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Triac, Transistor, or relay?

Discussion in 'Electronic Basics' started by Baronvonrex420, Sep 29, 2003.

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  1. I want to use the outputs from a 7 seg driver (74LS47 OR 7447) to light a LED
    display AND to send a larger current (12vdc) to an LED array. Each seg of the
    array is comprised of 6 LEDS, 2 strings of 3 in series. I would assume I
    couldn't do this with the driver IC alone.

    What is the best way to accomplish this? Triac, Transistor? Cost is a major
    factor.

    Also, please explain how I would route the pins, for instance, base to driver
    emitter to ground, collector to power? Any additional components required such
    as resistors or caps?

    I need to get this project completed, so apologies in advance for the
    newbieness!

    Thanks,

    Greg
    http://www.rts-group.com
     
  2.  
  3. CFoley1064

    CFoley1064 Guest

    I want to use the outputs from a 7 seg driver (74LS47 OR 7447) to light a LED
    The ULN2003 is made for your application. It's a 16 pin IC with 7 darlington
    drivers, transistors and resistors built in. If you put a logic 1 at the
    input, the darlington output goes on and is capable of sinking enough current
    to drive your LED displays. The IC is relatively cheap, and commonly
    available. This will certainly be a better solution than discrete components,
    even if you cost the transistors and resistors at zero. Make sure you use
    current limiting resistors on your LEDs.

    http://www.allegromicro.com/datafile/2001.pdf

    Good luck.
    Chris
     
  4. Dana Raymond

    Dana Raymond Guest

    You didn't specify what the total current per segment would be. I'm assuming
    that its over 24mA (SN74ls47) and 40mA (SN7447). If not, then you can use
    the driver directly as the outputs can handle up to 15V.

    If you exceed the segment current, then there are a variety of ways to go.
    If the total segment current is less than 120mA then you can use 3 SN7447
    chips. One for the 7-seg display, and the other two for each of the strings
    of 3 leds. Or you can use the ULN2003 driver (as suggested by another).
    However, don't forget to switch over to the SN7448 or SN47LS48 as it will
    drive the segment inputs of the ULN2003 correctly.

    First, answer the questions, and we can then proceed.

    Dana Frank Raymond
     
  5. John Fields

    John Fields Guest

    ---
    Assuming 20mA red LEDs:




    +12
    |
    |
    E
    LS47 SEG OUT->---[1K0]---B 2N4403
    C
    |
    +----->>------+------+
    | | |
    [270] [510R] [510R]
    | | |
    [LED] [LED] [LED]
    |K |K |K
    | [LED] [LED]
    | |K |K
    GND GND GND

    The 270 ohm resistor will dissipate about 10V*20mA = 200mW, so you'd be
    OK with a 1/4 watt resistor, but you might want to use a 1/2 watter just
    to keep its temperature down. The 510 ohm resistors will dissipate
    about 8V*20mA = 160mW, so 1/4 watters will be fine.


    If you wanted to use high-efficiency 2mA LED's like HLMP-4700's, you
    could get rid of the transistor and the base resistor and just do this:

    +12
    |
    +----->>------+------+
    | | |
    [5k1] [3k1] [3k1]
    | | |
    [LED] [LED] [LED]
    |K |K |K
    | [LED] [LED]
    | |K |K
    | [LED] [LED]
    | |K |K
    LS47 SEG OUT->---+----->>------+------+

    All the resistors can be 1/4 watters, no problem.
     
  6. John Fields

    John Fields Guest

    ---
    Oops, kinda...

    It'll work, but I forgot to keep the base from floating.

    This is better:


    +12 +12
    | |
    [10k] |
    | E
    LS47 SEG OUT->-+-[1K0]---B 2N4403
    C
    |
    +----->>------+------+
    | | |
    [270] [510R] [510R]
    | | |
    [LED] [LED] [LED]
    |K |K |K
    | [LED] [LED]
    | |K |K
    GND GND GND
     
  7. John Fields

    John Fields Guest

    ---
    Hmm...
    Maybe the third time will be the charm.

    +12 +12
    | |
    [10K] |
    | E
    LS47 SEG OUT->-+-[2K]----B 2N4403
    C
    |
    +----->>------+------+
    | | |
    [510] [300] [300]
    | | |
    [LED] [LED] [LED]
    |K |K |K
    GND [LED] [LED]
    |K |K
    [LED] [LED]
    |K |K
    GND GND
     
  8. Thanks all! I have a couple of great plans and a couple of good backups to
    work with, I always appreciate a "Plan B". I can get this project off the
    ground now!

    Greg

     
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