How to wire 5x 3W leds in series with led driver.

Discussion in 'General Electronics Chat' started by semmyroundel, Oct 28, 2014.

  1. semmyroundel

    Thread Starter New Member

    Oct 28, 2014
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    Hi all, I'm trying to make an algae scrubber for my marine aquarium, this needs 5x3W 660nm deep red leds wired in series.
    The tech says forward voltage 2.4-2.7v, so would I be right in saying that a 12v 18w driver would be sufficient to power these in series?
    Also, what if I use less, say two, three or four leds? Do I need to introduce a resistor to each led, and what about 6 leds, do I need to get a driver that delivers 18 or 24volts?
    Many thanks.
     
  2. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Why are you restricting yourself to wiring them in series?
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    5x2.7 = 13.5V. So you either need a 15V+ constant-voltage supply with an appropriate wire-wound or ceramic power resistor, or you need a constant-current Led driver power supply that has a minimum compliance of 15V adjusted to deliver P/E = 3/2.7 = 1.1A. Either type of supply would have a minimum power rating of I*E = 15*1.1 = 16.5W.

    Or you could use two series strings of three, in which case halve the voltage and double the current...
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Yes
    Only one resistor is needed to limit the current to the series string. An 18V driver should be OK, as this exceeds 6 x 2.7V.
     
  5. MikeML

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    The OP proposed using a 12V supply, which does not have sufficient voltage to drive even five LEDs...
     
  6. mcgyvr

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    Oct 15, 2009
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    Do you want dimming?
    Do you have a reef controller that you want to control this LED with? If so whats its dimming signal?

    A few choices..
    1-Meanwell LDD + AC/DC power supply (can accept 5V PWM dimming)
    2-Meanwell LPF (can accept 0-10VPWM or 1-10V analog or 100k pot dimming)
    3-AC/DC power supply + high wattage resistor (which also equals heat + instability if Vf fluctuates)
    4-Other constant current drivers.

    An LED driver needs to have an output voltage greater than the sum of the Vf of the leds in the string and a current rating at or below the maximum current rating of the LED (700ma is good for those 3W LEDs.. (350mA if using a blue/sub 450nm LED)
     
  7. wayneh

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    Sep 9, 2010
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    He also asked about using 6 LEDs:
     
  8. semmyroundel

    Thread Starter New Member

    Oct 28, 2014
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    Hi all, thanks so much for your prompt input. The reason that I was going to wire in series was that the commercial versions of this project are done so, in fact on the forum dedicated to making these scrubbers, they frown upon wiring in parallel, I don't know why, I assumed it was something to do with the nature of the way the leds are driven so I didn't question it.
    I bought a 12v 18w driver and now I'm wondering because of the smaller size of the scrubber than was originally intended, due to my sump size restrictions, I will use 4 leds.
    So my questions should now be:
    Should I wire them in parallel, or series?
    If I use this driver, what resistor do I need (if any at all) and do I need one of them (I'm assuming this to be the case if in series) or four individual ones if in parallel?
    What does 2.4 to 2.7 forward voltage mean? I'm a qualified electrician and understand va ratings for lamps and power factor in discharge circuits, but have never heard of these terms.
    I'm not dimming.
     
  9. BobTPH

    Active Member

    Jun 5, 2013
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    Link to the driver, please.

    Bob
     
  10. wayneh

    Expert

    Sep 9, 2010
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    It means that, when they are operating on the specified current, they will drop that voltage across themselves. For instance you might say that an automotive bulb has a Vf of 12V, since it drops 12V when operating normally.

    The Vf is not a terribly reliable number for design purposes, but it helps you match a power supply voltage to a series string.

    Current flow is a far more important design consideration. Unlike the auto bulb, an LED does not self-limit the current it draws and can be easily damaged if current is not controlled externally.

    Most likely - depending on your driver - you will use a series arrangement. Then you need to control the current in just one string. Otherwise you need a current controller for each parallel string.
     
  11. MikeML

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    Oct 2, 2009
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    If you wire several LEDs in series (so that the same current flows through all of them), and you use your DMM set to DC Volts to read the voltage across each individual LED (one at a time), when operated at the rated current, the voltage across each LED will be somewhere between 2.4V and 2.7V. If you tested a 100 LEDs, and plotted their Forward Voltages, they would distribute on a Bell Curve, just like grades...

    If you grab four LEDs at random, wire them is series, hook them to a variable lab DC supply, and very slowly and carefully turn up the voltage (while watching the current meter to keep the current below the LED's max. current rating), you will find a point where the four LEDs light up, and the current is near the rated max. The total voltage could be as low at 4x2.4= 9.6V or as high as 4x2.7V = 10.8V, or anywhere in between, based on the random selection of the LEDs. You cannot operate them this way because as the temperature changes, the curent will increase, and you will burn them up.

    Now, if you hook the four LEDs to a 12V high-current constant-voltage power supply, you will instantly vaporize them. You need to put a resistor between the 12V supply, and the four LEDs to prevent that!

    Unfortunately, the specific value of resistance required to limit the current through the LEDs (think of ballast in the context of a florescent tube) is dependent on the exact total of the forward voltage drops of the LEDs. This is not the preferred way of doing this.

    It is much easier to buy a LED driver constant-current type of power supply, which has a pot on it to set the max. allowed current for your specific LEDs, and the power supply voltage magically takes care of itself with no resistor required...
     
  12. semmyroundel

    Thread Starter New Member

    Oct 28, 2014
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  13. semmyroundel

    Thread Starter New Member

    Oct 28, 2014
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    Geez, not enough current capability, shoulda looked more closely (4x700ma =2.8a)
     
  14. MikeML

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    Oct 2, 2009
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    What is the max allowed current for one of your proposed LEDs? Post a link to the LED's data sheet.
     
  15. MikeML

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    That thing is not even a "LED driver" in my understanding.

    What a dumb set of specifications...

    I make it out to be a constant-voltage 12V supply, designed to drive a "LED lamp replacement", NOT for driving series-connected individual LEDs.
     
  16. semmyroundel

    Thread Starter New Member

    Oct 28, 2014
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    No, that was my mistake, I just added up the wattage of the leds, doh! The reason I thought it might work is that I routinely instal self adhesive led strips to under-cabinet lighting with many leds on it and just wire in a regular driver such as this.
    Spec:

    2x 3W Deep Red Led for DIY lighting fixtures, aluminium flat bars and heatsinks.

    High energy efficiency, long life, high reliability and low maintenance costs.

    Applications: Plant Growth, Increase Flowering, Autumn, Sunset, Chlorophyll A, Food Production, Antioxidants, Aquarium, Tank, Cabinet, Increase Poultry Reproduction and Reduce Feed and Aggressiveness.

    Technical Parameters:Quantity: 2 pcs
    Emitted Color : Deep Red
    Intensity Luminous: 70-80LM
    Viewing Angle : 120 Degree
    DC Forward Voltage (VF) :DC2.2V~ 2.6V
    DC Forward Current (IF) : 750mA
    Wavelength: 660NM
     
  17. adam555

    Active Member

    Aug 17, 2013
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    If you are going to put the LEDs in series you don't need to multiply the current by the number of LEDs; it would be 700mA for the 4 LEDs in series.
     
  18. semmyroundel

    Thread Starter New Member

    Oct 28, 2014
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    Thanks for your reply, I don't understand how it's 550mA short though, I've done the math and can't get to 550mA short (once I posted the spec I can see they're 750 mA) so this driver says 1.5A
     
  19. adam555

    Active Member

    Aug 17, 2013
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    Forget about the 550mA; a mistake I already erased... read the primary's 0.15A instead of the secondary's 1.5A
     
  20. adam555

    Active Member

    Aug 17, 2013
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    Yesterday I was reading the datasheet for that type of LEDs, an those 2 voltages -at least in that model- where for the typical voltage and maximum voltage drop; not the minimum and maximum.

    So I was thinking, if 2.2v is the typical voltage, 12v for the 5 LEDs in series you had originally intended would come to 2.4v each; which shouldn't be too dim.

    Have you given it a try yet?
     
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