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Tachometer Woes

Discussion in 'Electronic Repair' started by [email protected], Dec 12, 2004.

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  1. I posted a more detailed message in .design and haven't got a response
    yet. But anyway, to keep this short and sweet. Is there any trick
    that people use to test and diagnose DC tachometers? I have a 4
    channel tach that is powered by 24VDC. Each channel has a VCC, Vout,
    and gnd. It's supposed to be an open collector output, but when I
    connect my external circuit all I get is a steady 24VDC on the Vout
    with a lot of noise on top of it. Does this inherently mean
    something? Are there any tricks I could attempt to find out what's
    going on? Thanks.
     
  2. James Sweet

    James Sweet Guest

    Do you have a bypass capacitor (such as a 10nF mica) bypassing the power
    pins on the chip?
     
  3. Franc Zabkar

    Franc Zabkar Guest

    I don't know what kind of external circuit you are using, but I would
    first test the tacho with a resistor (10K?) between Vout and Vcc.


    - Franc Zabkar
     
  4. G

    G Guest

    Sounds like this 4 channel tach is set up to forward the tach info onward
    via that Vout terminal. Does the tachometer function of each channel work?
    If so, is your external circuit configured to supply and/or work with the
    open collector Vout correctly? You can do a basic check by scoping the
    input to that open collector circuit and see if it's trying to be
    switched......then seeing if bias is required to be supplied by your
    external circuit (not untypical).....it could be that your external circuit
    is Hi-z and/or capacitively or incompatibly logic coupled. Once you see
    how the output of the tach circuit is configured, you can build a simple
    external circuit to see if it will switch (assuming you find valid
    switching voltage at the input to the open collector circuit......for
    example at the base if a simple bipolar common emitter circuit.

    What kind of a unit is this....just curious.

    Gord
     
  5. He shouldn't see 24 V on an open collector output.

    So what is your external circuit?

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  6. James Sweet

    James Sweet Guest


    Not even if his external circuit has a pullup resistor?
     
  7. Thanks for the responses. First, no, I do not have a bypass cap
    attached (there are no ICs involved here). My external circuit is
    simply a 1kOhm pullup resistor between Vcc and Vout, and I'll I'm
    seeing on Vout is 24VDC with lots of noise. Yes, I am turning the
    tach while looking at the output on a scope. I've tried other
    resistor values with the same results. The datasheets for the tach
    (Deuta Werke type DF9) say to do exactly what I'm doing.

    More details for you: 3 of the 4 channels work properly, and have
    worked properly for many years. These tachs are installed on a public
    transit fleet in which the manufacturer never used the 4th channel.
    We now need that 4th "spare" channel for a new modification that needs
    a vehicle velocity input. We were going to read the tach pulses in a
    micro controller and compute velocity, long story... All of the
    vehicles I've looked at so far have this problem on channel 4, so it's
    not that I've found one yet that works.

    One thing we are worried about is that the 4th channel may not be
    working because it was left floating for so long. Is that a
    possibility?

    In case anyone is wondering, the 3 other channels are connected to a
    PCB with the pulse detecting circuits, I have no idea what the exact
    circuit is, so I can't duplicate it. Eventually I need to find out
    though.

    One other question, as an alternative solution we were considering
    connecting our new equipment in parallel to one of the 3 other known
    working channels. Is this bad? The input resistance of the new
    equipment is very high, around 10 MOhm. Do you think this is a doable
    solution if we can't figure out what's happening with the 4th channel?
     
  8. Yes, the 3 other channels work fine. I don't know what kind of
    circuit they are connected to though, it's all on a PCB.

    I just followed the datasheet and used the appropriate pullup
    resistor.
    Deuta Werke type DF 9, 24VDC, 4 channel output, 110 pulses/rev
     
  9. Yes, of course of if his external circuit has a pullup to 24 V, but he didn't
    say what was in his external circuit. The noise is most likely from there.

    We need more info.

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  10. Or, perhaps it really doesn't exist - the manufacturer realizing you
    never used it, left if off. :) Can you trace the circuit inside the
    tach to see where each channel goes?

    I don't think a floating open collector output would cause any problems.
    As long as each channel sees what it expects, that should not be a problem.

    10M ohm is essentially an open circuit as far as it is concerned.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
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  11. Sam,

    Yes, you might be correct about that. I have finally made a contact
    with someone in Germany about this problem and I'm awaiting their
    response. The device is sealed (for rough outdoor use) and I suspect
    the circuits are epoxyed inside, although I should break one open at
    some point.

    The parallel connection idea is still an option, at least to keep
    further costs down.
     
  12. James Sweet

    James Sweet Guest

    I originally thought this was referring to an IC, not a sealed circuit. It's
    quite common for a manufacture to use a standard casing for a device and
    simply not internally connect pins that are not needed. As Sam said, it's
    entirely possible that this is only a 3 channel tach, and that 4 channel was
    a different model in the same box.
     
  13. Franc Zabkar

    Franc Zabkar Guest

    If you need to ensure that your add-on equipment will not affect the
    original installation, especially if it is powered from a different
    source, then isolate it via an optocoupler in series with your pullup
    resistor.


    - Franc Zabkar
     
  14. Thanks for the suggestion. Another of my co-workers was talking about
    this too. We are going to wait for the Deuta engineers to respond to
    us before proceeding.
     
  15. Yes, this is a totally sealed unit with an IP rating of 67. FYI, we
    mount them to the outer hub of a light rail commuter vehicle on the
    center axles. I'm waiting for Deuta to respond to me with a response
    to exactly the issue you pose below. The stampings on the tach lead
    me to believe the 4th channel IS connected though because it is
    clearly stamped "collector with R = 1k5 ch 1,2,3,4", but since I
    haven't opened one up I can't say for certain.
     
  16. Franc Zabkar

    Franc Zabkar Guest

    Could the 4th channel represent some special function? For example,
    could it be generating a single [hard to detect] pulse per revolution
    as a timing mark? Is there any special phase relationship between the
    three working channels? For example, are channels 1-3 spaced 120
    degrees apart or 90 degrees, or are they in sync?


    - Franc Zabkar
     
  17. Well according to the datasheet I have, channel 2 leads channel 1 by
    90 deg. Same with channel 4, it leads channel 3 by 90 deg. So
    basically according to this timing diagram, channels 1 and 3 are in
    phase and so are channels 2 and 4, but their outputs are inverted to
    each other. One is a trailing edge the other is a rising edge and so
    forth. Channel 4 should be just another output according to the
    datasheet and there is no discussion anywhere of it being a special
    output. Does this phase issue change anything from my point of view
    in connecting channel 4?
     
  18. Not based on the way you were testing, only how they are used.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
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    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header is ignored.
    To contact me, please use the Feedback Form in the FAQs.

    ..
     
  19. Franc Zabkar

    Franc Zabkar Guest

    Wouldn't that make them 180 degrees out of phase?
    The reason I asked about phase relationships was to get an idea how
    channel 4 fits in with the others. If the phase relationships between
    channels 1,2, and 3 had been 120 degrees, then that would have
    suggested that channel 4 was a special channel. A 90 degree phase
    difference suggests that it is identical to the others, ie it suggests
    that there are four sensors equispaced around the circumference (I
    think).

    - Franc Zabkar
     
  20. The datasheet also has text that indicates 90 deg between channels 1-2
    and 3-4.

    I've been corresponding with a Deuta engineer in Germany for the past
    2 days and even he is perplexed about this. Apparently we performed
    all of our connections correctly and used the loading resistor
    properly.

    I'm starting to wonder if a middleman distributor or other company
    possibly disconnected this channel before we actually received these
    units and installed them.

    I've been assurred by the Deuta engineer that leaving un-used outputs
    floating is okay.
     
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