tacho wire

by Mark Hills in 2 Stroke Technical Questions 7 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.)

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Re: tacho wire

by Bill Hertzel » 7 years ago

Bill Hertzel

13 1.95k 7.96k

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 Mark Hills » 7 years ago

Mark Hills

2 4 0

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 Bill Hertzel » 7 years ago

Bill Hertzel

13 1.95k 7.96k

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.

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Re: tacho wire

by Mark Hills » 7 years ago

Mark Hills

2 4 0

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 Bill Hertzel » 7 years ago

Bill Hertzel

13 1.95k 7.96k

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.

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tacho wire

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