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Current reversal in push/pull?

Discussion in 'Electronic Design' started by Jeffery Dean, Sep 18, 2005.

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  1. Jeffery Dean

    Jeffery Dean Guest

    I have an app where I am driving a solenoid using a dual op amp in
    bridge configuration to double the effective voltage from a single
    positive supply.

    In terms of current flow, can someone please clarifiy how this is
    different from using a single amp?

    I need the emiited field to be unidirectional and not reverse.

    Is this possible with a push/pull driver?

    Thank you,

    Jeffery
     
  2. Fred Bloggs

    Fred Bloggs Guest

    The bridge drive alternates the polarity of voltage across the solenoid-
    so the current will attempt to reverse. If you put the solenoid inside a
    full-wave rectifier bridge with (+) and (-) terminals connected to
    solenoid and AC terminals connected to opamp outputs, then the voltage
    doubles and is rectified- but you lose two diode drops.
     
  3. Phil Allison

    Phil Allison Guest

    "Jeffery Dean"
    ** HUH ???

    Scotty needs to hear about this.





    ........... Phil
     
  4. Pooh Bear

    Pooh Bear Guest

    Actually, your push-pull ( actually a bridge configuration ) is used
    quite specifically in order to be able to reverse current flow. It
    doesn't actually double the voltage available, merely offers a
    bi-directional output.

    Single output gives a swing of 0 to Vsupply.
    Dual output gives 0 to +Vsupply or -Vsupply.
    Tie one connection to the solenoid to ground or supply ( whichever
    makes most sense ). Drive it from one op-amp only.

    I hope you have catch diodes on the op-amps output to deal with the
    inductive 'flyback' pulse when the solenoid is de-activated btw.

    Graham
     
  5. Fred Bloggs

    Fred Bloggs Guest

    You can try this- assuming any other problems- there is no diode drop:
    View in a fixed-width font such as Courier.

    ..
    .. 2V
    .. ---
    .. | |
    .. | |
    .. | |
    .. V | |
    .. --- -- --
    .. | | + -
    .. | | >----|+\
    .. -- -- | >-|>|---+-KKKKKK--+----+
    .. +-|-/ | | |
    .. | | | |
    .. +------------+---|<|---+ |
    .. | |
    .. | |
    .. | |
    .. +--[R]-+-[R]---+
    .. | |
    .. | |
    .. +-|-\ |
    .. | > -+
    .. +-|+/
    .. |
    .. ---
    .. gnd
    ..
    ..
    ..
    ..
     
  6. I canna repeal the laws of ffisiks..........


    martin
     
  7. Pooh Bear

    Pooh Bear Guest

    The voltage *does not* double !

    A single driver will product a drive voltage = Vsupply.

    A bridge will produce a drive voltage of Vsupply or -Vsupply.

    A bridge driver arrangeemnt with rectifier bridge will produce a drive
    voltage of Vsupply - 2*Vf. I.e. *less* than the single driver.

    Graham
     
  8. Fred Bloggs

    Fred Bloggs Guest

    Maybe we're taking about two different things- the differential voltage
    across the solenoid is 2x the voltage drive of the op-amp pair: one
    output goes to Vin and the other to -Vin, this makes a magnitude of
    2xVin across the solenoid. A safe way to do this is:
    View in a fixed-width font such as Courier.

    ..
    .. 2V
    .. ---
    .. | |
    .. | |
    .. | |
    .. V | |
    .. --- -- --
    .. | | + -
    .. | | >-+----|+\
    .. -- -- | | >-|>|---+-KKKKKK--+----+
    .. | +-|-/ | | |
    .. | | | | |
    .. | +------------+---|<|---+ |
    .. | |
    .. | |
    .. | |
    .. +------------------[R]-+-[R]---+
    .. | |
    .. | |
    .. +-|-\ |
    .. | >--+
    .. +-|+/
    .. |
    .. ---
    .. gnd
    ..
    ..
    ..
    ..
     
  9. Fred Bloggs

    Fred Bloggs Guest

    I suppose if he is just railing them out- this will work well too:
    View in a fixed-width font such as Courier.

    ..
    .. 2V
    .. ---
    .. | |
    .. | |
    .. | |
    .. V | |
    .. --- -- --
    .. | | + -
    .. | | >-+----|+\
    .. -- -- | | >-|>|---+-KKKKKK--+----+
    .. | +-|-/ | | |
    .. | | | | |
    .. | | +---|<|---+ |
    .. | | |
    .. | | |
    .. | | |
    .. +----------------------+ |
    .. | | |
    .. | | |
    .. | +-|-\ |
    .. V | | >--+
    .. - >----+---------------------|+/
    .. 2
    ..
    ..
    ..
    ..
    ..
    ..
     
  10. Fred Bloggs

    Fred Bloggs Guest

    Make that :
    View in a fixed-width font such as Courier.

    ..
    .. 2Vdd- some things
    .. ---
    .. | |
    .. | |
    .. | |
    .. V Vdd | |
    .. --- | -- --
    .. | | | + -
    .. | | >-+----|+\
    .. -- -- | | >-|>|---+-KKKKKK--+----+
    .. | +-|-/ | | |
    .. | | | | | |
    .. | | | +---|<|---+ |
    .. | | -Vdd |
    .. | | |
    .. | | |
    .. +----------------------+ |
    .. | | |
    .. | | |
    .. | +-|-\ |
    .. V | | >--+
    .. - >----+---------------------|+/
    .. 2
    ..
    ..
    ..
    ..
    ..
    ..
     
  11. Pooh Bear

    Pooh Bear Guest

    Your circuit understood. I 'assumed' that the OP was driving the op-amps to
    rail ( which would make sense ).

    In any event the gain can be simply increased - if that's what's needed - by
    adjusting the feedback but a newbie might indeed not realise that.

    Usual problem - not enough detailed info from OP.

    Regards, Graham
     
  12. Fred Bloggs

    Fred Bloggs Guest

    I just noticed he said a *single* positive supply-"I have an app where I
    am driving a solenoid using a dual op amp in bridge configuration to
    double the effective voltage from a single positive supply." So he's
    sunk. I can't even understand them half the time- the basis for their
    question is so off-the-wall they are unintelligible.
     
  13. Pooh Bear

    Pooh Bear Guest

    If we knew the relay voltage and the supply voltage it would make more sesne.

    As you say, with a single supply there's a limit to what's do-able here.

    Graham
     
  14. Not as funny, now that James Doohan is dead. :(
     
  15. I read in sci.electronics.design that Michael A. Terrell
    I don't agree. For a public figure to be remembered with affection can't
    be bad.
     
  16. I disagree, Scotty, did one hell of a lot for getting youngsters
    interested in engineering. I hope that he would be happy for people to
    use his quotes.




    martin
     
  17. Ditto for non-public figures ;)
     
  18. We've been here before Fred, a year or so ago.
    A transient capacitive voltage doubler, generating
    2xVs to pull the solenoid in, dropping back to 1xVs
    for holding.
     
  19. Tony Williams wrote...
    Nice. I handed my technician a design for a box with ten
    small microwave relays -- except the crazy relays want tight
    connections I suppose, and each one takes 5W of coil power!
    That's 50W for the set, and it's got me worried about heat.
    I can't add a fan, because it's going on a quiet optics bench.
    My tech's waiting for parts, so I've got a few days to improve
    the coil-power scene.

    Tony, did you come up with a slick circuit?
     
  20. Yea Tony, let's see that X2 relay driver again. I can do it with a FET
    bridge, diode and 2*V storage cap but yours was much slicker.
    Regards,
    Harry
     
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