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Help using TIP42c

Discussion in 'General Electronics Discussion' started by Keng, Apr 12, 2013.

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  1. Keng

    Keng

    71
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    Apr 12, 2013
    Hi guys, I have troubles using a PIC with + output signal, becuase I need + high output, 12volts, I add a 2n3904 to the output of the pic to drive the TIP42c but I cant get that the TIP drive a 12volts bulb.

    I have everything well connect it.

    Any tip?

    Thanks
     
  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    This should do it:

    ckt.JPG

    R3 represents the bulb.

    Bob
     
  3. Keng

    Keng

    71
    0
    Apr 12, 2013
    Thanks, let me try it...
     
  4. Keng

    Keng

    71
    0
    Apr 12, 2013
    Just perfect... It is working now...

    Now I am to check how many amps I can get from this TIP... Thanks
     
  5. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Great! Glad to be of help.

    If you need much more than 1 amp you will have to lower the 470 Ohm resistor.

    Bob
     
  6. Keng

    Keng

    71
    0
    Apr 12, 2013
    This transistor is rated to 6amps, how low can I low the resistor to have around 5amps?

    I will make some test right now...
     
  7. Keng

    Keng

    71
    0
    Apr 12, 2013
    What if I jump the R2?
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
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    Jan 21, 2010
    Then you will almost certainly destroy one or both of the transistors.
     
  9. Keng

    Keng

    71
    0
    Apr 12, 2013
    Ok, I dont want that
     
  10. BobK

    BobK

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    Jan 5, 2010
    The minimum gain of the TIP42C is 15, so to get 6A, you will need 400ma of base current. That would require about a 27R base resistor. But that current is too much for a 2N3904.

    A 2N2222 would work, and you would have to reduce it's base resistor to abo 680R. And both transistors are going to get hot. You will definitely need a heat sink on the TIP42C. It will be dropping about 1.5V at 6A thus dissapating 9 Watts.

    A MOSFET would be a far better choice overall, and if it is just a bulb you are driving, use an N channel logic-level MOSFET and put it between the bulb and ground. That way it can be driven directly from the micro with no extra transistor.


    Bob
     
    Last edited: Apr 15, 2013
  11. Keng

    Keng

    71
    0
    Apr 12, 2013
    Thanks Bob, I will like to change it to a Mosfet N-channel but I preffer that the circuit had a + output,, so, I have to keep it with the 2n3904.

    I have 2n2222 that I will preffer to use, so, I have to use a 680ohm resistor to the base of the 2n2222 and a 470ohm to the tip42c base to have a "perfect" match?

    Thanks
     
  12. Keng

    Keng

    71
    0
    Apr 12, 2013
    BTW what P channel logic-level MOSFET will I use to drive a 90 watts 12volts halogen?

    Thanks again
     
  13. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    No, you would have to use a 25 Ohm on the tip42c to get 6 amps.

    If you switch to a P channel MOSFET, it will not help for it to be logic level since it's gate is referenced from the + supply and you will need a level shifter anyway I have actually never heard of a P channel logic-level MOSFET, though they may well exist.

    Bob
     
  14. Keng

    Keng

    71
    0
    Apr 12, 2013
    H again Bob, I am not very clear, 680ohm resistor to the base of the 2n2222 and a 25ohm on the Tip right?
     
  15. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Yes. 680 between the uController and the base of the 2N2222, then 25 Ohm between the collector of the 2N2222 and the base of the TIP42C. And a heat sink on the TIP42C. You will probably melt if if you try to run 6 Amps without a heat sink.

    Bob
     
  16. Keng

    Keng

    71
    0
    Apr 12, 2013
    Great... I am making test...

    Look, if I want to try two 2n3904 in parallel, I have to increase the resistor from the uController?
     
  17. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    It is not a good idea to parallel BJTs, their characteristics will differ enough that one will carry most of the current, heating it up, which causes it to carry even more of the current (thermal runaway).

    Bob
     
  18. Keng

    Keng

    71
    0
    Apr 12, 2013
    Thanks again Bob...

    Another question, unrelated with the tip...

    I need to use signal from 12volts and 24volts to the PIC, can I use to do that, I think is called resistive divider, I am right?
     
  19. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Yep, a resistive divider would work. To handle 24V, assuming the micro is running at 5V, you would need a ratio of about 4 to 1.

    Bob
     
  20. Keng

    Keng

    71
    0
    Apr 12, 2013
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