Help sizing components

Discussion in 'General Electronics Chat' started by spinnaker, Dec 28, 2009.

  1. spinnaker

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    I need to make another order to mouser to finish up my bench supply project but figured I might as well order some of the supplies for my second project which will be a PIC controlled solar charger.

    For now it will be just in the experimental stage. I will be real happy if I can get the PIC to sense the input voltage and turn the light on at a given voltage level (probably zero) at "night" and switch off the battery if the voltage goes beyond a certain level "daytime" charging. All from my test bench.

    I can always get into a supply for the PIC, proper charging techniques etc. etc. at a later date. But for know it would be just nice to experiment with the PIC and some basic interfaces.

    To turn the light on and off and to disconnect the solar panel from the battery I was considering using a solid state relay. During the best part of the day, my panel but out just a bit more than 2 amps short circuit.
    Would this mean I would need at least a 2 amp relay? Once you get over an amp they start getting a bit expensive.

    My supply will be 12V,

    I took pictures of the light I need to light. It looks like there are 36 circuits of 3 leds in series. I don't have any other information.
    What is the typical forward voltage and current of each of these LEDs? I found something to suggest that they they are around 3.5V and 20 ma? Is that correct? How do I calculate my current draw for such an array? Is it simply 36 X 20ma = 720ma?

    Would I need a current limiting resistor for such a large array?
     
  2. SgtWookie

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    Did you have a particular PIC in mind?

    You'll probably be better off to use power MOSFETs. They'd be much cheaper.

    You might consider getting some logic-level N-ch power MOSFETs, and maybe a few P-ch power MOSFETs.

    These N-ch MOSFETs are interesting, and cheap:
    http://www.mouser.com/ProductDetail...=sGAEpiMZZMtvcUztdGSumAHfsy4NaSCXaqh6V6tWGmo=

    You might use these P-ch MOSFETs for the charger portion:
    http://www.mouser.com/ProductDetail...=sGAEpiMZZMtvcUztdGSumApX9f0VyyIcZUIZ3nJygFU=

    It's hard to say if they were getting regulated power, as you haven't shown us how they connect into the charge controller board, and we haven't figured out how the charge controller board works, or worked.

    But, white LEDs generally have a Vf anywhere from 3.3v to 4v, and those size LEDs usually are rated for 20mA to 25mA current.

    Just a single resistor would be pretty wasteful, and would get quite hot.

    There was at least one inductor on the controller board. I don't know if that was used just for charging the battery, just for controlling the LED current, or possibly both.
     
  3. spinnaker

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    Not let. I figure something in the 18F family. Gives me a bit more memory for C. Something with at least 2 analog channels and enough i/o to run an lcd and a few inputs for setup buttons.




    I actually considered a transistor switch but you turned be on to the solid state relays in a previous post. They looked interesting and easy to use so I thought I would give them a try but I will give the mosfets another look.


    And it is going to be hard figuring that board out without a schematic. Even if I wanted to work backward from the board itself, I doubt it would do much good since we don't know what 90% of the components are due to all of the coating.

    So I think we are starting from scratch. So the question is, how do I light the panel? And do it so the panel lasts more than a week?

    I hook the LED panel up to a 12 VDC supply, connected my current meter and the panel was drawing about 1 amp. BTW Lights are nice and bright.

    Which is curious and maybe a hint to what is on the original charger/controller. It has a .5 amp fuse. So 1 amp would have blown that fuse if the light was connected to the battery right?
     
  4. spinnaker

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    Don't know if this is any help but I took a closer look at the original controller. It looks like the light is connected to a tiny chip PT4115 08A81C. This chip is connected to the inductor that you mentioned. The positive lead of the light is also connected to the "inductor". Does this make any sense?


    Ah you are right wise sage.

    http://www.datasheetdir.com/PT4115+LED-Drivers

    OK maybe not the exact circuit I need but it is late and I need to get some sleep!!!!!
     
  5. SgtWookie

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  6. spinnaker

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    Looks to me like 2 .32 ohm resistors in parrallel which would be .16 ohm total.

    I'd try running the math but looks like microbridge's site is down right now.


    This is going to be harder than I thought. :)


    I could not find the PT4115 on mouser or Digikey. Perhaps it is my inexperience but I could not find anything compatible either. I sure hope we can find another package type than what is on the board. That thing is tiny!!! Hard to believe it is capable of 30W.

    I am wonder if I should just buy one of these for the light panel.


    Exactly what is the LED driver doing? Is it stepping down the voltage?
     
  7. spinnaker

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    Ah the driver provides a constant current source. Makes sense.

    Very interesting article here.

    Now I just need on to run my panel. :)

    I will need at least something that gives me 900ma. Did I do my math right?
     
  8. SgtWookie

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    How did you determine that? Did they have R32 or R320 on them? That would be an E192 value:
    http://www.logwell.com/tech/components/resistor_values.html
    The mean current through the LEDs is determined by:
    100mV/Rs
    So, if you have two 0.32 Ohm resistors in parallel, that's 0.15 Ohms; 100m/0.16 = 625mA nominal current.

    What makes you say that? :)

    I couldn't find the PT4115 stocked anywhere either.
     
  9. SgtWookie

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    Yep.
    I hope you didn't run the LEDs very long directly from the battery. You probably shortened their life by doing so.

    Ahh, just need to run it?

    Nope!
    625mA is what you need.
     
  10. SgtWookie

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  11. spinnaker

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    2 resistors each with black, orange, brown stripe.



    Not to sound stupid but what is 100mV? 100Millivolts? how was that determined?



    OK correction. Hard for me. Easy for you. :) You are the Jedi master. I feel like one of those sand watchamacallits. :)


    [/QUOTE]

    OK so what are my solutions? :)



    No probably not more than a few seconds.




    OK Power :)

    You worked backward from the sensing resistor to get this? I used 36 * 25ma. If these really were 25ma LEDs, is that correct? If I am correct then I guess these LEDs draw 17.3 each? Or are we just limiting the current to that figure?


    OK then where do I find something that will work? Most of the drivers I see on mouser are pretty low current.

    Unless I searched wrong.

    I used maximum supply current and as my search parameters.

    I am assuming I just really need a constant current at 625ma? What is so special about the LED drivers?
     
  12. spinnaker

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    Thanks!!!

    This one

    Would be easier for me to work with with the DIP.


    I will need to select an inductor. I'll look tomorrow.
     
    Last edited: Dec 29, 2009
  13. spinnaker

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  14. SgtWookie

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  15. spinnaker

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    What about one od these?

    A bit more expensive but maybe better for a noob like me. Plus I won't have to hassle you with help in the design from the chip you selected for me. :)
     
  16. SgtWookie

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    The BuckPucks are nifty, but they require "headroom", or voltage above the LED's Vf in order to work properly. The minimum headroom is 2v for the 350mA units, and increases for the higher current units.

    Since your LED array is comprised of multiple strings of three LEDs in series, and we don't know for certain what the Vf of the array is when supplied with the proper current (typical Vf for white LEDs is anywhere from 3.4v to 4v) it would be rather risky to try to use a BuckPuck.

    I looked at the NCP3065, but it would be far more complex for you to use than the others I linked to.

    The two others (non-DIP) only require a single current set resistor, an inductor, and a couple of capacitors.
     
  17. spinnaker

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    OK I will try this one a try. It is one od the ones your recomended but on mouser.

    For C1, it looks like the are recommending 2.2 uf for an Vin of 12 VDC.

    I am going to have to take a close look at my selection as they are recommending a X7R, X5R, or better dielectric.


    Choosing an inductor will be more difficult. Their chart only goes up to 15 diodes. What can I do here?


    They recommend a B3100 for D1. Do you concour?


    They seem pretty clear on PCB mount I was planning on using a small per board. Will I have problems?
     
    Last edited: Dec 30, 2009
  18. SgtWookie

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    Here's an inductor for you:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=TE2151-ND

    It's larger than the one in your controller board, but it's very low resistance. Trying for efficiency, here.

    Use a 4.7uF X7R cap for the input.
    Use a 100nF (0.1uF) cap on the ADJ input. Later, you can use that input to turn the LEDs off when the battery starts to get low.

    As far as diodes, you could go with a 1N5822: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=1N5822-E3/54GICT-ND
    It's a 3A Schottky diode rated for 40v with a low Vf.
    You could use a B3100 if you wanted to, but since this is a low voltage application, going with the lower voltage rating means a lower Vf, thus less power loss in the diode. The most efficient way to do it would be a synchronous rectifier, but that would be getting complex.
     
  19. spinnaker

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    Yeah I was thinking I could use the Adj pin to turn off the light too. And it would avoid the use of the MOSFET correct?


    Thanks for the selection for the components. Hopefully I will be able to find similar on mouser. What made you go with 4.7uF?


    I was thinking of making the light / power supply self contained. I was going to place the power supply inside the light. It s really too tight for the controller and battery anyway. But if I put the power supply inside the light and the controller outside, I would need to run 3 wires to the light as opposed to 2. I would be nice to be able to just attach the light to a power source and have it light without worrying about the control. Though I suppose one less mosfet makes things more efficient?

    How would I use it as a control PIN from the PIC? The datasheet says

    Note that the ADJ pin is not a logic input. Taking the ADJ pin to a voltage above VREF will increase output
    current above the 100% nominal average value. (See page 18 graphs for details).

    Would I set my PIC i/o pin as an input when I wanted the light on and an output low when I wanted to turn it off?
     
  20. SgtWookie

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    I really haven't looked at how much current it would take to bring that input to ground, but it's shown being driven by a uC directly - so it can't take much.

    Duh. Forgot you were getting stuff via Mouser.
    Datasheet. It's due to the low differential between input and output voltages.

    Any time you go through a semiconductor, you'll have some power dissipated as heat due to the resistance of the semiconductor.

    That's basically the idea.
     
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