Roll Your Own CDI

Are You Going to Roll Your Own CDI?

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Everything you have drawn so far is spot-on. I can try and make a small chisel later today and see what I can whittle away. I estimate 50/50 odds on getting the info we need but I am kinda determined to see for myself so I will be extra careful. I'll be back.
 
So, the schematic of the GT5 CDI so far is:



If we can ID the two unknown components that are across that resistor, then we have completed the reverse engineering of it!
 
rohmell said:
I will be building two more MBs during the colder months here in the US and will give that circuit a whirl on those bikes.
Click to expand...
these pots mentioned ,would they be the same as a volum adjuster in a radio,if so u could adjust the spark like on a harley or model-T ?
 
The chisel work has been quite a task for my old eyes. The resistor's epoxy was stuck to the potting material, so no color bands. Gonna have to meter it. I am nervous to try to get that red blob off but some of the three components are revealed. The cap has been placed from where it popped off as well as the diode right next to it. The far left component has been shattered. I should probably stop and mail what remains to you?
 

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Rode for 15 miles yesterday with a new factory carb. Had a small stutter for a lil bit and figured it was some crap caught in the carb that finally broke free. Set out on my commute this morning ang 1/2 mile out from home she dies...hard. Pedal back and drive car to work. Checking my CDI first because I made it and want to be sure it's ok. I didn't totally air-tight my box and I found some condensation inside as well as telltale signs on some of the diode leads. Then I saw it, burnt my 82ohm resistor. Changing resistor and sealing the box then we try again.
 

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Perhaps it would be a good idea to upgrade both resistors to 1 watt.

The 18 and 82 ohm resistors are in series with the SCR and therefore has to dissipate some energy and leaves less for the ignition coil.

Maybe it is better to start looking at the GT5 CDI schematic, where there are no resistors in series with the SCR, and more energy can be delivered to the ignition coil.
 
Ratchetbirds said:
Rode for 15 miles yesterday with a new factory carb. Had a small stutter for a lil bit and figured it was some crap caught in the carb that finally broke free. Set out on my commute this morning ang 1/2 mile out from home she dies...hard. Pedal back and drive car to work. Checking my CDI first because I made it and want to be sure it's ok. I didn't totally air-tight my box and I found some condensation inside as well as telltale signs on some of the diode leads. Then I saw it, burnt my 82ohm resistor. Changing resistor and sealing the box then we try again.
Click to expand...

What are you using for a coil?
 
I am using a chinese pitbike coil. I replaced my resistor and re-installed. Ran for 3 seconds then dead again. Ya know, I have put about 50 miles on this CDI with no problems yet. Nothing was obviously burnt this time. I will probably change out the SCR tomorrow. If that doesn't do it I will try changing the ign coil.
 
Sorry to hear that you are having problems with the CDI.
You do know how to test an SCR, right?
Or do you need a schematic for a quick little SCR test fixture?
 
rohmell said:
Sorry to hear that you are having problems with the CDI.
You do know how to test an SCR, right?
Or do you need a schematic for a quick little SCR test fixture?
Click to expand...

Oh man, a schematic is worth it's weight in gold. I am a documentation geek. When I start a project I usually make a document file, so yes please. I am not even sure how an SCR works. I am a fluid power tech, so electric, electronic and fluid schematics follow alot of the same paths. I built a programmable ignition from a kit for my MG Midget. It's all surface mount. My eyes hurt for days after assembling that but it works like a champ.
 
Couple other things to report. I read the ohms on the resistor on my board and at the lowest setting on my DVOM (20) it read 09.8. It is deffinately 1/2 watt sized. I have another cdi I can rip into. If anyone out there has a dead unit they want to send to me to be opened I would welcome them. Just PM me. The more we can examine the better. I still have a strong suspicion that moisture is what killed my CDI. I hope to find out tomorrow.



Edit: I shoulda posted in your new forum. I cannot delete this post here but I am copying it to the new forum.
 
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Initial condition, lamp will be off.
Press the NO switch, and lamp will light.
Release NO switch, lamp will still be lit (SCR has latched).
Press NC switch and lamp will turn off.
Release NC switch and lamp will still be off.

Instead of building the circuit, you can connect it up using jumper clips and connect/disconnect a jumper clip to simulate the ope/closing of the switches.

You can find many variations of this circuit on the Internet, some with the lamp on the anode side instead of the cathode side, some using a motor for a load, some using the motor or lamp as the current limiter for the gate, etc.
 
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Awesome, Thank you. I replaced the SCR and tested today. Still dead. I have a real nice 12 volt dc power supply I can use to test the scr.



Forgot to ask what size bulb? Maybe an auto tail lamp bulb?
 
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Ratchetbirds said:
Awesome, Thank you. I replaced the SCR and tested today. Still dead. I have a real nice 12 volt dc power supply I can use to test the scr.



Forgot to ask what size bulb? Maybe an auto tail lamp bulb?
Click to expand...

A small lamp would be better. When the SCR latches, it has to pass all of the current that is required for the lamp, so if you use a big lamp, the SCR should be on a heatsink.
I would use something like a 194 lamp.

Here is a chart with common auto lamps:
Truck-Lite - Bulb Specifications
 
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