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tacho wire

in 2 Stroke Technical Questions 8 years ago
Hi, recently got a new 582, see picture. Is the gray wire, at tip of red arrow in picture, going to give me an rpm / frequency signal? Will the output be a multiple of the rpm or will it be straight rpm? Does anyone know what the signal of this wire would be, volts & amps? Regards Mark
P5151923_anno.jpg (You do not have access to download this file.)

  • Re: tacho wire

    by » 8 years ago

    The Grey/Tach signal will pulse six(6) times per revolution. Very common for 12 pole alternators.
    The current capacity is not specified but is very small, a few mA, and the voltage varies with rpm.
    Any Tachometer that uses, or can be set to 6 pulses (selector switch) will be happy with the pulses.
    Tachometers expect the pulses to be very messy and they are ready to accept anything from 3 to 3000 volts.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
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  • Re: tacho wire

    by » 8 years ago

    Hi Bill
    Thanks for your reply. Are you sure the voltage could vary from 3 volts to 3000 volts? This seems like a huge range and beyond the carrying capacity of the small gray wire. The reason I ask is that I want to get an Arduino to log engine rpm and therefore want to know the volts and amps of the rpm signal so I can arrange things for the Arduino to read them.
    Regards
    Mark
      
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  • Re: tacho wire

    by » 8 years ago

    I am not suggesting a 3000-volt square wave.
    Place your fingers across the 12-volt battery terminals of a parked car. Feel Nothing?
    Try it again with the engine running and some cars will give you a shocking surprise.
    Typically the signal will be a ~12-volt square wave with huge inductive spikes on the corners or a 20-30 volt sine wave.
    Some tachs just lay the sense wire next to a spark plug lead and use the induced voltage spikes.
    A Generic Tach designed for No engine in particular, must be prepared for the extremes.

    Google "Tach Input Circuits" to get some examples.
    You will typically see a large input resistor to limit the current, followed by a pair of clipping diodes to limit the voltage swings and square up the edges.
    A small Capacitor cleans up the high-frequency components and absorbs the voltage spikes.
    And voltage divided sizes it to the desired range.

    The 582 is the great great grandchild of an outboard motor. Think, Rotax > BRP> Ski-Doo > Jet Ski > Boat Motors
    The tach signal is a wire taped into the alternator windings before the rectifier and the regulator.
    The alternator is 12 pole so you will see 6 AC cycles per revolution at 6-30 vac with induction spikes when the plugs fire.

    A half a dozen passive components will clean this mess up to something a TTL input can handle.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
    Report

  • Re: tacho wire

    by » 8 years ago

    Hi Bill
    Thanks for your reply. Does the attachment look something like you are talking about? Please be gentle with me, I'm a mechanical engineer and only had a half year of electronics knowledge and this was twenty years ago.
    Regards
    Mark
    tachsketch.pdf (You do not have access to download this file.)
      
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  • Re: tacho wire

    by » 8 years ago

    Not too bad for a Mech! ;)

    Try Something like this....



    You may need to reduce the 10K resistors, but it this should be a safe starting point.

    Bill.
    Tach_Input_Circuit.jpg (You do not have access to download this file.)

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
    Report
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