I Need HelP!!! How to connect 25 led together.

Discussion in 'General Electronics Chat' started by CanalOcho, Dec 29, 2009.

  1. CanalOcho

    Thread Starter New Member

    Dec 29, 2009

    i saw the post on the web and it looks like you know a lot about leds jejeje. im making a led ligth for my video camera and it doesent work, it took me a lot of work and a lot of time but nothing hapens.
    the guy from the shop i bougth them toled me to do what i did, but im guessing he knows less than me.

    i left the first + connection, or long leg, free, and then connected - with the next + and so. then he toled me to conect the hole 25 led circuit to a 9 volt batery and that would work, i did it, and nothing happend, but now that ive seen your posts im thinkin hes crazy, i tested each LED again with two AA bateries and they all work individualy, what can i do to make them work with a small sorurse of power?? because its a small box.

    well i hope you can help me out, thanks a lot.

    PS: sorry for my english, im from Uruguay
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    What voltage are you using to power them?
    Have you any voltage limiting resistor in line anywhere?
    Did you follow the anode to cathode to each led? (current only goes one way through a diode) if any of your leds are reversed, it will not function.
    Do you have any specs on the leds? voltage?
  3. CanalOcho

    Thread Starter New Member

    Dec 29, 2009
    Thanks for your fast reply, im using a 9 volt batery, but i dont have a clue of what im doing jejeje, i know how to put a tipical circuit together, with normal ligths and stuff, but i never didi this with leds, i checked the circuit and it doesent have any led in the wrong way their all + to - and the first positive leg is free and the last negative leg is free, when i conect them to a batery, nothing happens.

    im not using any resistor or anything its just the leds and the power source, a 9 volt battery.

    thanks again....
  4. thyristor

    Active Member

    Dec 27, 2009
    Each LED will drop about 2v or so. Ergo, to put 25 LEDs in series and expect them to light would require a power source of around 50v minimum.

    In order to light 25 LEDs with a 9v battery would mean that you can only connect 4 in series. But if you make 6 series chains like this (of 4 LEDs each) and wire the 6 chains in parallel, you may get the LEDs to light.

    I say "may" because, if the forward voltage drop of the diodes is more than 2v, you will have exceeded the 9v source voltage and they won't light.

    Better to wire 8 parallel chains of 3 LEDs in each chain. Connect one end of this combination to the negative battery terminal and connect the other end of this combination to a 1/2 watt, 33 ohm resistor and connect the other side of the resistor to the battery's positive terminal. This accounts for 24 of the LEDs... if you want the 25th one, connect it to the battery via a 680 ohm resistor.

    The LEDs will not all be of identical brightness owing to the variation in forward voltage drop. To get exactly equal brightness from each LED will involve using a constant current source circuit.
    Last edited: Dec 29, 2009
  5. SgtWookie


    Jul 17, 2007
    What kind of LEDs are they?
    If they emit white light, they probably have a forward voltage of somewhere between 3.4v to 4v.
    If they are infrared, they may have a Vf of 1.2v-1.8v. If they have two IR emitters per package, it will be about double that amount.

    If you give us the typical voltage rating @ current for your LEDs, we will be able to give you a better answer.
  6. SIcam

    Active Member

    Aug 9, 2008
    Running LEDs without a Current Limiting Resistor is a bad idea as it could burn out early.

    Its sounds like you need a fast fix so here it is. Connect battery on one end of your 25 led link. Using the other end of the battery wire loosely, connect it across 8 of the leds - see if they light. If not, connect it across 7 LEDs, if not try 6, 5, 4. Do not go below 4. Do not hold the power to them for long.

    You might find that you can leave the rows of 5 LEDs connected but break each jumper to the next row. With the battery disconnected now connect the ends of each row as a single column. Now you have 5 rows of 5 LEDS in series with the postive on the end column on one side and the negative on the end column on the other side.

    Hope this makes sense. You need a resistor or LEDs could fail.

    This photo shows what I was getting at. You will need to change the direction of all LED's in every other row.
    Last edited: Dec 29, 2009
  7. Wendy


    Mar 24, 2008
  8. Audioguru

    New Member

    Dec 20, 2007
    Since he used two AA cells (new and alkaline?) to test the LEDs then they must be close to 3V.

    A 9V battery will drop to 6V when it is almost dead so two LEDs in series and in series with a (3V/10mA=) 300 ohms resistor will have 10mA if the LEDs are 3.0V each and the battery is 9.0V. The current will be only 4mA when the battery is 7.0V.

    If the LEDs are actually 2.8V each then the current is 11.5mA when the battery is 9.0V and is 5mA when the battery is 7V.

    Make 12 strings of two LEDs in series with a 300 ohm resistor. The total current when the battery is new is 120mA and the battery will drop to 7V in about 1.5 hours.
  9. ProfHesher

    New Member

    Dec 29, 2009
    You should use sepparate current limiting resistors in each of the series LED strings. This will prevent current hogging that can be caused by the forward voltage drop (VF) varying from one LED to the next. This will prevent some of the LED strings from appearing brighter than others and help the array last longer.
  10. CanalOcho

    Thread Starter New Member

    Dec 29, 2009
    thanks to your help, everything worked, its so powerfull, and i i've left it on for a long wile and no problems, thanks a lot to all.

    See you arround guys, thanks again.

    juan pedro verdier
  11. SIcam

    Active Member

    Aug 9, 2008
    So how did your finally connect it?

    Can you also post a photo of the finished product for reference to those who might search this post in the future?

    Good Deal!