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Make this DC CDI Circuit for Motorcycles

Make this DC CDI Circuit for Motorcycles

The circuit presented here is for a DC-CDI which are used in motorcycles. A DC-CDI is the one in which the high voltage (200-400VDC) is converted from 12V supply voltage.

Researched and Submitted by: Abu-Hafss

Studying the circuit, we see that it has two parts i.e. the CDI unit, enclosed in the pink box and the remaining circuit on the left is high voltage converter.

DC CDI Circuit for Motorcycles
The working of the CDI may be found in this article.

The circuit on left is a high voltage converter based on a blocking oscillator. The components Q1, C3, D3, R1, R2, R3 and transformer T1 forms the blocking oscillator.

L1 is the primary coil and L2 is the feedback coil. C1, C2 and D1 are DC voltage smoothing components.

How it Works

When the circuit is powered on, R3 provides forward bais to the base of Q1. This turns on Q1 and current starts flowing thru the primary coil L1 of the transformer.

This induces voltage in the secondary or the feedback coil L2.

The red (phase) dots in the transformer symbol indicates that the phase of the voltage induced in L2 (and L3) is shifted 180°.

Which means when the bottom side of L1 is going negative, the bottom side of L2 will be going positive.

The positive voltage of the L2 is fed back to the base of Q1 thru R1, D1, R2 and C3. This causes the Q1 to conduct more hence, more current flows thru L1 and ultimately more voltage is induced into L2.

This causes L1 to saturate very rapidly which means no more changes in magnetic flux and hence no more voltage is induced into L2.

Now, C3 starts discharging through R3 and finally Q1 is switched off. This stops the current flow in L1 and hence the voltage across L1 comes to zero.

The transistor is now said to be "blocked". As C3 gradually loses its stored charge, the voltage on the base of Q1 begins reverting to a forward-bias condition by means of R3 thus switching on Q1, and hence the cycle is repeated.

This switching of Q1 is very fast such that the circuit oscillate at quite high frequency. The primary coil L1 and secondary L3 forms a step-up transformer and thus a fairly high alternating voltage (more than 500V) is induced in L3.

To convert it to DC a fast recovery diode D2 is deployed.

The zeners, R5 and C4 forms the regulator network. The sum of the values of the zeners should be equal to required high voltage to charge the CDI's main capacitor (C6).

Or alternatively a single TVS diode with desired breakdown voltage may be used.

When the output at the anode of D2 reaches the breakdown voltage (sum of zener values), the base of Q2 receives the forward bais and hence Q2 switches on.

This action steals the forward bais of Q1 thus stopping the oscillator temporarily.

When the output is dropped below the breakdown voltage, Q2 switches off and hence the oscillation resumes. This action is repeated very rapidly that the output is maintained slightly below the breakdown voltage.

The positive trigger pulse at point (D) in the CDI unit is also fed to the base of Q2. This is important to pause the oscillation because SCR U1 demands the current across its MT1/MT2 to be zero to be able to self-disconnecting.

Moreover, this increases power economy as all power supplied during discharging is wasted otherwise.

A special request from Mr. Rama Diaz to have multi CDI sections sharing a common HV converter circuit. Some parts of his request is quoted below:

Ok most engines these days don't have distributors anymore, they have a coil for each spark plug or in many cases have a dual post coil that fires 2 spark plugs at the same time, this is called "wasted spark" since only one of the two sparks is actually getting used each ignition event the other one just fires into the empty cylinder at the end of the exhaust stroke, so in this configuration a 2 channel CDi will run a 4cyl and 3 channel for 6cyl and 2 x 2 channel for v8 etc...

Almost all 4 stroke engines have 2 cylinders that are paired so only 1 coil (connected to 2 spark plugs) will fire at a time the other one/s will fire at the alternate ignition events driven by a separate trigger signal, Yes aftermarket ECU's have up to 8 completely separate ignition trigger signals....

yes we could just have 2 or 3 totally separate units but i would like to have everything contained in one unit if possible, and im thinking there would be some way to share some of the circuitry...

...so im thinking you could have one heavier current step-up section to provide the ~400v then have two (or 3) separate CDI coil driver sections with a separate trigger signal for each one to drive the coils independently....possible??

That way i could use 2 (or 3) dual post coils attached to 4 (or 6) spark plugs and have then all fire at the correct time in wasted spark configuration 🙂

This is exactly the way we often do it now inductively using simple transistor based ignitors but the spark strength is often not strong enough for turbo and high performance applications.

DC CDI  sharing a common HV converter circuit


The entire circuit shown above can be used. The CDI unit enclosed in pink box can be used to drive one dual post ignition coil. For 4- cylinder engine, 2 CDI units; for 6-cyl, 3 CDI units can be used. When using multi CDI units, the diode D5 (encircled in blue) has to be introduced to isolate the C6 of each section.


Since the frequency of the oscillation is fairly (more than 150kHz), ferrite core transformers are used. A tiny 13mm EE core transformer can perfectly do the job but, handling such a small component might not be easy. A little bigger may be selected. Enameled copper wire 0.33 - 0.38mm for the primary (L1) and 0.20 - 0.25mm for the secondary L2 & L3.

The picture shows the bobbin's top view.

ferrite transformer winding details
For primary winding, start from pin no. 6, wind 22 neat turns in the direction shown and end at pin no. 4.

Cover this winding with a transformer tape and then start the secondary winding. Starting from pin no. 1, wind 140 turns (in the same direction as that for primary) and make a tap at pin no. 2 and then continue another 27 turns and end at pin no. 3.

Cover the winding with tape and then assemble the 2 EEs. It is advisable to make an air gap between the 2 EEs. For this a tiny paper packing may be used. Finally use the tape to keep the 2 EEs united.


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

77 thoughts on “Make this DC CDI Circuit for Motorcycles”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Wow, thanks for the quick reply! I’m excited to put something together now..
    Would you have a recommendation for a robust capacitor to use?

  3. I’m happy to have come across some proffessionals that know CDI! I have vintage motorcycle that is no longer supported with parts for ignition control! Its a little different becasue its an automatic and there are additional safety circuits that prevent starting or inhibit ignition!
    Schematic looks like this but it is “general” and I don’t know how to specify parts or values! Please help!
    Thanks You !

    • The part values doesn’t look too critical, You can use the following:
      1N4007, for all the diodes.
      2k2 1/4 watt, for the resistors.
      BT151, for the SCRs
      8050, for the transistor.
      If the schematic is correct then it should work perfectly with the above components.

  4. I make same ckt. And test on bick.
    I face two problem one is back kik and second is missing spark. What is reason behind this ? And how can I solved this problem?

    • which triac did you use? If you use any other triac than BT151 then the performance will get affected, secondly the capacitor must be of a good quality rated at 1uF /400V, because the capacitor is responsible for holding the charge so make sure it is a good quality one….and this circuit will not work with high speed bikes, it is recommended only for the normal city bikes.

  5. Hi there,

    Thanks so much for this circuit design. I never made transformer before. Im not sure which one is suitable. Can u kindly give me a link to each component that i need to built the transformer that you mention above. So that it serve me as an example if i need to find in my place or order it online.

  6. Dear
    After roasting a couple of Q1 (TIP31C / TIP41C), twice checking everything dfiscussed between Rama Diaz and Abbu Hafss, followed disscussion between Nathaniel and Abbu Hafss at another blog, tried TIP122 as well, but nothing seemed to be helpful. The Q1/R1 kept heating up. Then while working with an energy saver light I come across Transistor 13003, unwillingly tried the one which, of course, resolved the issue. Going to further replace it with Transistor 13005 for more satisfactory reaults. I think the fast switching capability of the transiator did the job where as datasheet shows inferior current ratings of the same. Should I go for the 13005?

    • sure, if you have practically verified the results and found MJE13005 to be a better choice then certainly you can go ahead with it. the current rating of this device is 5 amps, and that’s quite good considering its voltage capacity which is beyond 300V

      • Dear Swagatam,
        Thanks for your quick repose. I must appreciate your efforts, You guys are doing well.
        I’ve pratically tested 13003 and it’s current ratings is 1.5A. I don’t now how many Amperes may pass through this device during operational state. If current remains near about the ratings and there’s less chances of exceeding beyound the limits. It’ll be a cheaper choice for others as well. Please advise…

        • Thank you Ibtihaj, MJE13001 is actually rated at just 200mA according to the datasheet which looks quite low, therefore MJE13005 could be tried instead which is rated at 5 amps

  7. Hi Rama Diaz

    Sorry for my late response, as I am little busy these days.

    The pictures shows you have used correct components. The circuit is working and giving high voltage as required, this means the connections are also correct. The only problem is heating of the transistor Q1 which lead us to the transformer and the relevant feedback network. Apparently, the feedback components are okay therefore, the only doubtful device is the handmade transformer. I suspect it because I had to face similar issues with some of my handmade transformers until I got them from professional manufacturer in China.

    By the way, did you created air gap in the core of the transformer? Please see the picture below:


    Take a cardboard sheet (thickness about 0.3mm) and cut 3 pcs roughly to sandwich between the three legs of the E-cores as shown in the picture. Try it and see if it solves the problem. You might need to adjust the thickness because the air gaps are created on basis of calculations. You can google for transformer air gap calculation or formula for more details, if required.

    If you need the specifications of the transformer which I got from China, I shall be happy to share it.

    • Hi
      ok thank you, yes i used an air gap but only the thickness of the yellow transformer tape which is 0.07mm so this is one thing i will try. How about the hand winding? does it matter if i wound the secondary side randomly? or do all the wires need to be neatly packed together in perfect layers etc?

  8. Ok i made the transformer myself and was careful to make the correct number of turns L1 is very neat with all the wires aligned neatly one layer exactly filled up the full length of the bobin. Then for L2&L3 i just wound them more randomly but trying to keep it as even as i could. There is no noise or hiss at all. Below are pictures of all the components you mention and also C1&2 I think they are all correct but please verify!

    R1 https://photos.google.com/photo/AF1QipNPUo72TtClOEwF1qoVkGkROdYcFpXPRboUjTU4
    R2 https://photos.google.com/photo/AF1QipPtfMcZ2PiFa599jnFf6yL4Q9y6TlCAFngcQ9Ip
    R3 https://photos.google.com/photo/AF1QipPe8EYl-1Dg4ABvFVhEnSYn6YhMukIXbEPaY6UO
    C3&4 https://photos.google.com/photo/AF1QipPSonjjk_Ke3p_4wlRHuGyUXzcPXvg9kowLZg-w
    C1 https://photos.google.com/photo/AF1QipNZu1L-RT6FIThvaaAc1VsH35UcKtZMCKPeOMwa
    C2 https://photos.google.com/photo/AF1QipO1ZiwFmjmwPLFD1yrcKrfybfpd1I-jWQCL-4vV

  9. Hi Rama Diaz

    Yes, only L-C meter was required to measure the inductance.
    The measuring unit should be mH.
    L2 = 0.15mH or 150µH and L3 = 3.66mH or 3660µH are correct.
    L1 = 0.11mH or 110µH is slightly higher maybe you wound 24-25 turns.
    But it wo'nt have any adverse effects on the operation of the circuit.

    Did you wound the transformer by yourself or from some professional? Do you hear any sound in the circuit (hissing or faint whistle)?

    C1 and C2 are just smoothing capacitors for the input 12V, even if removed the circuit should work.

    If Q1 is getting hot that means, there is something fishy in the feed-back network (from L2 to the base of Q1). Please re-check the values of R1, R2, R3, C3 and C4.

  10. Hello Abu-Hafss
    Ok i finally got someone to test the transformer, he just used an L-C meter not a wave generator etc but it will give us an idea

    L1 22turns 0.35mm wire 0.11H (uH?)
    L2 27turns 0.22mm wire 0.15H
    L3 140turns 0.22mm wire 3.66H

  11. I have a problem I hope someone can help with. I have a motorcycle. It is 6V-DC with no powersupply. It contains a CDI and I would like to build one instead of dishing out $120-$200. so how would the schematic above change. lower voltage- higher current.. what values would change on the above schematic and is anyone willing to solve this for me and future viewers? Thanks for the time,


  12. Hmm, you need an LC-meter (Inductance & Capacitance) for that. There are some circuits in this blog but, you need an accurate device. You can also find accurate PIC microcontroller based LC meters on the net. If you build one, it would be valuable addition to your collection of electronic equipment.

    If not, take the transformer to some professional electronics engineer and request him to measure the inductance for you. It would be a matter of few seconds, just like measuring resistance.

  13. sorry for the lack of knowledge but how do i measure inductance? i only have basic equipment like multimeter etc.

  14. Q1 should be at ambient temperature under normal operation.
    The PCB tracks are okay. Most probably the problem is with the transformer. Remove it from the PCB, measure the inductance of the windings L1, L2 & L3 and share them with us.

  15. Hi Rama Diaz

    Glad to see your prototype working. I had spent about 4 months finalizing the circuit. Proper winding of the transformer is the key to perfect operation. Q1 and most probably R1 would get hot ONLY when the transformer is not properly wound. Please remove the transformer from the PCB and measure the inductance of coil L1, L2 and L3. Please share the findings.

  16. I only left it running for about 10 minutes so i worry that these high temperatures combined with the high temperatures of an engine bay will cause an early component failure leaving me stuck on the side of the road! What can we do to fix this? or do you think it will be ok? does the PCB track need to be bigger for Q1?

  17. here is a picture of the underside where Q1 is attached, you can see it started to burn the mask a little.


  18. Hi Abu
    ok so testing went well, I built a 4 channel unit incorporating the falling edge trigger circuits and 300v primary voltage, i will post some links below for picture and a video. I found from about 5000rpm the spark started to get weaker. I was testing on a very large gap and if i made it a little bit smaller it would go all the way to 9000rpm.

    Something i did notice while testing is a few components C1, C2 and Q1 all got really hot! not to the point of smoking but hot enough to burn your finger if you touch them. I was only testing one channel so i worry if we had all 4 channels working would the heat be increased?? and/or would the spark strength be reduced??

    one last observation is once the unit is turned off all the capacitors remain fully charged and they pack quite a punch!!… ask me how i know 😉 is it safe to short them with a piece of wire or can we incorporate a way to have them discharge slowly by themselves??

    All in All very excited to see it working!!




  19. Hi Taken me a LONG time but i am finally making this!! i even went to the trouble of getting a nice little PCB printed up 🙂

    I have one critical question about the transformer my PCB is laid out like the circuit diagram and in this way winding L2 (bottom)will have 27 turns and L3 (top) will have 140 turns… is that correct??? or should they be the other way around??

    • Hi Rama Diaz, Long time no see!

      Yes, you are correct. If you follow the instructions above, you will get it in order.

      But it does not matter if the L2/27 turns go at top and L3/140 turns comes down. The main thing is the shifting of the phase as indicated by the red dots on the schematic. And the output of the 140 turns should go to the fast diode and the output from the 27 turns should go to the resistor R1.

      If you are confused over it, simply ignore and just follow the instructions in the article. Moreover, it would be better if you could share pictures of your PCB from both sides so that we can give precise guidance.

    • thanks gozo, I am sure you must have read and understood the article fully, it does not include an automatic advance/retard facility.

  20. Thanks for the reply Swatagam.
    Lets say i just want to buy this kind of transformer instead of building it. Can u recommend some models that suitable for PCB build. I usually buy components from element14 and RSonline. So, if i can get the transformer from there, that is best for me. By the way, my application will be on 4-stroke 110cc motorcycle.

    • Kazuki, This is a custom built design so I don't think you can get it readymade from a shop or component dealer, it will need to be built at home or made to order as per the given data

    • Kazuki San, Swagatam is correct. Those transformers are made to order. They are not available with component dealer. However, the supplies of the transformer are easily available. Procure them and then can assemble the transformer easily at home/work-bench in 30 min. I have provided detailed instructions for assembling the transformer. Go ahead, we are here to help you. For your reference, following are the pictures of the EE core and Bobbin:


  21. I have already clarified the same……
    CDI's in smaller bikes can issue sparks with voltage around 180V at the primary of the ignition coil (transformer). Whereas, heavy bikes need more powerful sparks so the primary voltage at the ignition coil is selected like 300-400V.

  22. It's the size of ferrite core. But as I mentioned, it would be too small to handle. You may use a slightly bigger core.

  23. Dear author,

    I want to know whether we can use standard transformer instead of center tapped transformer like u did in your CDI circuit above? Mind telling us?

    • Dear Kazuki, it is a ferrite core transformer and it will need to be exactly as mentioned in the article, ordinary iron core type will not work

    • yes these are magnetic cores suitable only for high frequency applications…smps units usually have a few of these in their circuits…I would recommend you to consult a professional ferrite core winder, because this part could be crucial in determining the performance of the unit, a wrongly constructed transformer could make the design inefficient or produce unpredictable results

  24. You have misunderstood the main concept of CDI. The 300V stored in C6 do not issue sparks directly. The 300V are converted to 30,000V or more by the ignition coil which basically a step-up transformer.

    CDI's in smaller bikes can issue sparks with voltage around 180V at the primary of the ignition coil (transformer). Whereas, heavy bikes need more powerful sparks so the primary voltage at the ignition coil is selected like 300-400V.

  25. Actually, I have not yet gone through the circuit description, so I won't be able to assist you much unless I myself read and understand the design which I am not able to do due to lack of time, you can ask the same to Mr. Abu-Hafss who is the author of the above design…just post the question under any of his comments, hopefully you will get the required reply soon from him

  26. …."The zeners, R5 and C4 forms the regulator network. The sum of the values of the zeners should be equal to required high voltage to charge the CDI's main capacitor (C6)"

  27. Hi guys! I have a 1978 Honda CB400T that the CDI has crapped out on. New parts are no longer available, as Honda quit making them quite some time ago. I think it's fantastic that you have posted these schemata, and would love to make one, but I'm trying to figure out whether the CDI unit needs to be DC or AC. How do I tell?


  28. Rama Diaz

    In response to your following query:

    "Is it possible to configure it somehow so that the spark occurs at the end of the trigger signal? "

    You can add following circuit between each trigger signal and the CDI's trigger input:


    It will generate a triangular pulse right at the end of the original triggering pulse. And triangular pulse is equally capable of triggering spark in the CDI.

  29. Swagatam

    I could not find Rama Diaz's reply to my post of December 18, 2015. And I see that my response to that reply has also been deleted.

    • Abu-Hafss, I think it's posted in the other platform (WordPress) to which I had recently transferred my site, but due to technical difficulties I had to switch back to this blogger platform (Powered by Google).

      I am sorry all those comments which were posted during 1 April and 12 April are lost and cannot be retrieved back.

  30. Hi Rama Diaz

    All resistors are 1/4W except R1 which is 1W.
    All zener diodes should be 1W.
    All polarized capacitors to be rated 25V.
    All non-polarized capacitors can ceramic but better polypropylene film capacitors which are usually rated 100V.
    All diodes are normal diodes except D4 which is Schottky and D2 is fast recovery diode (rated 1000V).
    C6 is 1.5µF, 600V.

  31. Hi I have almost all the parts to make this but i do have some questions about some of the component sizes and v/watt rating some of the diods for instance are very small does this matter? and the resistors and little capacitors also very small, which ones will need to be of a bigger size/ rating? i see one resistor is listed as 1W all the others i have no idea. Also the closest big capacitors i could find are 1uf 630v they should be ok?

  32. Hi Im back so i am moving closer to building this project, my signal is selectable either like "A" @ 5V or it will be an inverse of A but without any voltage pullup, just signal going from open to LOW (never will it be like "B") So we can use either one but it is important that the spark action always happens at the end of the signal.

  33. Swagatam Majumdar
    hi, im realy interesting with your CDI idea. i dont have transformer according this article, but can you give me formulas or something to calculate how much turn to make transformator that produce 400v?

      • Farhan Sahib

        It is clearly mentioned in the article that the circuit is capable of producing more than 500VAC. In fact, I observed it to go to about 725VAC.

        If you want 400VAC, simply select values of Z1-Z3. You can use 4 x 100V/1W zeners or 1 x 400V TVS diode.

        The construction of the transformer is explained in details, in the article. Please read it thoroughly before starting the project.

  34. Excelent thank you!… yes 80000 rpm is quite high but that is rpm that many 6cyl race engine will use so i picked that as the practical maximum frequency.
    I was more concerned about the current draw on the transformer and other primary components, but the capacitor charge time is also a factor.

    Is it possible to configure it somehow so that the spark occurs at the end of the trigger signal? that is more in line with how most other ignition systems operate so the timing calibration will be configured to align more accurately that way.

    One more question the voltage rating of the capacitors (c6) is that an ac rating or dc rating?? the same cap is often listed with very different values for each



    • First please, let me know the shape of the pulses from the ECU. See the figure in the link and let me know if it is A or B.


      If it is type B then it will be easy to trigger the sparks at the end of the positive pulse. But if it is type A, it will be very difficult.

      Since the capacitor has to be charged by 250-400VDC, the capacitors should be rated about 600VDC or more.

  35. Hi Rama

    You are most welcome.

    A single cylinder CDI-unit would work up to 9,000RPM or more without any need of ignition mapping.
    400Hz means 400 x 60 = 24,000RPM, practically a 6-cyl. or 4-cyl. would never achieve such high RPM. It should be shared equally by each CDI-unit which means 8,000RPM per unit, though 8000RPM is practically quite high. Since all 3 CDI-units would be isolated, each capacitor C6 would get enough time to get charged and would not be much affected by the firing of other 2 capacitors. If you feel they are not working, you may change the values of C6 to 1µF/500V and/or reduce the charging voltage to 300V instead of 400V…….this will enable C6s to get charged more quickly.

    Yes, the ignition coils have two terminals. The negative has to be earthed and the positive has to connected to point (E) of the CDI-unit.

    As far as the duration of the trigger is concerned, I think it should be proportional to the RPM. A single cylinder CDI-unit would work up to 9,000RPM or more. For a 4-cyl. or 6-cyl. the RPM does not go that high therefore, even if the duration of the trigger is changed, it would not have any adverse effects.

    The CDI-unit will fire as soon as positive (threshold) voltage is received at point (B) of the unit. I have successfully tested it with regular fluctuating 5V signals. For a squared-wave signal, of course, the unit will fire right at the beginning of the signal.

    Yes, a good quality polypropylene capacitor should be used for CDI.

  36. Rama DiazDecember 14, 2015 at 8:52 PM

    Thank you!!!
    This is brilliant and it will get built! I like the simplicity of just using a flipflop to drive the Hf generator that is nice! Unfortunately i will be away the next 2 months but as soon as i return i will start.

    some questions; on a 6cyl with 3stage CDI this will operate at total spark outputs of up to 400Hz. Will the primary driver support this much draw??

    Also Automotive coils connect using 2 wires normally, one is constant power and the other is the switched signal in this case should one just be earthed? and if so which one?

    does the duration of the trigger signal matter? if so what should i set this to? (aftermarket ecus have this option)

    Does the coil fire at the start of the signal or the end??

    And lastly does it matter what type of capacitors are used?

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