pnp transistor..need help

Discussion in 'Embedded Systems and Microcontrollers' started by daredavel, Feb 28, 2010.

  1. daredavel

    Thread Starter Member

    Feb 22, 2010
    32
    0
    hello sir! i have problem testing the 2n3906 pnp transistor..i dont know what is the correct pin config of the 2n3906..(E-B-C) or (C-B-E)..i've tested both config..but nothing happened..the bad part, my pic16f77 was damage..i don't know why..if it has any connection to the transistor.. i also have tested it in my breadboard, without the pic to make sure first that i have correct pin assignments..still i dont know how it operates..i cant make the led lit..i can figure out the correct pin assignments.

    here is my schem:
    [​IMG]


    Here's the setup i've tested on my breadboard to test the transistor:
    5v supply
    [​IMG]
    [​IMG]
     
  2. MMcLaren

    Well-Known Member

    Feb 14, 2010
    759
    116
    Hi Daredavel,

    I wonder if those transistors shouldn't be NPN type in that sinking driver configuration, yes, no?
     
  3. daredavel

    Thread Starter Member

    Feb 22, 2010
    32
    0
    @Alberto: My friend told me to its better to use pnp in multiplexer sir..and the the circuit is already running in the simlation using proteus.

    @MMcLaren: i think so sir..because the multiplexer part was done by my friend..and also the programming..i'm the one in charge with the pcb and to make it running in actual..
    low volts are req to activate the pnp transistor.. i haven't studied the program yet, but how can pic16f877 produce a logic 0? to activate the pnp? thank you so much!
     
  4. daredavel

    Thread Starter Member

    Feb 22, 2010
    32
    0
    by the way sir, how can you output a logic 0 in pic16f877? to control the transistor? and Why is pnp is commonly used in multiplexer instead of npn? thank you sir!
     
  5. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    As others have said you are better off using a NPN transistor if you are doing low side switching (transistor between load and earth). PNP transistors are generally used for high side switching (with the transistor between the supply rail and the load).

    Get rid of R28, R29 and R31. They serve no purpose other than increasing the current though the transistors.

    Also add 1K or 470Ω resistors in series each of the transistor bases to limit the base current and protect the PIC ports.

    Of course you will need to change the software so that a logic 1 turns on a LED or segment rather than logic 0 as it does now.
     
    Last edited: Mar 1, 2010
  6. daredavel

    Thread Starter Member

    Feb 22, 2010
    32
    0
    if i use a npn sir..without the pull-up resistors or putting a pull-down resistor, my simulation goes haywire..if i connect the pull-up resistors R28,R29,R31..the simulation is working fine.. i dont know whats the reason behind..??
     
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