led circuit

Discussion in 'The Projects Forum' started by WILLIAM RYAN, May 15, 2010.

  1. WILLIAM RYAN

    Thread Starter New Member

    May 15, 2010
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    What circuit do I need to light 5 x Superbright 3mm LEDs ?
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    If you will post the current required by each LED and the forward voltage drop, as well as the voltage of the power supply you plan to use, designing a circuit will be easy.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Bill Marsden has quite a bit of info in his blog, here: http://forum.allaboutcircuits.com/blog.php?b=378

    Basically, you need to start off by finding out what their typical forward voltage (Vf) @ current rating is. This is usually supplied on datasheets.

    If you don't know what the current rating is, you can assume 20mA for super-bright LEDs.

    You can measure the Vf using a multimeter, a 12V DC supply, and a 470 or 510 Ohm 1/2 Watt resistor in series with your LED.
     
  4. WILLIAM RYAN

    Thread Starter New Member

    May 15, 2010
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    Each Led current is 30 mA.

    TypMax Forward voltage:Vf3.2V3.6V forward voltage drop 3.2V Min to 3.6V Max

    Power Supply 5 V from 3 x 1.5V batteries.

    thanks for your response
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    If you use three 1.5v batteries in series, you will have 4.5v available.

    If the batteries are NiMH or NiCD, you will only have 3.6v available, which will not be sufficient.

    You will need to wire your LEDs with separate resistors for each.

    I suggest that you use a typical Vf of 3.4v @30mA.
    The generic formula for LED current limiting resistors (Rlimit) is:
    Rlimit >= (Vsupply - Vf_LED) / DesiredCurrent
    So, using your values:
    Rlimit >= (4.5v - 3.4v)/30mA
    Rlimit >= 0.9v/0.03A
    Rlimit >= 30 Ohms
    Next, we consult a table of standard resistance values. One is here:
    http://www.logwell.com/tech/components/resistor_values.html
    Refer to the E24 (green) columns. We can see that 30 Ohms is a standard value (300/10)

    You will need five 30 Ohm resistors, one per LED.

    I suggest also using a switch so that you can turn them on and off easily.

    [​IMG]
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    What Wookie has shown is the simple way, which I would go for. There are more complex ways to do the same thing, some circuitry required. If you need to use the battery voltage shown and want to drive the LEDs let us know, there is a very simple 555 circuit (the LM555 is a type of integrated circuit) that can do it.
     
  7. WILLIAM RYAN

    Thread Starter New Member

    May 15, 2010
    3
    0
    Thanks,very much appreciated
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Maybe the absolute max current allowed for the LEDs is 30mA. Then they will last much longer at 20mA or 25mA.

    Three AAA alkaline cells produce 4.8V when brand new. With a max current of 25mA for 5 LEDs then the battery voltage will be only 3.6V and the LEDs will be extremely dim in 4.5 hours. The LEDs will dim slowly over the entire 4.5 hours, most dimming in the first 2 hours.
     
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