LED Limiting capacitor question?

Discussion in 'The Projects Forum' started by agenttang2001, Nov 3, 2010.

  1. agenttang2001

    Thread Starter New Member

    Nov 3, 2010
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    Hey guys, im new to circuits, i have a question im making an array of 4 blue LED's with a 3volt drop across each running at 20 mA. now my question is the 12volt battery source fluctuates and runs about 12.5 voltish so what would be better to use a limiting capacitor or will a resistor will be sufficient? any help will be appreciated thanks
     
  2. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    I am not quite sure how you would use a capacitor to limit current to LEDs athough if you put one in series it certainly would limit the current, like none at all. Stick to the resistors for current limiting. There some great links on this site for LEDs, just try doing a search.
     
  3. agenttang2001

    Thread Starter New Member

    Nov 3, 2010
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    Thanks for the help! yeah i did some research on resistors and know about those but i didnt know if you could use a capacitor do limit. would it be a good idea to use a capacitor in line with a resistor to steady the current? or would a capacitor be more used for timing?
     
  4. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    As a capacitor will not allow DC current to flow if you put it in line with the resistor you will stop the circuit from working.

    Because you are powering this with a battery there is no need for a capacitor to "steady" the current.

    I suggest you have a rea of the topics across the top of the screen, these will give you a better understanding. Also start at Volume 1.
     
  5. Wendy

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  6. agenttang2001

    Thread Starter New Member

    Nov 3, 2010
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    Hey thanks guys i didnt even see those lol i was searching the posts. i will check them out!:)
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Keep in mind that a lead-acid battery will measure around 12.7v to 12.8v when fully charged (depending on type), and will be fully discharged at around 11.4v to 11.5v at room temperature (25°C).

    You really don't want to completely discharge a lead-acid battery, as it's very hard on them. Below 80% of full charge (around 12.5v) they start sulfating, where the sulfur part of the sulfuric acid combines with lead oxide and gets deposited on the plates; weakening the electrolyte strength and inhibiting the ability of the battery to accept or release a charge.

    For operation from a battery, you should only use a couple of those LEDs in a string with a resistor to limit the current.

    Blue LEDs typically a higher Vf than 3v at their rated current - more like 3.6v to 3.9v.

    You should post the part number of your LEDs, or a link to the specifications. You want to look for Vf(typ) at their rated continuous current.
     
  8. agenttang2001

    Thread Starter New Member

    Nov 3, 2010
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    Wavelength 470nm
    Typical Voltage 3.0v (Max 3.6v) at 20mA

    here is the information that i got for my leds. also could a regulator work on such a circuit? oh and also the battery is in a car. thanks!
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Use a volt meter and check your battery while the car is running. You will see 13.8-16 or more volts depending on the state of charge of your battery and the operating speed of the motor.

    You must make sure that the circuit will not allow more than the max current at the max voltage the vehicle can attain, higher voltage, car running.

    In these situations you almost have to use a current limiting regulator. Using such a thing would eliminate the need for a series resistor. The three legged regulators are a breeze to wire up and use, and are priced right, probably save money if you bought resistors at corner 'Electronics store' prices by using one.
     
  10. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    You can use a LM317 as a current regulator to run the LEDs at around 18mA, depending on the source of the LEDs, at 20mA continuous, some may die, some may not, I add a little margin especially when getting the "grab bag" types of LEDs for projects with my kid.
     
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