LED red and blue lighting in series

Discussion in 'The Projects Forum' started by bobbyandck, Jan 10, 2014.

  1. bobbyandck

    Thread Starter New Member

    Jan 10, 2014
    Dear all,

    I'v been looking into this for some time but I don't seem to find the answer to my questions. As I am a beginner, I'll be rather detailed so that you can better see what are the things I got wrong.

    I have the following:
    A transformer giving 0.9A at 18-40V
    Red Leds using 0.75A at 2.2-2.6V
    Blue Leds using 0.7A at 3.6-3.8V
    (*Thank you MrChips for the correction, It is indeed all Amps and not mA)
    The point is to put as many red as possible but also some blue.

    Mounting it in parallel seems not possible as 0.75A+0.7A>0.9A
    So the only solution is mounting in one series but then I have the problem of different voltages (I guess the difference between 0.7A and 0.75A is too little to be of any major consequence, so I calculated on 0.7)...

    I was imagining the following series setup:
    10 Red followed by 3 Blue, that would give a total of 32.8-37.4V which gives a little space in terms of transformer variation.

    - First question, is this at all possible or will I simply burn the lower voltage red leds ? or should I mount them first blue and then red with a resistance in the middle ? or is that not a problem except perhaps a little less light ? (I've looked on various websites and they either say never to mix voltages or that you can do whatever you want, so I guess the answer is somewhere in the middle)

    I now try to calculate the resistance at the beginning (positive side) of the series. Using (Vinput-VƩleds)/Amps gives me between 3.5 and 9.6 ohm, which I guess is the size of the resistance I need. I therefore looked on a local online supplier (www.distrelec.ch, that's in switzerland) the closest I found was one of 10Ω with a nominal power of 3W. And that is where I get lost again.

    According to what I found on websites a 10Ω resistance at 3W only allows 5.48V [√(P*R)] and 0.548A [(√P)/R], which is quite a lot below my max of 37.4V at 0.75A... does that mean that the difference is too little to need a resistance or am I going to blow up my whole setup by even attempting something like this ?

    Thank you in advance for you time and assistance.
    Last edited: Jan 10, 2014
  2. MrChips


    Oct 2, 2009
    Just some quick comments while I am passing through.

    Did you have your units mixed up? Is the current draw 0.75mA or 0.75A?

    Yes, you are in danger of blowing the LEDs.

    It doesn't matter which way you string LEDs and resistors in series. The same current goes through all elements wired in series.

    LEDs require constant current, not constant voltage. What you need is a constant current supply. A 10Ω resistance is not sufficient to protect your LEDs from over current.

    Someone else will come along and guide you on how to drive your LEDs. Hang in there.
  3. elec_mech

    Senior Member

    Nov 12, 2008
    Welcome to AAC.

    As MrChips pointed out, your LED current draw specification looks off. I'm assuming the LEDs are drawing 75mA and 70mA, not 0.75mA (0.00075A), 0.7mA (0.0007A), 0.75A (750mA), nor 0.7A (700mA).

    Also, you state your transformer can output 18-40V at 0.9A. That is quite a range of voltages at a constant current. Does the transformer have taps at different voltages or is this a wall wart with selectable voltages? In either case, the current output usually changes with the voltage output (net power in watts remaining the same). So if the transformer is rated at 0.9A at 18V, it will probably only output ~0.4A at 40V. This is important to know.

    We need to know more about the LED and transformer ratings, but if we assume the LEDs are rated at 70-75mA and the transformer has an overall output of 16W (18V x 0.9A), then we can do the following:

    Ten red LEDs in series with three blue LEDs: 10 x 2.2V + 3 x 3.6V = 32.8V.

    We use the lowest LED voltage to be safe because this will give us the largest current-limiting resistor to use which will better protect the LEDs. Once assembled, you can test the actual current draw and decrease the current-limiting resistor if needed.

    As you concluded, 5mA probably won't make much of a difference, so we'll select 70mA to be safe.

    Assuming you set the transformer to 40V, then:

    40V - 32.8V = 7.2V - this is amount of voltage we need to drop across the resistor.

    Ohm's law tells us:

    E = IR

    7.2V = 0.07A x R

    R = ~103Ω

    3Ω won't make a huge difference, so let's say 100Ω since this is commonly available.

    Now you need to make sure the resistor is rated to handle the current across it.

    P = VI

    P = 7.2V x 0.07A = 0.504W

    Alternately, P = I x I x R = 0.07 x 0.07 x 100 = 0.49W

    So the resistor will need to dissipate 0.5W. A 0.5W resistor would be pushing the resistor to its limits, something you never want to do. A general rule of thumb is to select a resistor with at least twice the power rating you need. In this case, a 100Ω, 1W resistor should work fine, but no harm in getting a larger wattage rating if you so choose.

    As MrChips mentioned, it does not matter what order you put the parts in - they are all in series so the same current is passing through all of them.
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    700mA (.7A) is VERY common now for high power 1-3W LED's

    The different voltages in series doesn't matter..
    BUT your power source must be greater than the sum of all the max forward voltage ratings..

    And to use resistors for current limiting high power LED is usually silly/inefficient.
    Look into constant current drivers..
    Like the meanwell LPF series or you can use the LDD series with an external constant voltage power supply..

    Have fun with your grow light. Hope you used the correct wavelength (nm) range for those leds..
  5. bobbyandck

    Thread Starter New Member

    Jan 10, 2014
    First of all thank you mech_elec for your detailed response.

    Thank's to your explanation, I now know that I can mix voltages in a series, and that the load that the resistor needs to take in charge (P) is not the full load of the circuit but only whatever is left for the resistor (it makes sense now that I know it...:)).

    I double checked the base data and I get the following:
    Output Voltage: 28V-38V
    Current: 900mA
    (I guess that as it is a constant current transformer, the voltage should average around 30/0.9=33.33V)
    Forward Voltage:3.4V~3.8V
    Forward Current: 700mA
    Forward Voltage: 2.2 ~ 2.6Vdc
    Forward Current: 750 mA

    Having looked on various websites who always mention between 20 and 90mA, the 700-750mA does seem off, but the vendor mentions that they are high power 3W leds with the specifications mentionned above.

    With that and following the 10r+3b setup we have 7.2V (as you calculated) that the resistor needs to take in charge.

    So 7.2V/0.7A=R=10.28Ω≈11Ω
    P = 7.2V x 0.7A = 5.04W
    P = 0.7 x 0.7 x 11Ω = 5.39W

    Does that sound right ?

    Thank's again for the help.
  6. bobbyandck

    Thread Starter New Member

    Jan 10, 2014
    mcgyvr, you're quite right it is for a grow lamp, I chose blue 460nm and red 660nm, it is just as an occasional use, to help my plants that get really little light in the winter. I guess that if the colour is slightly off, it won't matter too much, and as apparently every plant responds differently, I imagine it is a bit of a guess that it will work with mine...

    sorry, I forgot to mention in my first post that it is a constant current driver, but then what would be the most efficient way of limiting the power ?

    Thank you
  7. elec_mech

    Senior Member

    Nov 12, 2008
    Whoa, those are big puppies. Having no experience with high powered LEDs, I would defer to mcgyvr's advice - look for a constant current supply. A current-limiting resistor with that much current is going to be considerably inefficient and you'd need a resistor rated for at least 10W.

    Also keep in mind you'll be pushing the transformer to its limits. 13 LEDs x 3W = 39W; transformer = 30W. Each LED isn't quite 3W (2.6V x 0.75A = ~2W), but keep this in mind moving forward.

    Hopefully mcgyvr will go into more detail about constant current supplies. I found this, but I have no idea how to set these up.
  8. BobTPH

    Senior Member

    Jun 5, 2013
    You cannot make a 0.9A constant current source output 0.7A. You need a constant current driver that can be set to 0.7A.

  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Assuming the sum of the max forward voltage drops is 37.4V (I didn't do the math) and you want to run them close to 700mA then I would suggest this driver. Meanwell LPF-25D-42
    Its rated to 23.1 to 42VDC output and 600mA.
    You can get them online for about $26.. Pick up one of those and a male outlet plug.
    If you would like the ability to adjust the brightness then also pick up a 100K potentiometer for the dimming signal input.
    In general when picking one you want a DC output greater than the sum of the max Vf and slightly below the max current rating. That about it for proper sizing..

    Thats all you need.. Wire all the 13 leds in series to the output, slap a plug on the input wires and voila..magic. Of course you need substantial heatsinking as these LED's get hot quick and without a decent heatsink they will go up in smoke in minutes or less..

    I offer help on these types of setups ALL the time.. Its VERY common in the saltwater reef world.. Some of us are using a whole bunch of these LEDs mounted 100+ 3W on star metal clad PCB's and bolted to big heatsinks to replace metal halide lighting over coral reef tanks.. Its done all the time..
    My last fixture was done using 2 x 100W leds..
    Also built a grow light for the wifes veggies out of a single 50W blue LED and 5 x 20W red LEDs.
    I always recommend meanwell constant current supplies as they are high quality and just run forever.

    Last thing.. NEVER look directly at the LED's when they are on.. They WILL blind you. It only takes a few seconds to cause possible permanent injury. Even just 2 seconds and you will be seeing spots for a good hour.. trust me.
    elec_mech likes this.
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
  11. bobbyandck

    Thread Starter New Member

    Jan 10, 2014
    First of all, sorry for my lack of replies.

    I've finally gotten to assembling everything, including the led driver mcgyvr suggested. And it seems to work perfectly.

    The only exception being the potentiometer (Vishay 1W 100kOhms) which doesn't seem to do anything at all, but I'll need to investigate that further

    I'll now try leaving it on for longer and longer periods and see how it behaves.

    I'll keep you updated, but so far so good.

    Thank's for all the help !

    Best regards
  12. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    You just have it wired wrong.. It has 3 terminals.. you need one end in the dim+ and the wiper in the dim- Then it will dim accordingly.