Voltage used up by a transistor and shift registers

Discussion in 'The Projects Forum' started by yash02, Apr 10, 2015.

  1. yash02

    Thread Starter New Member

    Nov 19, 2014
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    i am on a project 8x8x8 LED cube and I am stuck on the value of register i should use.

    Talking about single layer
    I am using 2N5296 transistor for common cathode and 8 shift registers for controlling 64 anodes.The LED I have used are from robocaze.com which are 3mm Blue LED undiffused.

    Now the problem arises between shift register and anodes of LED I need to connect resistor and I want to calculate its value.But I dont know that what amount of voltage and current are used up by the transistor and the shift register (74HC595).

    If someone can help me figuring it out then I will be very thankful to him.

    Datasheet Links :-

    Transistor 2N5296

    Shift Register 74HC595

    Thanks in advance

    -YASH LOTAN
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    How much LED current will you run through each 2N5296?
     
  3. yash02

    Thread Starter New Member

    Nov 19, 2014
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    LED can tolerate a max of 35mA. I wish to run 64 LED array at 20mA i.e., 20mA * 64 = 1.28A which is the current I want from each transistor.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,805
    1,105
    Provided one shift register will be driving only one LED at a time, value = (Vdd-Vf-Vsat)/0.02 Ohms, where Vdd is the shift register supply voltage, Vf is the nominal forward voltage of the blue LED and Vsat is the saturation voltage of the 2N5296.
     
  5. yash02

    Thread Starter New Member

    Nov 19, 2014
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    The saturation voltage is max 1V so much I should take to calculate resistor value?
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    Attach the schematic will be more easy to see the whole picture and to help the discussion.
     
  7. yash02

    Thread Starter New Member

    Nov 19, 2014
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    [​IMG]

    I want to know the value of R1.
    Q1 is 2N5296 Shift register output is 5V and 40mA(i.e. from arduino mega 2560 digital pin)
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    Your linking of 2N5296 is wrong, here is a correct datasheet.

    I still can't see the whole picture only can guess, the output of register can't offering enough current for 8 leds, I_led = 20mA * 8 = 160mA, for the saturation current of Q1 that the hFE should be count as 1/10, and b of Q1 should providing Ib = 160mA/10mA=16mA.
    Calculating the values of R1:
    Assuming that the output voltage of register is 5V and labeled it as Vq.
    I_R1 = (Vq - Vbe)/16mA
    I_R1 = (5V - 0.7V)/16mA
    I_R1 = 4.3V/16mA
    I_R1 = 268.75Ω = 269Ω

    269Ω is for the full duty cycle, but the leds display is using scan, so you can using the values less more than that.
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    1,105
    A single 5V shift register output can drive only one blue LED; not three in series.
     
  10. yash02

    Thread Starter New Member

    Nov 19, 2014
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    corrected schematic
    [​IMG]
    i want to know the value of R1
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,805
    1,105
    You're still showing 3 LEDs in series. That won't work unless your shift register can output > ~9V at 20mA.
     
  12. ScottWang

    Moderator

    Aug 23, 2012
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    Does it like lp stylus jumping?
     
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