Scooter CDI help

[skydeaner]

I've attached a PDF of the scooter's CDI I have been dicking with. 

In the scooter's magneto, there are two coils involved in this, both of the coils have one end grounded, the pulse coil is very small in width and is the red/white wire in the pdf which should control the timing of discharge, and then the black/red wire is to a large coil that provides power somehow to the unit, it is called a charge coil, so maybe for charging a the big output cap?  The output wire is the orange one, and the "big cap" i'm referring to is right before the output.  I'm not sure about it's value, I just looked at some diagrams and labeling info and decided on the label it has...  But it is about 1.2in wide, .8in high and .4in deep in size, it is a red/orange epoxy dipped film cap.  in which i can't see the label clearly except for that it is 400v from shizuki.. 

Anyways here is my question.  The SCR got damaged some how, all other components test fine.  I have ordered a replacement SCR and it will be here in a few weeks from china, but in the mean time I would like to replace it with something else maybe something I can get locally or something I have laying around so I can still ride the scooter. 

What? could it possible be replaced with?  I have tons of N-channel fets, some NPN transistors, and radioshack lol.  Maybe some other items, I have tons of dead electronics laying around I can pull from, but none with SCR's lol. 

Let me know if there is any possibility.  Or any other info you  need to know.  I'm sorry about the sloppy circuit drawing, I'm just learning to use NI Multisim. 

[skydeaner]

I fear i butchered that schematic horribly.  This picture shows almost exactly what the schematic should be

[skydeaner]

Oh and instead of a constant 12v coming in I have about a constant 33vac  if that makes a difference for the Gth of the mosfet or whatever.

[skydeaner]

Ok so using a mosfet I was able to get what looks like adequate spark on the plug.  I still can't get the damn thing to start now.  I wonder if using a mosfet effects the timing a great deal?  Should I need to adjust the size of the capacitor because of this?

[mc]

I'd hazards a guess the ignition coil is breaking down, and causing a spike on the LT (low tension) side.

If this is your bog standard china scooter/quad/50-100cc CDI system, I'd buy a new CDI unit, Coil, and spark plug. Chances are the spark plug has worn, which then increases the voltage required to create a spark, to the point where the Coil starts breaking down internally, and can send spikes back into the CDI unit (probably what killed the SCR). Even when everything is working well, it's a pretty woeful ignition system.

I've dealt with a few of these now, and in the end I've always ended up replacing everything, as in the UK you can pick up all the bits for £40-50.

[skydeaner]

ah, thanks for the response, finally someone that has some experience in these things.  I forgot to mention that I killed the scr by hooking a 12v+ lead from the battery to the gate line on it LOL.  At least I am pretty sure that is what killed it.  That would wouldn't it?  I had the key stuck and instead of jumping the black/yellow wire to start it with the key in the "running" position I accidentally jumped it to the red/white (pulse coil) wire.  And after that, even when i got the key unstuck, no bingo. 

Also, this is a 20 year old yamaha cv80 scoot.  None of the current cdi's have proper pin placement, i would have to find a pinout for them and all that.  The spark plug is new, the ignition coil is still pretty old, but I have never had an issue with it, so I doubt it would start now. 

Should I just grab a random CDI, like the one for the scoots you were mentioning and try to figure out the pinout on it?  I've already ordered an SCR, but if there is anything else wrong with the circuit, like the big cap being bad I have no way to test that, so it might be no joy even after getting that in.  Also, the circuit above isnt exact to mine, I have 3 resistors and an extra 25mf 25v electrolitic cap along with the large non polarized output cap.

[mc]

Given it's not some cheap chinese rubbish, I'd attempt to repair it, espcially since you know how you killed it.
The chinese stuff is mostly copies of Japanese stuff, but without as much attention to detail or longetivity. Some stuff is that much of a copy, parts are interchangeble!

If the capacitor has gone bad, then you won't get a good spark, as getting a spark relies on the capacitor having a good charge.

I've not got much experience with bike stuff (I only deal with a couple quads and the occasional scooter, which are generally of newer chinese origin), so can't really comment on what is likely to work in terms of replacements.
Might be worth seeing what is available secondhand?

[skydeaner]

Well Just so everyone else knows LOL.  I got a solid spark with a very sensitive gated mosfet.  One that had a gate threshold of around 2v max and was rated up to 600v that I just happened to have laying around.  Where the hell I got something like that, who knows.  It also had a diode integrated into the gate for protection, which I thought was cool.  Little black thing right before the chip on the gate pin.  I had to make my own circuit similar to the one above, but different in a few ways. 

Anyways.  Even with a consistent spark it just would NOT time correctly to fire.  So there is a possibility of someone being able to design a mosfet based one, but not for the amateur. 

I got my SCR in today, slapped it in and after I fixed a problem with the exaust it fired right up.  So, all is good in the neighborhood.

[mc]

Glad to hear you got it fixed.

One good thing about older stuff, is it's generally a lot more fixable. Most modern stuff you've got to destroy to get anywhere near the components, and then chances are they're unidentifiable.

[Paul Price]

If you replace the SCR with a HV MOSFET you won't likely have any luck.  The SCR needs to very quickly turn on and turn off. A MOSFET will slowly turn on to the tirgger waveform and not turn off when the oscillations created by the HV discharge to the ignition coil changes directions in only a few microseconds.  Result:  poopy sparky. No large voltage is generated by a long, slow rise  pulse to the coil. Radio shack sells SCR's, you just got to buy one with 400V or better reverse voltage rating. I have built many of my own CDI units of my own design for my older cars and making one small enough for a scooter is a tough game, but I am sure a SCR is an only choice to kick the spark coil with a 400V shot of juice and then have the self-turnoff timed correctly by the coil after a spark is delivered.

[NiHaoMike]

If you replace the SCR with a HV MOSFET you won't likely have any luck.  The SCR needs to very quickly turn on and turn off. A MOSFET will slowly turn on to the tirgger waveform and not turn off when the oscillations created by the HV discharge to the ignition coil changes directions in only a few microseconds.  Result:  poopy sparky. No large voltage is generated by a long, slow rise  pulse to the coil. Radio shack sells SCR's, you just got to buy one with 400V or better reverse voltage rating. I have built many of my own CDI units of my own design for my older cars and making one small enough for a scooter is a tough game, but I am sure a SCR is an only choice to kick the spark coil with a 400V shot of juice and then have the self-turnoff timed correctly by the coil after a spark is delivered.

MOSFETs can easily switch well into the hundreds of kHz. Of course, you need the right drive circuit to do that.

[Paul Price]

That's what I am saying, in the circuit you have and without elaborate triggering modification, a MOSFET will not work. You have a much simpler circuit using a SCR.

[skydeaner]

shoot, my radioshack didn't carry them, but i did get one shipped in.  It still doesnt' run right.  I dunno wtf is goin on now.  I'm going to replace all the components on the board and see if that helps.  Maybe a diode isn't performing quite right?  Do diodes blow hard?  or do they trickle?   I know when I measure them with a MM on the diode checker setting I get INfinite resistance one way and what looks to be like 560ohms reversing the polarity of my leads.  It tests very similar to some other diodes I have laying around.  All of the resistors on the board seem to check out based on their color bands.


So hey, for the extra components I was talking about, the extra resistor and electrolytic cap.   There is that one 25v 25mf electrolytic that i have no clue what it is doing there, i have replaced it already.  And have tried different values.  The negative leg of it goes directly the gate of the SCR.  The positive leg gets connected to the gate through a 2.2k resistor, and then also has the pulsed line coming in to it through a diode and 1k resistor.  This is the only way the pulsed line gets to the gate, is through (a diode of course to rectify) the capacitor and that 1k resistor, and through the 1k then the 2.2k resistor.

What would be the point of taking a resistor from the gate line and putting a cap between it. and the gate line??

[SeanB]

Pretty much any SCR or even a triac will work here. Regular ones are best, a 600V 10A one is pretty cheap and easily available from any slightly used light dimmer.

The capacitor is to apply the fast going edge to the gate. Diodes can leak at high voltage yes, and will test out ok on a simple test but fail under load.

[vk6hdx]

I just built a CDI kit from Jaycar for my scooter as the one that came on the bike was RPM limited 

It uses this circuit from Silicon Chip - http://archive.siliconchip.com.au/cms/A_110499/article.html

Not sure if it helps as it also uses an SCR, but there are schematics and some good theory of operation there.

[skydeaner]

That site is amazing.  It explains everything I had questions about.  Well, there shouldn't be any problem with my CDI, maybe through this whole process wires aren't conducting correctly somewhere.  the spark seems pretty weak.  I went and tried different output cap sizes to see if it increases spark or decreases.  It seems anything extra overloads the ignition coil.  So I'll just leave the one they have.  Oh well.  Thank you all for the help, off to play some more. 

[Paul Price]

I doubt any of the components on the board or, for that matter, other than the SCR are defective or damaged. Something else is going on here you have not caught here.  The high voltage diodes are most likely good, and if you've replaced the 25uf cap on the gate with the + and minus polarity the same way, then that is not the trouble either.

There are two types of SCR's, sensitive gate triggered and large-voltage to trigger versions. Is the replacement SCR you got exactly match the original? Did you check to see if you have reversed the gate and cathode wires when soldering it in?

I know it is hard to troubleshoot a motorcycle because you gotta pump the pedals to get the circuit to work, but what are you seeing from the output of the spark coil? If you disconnect if from the spark plug and place a small screwdriver in the sparkplug connector .25-in from the chassis (try to fix it in place with duct tape .25-in from a metal ground point on the bike frame/engine.) Do you then see  blue or yellow HV spark at all then when cranking the engine?

Most likely, the only parts that could be damaged by use or tampering with the circuit would be the SCR or  C4, the .5uf to 3uf 400V capacitor delivering the punch to the spark coil. If the wire from the capacitor connection on the spark coil was shorted to ground the current surge could destroy the both the capacitor and the SCR or otherwise kill the SCR.  the other

components and resistors are very reliable and unless the printed circuit underbelly is damaged by corrosion, everything works fine.

If the diodes check open in one direction, the are most likely ok. Diodes like this very seldom fail except  by completely shorting(almost all failures) or opening up(very, very rare).
Have you checked the wiring to the CDI and to the spark coil for shorts to the bike chassis or else for open circuits? Have you checked the connector to the spark plug from the coil to ground for continuity? You should read a consistent coil resistance of tens of ohms to a few hundreds of ohms.

You need to see if the the 230VAC is getting to the CDI unit as well as the trigger pulse.  A oscilloscope would help, but a MM might give you an idea of the voltage levels.

You can check the spark coil by applying a good 12V bike battery to the wire leading from the CDI to the spark coil for a very short time. A quick swipe with a wire should produce a healthy spark most of the time you make a very quick on/off connection. To do this, I use a large bastard course metal file connected to the battery with a wire and  a small screwdriver to quickly rub (move so as to make the tip press against) the ridged surface of the file. The screwdriver is connects  to the coil with a gator clip, and a fast swipe creates a train of fast pulses to the spark coil to do a test.

Make sure the CDI wire going to the plug is disconnected for this test.

[skydeaner]

Thanks for all the suggestions and help everyone.

Well It looks like when I replaced the SCR in the CDI that took care of the problem completely.  Then the reason it wouldn't start was because the magneto wasn't lined up properly.  You see there is this little wafer called a woodruff key.  Well at some point in time through all of this trying to figure out what was going on.... it dissapeared LOL.  So I had to make another one and now that the timing is correct, it runs like new.  Actually much better than new because I increased the output capacitance and it sparks much much stronger now.  You guys have been extremely helpful.  I know this isn't a scooter and moped site, and I can't tell you how much I appreciate you taking the time to respond.

::END of Thread:: LOL