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led MATRIX CURRENT DRAW

Discussion in 'Microcontrollers' started by MrDEB, Oct 4, 2012.

  1. JonSea

    JonSea Well-Known Member

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    No. I used the IRLD014PBF as a low side switch.

    I STILL suggest you use individual LEDs as I outlined above. It's simpler from both a hardware and software perspective. It would also be brighter so there is no multiplexing involved.
     
  2. MrDEB

    MrDEB Active Member

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    I agree that it would be easier to use individual LEDs but this end project is to be mounted on a bike cycle thus durability is an issue.
    While waiting for a shipment of the TD62783APG's I will just put along with dim LEDs and develop code. Am looking at using Manchester code and interrupts.
     
  3. JonSea

    JonSea Well-Known Member

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    If your LEDs soldered on a circuit board aren't durable, you have a serious problem somewhere.

    If you're planning on multiplexing one LED at a time, physics dictates an LED on for 1/7 of the time will be 1/7 as bright as the same LED on full time. You can correct for this somewhat by driving the LED harder - I believe 40 mA is the maximum current vs 25 mA if supplied directly from a port pin, but even this only brings you to about 25% as bright.

    "am looking at interrupts" isn't an optional idea if you're multiplexing. The 10 mS (or whatever rate)cycling can't be stopped while you decode a message, wait for a switch to be pressed or whatever.

    Your big plans and your no-so-big skill set are not well matched here. I'm not trying to change your mind or put you down - I'm trying to save you from a scrapheap of failed efforts and a few hundred more forum posts asking for help.
     
  4. dave

    Dave New Member

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  5. be80be

    be80be Well-Known Member

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    There one thing i don't get is why did you pick the chip Jon said too. It's meant to use 15 volts or greater

    And the one I said is more able to use lower voltage 5 and up.

    How do you pick these parts.

    [video=youtube_share;mlsCzUNDtH8]http://youtu.be/mlsCzUNDtH8[/video]
     
  6. JonSea

    JonSea Well-Known Member

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    Yes, the TD62783 can handle up to 50 volts. Nothing in the data sheet puts a lower limit on Vcc.

    This is a similar range as for the ULN2003/ULN2803, which works down to 5 volts or less just fine.

    Selection of the high side driver isn't going to be the big problem with this design.
     
  7. JonSea

    JonSea Well-Known Member

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    The UDN2982 is in fact similar to the chip I suggested, handling up to 80 volts!

    It's an equally valid selection for a high-side driver, but doesn't seem to be in stock at DigiKey or Mouser. I've looked at this before, but couldn't fine stock any place.
     
  8. MrDEB

    MrDEB Active Member

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  9. JonSea

    JonSea Well-Known Member

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    Wow, it's like you don't see anything but what you want to see and you ignore the rest. It's amazing.

    I don't believe those two chips are going to work well together. If you feel you must have constant current, then buy a chip designed to control a matrix.

    Answer this: why do you need a constant current drive? You'll say: "to get uniform brightness."

    Why were your LEDs not of uniform brightness? Because you had more than one LED operating from a single resistor! You can't do that.

    This isn't to say constant current drive isn't a good thing for many applications, but trying to multiplex with the part you selected increases complexity immensely. In addition to multiplexing, which you are struggling with already, you need to shift control data to the constant current driver.

    I am truly out of this one. If I feel compelled to reply, I'll delete my account.
     
  10. be80be

    be80be Well-Known Member

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    JonSea I didn't ask you how you pick parts. And I'm not putting you down in any way.

    I would like to see how MrDeb make a decision
     
  11. MrDEB

    MrDEB Active Member

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    I made my decision by looking at numerous LED matrix projects. The Constant current sink is used as an example http://picprojects.org.uk/projects/lc/index.htm
    The http://www.electro-tech-online.com/custompdfs/2012/10/8992.pdf was suggested by Jon or Burt?.
    Hopefully the two components will work together, no reason they shouldn't.. Just need to figure out connections. The matrix cube pretty much explains it.
    IF it works then GREAT. If not well back to the drawing board so to speak.
    If you read the data sheet on the constant current sink it sounds like an "automatic" current resistor so to speak.
     
  12. JonSea

    JonSea Well-Known Member

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    please explain exactly what problem a constant current driver solves? Be specific.
     
  13. be80be

    be80be Well-Known Member

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    You no Mrdeb I made a Led cube code some else made I changed the hardware and chip

    It used the pic to source power and Npn to sink it some one on here cant remember there name took it and
    did the same thing your doing put it on a motor bike for turn lights And it worked very well.

    You post the parts you want to use and we could help you lay it out

    Or even better Mike said he would make the sch to show you how if you ask him.
    I would take him up on that.
     
  14. be80be

    be80be Well-Known Member

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    Watch the video these leds are high power like what your using and the voltage driving them is 5 volts.
    [video=youtube_share;T8dcpp4fiLI]http://youtu.be/T8dcpp4fiLI[/video]

    It's using NPN to sink with and anodes are tied to VDD

    I remove these from a toy there 2 red 2 blue and 2 green leds there in sets of two but you can see there happy at five volts.
     
    Last edited: Oct 13, 2012
  15. JonSea

    JonSea Well-Known Member

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    The matrix MrDEB apparently has has two LEDs in series for each dot. The typical Vf is 4 volts, but the maximum is 5 volts. He needs to design around the 5 volt worst case to be sure the dots will illuminate.

    What MrDEB is refusing to believe is that each dot must be driven via its own resistor. No exceptions to this rule - LEDs cannot be used in parallel from a single resistor. Thus, MrDEB's current measurements with 1 - 5 LEDs driven from a single resistor is meaningless. They won't work like that!

    MrDEB's situation is complicated a bit by the high forward voltage of the matrix he has, but just a bit.

    The first step which really must be understood is the basic circuit of a multiplexed display. Think way back to the beginning of this mess. Seven port pins driving the anodes of the rows via a resistor. Five cathodes connected to ground via a ULN2003. Keep this circuit in mind. Yes, I know the 5 volts from a PIC won't drive the matrix MrDEB is attempting to use. Just assume a matrix with Vf = 2.2v and the resistors sized for a current flow of 25 mA per LED.

    Now, lets say that all seven anode pins are high, and one and only one cathode, the first column, is conducting to ground. What have we got in this case?

    1. All of the LEDs in column 1 are illuminated.

    2. Each port pin is supplying 25 mA to its associated LED.

    3. The ULN2003 is sinking all the current through the LEDs to ground, namely 7 x 25 mA = 175 mA.

    Everyone is happy as long as exactly one cathode is active.

    What happens if we quickly alternate between cathode 1 active and cathode 2 active? The dots in the first two columns will be evenly illuminated. And if we quickly switch from 1 to 2 to 3 to 4 to 5 and continue the cycle? All of the dots will be evenly illuminated. They won't be as bright as when we had only one column illuminated because each column is only on 1/5 of the time.

    Be sure to notice that only one cathode is active at a time, and each port pin only drives one LED at a time.

    So how do we display a symbol on the display? Let's consider a T five dots wide and seven dots tall.

    1. With all cathodes off, we set only the anode connected to the top dot high (the pattern in the first column)
    2. We active the cathode in column 1 for a short period of time, say 10 mS as a guess.
    3. Turn off the cathode for column 1.
    4. Set the anodes for the patern in column 2, namely just the upper dot.
    5. Turn on the cathode for column 2 for 10 mS
    6. Turn off the cathode for column 2.
    7. Set the anodes for the pattern in column 3, namely all on.
    8. Turn on the cathode for column 3 for 10 mS.
    9. Repeat the process for columns 4 & 5.
    10. Repeat the entire process.

    Notice only a single cathode is every on and each anode supplied exactly one dot.


    But, but, but......

    Yes, MrDEB's displays are red and green. This just means there are ten cathodes instead of five. An additional ULN2003 is needed (and 5 more port pins to control it) but the same principles apply. Only one cathode is active at a time.

    But, but, but.....

    Yes, the display MrDEB has requires greater than 5 volts to operate. Add one of the driver chips the be80be or I listed to driven the anodes. This may flip around high/low on/off but the principles are still the same. One cathode active and each gate of the high side driven supplies exactly one LED.

    Do note that the current draw is (LED current) x (max number on in a column). With a current of 25 mA per LED, this could be as high as 175 mA, not agreeable to a 9 volt battery or even 6 AAA cells.

    MrDEB, this is the straight scoop. Don't get creative with things you don't understand.
     
  16. be80be

    be80be Well-Known Member

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    Jon I bet the whole problem is he has resistors on the columns not the rolls
     
  17. JonSea

    JonSea Well-Known Member

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    Your guess is as good as anybody's since he won't show a schematic that shows what he's doing NOW.

    the problem is he won't spend Half an hour understanding what needs to be done.
     
  18. MrDEB

    MrDEB Active Member

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    Present setup

    Here is a simplistic schematic I am using. I am only enabling one LED per colum (cathode) but multiple LEDs per row up to 5 (5 colums)
    The matrix has 7 ROWS(anodes) and 5 COLUMS)
    I did some experimenting this morning before heading off to church.
    Using a 5v (4.15 measured) and an NPN transistor ( tried a PNP as well) and only get 3ma without a resistor.
    Inserted a 120 ohm resistor and used an 8.48v (measured)battery pak I get 30ma.
    Lit up two LEDs in same ROW but different colums (1 red and 1 green) and measured 58ma aprox.
    conclusion only one LED per colum which I had planed from the start but up to 5 LEDs per row.
    BUT I need to use more than 5v. Problem now is I get the 7v on the base of the transistor. Switching a voltage higher that the Vcc of the controlling source using a BJE transistor is not the way to go.
    I understand how a matrix works and all that Jon posted.
    By putting the resistor on the colum and only enabling one LED per colum seems to work fine.
    Using a driver that was suggested TD62783APG (opposite of the ULN2803) will hopefully solve the problem of the usage of a transistor.
    Once I get this configuration working I hopefully can integrate the Constant Current Sink concept. Jon asked how will it help? see data sheet snippet I posted. I maybe wrong in using this but I won't know until I try.
    Hopefully I cleared up any misunderstandings
     
  19. MrDEB

    MrDEB Active Member

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    Pondering over the ONE cathode enabled instead of one Anode.
    Going to see what a 10ms delay really looks like but only on 5v.
    ONE cathode per colum instead of several which will basically put leds in parallel.
     
  20. JonSea

    JonSea Well-Known Member

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    [MODNOTE]Deleted bad language and abusive[/MODNOTE]
     
    Last edited by a moderator: Oct 15, 2012
  21. JonSea

    JonSea Well-Known Member

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    I'm done here. Sanity gone.

    Say bush bye. This account is gone.
     

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