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4017 Counter skips under load

Discussion in 'Electronic Design' started by [email protected], Jul 2, 2007.

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  1. Guest

    I have a fairly simple circuit that consists of a 4017 decade counter
    and nine relay/LED combinations. The relays are very small and the
    circuit operates just fine with no load, or when I connect an LED to
    the relay output. But when I try to operate the circuit under load
    (it is being used to fire nichrome ignitors), then the counter simply
    skips the loaded relay and moves right to the next output. So if I
    send the counter a series of 5 clock pulses, and put a load on the
    relay attached to output 3, then the count goes 1,2,4,5,6. The final
    count ends up being one off, so it's almost like the output in
    question doesn't exist. What I don't understand is why this is
    happening, since the relay is what bears the load, not the 4017.
    Shouldn't the output requirement on the counter be the same regardless
    of what the relay is switching?
     
  2. Joerg

    Joerg Guest

    Looks like power supply spikes, sub-optimal decoupling or a substrate
    diode gets hit. Time to crack out the storage scope, I'd say. Hang it
    onto VCC of the 4017, then to the pins.
     
  3. Yes, it should be, but only if you have used proper circuit physical
    circuit layout techniques to ensure that there is no ground or power
    bounce.
    Think "star grounding" and power decoupling.
    Very common trap for young players.

    Dave.
     
  4. Guest

    I installed the final power setup (on the tests I just had the board
    jumpered onto the power supply) and now it will fire the ignitors.
    The problem is that the counter is now behaving irratically. It will
    skip several counts when receiving a clock pulse and will jump around
    with the slightest change in ground (even connecting a single lead
    from the multimeter to any point on ground advances the count). I've
    tried several different caps (.01-10 uF) connected directly to the
    power leads of the 4017 with no luck. All of the info I can find on
    decoupling keeps talking about selecting the capacitor based on
    frequency. But I'm only sending a single pulse of about 25 ns every
    10-20 seconds. Does this mean I can use a larger cap? I'm seriously
    freaking out at this point cause I only have 24 hours to get this
    working. Thanks for the help!
     
  5. Joerg

    Joerg Guest

    How on earth do you get a 25nsec pulse out of a CD4017? That's like
    clocking a moped at 100mph.

    Anyhow, since things seem to become desperate over there I can only
    suggest to build it up again on an experimental board that has a ground
    plane. And no, you do not have to select decoupling caps based on
    frequency. A 0.1uF plus a nice 10uF electrolytic should do, more if your
    power supply is wimpy.

    Can you post schematic plus photo?
     
  6. Guest


    Sorry, I mistyped. The clock pulse coming into the 4017 from the PC
    parallel port is 25 ms in length. The counter then activates the next
    relay in sequence and the output stays active for 10-20 seconds before
    moving on to the next firing cue. I'll post a printout of the circuit
    design when I get home in a couple hours. I'm using a pretty good
    sized power supply, and I've tried other power sources as well with
    the same results. Should I be doing some sort of decoupling where the
    power enters the board as well? I noticed that the parallel port
    interface card that I'm using has some sort of diode/cap combination
    where the power enters the board. Would this help even things out
    further?
     
  7. John Fields

    John Fields Guest

    ---
    25ns is seriously pushing it. Increase your clock pulse width,
    make sure the driver is low-Z when its output is either low or high,
    and keep its output as close to the 4017 as possible.

    Also, if you're not already doing it, use a separate supply for your
    igniters and make sure you've got a catch diode across the relay
    coils.

    If you can't use a separate supply for your igniters, then you'll
    need to decouple the counter and its circuitry from the supply wit a
    diode and a big capacitor.

    can you post a schematic of your circuit somewhere?

    Alt.binaries.schematics.electronic will work, or any where you can
    post it and supply a link to.
     
  8. Rich Grise

    Rich Grise Guest

    Yes - Rich Sez: You Can't Overcapacitate Power and Ground Planes. :)
    (Unless you've got some temperamental LDO regulator or some such... ;-) )

    Do the relays have reverse diodes or RC snubbers across the coils? If not,
    put them there. You say the relays are sensitive enough to be driven by a
    4017 - small-signal diodes like 1N914 or 1N414x would work fine.

    Depending on what you're switching, it might not hurt to put another
    snubber at the relay contacts; this would probably be RC, depending.

    Good Luck!
    Rich
     
  9. Joerg

    Joerg Guest

    Ok, I might not be there at that time but others will be. As Rich said,
    mind the relays. I would not drive them directly from the chip, it does
    not like inductive loads and their spikes. At the least place some kind
    of buffer in between. The CD40106 for example.

    If the power supply is clean you don't need much filtering onto the
    board if the leads are not longer than a few inches. But the CD4017
    should be grounded and there should be a 0.1uF from its VDD pins
    straight to the ground plane.

    If you post, maybe also include a photo. That can help a lot. For
    schematics a scanned hand sketch is fine.
     
  10. Ah, the alarm bells start ringing right there, no pun intended.
    You probably have ringing on your clock line from the PC. Keep the
    lead from the parallel port as short as possible.
    Are you able to view the signal with an oscilloscope?

    Classic problem with PC parallel port driven equipment.

    Dave.
     
  11. Joerg

    Joerg Guest

    Good point. It could also be relay current spikes getting into that
    clock line. Might be worth using a coax. And no long lines of several
    feet or you get riniging like crazy on it.
     
  12. me

    me Guest

    Another problem might be the clock. What are you using for the clock
    signal that you can send five pulses?
     
  13. Joerg

    Joerg Guest

    AFAIU the OP uses a pin on the parallel port of a PC for that. Which
    makes me wonder about the levels. I think the parallel port is TTL and
    if he runs the 4017 at 5V it'll be quite sluggish and have weak outputs.
     
  14. Guest

    Unfortunately I don't have access to a scope. This is the kit that
    I'm using. The board itself is plugged directly into the parallel
    port. I then have leads soldered on to the board at the resistors so
    that I get a clean 12 volt signal from the parallel port. From there,
    that signal is run via ethernet cable to the firing controllers. In
    my initial tests, this setup worked fine with just an LED as the
    load. Because of safety reasons, I can't have the laptop and thus
    myself any closer to the controllers themselves.

    <img src="http://kitsrus.com/jpg/k74v2_1.jpg">
     
  15. me

    me Guest

    True enough, and I've seen some parallel ports that were marginal for TTL
    level highs.
     
  16. Ah, a picture tells a thousand words.

    The board should work just fine *if* you power your load from an
    entirely separate and *floating* power supply relative to the plugpack
    or power supply which powers the board. You've said the board works
    just fine with an LED load, so obviously something is amiss with the
    power supply wiring for the load.

    Dave.
     
  17. Joerg

    Joerg Guest

    That looks like something very different. No 4017 but a ULN2803 driver.

    Sure you are getting 12V from the parallel port? I never saw one that
    did that.
     
  18. rebel

    rebel Guest

    And you won't - they are TTL. But the 2803 et al are TTL-compatible input.

    Note the data sheet at http://www.analog.com.tw/DB/pdf/DS-2003-02.pdf shows a
    solenoid driver config on p1.
     
  19. ehsjr

    ehsjr Guest


    If you are using the same supply for the 4017/relay
    circuit and for the igniters, do this:

    +V -----+----------> To igniters through relay contacts
    |a
    [Diode]
    |
    +----------> To your 4017/relay circuit Vcc
    |+
    [470 uF] +---> To your 4017/relay circuit gnd
    | |
    Gnd ----+------+---> To igniters ground

    The idea is to stabilize the supply voltage to the 4017/relay
    circuit. Without the diode and cap, when the igniter fires
    the voltage to the 4017 suddenly sags.

    At the 4017 circuit board, add a .01 cap across the supply
    input.

    Ed
     
  20. Guest


    That is a parallel port interface kit that I bought some time ago and
    use for any applications where computer control is needed. It is
    completely different from the boards that use the 4017. I will post
    pics in the morning when I have enough good like to get a detailed
    picture. As for the parallel board, the 2803 went out, so I replaced
    it with an NTE 2018. I'm told that they are an identical match, both
    8 channel darlington arrays. Am I wrong in assuming that if the on-
    board relays are rated for 12 volts, as is the power supply, that the
    output from the 2018 is also 12 volts?
     
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