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how to drive 24V with 18V max darl. opto without extra supply???

Discussion in 'Electronic Basics' started by tom, Oct 12, 2004.

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  1. tom

    tom Guest

    Problem:

    need to drive 24V 600ohm coil relay.

    part on hand: 6n139t darlington output optoisolator, max V collector 18V.

    I assume that one should not present the collector with more than 18V, even when
    the opto is off. This means you need to have a voltage regulator, say 12V for
    the opto and drive an external transistor or mosfet whose collector or drain CAN
    float up to 24V.

    Is there a way to do this without a second power supply? I was thinking a stiff
    divider, but that seems silly.

    Thanks,

    Tom
     
  2. colin

    colin Guest

    You should have enough tolerence on the relay coil to pull in at less than
    24 volts and maybe you can select darlingtons wich can handle a bit more
    than 18v, depends if u wana risk it or not, make sure the emf is well
    clamped.

    Colin =^.^=
     
  3. Ban

    Ban Guest

    Tom,
    It would be better to have an optocoupler with a simple 30V open collector
    output, which can drive the relais or an external bipolar transistor
    directly, since the saturation voltage of a darlington is at least 1V. But
    you could do that also with your device and a couple of resistors. You do
    not necessarily need an extra supply, you can either use a zener to keep the
    voltage at safe levels or make a resistive divider. But be aware that a lot
    of power is wasted even when the relais is off. if you have several outputs,
    an extra +5...12V supply will make sense.
     
  4. Sure. Use a 15V or so zener between the 24V supply and the collector
    resistor. Then, drive another transistor with the collector off the
    optoisolator. It'll invert the sense.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  5. CFoley1064

    CFoley1064 Guest

    Subject: Re: how to drive 24V with 18V max darl. opto without extra supply???

    Something like this? (view in fixed font or M$ Notepad):

    VCC VCC
    + +
    1N4002| |
    24V - C|
    | ^ C|
    .-. | C|
    2.2K | | | |RY
    | | | |
    '-' | |
    | | |
    | '----o
    | |
    o---. ___ |/
    | '------o-------. .--------|___|- o|2N3904
    | | | | 10K | |>
    1N4744(15V)| .-o---o---o---o--. .-. |
    | | 8 7 6 5 | | | |
    /-/ | | 10K| | |
    ^ | 6N139 | '-' |
    | | | | |
    | | 1 2 3 4 | | |
    === '-o---o---o---o--' === ===
    GND | | GND GND
    | |
    + | |
    o--------------------' |
    |
    ___ |
    o----|___|---------------'
    -
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


    This has the added advantage of being non-inverting (relay on when opto on).

    Good luck
    Chris
     
  6. peterken

    peterken Guest

    Looks OK to me, except I would reduce the size of Rbe at the external
    transistor to say 1k something
    This for bypassing the leakage current of the opto device (upto 100uA if I
    see datasheet)




    Something like this? (view in fixed font or M$ Notepad):

    VCC VCC
    + +
    1N4002| |
    24V - C|
    | ^ C|
    .-. | C|
    2.2K | | | |RY
    | | | |
    '-' | |
    | | |
    | '----o
    | |
    o---. ___ |/
    | '------o-------. .--------|___|- o|2N3904
    | | | | 10K | |>
    1N4744(15V)| .-o---o---o---o--. .-. |
    | | 8 7 6 5 | | | |
    /-/ | | 10K| | |
    ^ | 6N139 | '-' |
    | | | | |
    | | 1 2 3 4 | | |
    === '-o---o---o---o--' === ===
    GND | | GND GND
    | |
    + | |
    o--------------------' |
    |
    ___ |
    o----|___|---------------'
    -
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


    This has the added advantage of being non-inverting (relay on when opto on).

    Good luck
    Chris
     
  7. colin

    colin Guest

    another alternative would be to use a comon base npn transistor, the base
    biased to about 5v by a simple 2 resistor dividor, the darlington the drives
    the emiter.

    when the darlington turns on it pulls down the emiter of the 2nd transistor
    driving it into saturation.

    a comon gate n ch mosfet could easily be used instead, with the gat siting
    at around 10 volts.
     
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