Darlington transistor switch

Discussion in 'Electronic Design' started by David M, Sep 24, 2004.

  1. David M

    David M Guest

    Hi,

    I have a relay that I want to drive from a PIC micro. The PIC has a
    5V supply - the relay has a 12VDC coil.

    The circuit is basically this (fixed pitch font):

    +5v +12v
    | |
    \ |
    / 10K pullup resistor $
    \ $ (12V relay coil)
    | /
    PIC IO pin ------- /\/\------------- | (Darlington Driver)
    5K6 \
    |
    Gnd

    Will this circuit work with the 5v supply on the base of the
    Darlington switching a load with a 10 to 12V supply ?

    Regards

    David
     
    David M, Sep 24, 2004
    #1
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  2. David M

    CFoley1064 Guest

    >Subject: Darlington transistor switch
    >From: David M news^#NO@SPAM^moorhouse#.net.nz
    >Date: 9/23/04 11:08 PM Central Daylight Time
    >Message-id: <>
    >
    >Hi,
    >
    >I have a relay that I want to drive from a PIC micro. The PIC has a
    >5V supply - the relay has a 12VDC coil.
    >
    >The circuit is basically this (fixed pitch font):
    >
    > +5v +12v
    > | |
    > \ |
    > / 10K pullup resistor $
    > \ $ (12V relay coil)
    > | /
    >PIC IO pin ------- /\/\------------- | (Darlington Driver)
    > 5K6 \
    > |
    > Gnd
    >
    >Will this circuit work with the 5v supply on the base of the
    >Darlington switching a load with a 10 to 12V supply ?
    >
    >Regards
    >
    >David


    Hi, Dave. It will work, but it's customary to use a pulldown for NPN and
    pullup for PNP. Try this (view in fixed font or M$ Notepad):

    VCCVCC
    + +
    | |
    - C|
    ^ C|
    | C|
    | |
    '--o
    Output Pin |
    PIC ___ |/ NPN
    o-|___|-o-| Darlington
    5.6K | |>
    .-. |
    10K| | |
    | | |
    '-' |
    | |
    | |
    === ===
    GND GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    By the way, don't forget the diode to protect the transistor.

    Good luck
    Chris
     
    CFoley1064, Sep 24, 2004
    #2
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  3. David M

    David M Guest

    On 24 Sep 2004 05:23:30 GMT, (CFoley1064) wrote:

    >>
    >>I have a relay that I want to drive from a PIC micro. The PIC has a
    >>5V supply - the relay has a 12VDC coil.
    >>


    >Hi, Dave. It will work, but it's customary to use a pulldown for NPN and
    >pullup for PNP. Try this (view in fixed font or M$ Notepad):
    >
    > VCCVCC
    > + +
    > | |
    > - C|
    > ^ C|
    > | C|
    > | |
    > '--o
    > Output Pin |
    > PIC ___ |/ NPN
    > o-|___|-o-| Darlington
    > 5.6K | |>
    > .-. |
    > 10K| | |
    > | | |
    > '-' |
    > | |
    > | |
    > === ===
    > GND GND
    >created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
    >
    >By the way, don't forget the diode to protect the transistor.
    >


    Thanks for your help Chris.

    The Darlington is a ULN28903A (npn array) which has a built in
    protection diodes.

    It's interesting that I need a pulldown rather than a pullup as I
    built a similar circuit a few years back. That circuit had a LM339
    voltage comparator driving the Darlington and used a pullup.

    Back to the bench !

    Regards

    David
     
    David M, Sep 24, 2004
    #3
  4. On Fri, 24 Sep 2004 16:08:14 +1200, the renowned David M
    <news^#NO@SPAM^moorhouse#.net.nz> wrote:

    >Hi,
    >
    >I have a relay that I want to drive from a PIC micro. The PIC has a
    >5V supply - the relay has a 12VDC coil.
    >
    >The circuit is basically this (fixed pitch font):
    >
    > +5v +12v
    > | |
    > \ |
    > / 10K pullup resistor $
    > \ $ (12V relay coil)
    > | /
    >PIC IO pin ------- /\/\------------- | (Darlington Driver)
    > 5K6 \
    > |
    > Gnd
    >
    >Will this circuit work with the 5v supply on the base of the
    >Darlington switching a load with a 10 to 12V supply ?


    Why do you want to use a pullup? Are you using an open-drain output
    (some PICs have them)? Do you want the relay to pull in if the micro
    is unplugged? If not, lose the part entirely. Follow the other
    suggestions CFoley made (especially the catch diode). Look at
    worst-case voltage drop of the darlington with full coil current and
    275uA base drive and make sure you are not close on that (760uA if you
    can get rid of the pullup).

    Best regards,
    Spehro Pefhany
    --
    "it's the network..." "The Journey is the reward"
    Info for manufacturers: http://www.trexon.com
    Embedded software/hardware/analog Info for designers: http://www.speff.com
     
    Spehro Pefhany, Sep 24, 2004
    #4
  5. On Fri, 24 Sep 2004 21:38:45 +1200, the renowned David M
    <news^#NO@SPAM^moorhouse#.net.nz> wrote:

    >On 24 Sep 2004 05:23:30 GMT, (CFoley1064) wrote:
    >
    >>>
    >>>I have a relay that I want to drive from a PIC micro. The PIC has a
    >>>5V supply - the relay has a 12VDC coil.
    >>>

    >
    >>Hi, Dave. It will work, but it's customary to use a pulldown for NPN and
    >>pullup for PNP. Try this (view in fixed font or M$ Notepad):
    >>
    >> VCCVCC
    >> + +
    >> | |
    >> - C|
    >> ^ C|
    >> | C|
    >> | |
    >> '--o
    >> Output Pin |
    >> PIC ___ |/ NPN
    >> o-|___|-o-| Darlington
    >> 5.6K | |>
    >> .-. |
    >> 10K| | |
    >> | | |
    >> '-' |
    >> | |
    >> | |
    >> === ===
    >> GND GND
    >>created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
    >>
    >>By the way, don't forget the diode to protect the transistor.
    >>

    >
    >Thanks for your help Chris.
    >
    >The Darlington is a ULN28903A (npn array) which has a built in
    >protection diodes.
    >
    >It's interesting that I need a pulldown rather than a pullup as I
    >built a similar circuit a few years back. That circuit had a LM339
    >voltage comparator driving the Darlington and used a pullup.


    Why don't you use a ULN2803A (8 drivers) or ULN2003A (7 drivers) which
    has the resistors inside? I'm not familiar with the part number you
    mention above.

    The LM339 has an open-collector output. Most PIC outputs are
    push-pull, some have one pin that is open-drain.

    Best regards,
    Spehro Pefhany
    --
    "it's the network..." "The Journey is the reward"
    Info for manufacturers: http://www.trexon.com
    Embedded software/hardware/analog Info for designers: http://www.speff.com
     
    Spehro Pefhany, Sep 24, 2004
    #5
  6. David M

    David M Guest

    On Fri, 24 Sep 2004 10:42:30 GMT, Spehro Pefhany
    <> wrote:

    >On Fri, 24 Sep 2004 21:38:45 +1200, the renowned David M
    ><news^#NO@SPAM^moorhouse#.net.nz> wrote:
    >
    >>On 24 Sep 2004 05:23:30 GMT, (CFoley1064) wrote:
    >>
    >>>>
    >>>>I have a relay that I want to drive from a PIC micro. The PIC has a
    >>>>5V supply - the relay has a 12VDC coil.
    >>>>

    >>
    >>>Hi, Dave. It will work, but it's customary to use a pulldown for NPN and
    >>>pullup for PNP. Try this (view in fixed font or M$ Notepad):
    >>>
    >>> VCCVCC
    >>> + +
    >>> | |
    >>> - C|
    >>> ^ C|
    >>> | C|
    >>> | |
    >>> '--o
    >>> Output Pin |
    >>> PIC ___ |/ NPN
    >>> o-|___|-o-| Darlington
    >>> 5.6K | |>
    >>> .-. |
    >>> 10K| | |
    >>> | | |
    >>> '-' |
    >>> | |
    >>> | |
    >>> === ===
    >>> GND GND
    >>>created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
    >>>
    >>>By the way, don't forget the diode to protect the transistor.
    >>>

    >>
    >>Thanks for your help Chris.
    >>
    >>The Darlington is a ULN28903A (npn array) which has a built in
    >>protection diodes.
    >>
    >>It's interesting that I need a pulldown rather than a pullup as I
    >>built a similar circuit a few years back. That circuit had a LM339
    >>voltage comparator driving the Darlington and used a pullup.

    >
    >Why don't you use a ULN2803A (8 drivers) or ULN2003A (7 drivers) which
    >has the resistors inside? I'm not familiar with the part number you
    >mention above.


    Sorry - my typo - should be 2803A.

    >
    >The LM339 has an open-collector output. Most PIC outputs are
    >push-pull, some have one pin that is open-drain.


    Yes, I now understand. Electronics is a hobby for me.

    Thanks & Regards

    David
     
    David M, Sep 24, 2004
    #6
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