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Running multiple connections on circuit board ... good or bad?

Discussion in 'Electronic Basics' started by Harry Muscle, Dec 23, 2003.

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  1. Harry Muscle

    Harry Muscle Guest

    Is running multiple connections from one point to another a good or bad
    thing to do on a circuit board. I'm working on a layout of a simple
    temperature control circuit and I was planning on having the ground run in a
    circle, such that almost each point on the circuit would have at least two
    different routes back to the ground connection. Is this a good thing to do?
    The more routes the better? Or is there a limit? Do the same rules apply
    to non ground connections?

    Thanks,
    Harry
     
  2. This does not normally cause problems, and the full extension of the
    idea is to have a ground plane. The exception to this involves either
    big or fast currents that will cause voltage drop on their ground
    lines or two or more points that need very accurate ground reference
    voltages. In the first case, you use a separate ground trace to carry
    the current, to keep that drop out of the rest of the grounds,
    connecting at a point that does not carry current. In the second
    case, you connect those nodes together separately from the rest of the
    grounds, connecting them at the quietest point in the ground system.
     
  3. Harry Muscle

    Harry Muscle Guest

    I'm guessing a relay and a fan would fall under the "big current" category.
    What about a reservoir capacitor (used to smooth out the DC) of 1000uF?
    Should I give it a se te route back to ground?

    Thanks,
    Harry
     
  4. They are certainly in the category of noise makers rather than noise
    sensitive components. I would give their currents as separate a path
    as possible, including cancellation of those current's magnetic
    fields, by folding the current path into a parallel pair.
    The currents between capacitor and steady loads is normally part of
    the quiet stuff, while the rectifier currents that charge the
    capacitors ar part of the noise producing stuff. Treat the rectifier
    currents like you do motor and relay currents. Run them to the
    capacitors on separate traces than the grid that connects the
    capacitor to its loads so that voltage drops caused by the charging
    pulses do not get added to the capacitor voltage as seen by its loads.
     
  5. Harry Muscle

    Harry Muscle Guest

    You mean by running the + and - lines beside each other, correct?
    Got a quick question about running the connection to my caps. Which of the
    following two is better:

    RECTIFIER------CAP----VOLT REGULATOR-----LOAD

    or

    CAP----------|------------VOLT REGULATOR-----LOAD
    |
    RECTIFIER

    The bottom option would provide seperate leads to the reservoir caps, but
    I'm not sure if that's the proper way of doing it?

    Thanks,
    Harry
     
  6. Yes. Run a pair of traces from the switching point with a bypass
    capacitor across those rails at the switching device, with the catch
    diode close to the switch.
    I like the first one better, because it separates the big charging
    pulses from the steady load currents to the regulator. If you show
    all the traces, you end up with the rectifier having two lines to the
    capacitor, which is right at the input of the regulator, a second
    (smaller) cap right at the output of the regulator, and the common
    ground node for these two caps as the tie in point of the regulator to
    the circuits that need clean, regulated voltage. That way, the big
    charging pulses from the rectifiers cause drops on traces that are not
    between any devices that have to agree on what zero volts is.
     
  7. Addie

    Addie Guest

    I was just reading this out of interest, and I am a bit mistified about
    this 2nd option. What purpose would a capacitor have before
    a rectifier. Surely the capacitor *must* be after the rectifier to make
    any sense, and not just a matter of someone's "personal preference" ?
     
  8. The point is that the rectifiers, capacitor and regulator share a
    couple nodes, but there are several ways to run traces to get them
    connected together. If traces had no resistance or inductance, any
    way would work the same as any other way.
     
  9. Chan

    Chan Guest

    o.k. seems like I'm out of my depth on this one. Still remember my power
    supply circuit ? I wanted to draw the component names as you suggested
    to make the discussion easier, but I haven't done it yet. I didn't quite
    understand why we need to guard against oscillation. Is this oscillation
    picked up from the air or mains, or is it self generating and can be
    calculated. The other thing I/you was not sure about was whether the
    resistors near the emitter follower transistors where drawn correctly /
    or had correct values. Do you think it is worth while to simply measure
    some voltage drops across components for varying load voltages/currents ?
     
  10. I don't recognize your name. Are you talking about this post?
    http://groups.google.com/groups?selm=&output=gplain

    Any time you have gain greater than 1 and feedback, there is a
    possibility of oscillations (regenerated echoes) being generated.
    Before going further than that, I need to know what you are talking
    about.
     
  11. Chan

    Chan Guest

    http://groups.google.com/groups?selm=&output=gplain

    sorry, I forgot to mention which posting, you have guest right.
    I will post the circuit again. I think the capacitors around the
    op amp are simply standard practice to avoid oscillation, and
    are not something in particular for a power supply.
     
  12. Perhaps, but a power supply regulator is a big, power amplifier with
    lots of feedback, whose job it is to hold a voltage stable, or switch
    to smoothly and accurately regulated current in the event of an
    overload, in spite of all variations in load from any value of
    resistor between infinity and zero, inductors, to capacitors, (or
    resonant loads made of inductors and capacitors) to pulsing circuits.
    Their stability is quite a design problem, especially if there is a
    fast spec on how quickly the voltage must get back to setpoint after a
    big load current spike.
     
  13. Chan

    Chan Guest

    You may be right. I have a book (how to use op amps) which says
    on p.9 that it is very unusual for the 741 to "misbehave", so perhaps
    the 10nF from negative op amp input to op amp output is to remove
    any ac component from the controlling feedback loop.

    Zc = 1/jwc = 1/(6.28*10n*f)

    I believe you chose f = 20KHz (why?) which gives us
    Zc = 8 ohms
    Hence Zc << than transistor feedback loop which is at least
    hie of 3 emitter followers.
     
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