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tacho wire

  • Mark Hills
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6 months 2 days ago #15947 by Mark Hills
Mark Hills created the topic: tacho wire
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

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6 months 2 days ago #15952 by Bill Hertzel
Bill Hertzel replied the topic: tacho wire
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.

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5 months 4 weeks ago #15987 by Mark Hills
Mark Hills replied the topic: tacho wire
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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5 months 4 weeks ago #15988 by Bill Hertzel
Bill Hertzel replied the topic: tacho wire
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.

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5 months 3 weeks ago #16023 by Mark Hills
Mark Hills replied the topic: tacho wire
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

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5 months 3 weeks ago #16028 by Bill Hertzel
Bill Hertzel replied the topic: tacho wire
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.

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1 week 3 days ago #17495 by Daniel
Daniel replied the topic: tacho wire
Would this circuit work for a Bosch Points Ignition system?

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1 week 3 days ago #17496 by Bill Hertzel
Bill Hertzel replied the topic: tacho wire
It might be OK as shown.

For any points type ignition, you might Increase or eliminate the 47K resistor and reduce the second 10K resistor down to 1k or less.

You really will need an oscilloscope to see what you are getting, otherwise, it is just a trial and error shot in the dark.

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