This is a second try with shortened file names in my photobucket links, which should fix the missing pictures problem...

Warning: This post is long and has lots of images.

I while back I had some intermittent electrical problems on my GY6 150cc. With help from this forum and buggydepot I solved the problem, but was puzzled by widely varying and conflicting reports on expected voltages in and around the stator and CDI. I did a little poking around and was very surprised at what I learned. I used an oscilloscope to measure various ignition waveforms using two different brand CDI modules. Both CDI modules work great. The amazing thing is how differently they work, and how different the voltages are between the two. No wonder there is so much conflicting information on the internet!

Disclaimer: I don't have a buggy (yet). I have a 150cc GY6 ATV quad. The engine is identical. The wiring is nearly identical (differences are in the battery charging and lighting areas). So why am I here? Cause this is the "best" place to find info on GY6 150cc engine (I'm including buggydepot here as well). CDI #2 desribed below is out of a buggy.

In linking in the waveform images to buggynews a lot of resolution is lost from the original. Hence you will have to connect the dots a lot, but I think there is enough info left in the pic's to get the idea.

Of course you don't need an oscilloscope to fix the electrical system on an engine. A voltmeter is fine, but it helps to see what the waveform should look like so that you can then interpret the voltages you get.

This is a picture of the two different CDI modules. CDI #1 is made by LiHau, CDI #2 is marked with the brand name wuFLP. As I mentioned before both work fine.



The following picture shows the output of the ignition power winding on the stator with the engine running. Note how different they look. Also note that the stator output is not a sinisoidal AC waveform like one might expect, but has a large DC component and a complex AC component as well. Note that CDI #1 is mostly negative while CDI #2 is mostly positive. I have no idea why the waveforms are doing what they are doing, but the +250v positive spikes in CDI #1 are when the spark plug fires.



So if person "A" posts that their engine won't start, and they are measuring the voltage on the stator ignition winding at so and so volts while cranking. Then they ask if this is correct. Person "B" (wanting to help) goes out to measure their engine. But to keep it from starting (to measure real cranking voltages) they turn on the kill switch without realizing what that does to the stator voltage. The next picture shows what happens:



Could the two waveforms above be any different? Who would expect the kill switch hooked to the CDI would so drastically affect the stator output voltage?

Below in the next picture is what the stator ignition winding output voltage is when it is disconnected from the CDI. Now we see something approaching a sinisoidal waveform which one would expect from a magneto. If you measured this voltage with a voltmeter you would get about 85 volts AC).



The next picture show the kill switch connection voltages for the two CDI modules. At least these both pulse positive to some significantly high voltage. The kill switch shorts this line to ground to stop the engine from running. Note that CDI #2 waveform looks identical to the stator ignition power waveform (when connected to the CDI). They are not the same connection however. They don't ohm out as being the same.



Next is the trigger pulse waveform from the stator. This waveform provides the timing info to fire the spark plug. Because it is a low duty cycle set of pulses it is difficult to measure with a voltmeter accurately. I have several voltmeters, and the cranking voltages measure from 0.1 to 0.3 VAC depending on which meter I use. There is only one waveform in the picture because (for once) both CDI modules show the same waveform, and it is the same whether hooked up to the CDI or not.



You can use the following simple fixture to verify the trigger voltage is high enough to trigger the CDI. The pair of LEDs don't conduct in the forward direction until the voltage is above 2.1 volts (for green LEDs - it is about 1.8 volts for red). The LEDs don't put out noticable light until the current is above tens of milliamps so with the drop acrodd the resistor the trigger voltage must be above 2.5 volts if the LEDs flash. While cranking the LEDs will flash at 10 times per second. At idle they will flash about 2 1/2 times faster. You must use a pair of back to back LEDs - a single LED will be damaged by the reverse voltage of the trigger output in the non-conducting direction. You can get these parts at radio shack for a couple bucks.



Finally, the output of the CDI to the ignition coil is next. Once again they are very different. In this picture the resolution lost is significant. CDI #1 is actually a relatively wide rectangular pulse compared to the fast AC transient in CDI #2. CDI #1 is driven 150 volts positive. CDI #2 is driven 250 volts negative, then rings back and forth at roughly 30 KHz.



The lesson I learned from all of this is that you can't determine anything by measuring voltages out of the stator with the CDI hooked up. If the voltage is low (for example) is the CDI bad or the stator? Or is the kill switch? Or do I have yet a third CDI design that is different still? CDI's are just black boxes that take a current limited AC voltage source (stator) and convert it to spark energy timed from another smaller stator winding. There are a zillion ways to do this, and apparently everyone hasn't copied one another, with different methods being used by different CDI vendors.

Great job!

You might be interested in the timing & performance differences I found in 6 CDIs that I tested:
http://www.buggynews.com/topic4611.html

I hope you repost the schematics, too. WAY easier to understand than the factory schematics.

Hi, nice post!
I´m trying to develop a spark simulator. Say, I have a stator, a coil and a DC motor.
I mounted all yesterday. It´s not working at all.
So I have some questions. I see the signal out the stator as a kind of sine wave of 260Vpp whitout pluging the coil. Measured each cable to GND.
Both outs have exactly the same signal. It´s that OK?

I have no signal coming out of the secondary coil.
What resistence it´s suppose to find on the primary and secondary coils? (I measured 35ohms and 6000ohms)
How about the stator? (I have 600 ohms to GND on one cable and the same on the other one).

Any comments will be helpful.
Bye, Gustavo.
www.microtex.com.ar

GREAT STUFF. Have you run any tests on performance CDI's?

It would be interesting to again see the different results.

Stu

Physical:
Your fixture will have to mount the stator in a base. The flywheel will be spun by the DC motor. The pickup for the flywheel magnet will need to be mounted on an arm close to the flywheel. Tolerances are tight so the stator does not contact the flywheel and the pickup can sense the flywheel magnet.

Electrical:
The CDI power will need to come from the red/black single plug out of the stator. The CDI return will need to be connected to the stator ground. The flywheel sensor will need to be connected to the stator ground and CDI trigger input. The coil will need to be connected to the CDI output and stator ground. The spark plug will need to be connected to the coil output and stator ground.

OK, let me explain why I´m doing this...

I development a digital lap timer / rev counter like Mychron 4 and a rev counter.
I have tested it on many karts and they didn´t work because the spark noises. So I bought a car racing coil and make it work with a transistor at 40000RPM. Then tested my design again and work OK. Finaly I went to the tests kart again and failed.
So, what I´m trying to do is to have the very closer real situation on my workbench. I bought a kart ignition system without any CDI, just because they are not able to use on the Argentinian kart racings.

I take some pictures to show you what I have done.

The rotor is not perfectly centered, I don´t know how much can that modify the signal.




Thank you.
Gustavo.

Your ignition is set up differently than our buggies. We do not use the stator output to signal the coil to fire. There is a sensor that picks up movement of a magnet on the flywheel. The CDI uses this signal to gate the voltage created by the dedicated stator phase to the coil.

Gustavo,

Nice pictures. And you got it to work the first time!

As pghruby said the stator doesn't drive the ignition coil directly. In fact, it can't. It can't put out enough power in the very short time needed to fire the spark plug.

As an analogy, a candle and a firecracker ouput roughly the same amount of energy over both their short life spans. The candle burns more slowly over a longer period of time, but the firecracker does it all at once. The purpose of the CDI is to store the slow but steady candle like energy coming from the stator and then get rid of it all at once (firecracker style) into the ignition coil.

The CDI stores power from the ignition power stator winding over an entire revolution of the flywheel (which is multiple pulses because of multiple magnets and multiple coils). This energy is stored up on a large capacitor, until a timing pulse tells the CDI to discharge all its stored energy all at once into the ignition coil. You cannot bypass this process and drive the ignition coil directly.

If you want a really good technical explaination on how a Capacitive Discharge Ignition (CDI) system works, try going to www.freescale.com and searching for AN4006 (application note 4006). Warning - your PDF reader must be up to date or the download will hang. The design they talk about is a "performance" CDI which controls the spark advance with a microprocessor. I'm sure the generic CDI's more generally available are not quite so sophisticated, but there is a lot of info to look at none the less.

Lynn Edwards

I read the info in your links. I was fascinated! I haven't done any comparisons between timing advance between the two CDI's. That is a project for this weekend.

RE: Schematics: I assume you are referring to the schematics in my photobucket directory that the moderators and pghruby so kindly pointed to, after my failed attempt to post pictures in my first try at this thread.

Unfortunately they don't scale down well to fit in the buggy news posting format, and they are a bit off topic since they are a schematic of my 150cc GY6 ATV quad. Although similar to buggies, there are differences in the charging and lighting areas that might be confusing to some. Anyone is welcome to them - just go down a few topics to find my messed up post on this same subject and read the photobucket links. I traced out the whole wiring diagram over a weekend because there wasn't any available.

Lynn Edwards