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Coil driving

Discussion in 'Electronic Basics' started by Nuby, Oct 13, 2007.

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  1. Nuby

    Nuby Guest

    What's wrong with this picture?

    TTL control lines with small current sinking capability (say 20ma)
    need to activate electromagnetic coils (50VDC, 300ma). Each control
    line gates an SCR which runs Vcc into a coil. The group of coils has
    a common disable in the form of a big relay that cuts the common
    connection to ground.
  2. JeffM

    JeffM Guest

    Are you planning to protect the semiconductors from inductive
  3. Guest

    You're missing an interface driver. Perhaps a common ULN2003 would
    help. If you're running DC with SCRs, it will turn on one time and
    latch. Is that the goal? Lifting the ground as a master reset will
    complicate the TTL interface. Can you interrupt the positive instead?

    What are you really trying to do?

  4. default

    default Guest

    Maybe nothing. Assuming you have the correct polarity to gate the
    SCR's into conduction.

    A snubber across the coils isn't a bad idea, but SRR's tend to be
    pretty rugged. I'd probably just go for it, and see if anything
  5. Jamie

    Jamie Guest

    You should be using opto-isolators that will drive the SCR etc.
    You must insure you use the proper relay coil for this. With
    SCR's, it would be a DC coil. with an TRIAC , it would be an AC coil.

    This only assumes that you're using AC as the load source.
    if DC is going to be the source. The main relay you speak of must
    not be an SCR. it has to be something like a real relay, GTO or
    FET switch other wise, you'll get latch up until power is removed.

    Actually, you can get SSR's (Solid State Relays) that can be driven
    from a TTL signal.
  6. Eeyore

    Eeyore Guest

    In overall concept it's ok but there are some practical considerations.

    a. Do you really mean actual TTL ? TTL has annoying limits on output voltage
    swing and 20mA may also be an inadequate trigger current for the thyristors.

    b. The common disable would make more sense in the 50V DC supply positive input.

    c. As others have noted, you need protection from inductive kickback when the
    loads switch off.

  7. Eeyore

    Eeyore Guest

    Why ?

    Ne never mentioned AC, why confuse the issue ?

  8. Nuby

    Nuby Guest

    Not the end goal, but an acceptable method.
    I don't see the connection/problem. Can you explain?
    I can, but having a common connection to Vcc and distinct/controlled
    connections to ground appears slightly perverse. I can afford to
    ground the frame, which simplifies the harness, but will not connect
    the frame to Vcc.
    Control 100 miniature/high-density coils as cost-effectively as
  9. Nuby

    Nuby Guest

    Um, have you priced such isolators recently? I'm looking for a
    circuit as simple as possible.
    Hunh? None of that makes sense to me. What relay?
    What load are you talking about?
    The only purpose of the big relay is to interrupt the latch effect.
    Yes, but they are neither simple nor cheap. Why do I need such a
    complex component?
  10. Nuby

    Nuby Guest

    I think so. Typical outputs would drive 10-20 TTL loads. The
    actual control chip is a serial-to-parallel shift register.
    Interesting point.
    How can I determine how much of a problem this would be? The energy
    in each active coils is going to be pretty small.
  11. Eeyore

    Eeyore Guest

    What's this chip's part number ?

    Depends of course on the SCR you use. Did you have a specific one in mind ?

    A simple inverse parallel diode such as a 1N400x across each relay coil should be

  12. Eeyore

    Eeyore Guest

    Never mind Jamie, he talks a lot of nonsense, including posting things that are
    100% wrong. Every group has one. In this case he merely posted mainly random

  13. Nuby

    Nuby Guest

    We have not chosen a specifc one yet. An example would be the
    MM74HC164 family.
    Again we don't have a specific target yet. Any sensitive-gate SCR
    that can handle the (light) duty cycle would work for us.

    OK, but I still don't know whether the protection is necessary.
  14. Eeyore

    Eeyore Guest

    Ah, fine. That's not TTL, it's HCMOS. Much better.

    This would appear to do the job.

    It's nice and cheap too.
    It would be very silly not to fit it.

  15. Jamie

    Jamie Guest

    ^^^^^^^^^^^^^^ One should practice what one preaches.
    You're such an anus Mr. Ham...

    When some one gives incomplete information. It can lead to to many
    roads. Until that poster makes it clear as to what they're trying to do,
    which apparently has taken place now. People that have been down many
    avenues can come up with so many scenarios that it would make some one
    like you get all confused. Sorry for your disablity in this area.

    I guess that is what most would call "separating the men from the
    boys" when it comes to problem solving which you seem to be lacking.

    Have a good day sucker.
  16. Jamie

    Jamie Guest

    yes, it's required on the coils, other wise, you'll take out the SCR
    at some point, and may also take out the TTL device.

    The other problem is the (time on) gate signal. The coil is inductive
    which means you'll get low current in the SCR at the initial signal.
    If the Gate isn't kept on for the minimum time required that gets the
    SCR => it's holding current. It will not latch. This can be solved with
    a resistor across the coil if it becomes a problem.

    To know that, you would have to know the inductive value of the coil
    and perform some calculations for the time frame your signal will be on.

    Have a good day.
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