fbpx
Jump to Forum..

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.

      
    Report

  • 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
      
    Report

  • 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.)
      
    Report

  • 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
You do not have permissions to reply to this topic.
Related Topics
Title Category Replies
Wire size vs current incongruity General Tech Discussion 5
Safety Wire Points 912iS Technical Questions 2
Lane A and B Grd wires 912iS Technical Questions 5
replacing coil HT wire 912 / 914 Technical Questions 1
Longer Plug Wires 912 / 914 Technical Questions 12

info@rotax-owner.com