be using a 220 Ohm resistor, and what wattage resistor is required.
    Ohm's Law:
    I = E/R (Current = Voltage / Resistance)
    I = 5.4 / 220
    I = 24.545... mA - mighty close. Let's go with that, and find out the power dissipated in the resistor.
    P = EI (Power in Watts = Voltage X Current)
    P = 5.4 X 24.545... mA
    P = 132.545... mW, or rounded up, 0.133 Watts. Double it to find out the resistor rating you'll need:
    0.133 x 2 = 0.266 Watts
    That's just a bit over 1/4 Watt. You could easily get away with using a 1/4 Watt resistor. However, for maximum life and reliability, you may wish to go with a 1/2 Watt resistor.

    A couple of notes on resistors:
    1) They generally measure somewhat LESS than their markings. If you purchase a 5% tolerance 220 Ohm resistor, it may measure anywhere between 209 to 231 Ohms, and still be within specifications.
    2) I've found Radio Shack fixed resistors to be remarkably stable even after being stored under fairly adverse conditions (broadly fluctuating heat/humidity) for an extended period of time. This has not been the case with a fair number of supposedly mil-spec 1% tolerance resistors that I've had stored under the same conditions.
    3) While my experience has not been exactly scientific nor is authoratively conclusive, I'm willing to make the assertion that the fixed resistors which Radio Shack stocks have a much better than average probability of maintaining their resistance rating for a long period of time.
  8. Sidewinder

    Thread Starter New Member

    Jun 1, 2008
    Thank you SgtWookie, your explanations are top notch.