How to connect more LEDs in series

Discussion in 'General Electronics Chat' started by Sonoma_Dog, Oct 5, 2008.

  1. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
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    hi all, Currently i am using a 12 volt to drive LEDs. I can only connect 3 LEDs in series.

    I would like to connect more LEDs in series, maybe 10 or more, so I am thinking about steping up the DC voltage.

    if I do that, will i use more power?
    I only need to drive about 10-15mA to the LEDs.
    ,and what kind of circuit I need to use to step up the DC voltage ?



    Thanks all!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You should keep your voltage supply at under around 50v for safety. Once you get up over 50v, you're in potentially hazardous territory.

    If you try to "boost" your existing 12v supply to a higher voltage, you can exchange voltage for current, but there is a "tax" called efficiency. About the best efficiency you could hope for would be around 85%.

    You could consider building a higher voltage DC supply, maybe using a transformer with a 36VAC secondary. Rectified output would be just under 50v. You'll still need to regulate it. An LM317 could be used, as long as the output was not less than 40V different from the input voltage, and a good heat sink were used.

    But, one resistor for every three LEDs would be a lot cheaper than building a whole new power supply. It's up to you.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    Is there any particular reason for you to prefer serial connection over parallel connection of the LEDs?

    hgmjr
     
  4. chrissyp

    Active Member

    Aug 25, 2008
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    Hi
    The answer to hgmjr 's question is that current driving an LED array in series is the only way to do the job properly . Parrallel is not an option.
     
  5. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    I believe there are a couple of advantages for driving many in series. You can guarantee that the LEDs are illuminated to the same level . And you can use thinner traces on a PCB. You can probably put in more voltage overhead for less wasted power.

    These are my guesses anyways

    Steve
     
  6. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
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    another reason i wanted to connect LED in series is because it would make the wiring much easier.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    When paralleling strings of LEDs, one needs to supply the array with a regulated voltage, and each string needs a current limiting resistor or other means of controlling the current. If each string has an active device or circuit controlling the current, the voltage supply to the array does not need to be regulated, so long as the active device/circuit's wattage rating is not exceeded.

    Attempting to supply an array of LEDs using current regulation is an invitation to disaster via the "domino effect".

    If one string fails, the remaining strings must carry the failed string's current, soon causing another string to fail. The 2nd failed string's current is divided among the surviving strings, which are now likely carrying far more current than they were designed for. The remaining strings fail in rapid succession.
     
  8. chrissyp

    Active Member

    Aug 25, 2008
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    Hi
    I don't understand your reservation of current driving an led array.If you have say 5 led's needing 3.5v @ 1a and you supply the array with 17.5v @ 1a, if , an led fails in that array ,they all still get an amp , no domino effect.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    You are describing a "string" of LEDs in series that have the voltage and current controlled, not an array. An array would be multiple series strings connected in parallel.
     
  10. chrissyp

    Active Member

    Aug 25, 2008
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    Hi
    Ok , I will re word my original statement .Drive led's in a series,current driven string ,it is the only way to drive them properly.Parrallel driven systems are not an option.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    When the voltage is stepped up then much more power is used and much more current from the from the battery is needed.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Is there an echo in here? :rolleyes:
     
  13. chrissyp

    Active Member

    Aug 25, 2008
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    WHAT!!!
    The current of a series driven led string will always be far less than parallel, irrispective of the voltage
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    But voltage is extremely relevant. While LEDs are current devices, they drop a given amount of voltage, Vf, and you can't get around it. If the devices drop 3.5 volts each, and you have 12 volts for your power supply, you can only have 3 in the chain (3.5V X 3 = 10.5), if you have 4 LEDs there won't be enough current to turn them on properly. The only way around it is to increase the power supply voltage.

    If you have 24V then you can have 24V/3.5V = 6.8, or 6 LEDs, with the resistor using the .8 leftover to regulate the chain. 6 X 3.5 = 21V, so you figure out the resistor needed to limit the left over 3V to what ever current you want (say 20ma).

    Just to make life more difficult, LEDs don't really drop as predictable a voltage as we might like. Even within the same family there can be a variation of several tenths of a volt.

    Electronics and physics don't really care what we want, they just are, and we have to adjust our expectations to them.
     
    Last edited: Oct 7, 2008
  15. Audioguru

    New Member

    Dec 20, 2007
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    12 at 1A is 12W. 12V at 4.17A is 50W (LEDs with current limiting resistors, in parallel).
    50V at 1A is 50W.
    To use 12V and boost it to 50V/1A (LEDs in series and in series with a current-limiting resistor) then the current in the 12V battery is 4.9A (85% efficiency).
     
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