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Darlington question

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  • Darlington question

    Question: If a Darlington pair is turned off, is it safe to force its output high?
    Scenario: A combination action is driving the magnets on a SAM. I need to drive the same magnets from a separate reversible
    board. Can the outputs from the combination action and the reversible simply be connected together in a wired OR configuration?
    I think the answer is yes, but I'd like confirmation before I bake this into my layout.
    And are there drivers where this would be verboten?

  • #2
    I don't know what a SAM is, but,
    a wired OR would mean 2x darlington, or darlington and switch in parallel, driving the output high by the load.
    that is ok. if you accidentally activate the darlington the maximum current will be determined by the load.

    driving the output high by an active element such as a switch or power-connected darlington will go wrong. As long as you donot drive the darlington all is fine but if you accidentally activate it the current will only be limited by the explosion of the darlington.
    accidental driving is very common to happen at switch-on or switch-of of a circuit :-).

    anyways,... if your circuit is the 1st, parallel “wired or” all should be ok but if I read it well You're driving a coil, in that case there needs to be a reverse polarity connected diode in parallel with the darlington to protect against reverse current at coil switch off.
    Made my own organs in 1975 and 1981.
    Fender Rhodes since 1982.
    Hammond xm1 midi-module with Solton Bass/Treble Leslie for many years
    Hammond L122 with 1speaker Leslie cabinet since January 2020


    • #3
      SAM = Stop Action Magnet, i.e., a stop mechanism with two electrical coils to move the stop from on to off and vice versa. The answer depends upon the reverse breakdown voltage of the Darlington emitter to base junction is--check the spec sheet to see what the rating is.


      • #4
        Is it a high-side or low-side switching configuration?

        Low side is safe because you're only pulling the coil to ground. However, high-side switching should have protective diodes in each drive path to prevent the the inactive side from being back-driven. Otherwise, it's very tricky to ensure all the biasing works out safely to prevent blowing a junction.

        --- Tom
        Rodgers 660 with additional analog rack sets (practice), 36D/C in digital conversion, Yamaha CVP-107


        • #5
          Thanks for the responses. But I only ended up more confused,
          so I contacted the chief engineer at Syndyne. He assured me I could safely
          parallel the outputs of their combination action and their reversible boards.