Transistor pair to switch between Voltages

Discussion in 'Circuit Help' started by fatman57, Nov 13, 2016.

  1. fatman57

    fatman57

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    Cheers Steve! Much obliged...
     
    fatman57, Dec 24, 2016
    #21
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  2. fatman57

    fatman57

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    Please find final version below. Many thanks for all your help to date!!!!

    [​IMG]
     
    fatman57, Jan 5, 2017
    #22
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  3. fatman57

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    All good except you have drain and source sealed on your MOSFETs.

    The LEDs will probably be very dim.
     
    (*steve*), Jan 5, 2017
    #23
  4. fatman57

    fatman57

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    Thanks Steve,

    Sorry, but what do you mean by sealed? I used an online sketch service and it was what came out of the box if you are referring to the symbols.

    Will the LEDs be dim because of the values of the resistors?

    Much obliged
     
    fatman57, Jan 5, 2017
    #24
  5. fatman57

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    AAAAGH "swapped". Bloody autocorrect.

    Yeah 10k for the LEDs is a bit high. The one on the 5V input need to be about 1/3 of the value of the one indicating 12V if you want equal brightness.
     
    (*steve*), Jan 5, 2017
    #25
  6. fatman57

    fatman57

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    Again! I followed the same example as seen on post #19 - makes me feel a little embarrassed - not sounding too stupid what indicators should I look for to make sure I get it right (as in with regards to schematic diagrams)?

    Thanks! I will use an online calculator to assist in those values.
     
    fatman57, Jan 6, 2017
    #26
  7. fatman57

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I find that drawing in the body diode on MOSFETs helps a great deal in getting them around the right way. Then, knowing that in simple switching applications the source is connected to the same supply rail as the channel type, you'll get everything the right way around.
     
    (*steve*), Jan 6, 2017
    #27
  8. fatman57

    fatman57

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    OK thanks, I thought I had made it the same as post #19, does this picture correctly illustrate what you are saying:

    [​IMG]
     
    fatman57, Jan 6, 2017
    #28
  9. fatman57

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah, although typically you'd draw the p channel one up the other way.
     
    (*steve*), Jan 6, 2017
    #29
  10. fatman57

    fatman57

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    Please see my hopefully final version below:
    [​IMG]
     
    fatman57, Jan 12, 2017
    #30
  11. fatman57

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Looks fine.

    Note that they are P Channel MOSFETs, not PNP MOSFETs.

    We've also spoken about the resistors for the LEDs.

    Neither of the issues will stop it working :)
     
    (*steve*), Jan 12, 2017
    #31
  12. fatman57

    fatman57

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    Yes thanks Steve I just wanted to check it was the right way round. I have a circuit board prototype almost ready will post a picture if anyone is interested. Comments welcome.
     
    fatman57, Jan 13, 2017
    #32
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