Easy to use buck current regulator?

Discussion in 'The Projects Forum' started by spinnaker, Dec 11, 2015.

  1. spinnaker

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    I have a project already built.

    Look at related thread for details.
    http://forum.allaboutcircuits.com/t...ts-of-small-leds-or-fewer-larger-leds.118425/

    It is a LED light of 64 LEDs. It operates 12V / ~640ma. The current light is driven by a Luxdrive buck puck which works great. It regulates the current and provides 5V for the pic. It's only problem is they are a bit expensive. I want to have a backup and figure I could now build one myself.

    I do want to build a pin compatible version so a LED driver with multiple but lower voltage and current outputs won't do.

    I already have the 5V regulator handled with a LM2675-5. Very easy to use and low part count. I would like to find a similar buck chip to regulate the current to 640ma .

    Any recommendations?

    Bonus for

    1. Also supplying 5V so I can use just one inductor.

    2. It is adjustable with a trim pot.

    3. It is through hole.

    I do have some pt4115s on hand. Nice low part count but I they are fairly old technology and I purchased them from eBay so not sure how reliable they are. They do seem to work as I have already built a prototype. Plus the pt4115 is surface mount which is just a bit of a pain.
     
    Last edited: Dec 11, 2015
  2. Papabravo

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    A current regulator works by changing the output voltage and if you are driving LEDs don't they already regulate the current fairly well?
     
  3. spinnaker

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    Not sure I understand your question.

    Maybe I am using the wrong terminology?

    I am using the adjustable version. Part Number 3021-D-I-700
    http://www.ledsupply.com/led-drivers/buckpuck-dc-led-drivers
     
    Last edited: Dec 11, 2015
  4. spinnaker

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    Never toss anything out. I had a fried buckpuck around here for a long time. Had the idea to cut it open to see if I can get a clue as to what they did but it is gine. :(
     
  5. Papabravo

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    You seem to think that because a SMPS exists that regulates voltage, there must also be an equivalent circuit that regulates current. I'm not sure that is correct. You can make a current regulator from a linear voltage regulator by floating the ground. That said, I question the need to regulate current if you are driving LEDs. The LEDs and the series resistor determine the load current. Why do you need have redundant current regulation?
     
  6. crutschow

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    As PB noted, don't the LEDs have a built-in resistor (?) so you need to supply them from a 12V voltage source, not a current source.

    What is you input voltage that you want to regulate.
     
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  7. spinnaker

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    There is no current limiting resistors. That is the point of the buck regulator. The LEDs are being supplied by a 12V battery. I do not want to waste energy heating a current limiting resistors.

    There are switching regulators that regulate current. That is exactly what the buck puck does that I posted. It is adjustable up to 700ma. I think I have it set at 640ma equal to the PT4115 switching regulator it replaced. The buck puck has the added bonus of supplying 5V out on an auxiliary pin to supply components like an MCU.

    Maybe it is I am not making myself clear?
     
  8. Papabravo

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    You're confusing things by the careless use of terminology. The link you gave us is for a boost regulator, not a buck regulator. The input voltage is 5-28V DC and I'm not sure what an input margin of -3 reefers to. It is also not clear if the output is greater than the input and by how much, although it says the output can be up to 48 volts. You do realize that for an output current of 350 mA, you'll need an input current of 700 mA minimum, if the output voltage is twice the input voltage. Next the link is talking about 350 mA and you're talking about 700 mA. How can we have a reasonable discussion if you're pointing us all over the place.

    Can we back up and start with the datasheet for the LEDs?
     
    Last edited: Dec 11, 2015
  9. spinnaker

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    I mentioned it was similar not the same. I found the right device and corrected the link above. The part number I am using is 3021-D-I-700. The adjustable version.

    Sorry for getting the terminology wrong but apparently even Luxdrive does, at least in the name. It is called a BuckPuck after all.
     
  10. spinnaker

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    There are no resistors. Current is regulated by the LED driver.
     
  11. Papabravo

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    IC's called "Buck Current Regulators" actually look like "Boost Voltage Regulators" in disguise. So here is the deal. The input current to the "Buck Current Regulator" is greater than the output current. The input voltage is less than the output voltage, thus my initial confusion. You insist that there are no resistors, but that remains to be seen. Using the chips appears to require some resistance, the required current, and the number of LEDs in the string. So hows about a datasheet for the LEDs.

    Here is one for example:

    http://www.ti.com/lit/ds/symlink/lm3404.pdf

    Used as a current regulator, only the feedback circuits is changed from a Voltage Boost Regulator. I can't tell you if it is suitable for your purposes or not.
     
    Last edited: Dec 11, 2015
  12. spinnaker

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    No datasheet for the LEDs. It is an old light that we bought years ago. The kit came with a light, battery, controller and solar panel. I rebuilt everything except for the light. Tried to get data on the light but the manufacturer either could not or would not supply any information. So the best I could do is to go by what I had. And that was the PT4115 on the existing controller that was fried. According to the datasheet for the sensing resistor used, the current was 640ma with a 12 V supply.

    I do want to eventually rebuild the light with known LEDs. A related post is linked above.

    There are no resistors in the light. It is just an array of LEDs. There are no resistors in my new controller. The only thing limiting current is the buckpuck.

    Here is the back of the existing LED array. Those are all LEDs.

    [​IMG]

    Here is the front
    [​IMG]


    Sorry I don't have it framed better but the only pictures I have. The light is lighting our sign right now.
     
    Last edited: Dec 11, 2015
  13. Papabravo

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    Good luck
     
  14. Lestraveled

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    Maybe this is what you are looking for. It is a buck regulator that is configured to operate in constant voltage or constant current. When the switch is in the down position it will regulate constant voltage. In the up position, constant current. Look at the data sheet for the LM2576 for more detail. In the constant current mode the variable resistor plus R8 sets the current. Current out is equal to 1.2 volts divided by resistance. I'll provide more data if you are interested. The circuit works well and will easily handle your current requirements.

    [​IMG]
     
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  15. spinnaker

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    All I am asking for is a recommendation on a chip to use for a constant current of ~640ma. I am not looking for someone to design it for me. The fact that I do not have a datasheet for the LEDs should be irrelevant. The specs are constant current of 640ma with 12V in. That should be enough.
     
  16. Lestraveled

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    Ah spinnaker.......look at post #14
     
  17. spinnaker

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    Thanks I did not realize that the LM2576 can be put into a constant current configuration. I did know the 34063 had that capability so I guess it stands to reason.

    What is the current range of this particular configuration?

    One thing I really like about the TI chips is their calculator / designer. Makes it a lot easier for an analog handicap person like me. Do you know if the calculator works in the current configuration? I went out looking for it and can't even find the voltage mode designer now. I could have sworn there was a link off of the product page.
    http://www.ti.com/product/LM2576

    And I just had the Adj in my shopping cart at mouser the other night but removed it before I ordered! I was going to use it for 3V but went with the 3.3v fixed instead. :)
     
  18. Lestraveled

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    The LM2576 is a 3 amp regulator. If you wanted the circuit to regulate at .64 amps then you would use a 1.87 ohm resistor instead of the variable resistor and R8. If you didn't need voltage regulation then simply remove R 5,6,7 and the switch.
     
  19. Lestraveled

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    Here is a simpler version with the voltage mode removed and over voltage protection added.

    [​IMG]

    The current out is set by R8. current out = 1.2/R8
    Max voltage out is set by the zener diode D1. Max voltage out = 1.2V + zener voltage.
     
  20. spinnaker

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    Thanks that is a lot easier. I see that R8 is a 2W. I assume that is going to dissipate a lot of heat? Especially since it is in series with the load. The whole purpose of me wanting to use a switching current limiter is to be efficient as possible since I am powering with a 12V battery.
     
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