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# CDI trigger circuit parts

#### MarkySparky

Aug 12, 2022
10
Hi,

I'm reconstructing a CDI trigger circuit for an old Hit-n-miss engine.
There is a small coil that I'm calling the Trigger-Coil. On the flywheel there is a magnetic assembly consisting of two magnets separated by a few inches. There is a Strong magnet North UP followed by a weak magnet with South UP. I assume that this is a trigger signal and a reset signal to close the SCR for charging again.

I am trying to figure out what components to use in conjunction with this coil to fire my circuit. I cannot change the coil to a Hall effect sensor - I have to keep the coil and magnets. I've seen a few different designs for trigger circuits but I don't UNDERSTSAND the selection and sizing of the components.

My circuit has a 12VDC source that steps up to a 350VDC to charge the capacitor part of my circuit. currently, I can trigger the gate if I hit it was 12VDC (just tapping the SCR leg with 12VDC). The spark plug sparks. I need help with the trigger now...

Do you think that the Trigger-Coil has 12VDC going to it at all times and that the magnet passing by changes the voltage and fires the Gate or do you think that the Trigger-Coil and magnet generate a voltage spike by themselves that trigger the Gate to fire?

- Mark

Last edited:

#### Bluejets

Oct 5, 2014
6,199
I have doubts that any of the existing ignition system would have been CDI.
However, I have been involved one way or another with CDI systems for model engines for many years.
The latest uses of-the-shelf items, much simpler, cheaper,quicker to get up and running and above all, works well.

Sounds like you would be inline for one of the units like I have made modifications to, to enable triggering from a hall effect switch.
Now here is the interesting part for you if I read your requirements correctly.
The "black box" which is central to my work is originally triggered by a pick up coil which has voltage induced in it by a passing magnet.
I have made these sensor coils using the coil from a small 240v relay and replacing the laminated solenoid with a solid steel core.
This had a magnet glued to the core as I wanted to use a plain steel metal vane as the trigger.
Just the coil with a passing magnet would work equally well.
This pick up coil was far too large esthically for any model engines, so the conversion to hall effect.

Now the 4 pin 12V DC version of the "black box" triggered by a pick up coil and driving a GY-6 coil, all come from the mini motorcycles (50cc etc.)
and available in many online stores such as Ebay.
Originally the black box and the coil could be purchased for around AUD$15 total but things changed after covid so double that now, still cheap as though. Anyhow some of the parts are....... stator pick up (if required) ..... https://www.aliexpress.com/item/32914061147.html GY6 cdi ignition coil............... https://www.aliexpress.com/item/33037811119.html 4 pin 12v DC CDI black box................. https://www.aliexpress.com/item/1005003571737448.html If you want to take a look at any of my system I can pop details/schematics in here for you. Some photos of what you have and any schematics would be a help to determine if this CDI is suitable exchange for what you have or if your system could be modified to suit. Finally a video of my system using a hall effect and a modified add on pcb. And the previous hall version also using my home made pick up coil. Last edited: #### MarkySparky Aug 12, 2022 10 Very exciting! I really appreciate your feedback. Part of me is interested in getting a working device but a bigger part of me is interested in knowing why/how the black box works. You are correct that the pickup coil is largish and would not look good as part of a model engine. Mine looks like it was taken out of a doorbell coil and the center shaft is a bolt and washer / nut to hold it to a brace while the magnets spin on the flywheel. I’d very much appreciate your schematics if available. I have been able to make 50% of circuit so far. I have an SCR and a capacitor in some Diles in resistors and the ignition coil and spark plug. I have been able to convert 12 V DC to 300Vdc and can trigger a spark just touching the gate of the scr to 12vDC. I need to make the trigger work with the pickup coil I have. I feel that I’m very close. I’ll work on getting some pictures later today. Thanks again! #### Bluejets Oct 5, 2014 6,199 Part of me is interested in getting a working device but a bigger part of me is interested in knowing why/how the black box works. Nope, of no interest to me what-so-ever these days. Plenty of CDI how-it-works jargon on the internet without my tearing one apart with an even remote chance of reverse engineering given it is a "potted device". I found details on requirements for trigger and went from there to now, that's it. As I said, built CDI's before and the work involved is simply just not worth the effort, given they never perform as well as the$15 unit complete in the mail, done.
My work is taken up building the miniature engines etc.

Aug 12, 2022
10
The CDI I’m reverse engineering is for an old hit and miss engine on my oil well. This well is my income now that I’m retired. If it doesn’t work then no [imath]this month. The black box for this engine is around[/imath]350-$500 because it’s such a speciality niche market. Functionally it is similar to the motorcycle CDIs. #### Attachments • 1E4DA236-FD6D-4774-A586-FD2C871AC049.jpeg 161 KB · Views: 2 • 5DEE0B5F-2B20-48D5-834A-103AD2F5FDF7.jpeg 76.2 KB · Views: 2 #### MarkySparky Aug 12, 2022 10 On the bench only, I have managed to take this AC circuit and simplify it greatly by purchasing a circuit to make 300Vdc from a 12vDC battery. I have a solar charged battery out at the well I intend to use. I have also stripped off the magneto and the ac-dc diode bridge and the magneto itself. All I have left is the SCR trigger coil and the flywheel magnet left to figure out. #### Attachments • C3F8CF3F-3E3F-4F41-B1EB-84309858A939.jpeg 329.8 KB · Views: 4 • 4FD40F08-3334-4A19-B3F3-2364B0828758.jpeg 367.9 KB · Views: 4 • 812CD340-B684-40FE-B8AE-ACFB7BE6EF0B.jpeg 414.1 KB · Views: 3 • 4ED6D9E5-75CB-4AE6-8287-FD59D4D42138.jpeg 480.9 KB · Views: 3 • F11C374B-031D-4ACC-9DD9-211AEA7B41DA.jpeg 445.3 KB · Views: 3 • 7B1A74E5-DAE0-4838-B28A-AAD730487100.jpeg 381.1 KB · Views: 4 #### Bluejets Oct 5, 2014 6,199 Again, if it's an old engine, it would not be CDI. Likely either standard or impulse magneto, the later being used on the old hit and miss engines. Are you changing the existing system to your home brew CDI..?? #### MarkySparky Aug 12, 2022 10 You are correct that the old engine was a Magneto only. It was retrofitted about 15 years ago with a CDI. It has a charging magneto (AC) and inside the black box it has a bunch of diodes to make DC, step it up, charge the cap and fire. I am going to change the circuit to strictly DC. It works better in the cold winter. #### MarkySparky Aug 12, 2022 10 #### Bluejets Oct 5, 2014 6,199 More than likely the "black box" I use and referred to would be a much better option all round. Can be either hall or points fired with a small additional circuit or sensor coil fired direct. Run mine on voltages from 7.4v lipo up to 12v gel battery. Video required a download, not online viewing. Don't come into that these days. #### MarkySparky Aug 12, 2022 10 Well if you can dig up your schematics that would be great. #### MarkySparky Aug 12, 2022 10 Omg I just realized something. All of these resistors and caps are placed in the trigger to quiet down AC to DC transients caused from rectifying AC from a magneto. I have super clean DC that is magneto independent. I need a diode in parallel with the primary winding of my ignition coil for back EMF and a diode feeding into the gate. The Gate trigger voltage is small so once the flywheel gets up to a 2-4 rotations per second it is making enough voltage to trigger the Gate. IT WORKS !! I’m going to try a few different things to see what effects they have on the circuit. I’ll need a scope from work… and I’m going to get this thing up to 450-500 rpm and let it run for a few hours and take thermal images too. Then I’ll need some way of making this into a nice PCB that I can stuff in a little black box without potting material!! I’ll post progress notes, schematics and thermal data as I get it. #### roughshawd Jul 13, 2020 289 My dad(WWII vet) helped my Brother in Law(doctorate in finance) to get his boathouse motor going. It was one of the "HIt-n-Miss" Engines. The idea is that once it fires, a heavy fly wheel spins up to speed and a gravity activated cam disconnects the spark until it slows down and fires again. Another thing that happens is a compression release opens to allow the motor to spin also. Frankly the idea is all about timing, and all you need is the spark plug. The magneto drives the coil, which is usually built into the flywheel. You are trying to one up gravity propulsion... really all you need is a knee... #### MarkySparky Aug 12, 2022 10 You are correct. Typically that is how these things work however this ancient engine lost it very expensive magneto a long time ago and was retrofitted with one of these$500 CDI boxes that intentionally have a very thin trace on the circuit board that access a fuse link if and when the module starts firing around 530 to 550 RPM. The tricky part is the entire circuitboard is encapsulated in some hard glass epoxy. Once in a great great while our modules will die… That usually happens when a belt breaks and the engine goes into a runaway

So now I have designed this little CDI module on a bread board and it works great except for I need to be able to regulate and limit the input 12 V DC from the solar panels and batteries. You would think that the circuit would only draw as much as it needed but that is not the case. Now when I take an inverter and run it off of the batteries to produce 120 V AC and then plug in my wall transformer and feed my CDI module 12 V DC everything works perfectly. But running an inverter and a plug-in transformer it’s just silly

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