please sanity check LED multiplex current

Discussion in 'General Electronics Chat' started by skk, Oct 30, 2011.

  1. skk

    Thread Starter Member

    Mar 15, 2011
    31
    0
    I changed my mind yet again, and now I am going with 2 sets of multiplexed leds going at the same time. Please validate my design and assumptions.

    Circuit:
    I have 2 different multiplexed sets of LEDS. The first set is 3 7-segment leds blocks, each with 7 20mA segments plus 1 10mA dot. The other set is 8 10mA leds x 4. These leds are common cathode red/green dual 3 lead leds.

    Multiplexing:
    I am multiplexing using the common cathode of both sets. Common cathode is switched by a 74HC595 ( source 35mA max ) with a 2N2907 for 350mA max. The rows are sunk using 2 TPIC6C595 shift registers. Each one will sinc 100mA per row and 500mA total.

    My thinking is that since my 7-segments are on 1/3rd of the time, max that would be (20mA * 7 + 10mA ) * 3 or a pulse of 450mA with a 1/3 duty cycle to get 100% brightness. First, is this correct?

    Assuming that, then I am not going to get enough current out of my source register because it gives only 35mA, with 2n2907 transistor that is 350mA. But since I only need 450mA for 100% brightness then that is like 77% brightness at 350mA, so I can live with that.

    The second mutiplex set is 8 x 4 10mA LEDs, or 320mA < 350mA, so I am fine. On the sinc side, each line can drain 100mA each and 500mA total, so I am also fine.

    So now I have the current limiting resistors for each of the 20mA segments ( 1v drop). The 7 segment ones are on 1/3rd of the time, so I need 3 times the normal current or

    1v/ 20mA * 3 = 16.6 ohms
    and the 'dot'
    1v/ 10mA * 3 = 33.3 ohms
    and the 8x4 10mA LEDS
    1/ 10mA *4 = 25 ohms

    Am I doing this right?

    As for the base resistors on the transistors, that would be:

    4.3v / ( 20mA * 7 + 10mA ) = 28.6 ohms

    and the base resistor for the single LEDs

    4.3v / (10mA * 8 ) = 53.7 ohms

    Does this check out?

    Thanks









     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    It would really make things easier if you attach a complete circuit diagram.
     
  3. skk

    Thread Starter Member

    Mar 15, 2011
    31
    0
    OK, I uploaded the complete schematic ( attached ).
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Ok, I should have been more specific. Not everyone has always a schematic designer software at hand. Right now I'm accessing the site on a mobile device. No way I can open a .sch extension.
    Diagrams should be posted as jpeg or png pictures unless someone asks specifically for something else.
     
  5. skk

    Thread Starter Member

    Mar 15, 2011
    31
    0
    Here is the part of the schematic I was asking about. The 3 shift registers are at left. I am using STPIC65595s ( -100mA ) for the sinc and MM74HC595 ( +35mA) to drive the transistors. The emitter of the pnp is connected to vcc ( +5v ).
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Image is too small to read any text or numbers.
     
  7. skk

    Thread Starter Member

    Mar 15, 2011
    31
    0
    OK, lets try again with text.

    As I said before the shift registers for the sincing are STPIC6C595s and the sourcing shift register is MM74HC595 with help of 2N2907 transistors. In the circuit below I am running 4 columns of LEDs, each with their own transistors, each from different outputs of the MMHC74595. Only 1 of the 4 columns will be on at one time. The circuit below only shows 1 column:

    Code ( (Unknown Language)):
    1. [FONT=Fixedsys]
    2.  
    3. +----------+
    4. | STPIC595 |
    5. |          |--------/\/\/\/\----|<|---+
    6. |          |--------/\/\/\/\----|<|---|
    7. |          |--------/\/\/\/\----|<|---|
    8. |          |--------/\/\/\/\----|<|---|
    9. |          |--------/\/\/\/\----|<|---|
    10. |          |--------/\/\/\/\----|<|---|
    11. |          |--------/\/\/\/\----|<|---|
    12. |          |--------/\/\/\/\----|<|---|     vcc +5v    
    13. +----------+                          |      |
    14.                                       \     /
    15.                                        \   v
    16.                                     ---------- 2n2907
    17.                                          |
    18. +------------+                           |
    19. | MMHC74595  |                           |
    20. |            |--------------/\/\/\-------+
    21. +------------+
    22.  
    23. [/FONT]


    The other set of multiplexed LEDs is similar, but just has 3 columns, but each LED is a double ( 20mA ), except for the 'decimal point' which is a single ( 10mA). Only one column shown below:


    Code ( (Unknown Language)):
    1. [FONT=Fixedsys]
    2. [/FONT][FONT=Fixedsys]
    3. +----------+
    4. | STPIC595 |
    5. |          |-----/\/\/\/\----|<|-|<|--+
    6. |          |-----/\/\/\/\----|<|-|<|--|
    7. |          |-----/\/\/\/\----|<|-|<|--|
    8. |          |-----/\/\/\/\----|<|-|<|--|
    9. |          |-----/\/\/\/\----|<|-|<|--|
    10. |          |-----/\/\/\/\----|<|-|<|--|
    11. |          |-----/\/\/\/\----|<|-|<|--|
    12. |          |-----/\/\/\/\----|<|------|     vcc +5v    
    13. +----------+                          |      |
    14.                                       \     /
    15.                                        \   v
    16.                                     ---------- 2n2907
    17.                                          |
    18. +------------+                           |
    19. | MMHC74595  |                           |
    20. |            |--------------/\/\/\-------+
    21. +------------+[/FONT][FONT=Fixedsys]
    22.  
    23. [/FONT]


    So top diagram on 1/4th of the time, bottom on 1/3rd of the time. Assume forward voltage drop 1v per led and max current is 10mA. The double LEDs are then 20mA and Vf=2v. The STPIC595 will sync 100mA per pin 500mA for the package, and the MMHC74595 can source 35mA, with the 2n2907 good up to ~500mA. I just wanted my original numbers for the base resisters and the current limiting resistors verified.

    Thanks again.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here, I exported the Eagle schematic in b&w scaled at 150DPI, saved it as a .png and attached it.
    [eta]
    Gosh, the schematic doesn't resemble what you posted at all.
     
  9. skk

    Thread Starter Member

    Mar 15, 2011
    31
    0
    I hope I was not too simplistic in my text drawing, but I think I captured all the pertinent info. There are lots of components I was not asking about, and schematic is hard to read because I used a lot of buses. Anyway, the 3 shift registers are on the right. SINCEREG1 and SINCREG0 are the STPIC595s and the SRCREG is the MMHC74595. The two columns of common cathode LEDs form 4 columns of single LEDs 1 of 4 on at any time. The 7 segment displays at the right form the 1 of 3 multiplexed group. The anodes of both sets are driven by the MMHC74595 plus 2n2907 transistors. None of the resistors show values, because I still am not sure what values to use...
     
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