Drop Voltage for a Constant Current Supply

Discussion in 'The Projects Forum' started by jpguppy, Apr 25, 2011.

  1. jpguppy

    Thread Starter New Member

    Apr 25, 2011
    3
    0
    Hello All,

    I am finishing up a project using Cree LED's for a reef tank. I have a large majority of the electronics done except for one section: LEDs for aesthetics.

    I have a total of 3 separate strings that I am working with: Whites, Royal Blues and Cool Blue. All of these will be run on Meanwell ELN 60-48s. I had looked into a CAT4101 build, but it was going to cost just as much to make it as it was to buy the meanwells, so I went with them. The only thing I did not realize was my Cool Blues (6 LEDs total in the string) have too low voltage to run on the minimum 24V needed for the constant current power supply. I had considered buying a buckpuck to drive these but this requires a DC converter which is why I went with the Meanwells-converted straight from mains.

    First of all, at 350mA, the LEDs run at 3.2v each (19.2V total), which will more than likely be way too high of a current for their purposes (probably run at 100-200 mA). So, I'm looking for either a method to drop about 10V (to be safe) using these constant current regulators.

    Second, this setup runs with a dimmer effect at the beginning and end, ramping up/down the the light cycle as controlled by the microcontroller. Therefore, the current will be changing during this ramp up/down process.

    I had thought of using a Zener as a shunt, and stumbled across this link that talks about using a zener, resistor and transistor to regulate the voltage. Would this work for my purposes, or is there a simpler/cheaper method to drop an accurate voltage from variable 50mA-200mA?
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Could you point to the datasheets or source for your current limiters?

    The whole point of a constant current source is it doesn't really drop voltage (that is a voltage regulators job), but regulate current. Saying the LEDs don't meet a minimum voltage requirement (or is it the power supply?) doesn't really make sense.

    There are plenty of ways to do this, constant current regulators are not hard. Switching units run a lot cooler than analog versions, but analog versions are easy to build. Their problem is they get hot, and you have to plan for cooling.

    You will also have to plan on cooling for the LEDs themselves. At 3.2V Vf at 350ma they will dissipate 1.12W each. It may not sound like much, but it will burn your fingers, and the LEDs themselves if you don't heat sink them properly.

    If you have a fixed voltage power supply you are already more than halfway there. You could make a fixed resistor to drop 4.8V at 350ma, which would be 13.7Ω at 3W. You can use several high power resistors to do this. Since you don't care if the LEDs are driven to the max two 8Ω or 10Ω 1W resistors in series would work OK. Measure the voltage drop and use Ohm's law to determine the real current.

    Personally I would make a transistor current regulator. Power transistors are designed to connect to commercial heat sinks, overall a very inexpensive option.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    To the OP:

    Are you wanting to make this adjustable (as in a dimmer), or fixed?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Bill,
    He's using a microcontroller to control the ELN-60 supplys' built-in dimmer function.

    He just needs a way to drop around 10v.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Understood, but unless the OP changes the power supply voltage I suspect there will be no joy. If he can't change the power supply he is going to have to change the control device.

    It might be possible to get a wall wart with the needed extra voltage and insert it in series with the existing power supply voltage to provide the extra oomph, one for the LED controllers that need it of course.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    jpguppy, What size tank?.. Can you simply add some more LED's or rearrange your strings?
    By putting all 3 strings on 1 meanwell you aren't getting as much control over the color as you would with 2 or 3 meanwells...

    I'm very happy with my setup on my tank.. I have 1 string of 12 cool whites and 1 string of 12 royal blues. Each string on its own meanwell. That way I can dim the whites and blues on their own to dial in the color. I can also just turn off the whites and get a really cool moon light effect with just the royal blues dimmed down.
     
  8. jpguppy

    Thread Starter New Member

    Apr 25, 2011
    3
    0
    This is a setup for an SPS (High lighting demand coral) 29 gallon reef. Each color is on it's own respective power supply.This allows for individual control of each color. The whites and blues are fine as they are set up, it's just the 6 cool blues for additional light spectra/coral color enhancing.

    As stated earlier,the meanwells require a minimum of 24V to operate, hence my problem. I am just trying to be able to drop the extra voltage while running at 50-250mA, hence why I haven't just decided to go with a resistor setup, since the voltage will vary with current. I am planning on having these run from essentially 0% to 100% (with 100% being the maximum current that I decide within this range that meets my needs) depending on the time of day. SO at 'sunrise', they will be off, gradually increase along with the other lights over a set time, stay constant during 'noon', then gradually decrease until 'sunset'. This has all already been programmed.

    So basically, all I need is a somewhat constant voltage drop around 10V that operates between 0-250mA, as SgtWookie explained earlier.

    P.S.-Would have responded earlier had i known that there had been responses-apparently i didn't have it set up to email me when someone did respond....
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Personally I'd just get 2 or 3 more cool blue LED's into that string and call it a day.
    The best benefit is less of a spotlighting effect. The more LED's the more even the color looks.. and at 3-5 bucks an LED its cheaper/easier than having to rig something up.
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Or put that ELN-60-48 up on ebay (it will sell quick) and get an ELN-60-27
     
  11. jpguppy

    Thread Starter New Member

    Apr 25, 2011
    3
    0
    I looked into the ELN 60-27. It would be a good option other than the fact that it won't dim down to 50mA. They will only dim down to about 5-10% then shut off. Plus, the usable range would be very limited. I think I'm just going to go with adding two more LEDs. I've got a few extras of each kind, so I might just throw in some extra royal blues and see what happens. I will be running them on the low end, so i'm not really worried about messing any up by mixing colors on the same string.

    Thanks for everyone's input!
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I suggest that you add three more LEDs instead of two. The reason is that the Vf of the LEDs will decrease along with the decrease in current. Adding just two will not give you much of a margin, and I'm afraid that you won't be able to dim them as far as you would like to.
     
Loading...