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Thread: capacitor value?

  1. #1

    Default capacitor value?

    i'm using Diodes Inc. driver chip AL8861 to drive 2 LEDs per string.
    they recommend 4.7 to 10uF cap.
    "The minimum capacitance needed is determined by input power, cable‘s length and peak current. 4.7μF to 10μF is a commonly used value for most cases. A higher value will improve performance at lower input voltages, especially when the source impedance is high."

    my input voltage can be as low as 10 volts. LEDs are no more than 2" away from driver. peak current is 1amp.
    i've been using 10uF caps but they have greatly increased in price.

    when they say "improve performance", does that mean running at lower required current?
    can i safely use 4.7uF caps?

    thanks

  2. #2

    Default Re: capacitor value?

    Quote Originally Posted by mercrazy View Post
    i'm using Diodes Inc. driver chip AL8861 to drive 2 LEDs per string.
    they recommend 4.7 to 10uF cap.
    "The minimum capacitance needed is determined by input power, cable‘s length and peak current. 4.7μF to 10μF is a commonly used value for most cases. A higher value will improve performance at lower input voltages, especially when the source impedance is high."

    my input voltage can be as low as 10 volts. LEDs are no more than 2" away from driver. peak current is 1amp.
    i've been using 10uF caps but they have greatly increased in price.

    when they say "improve performance", does that mean running at lower required current?
    can i safely use 4.7uF caps?

    thanks
    - What is your power source?
    - What are you making?

    You could possibly run with much smaller caps and be fine if you don't care about current accuracy, response time to Vset changes, etc. .... or EMI. You do at some level need to be concerned about ripple current too.

  3. #3

    Default Re: capacitor value?

    car/boat battery with alternator or stator charging.
    portable light.

    exact current is not critical but don't want noticeable change in brightness.

  4. #4

    Default Re: capacitor value?

    Quote Originally Posted by mercrazy View Post
    car/boat battery with alternator or stator charging.
    portable light.

    exact current is not critical but don't want noticeable change in brightness.
    If the 4.7uF are less than 1/2 the cost of the 10uF you could use 2 of them. I believe capacitors in parallel ADD values together.
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  5. #5

    Default Re: capacitor value?

    Capacitors in parallel do add their capacity, so 2x4.7uF will result in 9.4uF. But they will most likely take more space than one single 10uF. In the case you described, generally, 4.7uF should suffice, but what is your shopping source ? It's absurd to say they are expensive. I can find 10uF @ 35V at 1 cent/piece, while 10uF @ 250V is 10 cents /piece at most.

  6. #6

    Default Re: capacitor value?

    can you please provide your source for 10uf/35v for 1 cent?
    thank you very much.

  7. #7

    Default Re: capacitor value?

    Quote Originally Posted by mercrazy View Post
    can you please provide your source for 10uf/35v for 1 cent?
    thank you very much.
    Likely he means aluminum electrolytic, not ceramic. As you probably know, aluminum electrolytic are pretty much useless at this switching frequency.

  8. #8

    Default Re: capacitor value?

    you're probably right.
    i've hunted my butt off and lowest i've found from a reputable source were 20cents at high volume.

  9. #9

    Default Re: capacitor value?

    Quote Originally Posted by mercrazy View Post
    you're probably right.
    i've hunted my butt off and lowest i've found from a reputable source were 20cents at high volume.
    There is still allocation in some ceramics.

    You can find single reel quantities in $0.13. Not sure what your volume, but I can likely get a reputable Asian source at 1/2 that with truly high volume.

    What voltage are you using the capacitors at? If some cases you are better off with a different technology where there is less capacitance versus voltage change.

    Even a good low ESR, long life 10uF/35V is not $0.01. General purpose sure.

  10. #10

    Default Re: capacitor value?

    up to 26 volts.
    i normally buy 5000 or full reels.

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