inductive pickup from igniton coil voltage reading

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hello people,

I attache a JPG picture of what I did in reading the pulse voltage from ignition coil. I try to make a tachometer from this concept, pickup signal of ignition process then integrate the signal, count and display. But the point is that I don't know how high voltage is when I pickup from the ignition line.

I did measure with the multimeter at the same point as show in the JPG, but the voltage displayed vary if I change the max. limit on the multimeter. I did not move the point on the measured line. In fact, at first I set the max. limit to 600V and it read 36V, then I think I can use max. limit of 200V but it read 3.6V. Max. limit of 20V, it read 0.36V.

Are any experts have explanation for me, please?

Komgrit S.

"Chill out with Atmel Corp."
- Scud88.

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oh
picture

Attachment(s): 

"Chill out with Atmel Corp."
- Scud88.

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Scud88,

If you are using an analog meter, such as the old Simson 260 stype meter, the meter indicator will have a tendancy to dampen the signal because of the meter movement response. The meter indicator is mass and has weight. The pulse is quite narrow. The pulse width is not wide enough to move the meter needle and display an accurate measurement. You will need to measure the "Peak" voltage of the ignition pulse, as this is what will actually damage the measuring input.

If you are using a digital meter, the same holds true with regard to the "Peak" pulse amplitude. The digital meter integrates the measured value over time and therefore displays a much lower "average" value.

The deference between the analog meter and the digital meter is that the analog meter integrates mechanically, and the digital meter integrates electronically.

In any case, you will have to figure out a way, say, by using a rectifying diode and several high voltage capacitors in series, effectively storing a DC voltage level on the series connected capacitors.

Then you will be able to measure the "Peak" DC voltage and continue on with the design analysis.

I hope this helps...

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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No use using a multimeter - use an oscilloscope. Every PC with a sound card has one.

I can see you've made little progress thus far with your tachometer. Have a look at the app notes for a lm2917 tacho chip.

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yup

I've got a lot of work to do since I decide to work on this little tacho

I got a job, worked for 4 months -- real busy
then got a new job -- really really busy than ever

of course as you say, little progress

"Chill out with Atmel Corp."
- Scud88.

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Kartman wrote:
No use using a multimeter - use an oscilloscope. Every PC with a sound card has one.

I can see you've made little progress thus far with your tachometer. Have a look at the app notes for a lm2917 tacho chip.

Kartman,

I check out LM2917 as you mentioned. It is pretty cool but I got a question that I have to scale down the ignition voltage from inductive pick up into voltage level about 250mV, correct? and How?

please suggest

Scud88

"Chill out with Atmel Corp."
- Scud88.

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I dare say your pickup is not inductive - it is capacitive and it is only necessary to put the wire parallel with the ignition wire for a couple of inches. If you wish for an inductive pickup, use a E & I ferrite core, pass the ignition lead through one hole and wrap about 100turns of wire through the other hole.

For the capacitive pickup, I tried a 10k resistor in series, a SA5.0 TVS diode to 0V, 10 K resistor to 0V and a 74HC14 inverter. Have a fiddle, you'll get something to happen. It worked on my go-kart. The output of the inverter was fed to a capture input on a mega128

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Kartman

oh

thank for correcting me :)

Quote:
I tried a 10k resistor in series, a SA5.0 TVS diode to 0V, 10 K resistor to 0V and a 74HC14 inverter.

I'm not sure I understand you the whole thing, about the abbreviation....SA5.0....TVS

Scud88

"Chill out with Atmel Corp."
- Scud88.

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Ask google - I just did a search on SA5.0 tvs and the first hit was good. The SA5.0 is a 5VDC transient surpressor diode - a super zener diode if you like. This is used to limit the voltage from the input circuit. A 400mW 4V7 zener works at a pinch.

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What about the idea of measuring rpm by looking at the frequency of alternator ripple on the battery? Its all 12v... seems easier.

Imagecraft compiler user

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is it varying? i'm not sure

"Chill out with Atmel Corp."
- Scud88.

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Get a buddy to rev the engine while scoping the + terminal of the battery with scope set on AC. Pretty simple experiment, huh?

Imagecraft compiler user

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what if i don't have a scope? :)

"Chill out with Atmel Corp."
- Scud88.

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many years ago when i worked with after market fuel injection computers, the tach pickup was from the trigger side of the ignition coil. some easy filtering got rid of the high voltage inductive spikes and what you get is a nice 12 volt pulse for your tach input. from there you just condition it to what ever level you need for your input.

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scud88 wrote:
what if i don't have a scope? :)

If your PC has a sound card, you've got a suitable 'scope. Do a Google to get some software.

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Maybe building a scope is a good project. Graphics lcd and an opamp with a gain switch and you've got a scope.

Imagecraft compiler user

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hmmmmm

good idea

how about the second hand scope?

"Chill out with Atmel Corp."
- Scud88.