need help with my circuit im a beginner

Discussion in 'General Electronics Chat' started by metalca, Sep 11, 2014.

  1. metalca

    Thread Starter New Member

    Sep 11, 2014
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    im working on a infrared illuminator, im powering it using a 9v battery, im using 20 leds, in 5 rows of 4 leds,
    the battery drains after 10 mins, im not sure if I have the right resistors or the right amount of resistors on the circuit,
    resistors I have
    0.25w
    270ohm
    carbon film
    ammo packed
    1/4 watt- tolerance 1%

    the leds I have
    10mm
    forward voltage; 1.5--2V
    backward voltage; 5V
    forward current; 20mA
    wavelength; 850nm
    angle; 30 degree
    luminous power; 300mw
    low frequency
    infrared ray


    that's the specs of the two parts im using, I eed help with amount of resistors or correct resistors, or if it cant be done what and how many resistors would I need for a 12v circuit, im new to this a ay help would be great.
     
  2. bertus

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  3. metalca

    Thread Starter New Member

    Sep 11, 2014
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    im a amateur only started few weeks back all the electronic talk an symbols are all chiese to me I haven't a clue as im only new, I thought I had the light made perfect then battery died after 10 mins an I tried another light same thing
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You have 5 strings of 4 leds with an 270 Ohms resistor.
    The battery voltage is about 9 Volts.
    The 4 leds take 4 X 1.5 Volts = 6 Volts.
    The voltage over the resistor is the difference = 9 - 6 = 3 Volts.
    The current through the resistor ( = the current through the leds ) = 3 Volts / 270 Ohms = 0.0111A = 11.1 mA.
    5 strings will take 5 X 11.1 mA = 55.5 mA.
    This could be quite a strain for the small battery.

    Bertus
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,077
    3,013
    I agree, it's that simple; too many lights and not enough battery.

    20 LEDs at 300mW each is 6W. A typical 9V battery might provide 400mAh to your lights until the voltage has sagged so far it will no longer work. If it really maintained, say, 8V (it won't), that would be 3.2W•hrs or about one half hour at 6W. Ten minutes is a little short, but not at all surprising.

    The batteries may have some useful life left in them for other uses, though.
     
    Last edited: Sep 11, 2014
  6. metalca

    Thread Starter New Member

    Sep 11, 2014
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    What about a 12v circuit how many resistors would i need on it.
     
  7. bertus

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    Hello,

    What will be the 12 volts powersource?

    Bertus
     
  8. metalca

    Thread Starter New Member

    Sep 11, 2014
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    4 AA batteries
     
  9. metalca

    Thread Starter New Member

    Sep 11, 2014
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    Sorry think its 8 AA batterys
     
  10. bertus

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    Hello,

    4 AA batteries will be 4 X 1.5 Volts = 6 Volts for alkaline batteries and 4 X 1.2 Volts = 4.8 Volts for the rechargable NiMH batteries.

    This will be much lower as the wanted 12 Volts.

    Bertus
     
  11. wayneh

    Expert

    Sep 9, 2010
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    Well, you can place 5-7 LEDs in series. If you want to use identical strings, 4 strings of 5 LEDs is the choice. Then you calculate the resistor as Bertus has shown, as would also be detailed in the link he provided.

    [didn't see the AA comment in time]
     
  12. metalca

    Thread Starter New Member

    Sep 11, 2014
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    Ill work something out thats for the help
     
  13. metalca

    Thread Starter New Member

    Sep 11, 2014
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    If i have a 12v powersupply (8x AA batteries) what resistors would i need an how many
     
  14. metalca

    Thread Starter New Member

    Sep 11, 2014
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    Im sorry to go on its just its new to me an i wouldnt be the smartest about lol. A 12v holds 8AA batteries if each batterys 1.5v thats 12v altogether then how do i work out what resistors i need. The link doesnt work for me
     
  15. wayneh

    Expert

    Sep 9, 2010
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    3,013
    First off, your cells may not be 1.5V. That depends on the battery chemistry and state of charge.

    But it's not hard to make the calculation: In any electric circuit, the total voltage drops across all the devices in series. So if you have 5 LEDs in series with a 12V battery, those LEDs will drop, say, 5 x 1.5Vf = 7.5V across themselves. The resistor must drop the rest, or 12V - 7.5V = 4.5V.

    Ohms' law, V = I•R, is used to calculate the proper R. In this case 4.5V = 0.02A • R and thus R=225Ω. That's to achieve 20mA. If that's the maximum spec for your LED, then you should aim below that, meaning you should use a higher value resistor to lower the current to 10-15mA. Your 270Ω resistors would be fine.
     
  16. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The leds are 1.5 to 2 volts.
    For the calculation of the resistor, I will use the lowest value as that will give the highest current.
    The leds are rated for 20 mA, lets take 10 % lower value to be safe, so 20 - 10 % = 18 mA.
    The supply voltage is 12 Volts, this woulld give a max of 12 / 2 = 6, but then there is no voltage left for regulation, so lets take 5 leds.
    5 leds will take 5 X 1.5 Volts = 7.5 Volts.
    The voltage across the resistor will be the difference betwen the supply voltage and leds voltage => 12 - 7.5 = 4.5 Volts.
    The value of the needed resistor will be 4.5 Volts / 18 mA = 250 Ohms.
    The closest E-series value is 270 Ohms, this will give you a current of 4.5 Volts / 270 Ohm = 16.66 mA.

    Bertus
     
  17. metalca

    Thread Starter New Member

    Sep 11, 2014
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    How many resistors would i need on my circuit then. With 20 leds. 5 rows of 4 leds
     
  18. bertus

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    Apr 5, 2008
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    Hello,

    If you want to use 4 leds in a string, the regulation will be better, but you will loose more power.
    For 4 leds you will need the following resistor:
    (supply voltage - n X ledvoltage) / wanted current =>
    (12 - 4 X 1.5) Volts / 18 mA =>
    6 Volts / 18 mA = 333.333 Ohms.
    The closest E-series resistors are 330 Ohms and 390 Ohms.
    The 330 Ohms resistor will give you a led current of 6 Volts / 330 Ohms = 18.1818 mA.
    The 390 Ohms resistor will give you a led current of 6 Volts / 390 Ohms = 15.38 mA.
    You will need a resistor for each string of leds, so 5 resistors.

    Bertus
     
  19. metalca

    Thread Starter New Member

    Sep 11, 2014
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    I will start learning my electonic components an calculations use guys are great ill order some resistors an give it a shot. If i order 330ohm resistors an use 5 with the.circuit i have
     
  20. bertus

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