How to process a funky signal.

Thread Starter

dandy1

Joined Sep 30, 2017
178
This is a signal from the primary side of a magneto style ignition coil, it is clear that there is a positive spike followed by a negative followed by another positive spike.

IMG_20230811_202945.jpg

Why this is so I have no idea. Vmax is consistent at 15v. It is apparent tho that the negative spike has greater (if not consistent) amplitude...

IMG_20230811_202144.jpg

Albeit the scope is cheap - there is some consistency - I have some inkling that there should be a 200v spike every cycle - just to give the secondary a chance of producing a spark. I am new to oscilliscopes so please advise any tips.

What circuit could i implement to read this signal with a mcu(atmega328pb)? Will the negative spike cause a problem in doing so?
 

strantor

Joined Oct 3, 2010
6,798
it is clear that there is a positive spike followed by a negative followed by another positive spike.

Why this is so I have no idea.
Because that's what the output looks like from a magnet being moved quickly across a coil. As the magnet approaches the center of coil it is generating a positive voltage and after passing center it generates a negative voltage.


It is apparent tho that the negative spike has greater (if not consistent) amplitude...
Maybe there is a diode or distributor elsewhere, something that is directing the positive pulse into a load (spark plug) and not the negative one; negative being more voltage because it is unloaded.

What circuit could i implement to read this signal with a mcu(atmega328pb)? Will the negative spike cause a problem in doing so?
I would use an optocoupler with a series diode to block the negative spike and a resistor divider to decrease the voltage seen by the opto.

EDIT: I just re-read, noticed you are talking about the second positive spike each rev. Yeah I don't know about that.
 

WBahn

Joined Mar 31, 2012
30,077
What is the bandwidth of your scope?

The kinds of voltages that a magneto can produce can definitely cause damage to some circuits, depending on how in interface to them.
 

strantor

Joined Oct 3, 2010
6,798
Here is some information about why your pulses look like that, and I think you have your scope probe backwards.


1691790842474.png


how is this odd waveform created? [...] the armature core in the spark coil and the pole pieces on the magnet(s) in the flywheel are specifically designed to produce this odd waveform. (“Poles” on magnets and armatures are pieces of metal from which the lines of magnetic force are directed.) If you look closely at the magneto in an outboard motor, you will find that there are more magnets in the flywheel and poles in the armature cores than would seem to be necessary. Johnson and Evinrude magnetos have two magnets in the flywheel, and three poles in the armatures. (Refer to figure 3-4 for details.) Other systems may have three magnets in the flywheel and two poles in the armatures. It is this difference between the number of poles in the magnets and in the armature cores that is responsible for the creation of this odd waveform, with two small negative voltage pulses around every large, positive voltage pulse that is generated
 

strantor

Joined Oct 3, 2010
6,798
thank you for the article, but defo have the right config on the probe - or could it be producing a negative voltage?
Probably is.

Single cylinder magnetos are designed in such a way that the spark plug always receives negative polarity sparks.
The first article was talking about engines with powered coils and the author was measuring the primary. I didn't realized unpowered ones were different.
 

Thread Starter

dandy1

Joined Sep 30, 2017
178
Probably is.



The first article was talking about engines with powered coils and the author was measuring the primary. I didn't realized unpowered ones were different.
either way, there should be good amplitude on the positive spikes to generate a signal for mcu? with diode to block negative voltages. Could i use a capacitor to hold up the voltage on the first positive spike which overides into the second positive spike as to alleviate 'debouncing'?
 

strantor

Joined Oct 3, 2010
6,798
either way, there should be good amplitude on the positive spikes to generate a signal for mcu? with diode to block negative voltages. Could i use a capacitor to hold up the voltage on the first positive spike which overides into the second positive spike as to alleviate 'debouncing'?
I would just discars the two positive spikes and catch the negative spike instead, by putting the blocking diode backwards and connecting the input side of the opto backwards too. Simpler.
 

Thread Starter

dandy1

Joined Sep 30, 2017
178
I would just discars the two positive spikes and catch the negative spike instead, by putting the blocking diode backwards and connecting the input side of the opto backwards too. Simpler.
So if i got this right, its opto anode to GND, opto cathode to diode anode to (RES) to SIGNAL?
 

strantor

Joined Oct 3, 2010
6,798
Consider that the big spike is (I assume) measured at "only" -200V with the spark plug connected, properly gapped, and consuming most of the generated power. If you replace the spark plug and don't gap it properly, the opto might see way more voltage. If you disconnect the spark plug (and try to crank the engine) there almost certainly will be enough voltage to destroy the opto. Electrically interfacing with the coil is not a very good solution. Applications like this usually use an inductive sensor or hall sensor to detect spark, for good reason. Not safe to be connecting your scope to it either.

EDIT: I was was picturing the magneto on my mower when I said that, and it doesn't have any accessible primary coil so I assumed you are connecting directly to the HV going to the spark plug. If that's not the case, if you're accessing some point between the magneto and a coil which steps it up to go to the plug, you're probably ok.
 
Last edited:

LowQCab

Joined Nov 6, 2012
4,076
The Big question is ........
Why are You trying to obtain a Tach-Signal directly from an Ignition-Coil ?
Something controls the Power to the Coil,
either a set of Points, ( hopefully not ), or a Magnetic-Pickup,
either of which would be a better signal source than the Coil.

Installing a Hall-Sensor, or even a Variable-Reluctance-Sensor,
would be far more reliable, and would create a very clean Waveform.
.
.
.
 

BobTPH

Joined Jun 5, 2013
9,003
EDIT: I was was picturing the magneto on my mower when I said that, and it doesn't have any accessible primary coil
Something controls the Power to the Coil,
either a set of Points, ( hopefully not ),
A magneto does not have a primary. It is simply a single coil that is activated by a magnet on the flywheel. Used on lawnmowers etc that do not have a battery and on aircraft engines for reliability and immunity from electrical failures.
 

LowQCab

Joined Nov 6, 2012
4,076
An actual "Magneto" Ignition-System/Distributor also has either a set of Points or,
a Magnetic-Pickup with a Switching-Module,
and if it is a Commercially-Produced Aircraft-Engine, it will have some sort of Tachometer-Output,
even if it's a mechanical Cable-Drive.

Regardless of all that, there are certainly other, more reliable methods, of obtaining a Tach-Signal.
.
.
.
 

Ian0

Joined Aug 7, 2020
9,845
There are low-tension magnetos that have points and high-tension magnetos that don't.
It should be possible to get an effective signal by capacitively coupling to the HT lead, by wrapping a pickup wire around it.
 

strantor

Joined Oct 3, 2010
6,798
A magneto does not have a primary. It is simply a single coil that is activated by a magnet on the flywheel. Used on lawnmowers etc that do not have a battery and on aircraft engines for reliability and immunity from electrical failures.
That was my initial understanding too but there are all kinds of pictures of magneto systems on the internet showing ones with coils. I am not familiar with all kinds of ignition systems, there seemed to be enough evidence that I might not know what I'm talking about so I made the amendment.
 
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