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Thread: Constant Current Power needed... I'm confused...

  1. #31
    Flashaholic* DIWdiver's Avatar
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    Default Re: Constant Current Power needed... I'm confused...

    Quote Originally Posted by JustAnOldFashionedLEDGuy View Post
    Spend the extra and get the https://www.mouser.com/ProductDetail...c08YAcZw%3D%3D ... this is the B model. It has 0-10V dimming wires. Unfortunately the data sheet does not show what happens when the dimming wires are 0V (i.e. you short the two dimming wires together), but normally they would go to the lowest level or 10%. What is not clear is whether that is 10% of the current set with the potentiometer but it should be. That would give you with the dimming leads shorted, 10% of 3.35A or 335mA. That is going to do a lot less damage than 3.35 or 6.7A. You will need 2-3 units in series to get up to 24V before the driver will work properly. But even two in parallel should not be damaged with 335mA.
    The B model does not have the internal adjustments. If you want both the internal and external adjustments, you want the AB model. There does not appear to be stock of these at american distributors at the moment.

  2. #32
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    Default Re: Constant Current Power needed... I'm confused...

    Quote Originally Posted by Dave_H View Post


    Interesting article (though a bit older) dealing with 2 parallel case:

    https://www.ledsmagazine.com/leds-ss...rings-magazine
    BTW not saying the method in this article is what the OP should necessarily use.

    Not sure the probability of LEDs failing short but that could throw parallel balancing off quite a bit. Different modes of degraded operation are possible but self-destruct should not be one of them!

    Likely there is an OTS parallel drive board/module which could balance currents or at least sense loss of string and reduce current or shut off; perhaps used in combination with multiple low-current drivers.

    An aside, I designed a discrete 48vdc/200mA linear regulator circuit for some LED strips with 14 series (2 parallel) times 3 (84 LED total) This could be driven by regulated 48v switching supply, common in telecom industry for IP phones. Measured vf total came in at 45-46v which left comfortable overhead, not much power loss (3v*0.2A = 0.6W). Considering the adapter is probably 90% efficient, overall efficiency of 80+% is not bad for this power level. Single-chip dc drivers are attractive but I wanted to customize some functions. For example, if the input voltage got too high (e.g. from using unregulated 48v "brick" adapter), it would switch to lower current to keep dissipation down, avoiding large heatsink on the MOSFET; or shut off completely.

    Obviously the designers of those strips thought they could get away with that series/parallel arrangement; matched LEDs on the same PCB. Or maybe that's why I got them surplus for such a good price...

    Dave

  3. #33

    Default Re: Constant Current Power needed... I'm confused...

    Quote Originally Posted by DIWdiver View Post
    That Mean Well power supply looks perfect. It's a solid brand, and I agree, buy it from a distributor like Mouser, DigiKey, or others. I plugged it into Octopart, and it looks like Newark has the best price at $72.32. Check their shipping, though, as it could swallow most of that savings.

    The first powerup is always scary. I'm coming up with no simple way to do it in stages.
    Well, I guess I'll move ahead with this plan. I hearing its a big project, and has some juice running through it. I will take care. Many thanks for the help gentlemen. I will post with my result/further questions. T

  4. #34

    Default Re: Constant Current Power needed... I'm confused...

    Dave_H,


    The majority of commercial and even residential fixtures are made with paralleled low power LEDs. We don't have the wide Vf variations we once had, though effective resistance is lower which makes things worse. COBs are already matched internally for Vf.


    The reason why you parallel the LEDs first, then stack them in strings is it averages out differences better and mismatches are per parallel section, not on whole strings.

  5. #35
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    Default Re: Constant Current Power needed... I'm confused...

    You could use something like this https://www.onsemi.com/pub/Collateral/NSI45060JD-D.PDF
    They are $0.89 at DigiKey for example.
    Look thru the spec (page 7 in particular)
    You can connect it with LEDs directly to AC. Well thru the bridge :-).
    You can use one bridge per connection point (all LEDs if they are plugged in one socket)
    Then make a strings of LEDs in series with this device in each.
    One goes to 100mA (you can adjust it with resistor between 60-100mA) but you can connect couple in parallel with one LED string to get more output.
    If you connect 3 those devices in parallel (see page 6) you can set any current between 180-300mA
    Don't be scared by 45V rating at the beginning of spec, it is max voltage between anode and cathode of this part. It means that input voltage can't be higher than 45V from voltage dropped by LEDs.

    For example, if you use 120V AC input, your LEDs are 9-12V I would put 10 of them in one string. Measure voltage, see how close it is to input, remove LED if is to high (LES are really droping 12V or more each) or add LED if is low and there is a space (e.g., you get 90V on 10 LED)

    Edit: Adjustment resistor goes between pin 3 (Radj) and cathode (pin 4), value you get from Fig 6 on page 4 (0.125W resistor). Not connected pin or anything above 1000ohm gives 60mA output, about 7ohm for 100mA. Looks like 33ohm should give 80mA and 68ohm 70mA (these are common resistor values)
    Last edited by arek98; 05-22-2020 at 09:04 PM.

  6. #36
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    Default Re: Constant Current Power needed... I'm confused...

    Quote Originally Posted by arek98 View Post
    You could use something like this https://www.onsemi.com/pub/Collateral/NSI45060JD-D.PDF
    They are $0.89 at DigiKey for example.
    Look thru the spec (page 7 in particular)
    You can connect it with LEDs directly to AC. Well thru the bridge :-).

    .....
    Perhaps the OP can comment on whether he wants to take on driver design at the component level. Looks like a
    small (or maybe not so small) PCB to mount these.

    This is a linear device which brings a number of design issues including thermal management; and dealing with
    line voltage requires correct component selection and safety issues e.g. fusing.

    In my electronic design and component engineering experience over time, I find you really need to read
    the datasheets and application notes carefully. For example maximum power per device is as low as 1.77W at
    25C with minimal heatsinking (~1/2 sq. in. of copper), which for many cases is unrealistic and needs to be derated. At
    100mA, means limiting voltage drop across each device to below 17v including variations in line voltage. Minimum voltage
    across device is about 4v, below which current drops off. This adds up to a narrow window for number of LEDs in series.
    Actual vf of the LEDs needs to be accurately known (or at least measured) for given current; 9-12v is a wide range.

    Not sure the device has thermal and short-circuit protection.

    I don't want to recommend too many specifics but looks to me like pre-made properly-certified ac-dc CC drivers are
    a strong approach. They offer flexibility, simplicity, efficiency, safety, and overall outweigh any benefits of linear
    drivers for this level of power; and may cost less in the end.

    And again, not objecting to linear drivers in some circumstances as noted from my 48v/200mA discrete design above. BTW
    Infineon also has some linear CC devices for this type of use.

    Dave

  7. #37
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    Default Re: Constant Current Power needed... I'm confused...

    Quote Originally Posted by arek98 View Post
    You could use something like this https://www.onsemi.com/pub/Collateral/NSI45060JD-D.PDF One goes to 100mA (you can adjust it with resistor between 60-100mA) but you can connect couple in parallel with one LED string to get more output. If you connect 3 those devices in parallel (see page 6) you can set any current between 180-300mA Don't be scared by 45V rating at the beginning of spec, it is max voltage between anode and cathode of this part. It means that input voltage can't be higher than 45V from voltage dropped by LEDs. Edit: Adjustment resistor goes between pin 3 (Radj) and cathode (pin 4), value you get from Fig 6 on page 4 (0.125W resistor). Not connected pin or anything above 1000ohm gives 60mA output, about 7ohm for 100mA. Looks like 33ohm should give 80mA and 68ohm 70mA (these are common resistor values)
    Setting each CCR to 100mA (its max.) using 7 ohms will result in much lower RMS current (looks like 50% or less) because 100mA limit is peak current, and full-wave rectified ac results in pulsed output. Application note AND8433/D referenced from datasheet details this. Higher-output e.g. 160mA CCRs are available but still leaves a lot of limitations (and calculations); still need at least 3 in parallel. This will also result in 120Hz flicker which may or may not be an issue. This configuration is similar to LED Christmas lights, 60Hz flicker is visible (and sometimes disturbing), but 120Hz is better (my experiment by adding a full-wave bridge). Looks like using switching-type CC driver module is still a better approach. Dave

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