Transistor switch for LED array

Discussion in 'General Electronics Chat' started by Phatness, Jun 9, 2013.

  1. Phatness

    Thread Starter New Member

    Jun 9, 2013
    2
    0
    Hello everyone, this is my first time building a circuit and I'm having trouble understanding transistors. I don't know which values in the datasheet tell me how many volts and amps I need to turn on the transistor.

    I am trying to create a 16x16 RGB Led matrix controlled with 8-bit shift registers (74HC595N). 1 output from the shift registers switches on a row of LEDs, if all columns are enabled that would be 48 LEDs.

    With a 5V power supply I think this gives a total of almost 1A over the row transistor. Could I use a TIP120 with the base voltage coming from the shift registers, or would I have to amplify the logic current first?

    I was also thinking about using 2 of the PN2222A per row so they each only had 24 LEDs which should require less volts and amps on the base pin...

    The columns are controlled by a 2N3904, only 1 LED should be on at a time in a column so I'm less worried about this transistor. I want to control all of the transistors with signals directly from the shift registers (with 1 resistor), is this reasonable?

    I haven't built any circuits before so I expect to be totally off, I've tried doing the math but I'm not really sure what values to use from the datasheets.

    My schematic for the 2 transistors per row (24 LEDs) idea.
    [​IMG]

    I'm basing this off of Kevin Darrah's 8x8x8 cube http://www.kevindarrah.com/?p=1424 which has 192 LEDs per "row" since my power needs are smaller I'm trying to reduce the complexity of the circuit.
     
  2. Evil Lurker

    Member

    Aug 25, 2011
    117
    23
    Thats a pretty neat looking toy.

    Personally if you are looking to control a bunch of transistors would suggest using something like ULN2004 (I can't remember might be a ULN2002) transistor array which can directly interface with 5v signals as the base resistors are built directly into the IC.

    Again, I can't remember exactly how many transistors are in the array, I'm thinking its either 8 or 10.

    Also each LED should have its own current limiting resistor if operating them in parallel because it can cause a cascaded failure if one or more goes out. Try and control them in series if possible using a higher voltage.
     
  3. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    Why is there a 100R resistor on the emitter of the PN2222A? It seems to me it would just drain the current away from the diode array. A capacitor would make more sense.
     
  4. Phatness

    Thread Starter New Member

    Jun 9, 2013
    2
    0
    Each LED has 100R resistors, the bottom part of the schematic is repeated once for all 48 LEDs, so I can individually control them with the 2N3904.

    Your right a capacitor would make more sense... I'm not sure I need anything between the emitter and LEDs, just a cap at the collector. I want to do PWM, the cap should help with that.


    What I am still unsure about is how much voltage/amps I need to supply to the base of the PN2222A (or transistor IC which I like) to supply the full 5V and 0.48A to the LEDs (I would like to supply 0.96A, I don't think the PN2222A can handle it, could/should I use a Darlington?)
     
  5. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    According to the datasheet, PN2222A is capable of a 1 A current. I suspect that it would get pretty hot.
     
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