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Having trouble lighting up LED in a breadboard.

  1. Aug 26, 2011 #1
    I've got some D.C power supply connected to my breadboard, and there is definitely current going through it as I've put some resistors in series and parallel and checked the values on the multimeter vs my calculations and it seems to be alright.

    Now I put in a 5mm LED in series with a 100 ohm resistor with 3 V and it didn't light up, I tried different combinations making sure that the calculated current didn't go too high, but I couldn't get the LED to light up and I know it was working because I tested it at the store before I bought them.

    I looked at the power supply box and it says it outputs 2.5A max, does this mean that 2.5A of current was running around in the circuit and possibly damaged the LED? I don't want to test out the other LED's until I'm sure of what's happening.

    The multimeter has two connection options, for currents less than 200mA and for currents over 200mA, and I can only seem to get a reading when the currents over 200mA option is used.

    Edit: Tested the LED with the multimeter diode test, and it lighted up, it's a blue LED, so the LED is definitely working, but just not in the circuit.
    Last edited: Aug 26, 2011
  2. jcsd
  3. Aug 26, 2011 #2
    Taking the voltage across the led 2v, then in your case the current i = (3-2)/100 = 10ma which is enough to turn the led on, but may be that you are connecting the led in a wrong direction, remember the anode(+) is connected to the +ve and the cathode (-) is connected to the -ve terminal of the supply.

    Concerning the 2.5 A of the power supply, it means that it can deliver up to 2.5 A depending on the load.

    This link can be useful for you:

  4. Aug 26, 2011 #3
    Most blue LEDs are made of GaN and have Uf ~ 4V, so they won´t work with 3V supply.
    Use a red LED (GaAlP, Uf ~1.8V) and you´ll be fine.
  5. Aug 26, 2011 #4
    I didn't recognize that it is a blue led!

    Yes blue led need 4 volts so it will not work.
  6. Aug 26, 2011 #5
    I actually tried several voltages, 3, 5, 6, 9 and it didn't work with any of those configurations.

    I'll see if I can take a picture and post it, maybe I've connected something up incorrectly.

    However, the multimeter manual says for the diode test, the voltage used is 2.4V, and that seems to light up the LED, so it appears the blue LED works even if it's not at 4V, though it's not all that bright.
  7. Aug 26, 2011 #6
    I'd agree that the voltage is probably the issue, but since you've tried multiple voltages without luck, do you have the polarity correct? The flat on the LED should go towards the common (-) of the power supply.

    If it turns out that you've had it connected backwards, you may have damaged the component with the higher voltages. LEDs don't like high reverse voltages.
  8. Aug 26, 2011 #7
    Yes it was correct, the flat side was towards the (-); the shorter connector pin in the LED. I've gotten LED's to work before, so as to why this is not working now is confusing me.
  9. Aug 26, 2011 #8
    Got it working, it appears I hadn't understood how the connections in the breadboard actually went.

    It's the same blue LED, but it is actually working on 3V, I have two resistors: 510 ohms and 1000 ohms connected in parallel, giving around 333 ohms, so there's around 9mA going to the LED.

    So how is the blue LED working on 3V if its voltage drop is greater than that?
  10. Aug 26, 2011 #9
    The brightness will change if you apply a greater voltage. With smaller voltage it may light but with brightness less than that in case of the required voltage.
  11. Aug 26, 2011 #10
    I see, thanks for the information.
  12. Aug 26, 2011 #11
    Sorry, I looked at the first datasheet I found and did not doublecheck. Uf of a blue led can be as low as 2.4V
  13. Aug 27, 2011 #12
    Why do you have to connect LED's across a gap on a breadboard? I tried connecting 4 LED's in series, and noticed only two of them turned on, those were the ones that went across the middle gap between the rows and the gap to the ground column.

    In fact, to get them all to light up, it almost seemed that within the rows I had to connect them in parallel, and across the gap in series.
    Last edited: Aug 27, 2011
  14. Aug 27, 2011 #13
  15. Aug 27, 2011 #14
    I know how they are connected, I just didn't really understand why the LED's wouldn't work the way I expected them to.

    And now I've blown the 0.2A fuse in my multimeter by fiddling with the circuit -.- have to replace it tomorrow.
    Last edited: Aug 27, 2011
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