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Simple Capacitive Discharge Ignition (CDI) Circuit

Simple Capacitive Discharge Ignition (CDI) Circuit

In this post we discuss the circuit for a simple, universal capacitive discharge ignition circuit or a CDI circuit using a standard ignition coil and a solid state SCR based circuit.

How Ignition System in Vehicles Work

The ignition process in any vehicle becomes the heart of the entire system as without this stage the vehicle just won’t start.

To initiate the process, earlier we use to have the circuit breaker unit for the required actions.

Nowadays a more efficient and long lasting mechanism has been developed for the ignitions in vehicles, called the capacitor discharge ignition system.

It incorporate an electronic circuit precisely functions like the old contact breaker, but in a solid-state electronic manner.

Let’s learn the circuit operations with the following points:

Basically as the name suggest, ignition system in vehicles refers to the process in which the fuel mixture is ignited for initiating the engine and the drive mechanisms. This ignition is done through an electrical process by generating high voltage electrical arcs.

The above electrical arc is created through extreme high voltage passage across two potentially opposite conductors through the enclosed air gap.

As we all know that for generating high voltages we require some kind of stepping up process, generally done through transformers.

As the source voltage available in two wheeler vehicles is from an alternator, may not be powerful enough for the functions.

Therefore the voltage needs to be stepped up many thousand folds in order to reach the desired arcing level.

The ignition coil, which is very popular and we all have seen them in our vehicles is especially designed for the above stepping up of the input source voltage.

However the voltage from the alternator cannot be directly fed to the ignition coil because the source may be low in current, therefore we employ a CDI unit or a capacitive discharge unit for collecting and releasing the alternator power in succession in order to make the output compact and high with current.

Capacitive Discharge Ignition (CDI) Circuit for Two-Wheelers


PCB Design

CDI ignition PCB design


CDI Circuit using an SCR, a few Resistors and Diodes

Referring to the above capacitor discharge ignition circuit diagram, we see a simple configuration consisting of a few diodes, resistors, a SCR and a single high voltage capacitor.

The input to the CDI unit is derived from two sources of the alternator. One source is a low voltage around 12 volts while the other input is taken from the relatively high voltage tap of the alternator, generating around a 100 volts.

The 100 volts input is suitably rectified by the diodes and converted to 100 volts DC.

This voltage is stored inside the high voltage capacitor instantaneously. The low 12 voltage signal is applied to the triggering stage and used for triggering the SCR.

The SCR responds to the half wave rectified voltage and switches the capacitors ON and OFF alternately.

Now since the SCR is integrated to the ignition primary coil, the released energy from the capacitor is forcibly dumped in the primary winding of the coil.

The action generates a magnetic induction inside the coil and the input from the CDI which is high in current and voltage is further enhanced to extremely high levels at the secondary winding of the coil.

The generated voltage at the secondary of the coil may rise up to the level of many tens of thousands of volts. This output is appropriately arranged across two closely held metal conductors inside the spark plug.

The voltage being very high in potential starts arcing across the points of the spark plug, generating the required ignition sparks for the ignition process.

Parts List for the CIRCUIT DIAGRAM

R4 = 56 Ohms,
R5 = 100 Ohms,
C4 = 1uF/250V
SCR = BT151 recommended.
All Diodes = 1N4007
Coil = Standard two-wheeler ignition Coil

The following video clip shows the basic working process of the above explained CDI circuit. The set up was tested on table, and therefore the trigger voltage is acquired from a 12V 50Hz AC. Since the trigger is from a 50Hz source, the sparks can be seen arcing at the rate 50Hz.


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!

208 thoughts on “Simple Capacitive Discharge Ignition (CDI) Circuit”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. what is the maximum voltage can produced by normal bike engine alternator.. like pulsar 150 (2007 model), if alternator is faulty what is the minimum voltage required to produce such arcs?

    • It is normally rated to produce 100 to 200V AC for the CDI coil. 75V to 100V AC is the minimum requirement for a good spark with at least 200mA current.

  3. I have a problem with my Cdi motorcycle ..I want to modified the Cdi because my rpm around 7500 ..I want more ..normally which component I must be to change ? Please help me…

    • you can use this same design which is explained above, because if the RPM is 7500, your pick up coil will generate the triggering signals at this rate which will correspondingly trigger the coil at 7500 pulse rate.

      However you can do one change, just place the diodes with BA159.

  4. Hello, I had opened my household generator as it is not starting. I think it might be the cdi. I want to know if this cdi will work or not ? If yes then i will be happy?. In your schematic diagram the pickup connection is single wire but on my generator engine the pickuphas two wire, how will i make the connection ? The pickup box is magnetic thing inside as metal gets attracted to it. Thank you.

    • Prosenjit, yes it should do the job for you! Out of the two wires one could be the negative or ground wire, please identify with a meter which wire is common with the ground line of the generator, once identified you can then connect it with the common ground line and use the other wire as the trigger input. If you are unable to identify use each wire randomly and check which one works.

      • Thanks for your reply. Now i understood the wiring. I will surely try that out. One more thing what is inside that pickup box, does it has any magnetic switch as it is magnectic. Thanks.

        • sure thanks! The pickup box normally has an iron cored coil which is triggered by a magnet fixed with the engine wheel, as the wheel rotates the magnet moves past the pick coil, and this induces a magnetic pulse in the coil, and this pulse output is used as the trigger for the CDI circuit.

  5. Hello, I have a sea doo and the mpem (multi purpose electronic module) burnt a diode from a stator over charging it. I can not buy this board so i replaced the diode. The engine cranks and all works except I do not have spark. The board has a key security built in with no work around so I would like to build a cdi so I can bypass this unit. Its a 2 cylinder 2 stroke engine. Would any of the cdi schematics on this site work for me?

    • Okay great. It has a pickup coil and a charging coil so this setup will work with 2 opposing firing spark plugs?

      • OK, in that case if the charging coil is able to generate around 100V to 200V this can be configured with the points marked 230V, and the pickup output can be connected with the points marked 12V…both with respect to the ground line.

        Once hooked up and initiated, the High tension wire should start generating the required high voltage for the spark plug

        • This is all good news for me. Anyway I can send you a schematic showing what I have and what I want to replace with this cdi?

            • Thanks for showing the wiring diagram, however I am not able to identify the pickup coil and the alternator inputs for the CDI, it seems the CDI coil is supposed to be all electronic, not sure.

            • So what I want is to cut out the electronic module (which includes the cdi) and just use the 2 coils (magneto in top right of the diagram) and built a cdi to power the ignition coil and get spark. I have looked all over and can not find a replacement module so this is a last option. Is it an option? Thanks again for all the help.

            • Using a multimeter identify the alternator output which produces 100 to 200V, and also the 12V from a pickup coil. Once these two are identified you can quickly integrate with the above shown CDI design for initiating the sparks

            • So my local store only has these options for the SCR
              NTE5465 SCR
              NTE5645 Triac
              The direct converaion was NTE5466
              Will either of those do?

            • The SCR specs are too low, it will blow instantly, the triac specs look OK, it can be tried, might work

            • On the picture I sent you the link for it shows a YL and YL-BK leaving the magneto but does not show where they go. They go into a regulator rectifier and then those leads head back into the diagram. I used my meter and believe those yellow wires to be the AC but because they do not ground on the engine how would I integrate that into the CDI? Or do I need to use the circuit that uses a transformer and does not use the AC from the engine?

            • sorry it will be difficult for me to understand and trace your wiring diagram without a practical test, you may have to do it yourself.

              If you can just find the pickup coil that would be enough, the CDI circuit could be run with a battery and integrated with your system quickly

  6. Hello, friend. I need a cdi circuit for a kymco quannon 125, it burns to me original, I want to replace the burned parts, so I need a circuit of the capacitors, transistors, relays their strengths and specifications. Thanks in advance.

    • Hello friend, I am sorry at the moment I do not have any specific design for the mentioned model, so won’t be able to help in this regard.

    • HI OG, the discussed CDI is suitable for all general motorcycles, it may not work efficiently only in high speed racer motorcycles or other forms of enhanced bikes, you can give it a try, most probably it will work.

  7. By the way, DAVID

    You mentioned, you are new to electronics. So how are you going to handle it, even if you get some working circuit? Do you have soldering experience?

    • Hi Abu-Hafss, most readers get confused due to the wrong placement of the "load more" button, If possible I'll try to shift it over the comment box soon, that will make it easier for the readers to know regarding the hidden comments.

      WE will surely wait for the new article. Thanks for your interest and time!

  8. Hi Sawagatam

    I cannot see the following post (+ posts after this) in your blog despite of repeated reloading of the page:

    Thank you Swagatam + Abu-Hafss
    Thank you Guys for replying so quickly to my question!
    Abu-Hafss- Are you asking for a picture of the magneto itself or a picture of the coil and how it is wrapped under the protective sheathing? here is a link to one like mine i found on ebay. i.ebayimg.com/images/g/NaMAAOSwR29ZE5We/s-l1600.jpg the magnet looks like this https://i.ebayimg.com/images/g/1kEAAOxyRHdRzMNq/s-l500.jpg

    @DAVID — Yes I wanted to see the coils inside the magneto cover. Anyway, I understood your required CDI is 2-wired self-triggered type used on motor-bicycles. That is slightly different from the one discussed in this article.

    I shall post an article for that CDI but it might take a week or so, as I am bit engaged these days. I hope you can wait for a few days.

  9. Hello
    +Swagatam Majumdar,
    I was curious if this application can be used in a single spark small 66cc 2 stroke motorized bicycle? I have no battery and engine is "push started" by release of clutch which spins the "magneto" which i assume acts as an alternator would but on a smaller scale. I have not measured the output of the magneto to the CDI that comes with this kit but I wish to get the best performance possible. I have a faulty CDI that does not produce enough to get spark from plug that is necessary. It is a solid state CDI. do you think that I could open it up somehow and upgrade the components and or repair this thing so it would work?
    I apologizes all this circuitry stuff is pretty new to me.
    thank you for your dedication and your willingness to share your vast knowledge! You are very inspirational in my desire to further understand circuitry and the whole bit!
    Reguards, David

    • Thank you David for posting your question and trusting my knowledge. If you are new to electronics then definitely you would find it almost impossible to troubleshoot or repair any faulty electronic unit…moreover since it's a solid state version would have more sophisticated parts than what is presented in the above article.

      The CDI explained in this article looks an ideal choice for your requirement, and in fact can be used for generating sparks on any vehicle that may be equipped to generate a minimum 60V AC, and has a pickup coil.

      The 60V is used for activating the ignition coil which ultimately generates the high voltage for the sparks. The pick coil responds to the wheel rotation and triggers the CDI to generate the high voltage pulses at the same rate as the wheel rotation.

      Even if the pick coil is absent, the high alternator output can be suitably dropped and used as the triggering source for the CDI coil.

      So basically if your bike possesses the above then you could easily exploit the above design in your favor.

    • Hi David

      The above circuit should work with your 66cc bike without any problem. However to ensure correct operation, it is advisable that you send us a photo of inside of your magneto.

    • Thank you Swagatam + Abu-Hafss
      Thank you Guys for replying so quickly to my question!
      Abu-Hafss- Are you asking for a picture of the magneto itself or a picture of the coil and how it is wrapped under the protective sheathing? here is a link to one like mine i found on ebay. i.ebayimg.com/images/g/NaMAAOSwR29ZE5We/s-l1600.jpg the magnet looks like this https://i.ebayimg.com/images/g/1kEAAOxyRHdRzMNq/s-l500.jpg

    • if you need a picture of the coil of the magneto let me know and i will cut off the sheathing on an extra one i have and send you a link.
      thanks again for your assistance!
      one more Q: can all of the components that are required for this build be purchased at radio shack? Or is there a better place to acquire these types of special parts that you guys go to for items such as these?

    • Thanks David, the ebay image is not opening for me.

      the 56 ohm is a 1/2 watt resistor….however the value is not so critical, you can use any nearby value, such as 68 ohm, or even 100 ohm will do.

      it can be as shown below:


    • 60 ohms will work but try to get a CFR type as referred in my above comment, a wirewound type would also work but it's not necessary

  10. Hi Swagatam,

    I really enjoyed this article though I felt a bit lost in certain parts because even though I like electricity and electronics all I know I've learned "the hard way" ':). But my question is how do certain cdi's defer from other brands? Like I would like to apply this on your article to my yamaha dt125r but my bike is a bit of a transformer because it no longer has many stock parts. Like for example here in my country is very hard or very expensive to get original or oem parts, so I managed to make the bike rub without a cdi but I feel it is not as precise as it should, and the trial and error cost me a few more parts. In case you are interested what I did was i took out the pin on the crankshaft that allows the flywheel to be in the correct position so I could advance it or retard it, and so I plug the source coil wire directly to the ignition coil and placed the piston at TDC and made a small mark on the crankshaft to know when it is in TDC and also on the flywheel for reference, then I tried to kickstart the bike (it is a 125cc 2stroke) and if it didnt work i moved it bit by bit to find where i would get a kind of good spark but sometimes I got a recoil from it and made the engine run backwards, when it was a bit off of TDC it was not a problem because it would just run back instead of forward x) But if it was too close to TDC the recoil would crank the kickstart so hard it would break a few things, but it was fun though. So back to my question I would love to find a way to make my own CDI for my bike what specs would I need of the bike itself so I could build one?

    Thanks in advance and best regards,

    • Thanks Robert,

      I too enjoyed your adventures with your motorcycle and it clearly proves your innovative nature and skills. Since automobile is not my main field of work, I tend to forget quickly anything that I try to grasp in this field. Therefore I could not understand much the mechanical modifications you did in your bike.

      However I am pretty sure that if your bike could work without a CDI then definitely it would do much better if the above explained CDI circuit was installed in your bike. Because the above design is an universal design and can be used for all regular bikes which are presently running on the roads throughout the world.

      No doubt about it, and you can easily do it by assembling the little number parts indicated in the diagram, and simply hook them up with your bike's electrical.

      I am sure you will be able to build the circuit and implement it successfully…. if you have any difficulty with the testing of the circuit, do feel free to post it here, I'll try to solve it out for you…

    • Hi Robert

      Answering your question, the shape of the CDI unit is not important. Basically, there are two types of CDI unit i.e. AC-CDI and CD-CDI. Two different CDI units, different in shape/size for two different bikes can have same circuitry.

      As you mentioned, if your bike has a source coil with high voltage output (about 180VAC) then the above circuit would definitely do the job provided the connections are correct.

  11. I'll check into the coil for the trigger… The max RPM's on this bike are 11,500. The four wires are + and – 25V primary coil and + and – for the trigger (according to some reading should be 3-5V). Assuming the (-) get tied to ground? What would be the minimum voltage needed for the trigger? Thank you, you have been a great help. I'm gonna give this a try, If it doesn't work at least I'll have fun trying it 🙂

    • 5V will be enough for triggering the SCr, but the other supply which is supposed to go to the capacitor and the ignition coil must be around 60 to 100V, otherwise the spark strength can get affected causing inefficient combustion of the engine.

      By the way 11500 RPM looks very high, not sure whether or not this CDI will be able to handle it.

  12. What size resistors do you recommend for this CDI? 1/4W, 1/2W, 1W or bigger? Also, I'm having trouble finding a 56 Ohm resistor locally, and alternatives? Thank you!

  13. What size resistors do you use for this CDI? 1/4W, 1/2W, 1W, Higher? Also, having trouble finding a 56 Ohm resistor locally, ant alternatives? Thanks!

  14. Hi i am hassam from kashmir . i have suzuki's very old model gs250 fw with 4 cylinder engine i got a some problem with its cdi its not working good can i replace it with some thing r can repair it ? I m very far away thats why i can't get it new one its not available here i m going to import it i gonna bebe so expensive what should you suggest me…thanks

    • Hassan, The above CDI is a universal model which is presently being used in reputed Indian motorcycle brands such as Bajaj, and Hero Honda, so you can build this CDI for your motorcycle also and use it for the time being until you find the original version for your specific model…

  15. Hi Swagatam, I am from the Philippines and my motorcycle's cdi got damage lately. I have an old honda C-100, thanks to your cdi design it is running again. I have used a 1uf/450V capacitor instead of 1uf/250V because there is no available 1uf/250V on the store. will it affect the performance of the cdi? and I also hear small clicking sound comming from the circuit, is that ok? I have already traveled around 250km already using your cdi design. Thank you Swagatam Majumdar.

    • That's great Christian, I am glad it's serving the purpose for you!

      If the max voltage from your alternator is below 200V then the 1uF capacitor can be rated at 200V without issues but if you are not sure and doubt that it might rise above 250V then you better change it to a 400V rated capacitor.

      Any kind of sound from a solid state circuit is not good, you can check if anything's sparking or arcing at any place within the pCB.

  16. Hi Vicente

    If I were you, I would have assembled on a 2" x 2" veroboard. But anyway, your connections seems to be OK. To me, the SCR looks doubtful.

    By the way, can you get the waveform of the pulser input? I doubt if the pulser coil on your bike issues negative pulse first, then positive pulse. If that is the case, you might need to make some modification in the circuit because this circuit is designed to fire on a positive pulse.

    Please see the difference between two wave patterns:

  17. Ok, thanks a lot.
    I'll try to improve my connexions, change several components, other pick up, etc.. and maybe, change motorcycle. I'll inform you.

  18. Hi, I used 1N4004, I think it's equivalent to BA159.
    Also when, the r.p.m. increases, the pickup pulse width decreases, really, but mounted the OEM CDI, the system works ok.
    Another question: how could I change the time since the pick up sends the signal, to arrives at the SCR? It is very interesting, beacuse I'would like to change or adjust the spark timming, in order that this circuit provides the spark too early, about 20 degrees before the original system, and this can be bad to the engine.
    I'll continue working on this project.
    Thanks for the circuit, to you and all the people that participates here.

    • Hi, 1N4004 is not a high speed diode, but anyway high speed diode may not be required.
      sorry I mistakenly related the charging of the capacitor with pickup pulse, actually the pick pulse is related to SCR triggering not the charging of the capacitor.

      Upon checking the waveform images, I found that increasing speed results in the increase of amplitude or the peak voltage output from the pickup, so how this could be affecting the SCR conduction is something that needs to be diagnosed.

      You can try feeding the pickup output to a small bridge rectifier and connect the output from the bridge to the SCR gate….see if this hack improves the results.

  19. Hi Mr. Swagatam!
    I use a BT151 650R but NXP.
    I tried to increase the value to 2uF/400 but not with one capacitor, just mounting four capacitors 400V in parallel to arrive at 2uF. But the problem persists.

    • NXP is Philips, so it's fine.

      the spark will be weak if the capacitor does not charge properly due to insufficient pulse width from the pickup.

      may be the 1N4007 are not able to handle the high speed pulses and not conducting quickly to charge the capacitor fully….you can try replacing all the 1N4007 diodes with BA159 and check the response.

    • Hi Swagatam and Vicente

      Since this CDI is an AC type (means HV AC is being supplied by source coil inside the magneto) the frequency of the AC voltage is not very high. 1N4007 should work perfectly.

      Vicente, could you please draw the circuit diagram of your prototype? And provide a photo of your circuit? Actually, the circuit is quite simple and should work if the connections are made correctly. For SCR, 2P4M also work perfectly in 50-100cc bikes.

    • Hi Abu-Hafss, yes a high speed may not be required since the RPM is not in the kHz range.

      We can assume the circuit built by Mr. Vincent to be correct since it is responding well at lower RPMs, and failing at higher RPMs.

      I think the pick-up coil output has something to do with the issue

  20. Hi sir.
    Recently I made this circuit on a 2stroke motorbike, 50cc, improved performances for amateur racing. At idle the system works ok, but when I try to rise r.p.m. the spark goes off, I just can increase about 500 or 1000 r.p.m. from idle.
    Whith the oem CDI system, it works ok, so the AC generator and pulse signal, I think its ok.
    To test it, I mounted the circuit on a protoboard to experimental circuits, It could be this the reason? I tried to improve the continuity of the wires, betwen the ignition coil and the capacitor and also mass, with double or triple wires, but I didn't reach any result.
    Comparing to the oem CDI, the spark if less powerful in this ciruit, I don't know if this could be the reason.
    Thank you.

    • Hi Vincent, which SCR did you use?

      make sure it is BT151 from Philips.

      If you think spark power could be the cause you could try increasing the value of C4 to 2uF/400V and see if that makes a difference.

  21. Hello Swagatam, I came to this very helpful blog, while researcing a CDI for a medium displacement two stroke bike, and I have a very high regard for your availability.
    Could you please elaborate a little, on the last statement of your last reply (July 30).
    The engine I will hopefully use the CDI is of this kind (high power to weight ratio), so could you hint to the posssible paths to take, to achieve such an improved (battery-less) ignition?
    Electronic advance, more current etc.
    Thanks very much.

    • It will work for all general bikes, not with special high speed bikes which are intended to be used for racing or for extra high speed operarions….

  22. You can try the following:

    ignore the ground connection of the CDI, meaning don't connect the indicated line of the CDI with the bike ground or body….and instead connect the two wires from the alternator with the 230V input and the ground line of the CDI

    this is what I believe should work, but I am not sure about the socket connections, you may have to connect the wires directly with the above explained CDI and not through the socket.

  23. Hi, i have a honda spada vt250 v-twin engine, i got a problem with my cdi some parts are burn and i dont know the value, do you have any idea with this or do you have some schematic diagram with it..? Please help me.. Thank you
    ps. the burned parts which i recognize is diode i dont know the value, and the other one is a circle shape and it is badly burned. Thank you..

    • Hi, if you can show me the pics of the burnt parts I could try to identify them, if these are diodes then most probably these could be 1N4007 diodes…if the size is bigger then it could be a 1n5408 etc

      If this circle shaped part has four legs then it could be a bridge rectifier…..

  24. Sir I want to make all Indian bike cdi unit can u provide me all solution like making n testing source….9015693522

    • Shinam, CDI circuit is provide in the above article, a CDI tester circuit is also included in this website you can use the search box to find the same.

  25. I mean that its better than scooters original cdi. Trigger pulse is only 5 volts in my scooter ( oskilloscope ). Therefore I made it without resistor. Its too low voltage for original cdi. This handmade cdi working very well. Thanks!

    • OK I am glad it's working for you! however a resistor is a must at SCR gate, even at 5V the SCR could destroyed if its gate is not protected with a resistor….you can use a 100 ohm resistor

  26. Thank so much. I made this like cdi by bigger thyristor and without R4 resostor. Its better than original.
    Pave from Finland.

  27. I have a 1980 4cyl 4 stroke kz 1000 that the ic igniter has become lost due to moving. I was wondering since it is a wasted spark system in which two plugs fire simultaneously can I build my own cdi to use in place of the ic igniter that was used originally? This question is brought about because of the rarity of factory replacements and the high cost of aftermarket replacements. If you have any ideas that will help please blueprint the for mentioned schematics, I will be more than grateful for any help.

    • There's no harm in trying the above shown design for your bike.

      you can try making two such circuits with two separate ignition coils…and connect the relevant connections with the appropriate sources from the bike.

      The system will certainly allow you to use your bike normally, if not at extreme high speeds.

  28. HI Swaga, just wondering the BT151 leg configuration>the BT151 Chip number label facing us: 1st leg-cathode(positive out) , 2nd leg-anode (positive enters), 3rd leg-gate..is this correct?

  29. hi swagatam, can we replace the bt151 with a 2n3055 instead? also what is your opinion of bedini radiant charging oscillators, have you tried to replicate one yourself with useful success?

    • Hi Fidel, BT151 is a thyristor, which is completely different from a transistor and therefore the two cannot be replaced with each other….
      no, I haven't yet tested the bedini charger, however i feel it could be true since it uses a flywheel concept and we all know how efficient a flywheel can be in terms of output vs input power

    • thanks for taking time to reply me mr swaga, I hope a man with such great knowledge in modern electronics concepts can build one test for yourself and give us your honest opinion based on the tests, regards!

    • it is important I also stress that we endeavor to investigate such devices at these times when the whole world is reliant on the $$$ guzzling oil and no one gives much thought about clean energy concepts seriously

    • I appreciate your thoughts Mr. Fidel, you are right, somebody must take serious initiative and prove the world about the potentials of these efficient machines…in my free time i would certainly want to go ahead and test one of those…

  30. Hi,
    I'm wondering If u have a printed circuit for this shematic.
    (I'm not good at all on that software to make circuit)
    If u have please share with us
    Otto V.
    Congrats for this page!

    • thanks otto, I had the PCB design some time ago, but have lost it now, so I am really sorry…. presently it would be difficult to post it back.

  31. Hi I have a 2005 suzuki gsxr 400 slingshot gk76 and can't find a cdi in south Africa and need to build my own if you can help whit that please

    • Hi, you can build the one that's shown in the above article, it's a universal type and is suitable for all 2/3 wheeler vehicles.

    • Hi Pieter

      Your GSXR 400 is a heavy bike with high RPM. I am afraid the CDI discussed here is suitable only for smaller bikes 70-125cc. It might work with your bike at low RPM but at high RPM it will fail to synchronize the sparking. The OEM CDI unit for your bike is based on a micro-controller which delay the sparks at high RPM.

      If you have no experience of programming/burning micro-controllers and want to get the job done without any hassle/frustration, I would suggest you to buy the original or compatible CDI from eBay or AliExpress. If you still want to DIY, let me know I'll forward you the link.

    • Hi Pieter, you can try the above circuit by replacing the CDI coil as per the specs, and under the supervision of an expert automobile engineer, the above circuit design is a universal design and can be used fr all conventional motorcycles with some modifications.

      here's the image of the CDI coil for your bike


      it has a twin coil it means you can try making two of the above circuits and integrate the triggers with the relevant pick up inputs and the outputs with two spark plugs.

  32. Not sure if my message went through as my computer is a little crazy at the moment.
    CDI unit C4 capacitor, is it polarized or non-polarized?
    I plan to try it on a Mercury outboard 402hp (40hp) motor, stator input is 180volt and trigger is 20volt, do you think it will be ok?

    • C4 is a non-polar capacitor, the circuit is intended for all 2 or 3 wheeler automobile engines, if your system similar to these engines then it'll surely work.

  33. If i want to use this ignition in a 4 stroke 90 degree V twin, can i just use 2 pickup coils 90 degrees apart, hooked to 2 separate ignition boxes? Thanks!

  34. can you tell me what voltage i should expect from the small 2 stroke ignitions used in made pales boat cars etc
    usually 23 cc to 35 cc
    Zenoah or chung yang
    also the zenoah engines use a primary and secondary coil where as the CY uses a single coil
    what are the differences between the 2

  35. you can use a momentary on switch to temporarily ground the signal from the alternator, disabling spark and therefore stopping the motor. kill switch 🙂

  36. If we alter some parts change for earn more voltages like 400 V to give ignition coil.
    like AC alternator 12v and 1amp change to direct DC 12 V battery input and circuit's change diods 1N007 change to 1N 5408 – 3amp doids.
    Does it run and genrate more voltages and more power?

    • increasing voltage or current won't do any good for enhancing the automobile performance because the spark plug gap is too small and will respond equally to a low voltage or a high voltage…..
      The ignition coil and the circuit is already powerful enough, to produce sparks 1/2 inch long….it implies that you could try altering the spark plug gap and make it about 1/2 inch long,,,,this alteration could help the cause…..because longer spark would mean longer area covered by the spark inside the chamber influencing bigger explosions.

  37. Dear sir,
    My bike is honda cd 125 and its 10000 rpm
    and its ignition build for 10000 rpm.
    i am customize engine for 15000 rpm and its require improved ignition.
    does above circuite use for this 15000 rpm

    • Dear Mansoor,

      yes according to me 15000 RPM will not be a problem for this circuit since it uses a fast, and advanced SCR for the switching.

    • Hi sir thank you for replying.
      actuall i mean to say that the above circuit output voltage is 230v to going on ignition coil,
      And ignition coil 230v to become 20,000 v.
      But it can possible circuit produce 400 V and forward to ignition coil.
      And then ignition coil 400 V to become 40,000 Volts?

    • I wana ask you one question.
      And question is
      Do you built any plasma ignition circuit?
      As you know plasma ignition very much powerful spark.
      and its circuit takes dc HV from battery.

    • Hi Mansoor,, It can be tried but forcing 400V into the coil could make it hot.

      making a plasma spark will require a much bigger coil….possibly I'll post the article someday in my blog

  38. hi sir,
    i want to build this circuit,but my question is how can i connect this,,
    is it alternator ang magneto are the same?
    i found a magneto coil to rectifier on my bike,where i connect this from your circuit?

    • hi nhark,

      yes alternator refers to the magneto in your bike…

      locate the 200V and the 5 V to 12 V sync pulse coming out from the magneto and connect them with the relevant terminals of the above circuit, also connect the shown outputs with the ignition coil and the coil output with the spark plug….the circuit will start working immediately.

    • Dear Nuraniku, it's not recommended, and moreover it would produce the same results as with a 1uF/400V….however, the capacitor will need to be a nonpolar type.

    • As Sawagatam said, it is not suitable because a 220µF capacitor would take longer time to charge as compared to a 1µF capacitor which would fully charge within nano seconds!

    • Hi, the body polarity is irrelevant here. You can use the above suggested circuit with your vehicle also….just make sure the "earth" lines of the circuit are only connected with the battery negative and not with the body of your vehicle.

    • Hi Abu-Hafss,

      It's one and the same, to me your diagram looks more confusing:) because in my diagram the bridge is well isolated from the transformers and makes it clearer to understand that it's been installed for powering the 555 circuit.

    • HI Swagatam

      Finally, I have managed to get the tester working with the fence charger configuration without the triac. Actually, I got deceived with the earth sign in your schematic. I separated the negative supply for the CDI + ignition coil and the triggering circuit. However, the sparks are quite weak. I am considering to replace the 2nd transformer with a 6V so that the output is around 400-440V.

    • Hi Abu-Hafss,

      That's great! In DC circuit, the earth symbol generally indicates the common negative rail.
      You can also try a 1amp transformer for the one which is connected with the CDI, this could also help to increase the spark strength.

    • Hi Swagatam

      Yes, the earth sign generally indicates the common negative rail, that's why I mistakenly assumed that the 555 is also grounded to that negative rail. Shouldn't I advise you to point out this important note in your article so that people like me may not get deceived.

      Right now, I am using 2 x 12V 250mA xformers. I think changing the 2nd xformer to 1A won't strengthen the spark because the sparks are released from the charge dumped in the capacitor. Therefore more higher voltage will result strong spark.

    • Hi Abu-Hafss,
      I'll remove the earth symbol so it doesn't confuse the new hobbyists.

      Higher voltage will produce longer sparks but will be weaker in power (not very bright), increasing amps will make the sparks thicker, stronger and brighter.

    • Hi Swagatam

      Ok, but in that case both the xformers should be changed………because the output from the first would the input for the 2nd. If only the 2nd is changed, the input wattage would remain the same and hence the output would remain the same.

  39. Hi Swagatam

    First I connected the triggering input to the output of variable frequency oscillator (max. 5V). When I could not get the sparks, I manually tapped the triggering input with 12V but still no sparks.

    • Hi Abu-Hafss,

      Something is surely wrong somewhere, I think for avoiding doubts you could try following the exact design that's been shown in my circuits, and also use the same ignition coil which has been shown in the drawings.

    • Hi Swagatam

      I checked the coil and found that the secondary has very high resistance in M-Ohms, maybe due to the high tension wire, though it worked perfectly in the bike. Anyway, I bought a new one same the one shown in your drawing. It had perfect reading of about 6k.

      However, as reported on Feb. 15th, when I fed 136V thru CDI to the new coil the voltage dropped to 40V and there were no sparks.

    • One more point………….
      When I switch on the circuit the LED connected to the pin#3 of 555 would indicate the oscillation. And sometimes when I connect the transformer, the oscillation would stop.

      In other words, sometimes when the transformer is connected to the collector of TIP122 and the +ve rail, the 555 would stop oscillating. And then when I disconnect the transformer, it would resume the oscillation after some time. This does not happens every time.

      However, the problem I reported earlier occurred when the 555 was oscillating with the transformer and the output 136V.

      Lastly, please remember the transformer I am using is 12V 100mA.

    • Hi Abu-Hafss,
      try the fence charger design without the 555 stage, let's first make the basic design work ad then you can proceed with the other versions.
      I think the second trafo should be rated a little higher, may be a 500mA will be more suitable.

    • Hi Swagatam

      The article you are referring involves 2 transformers whereas, my effort was to use a transformerless design.

      I had that point in my mind that current is not an important issue here. For that particular reason, the output is around 600V as long as the input voltage is between 12V-30V !!!

      Without the CDI, I have checked the capacitor gets charged within milliseconds. To connect to the CDI unit, I removed the capacitor + diode and connect the mosfet+coil directly to HV input of the CDI unit. (The CDI has its own diode and capacitor).

      At the HV input the DVM set at AC is showing 6-7 volts !!! How can that be??? The CDI unit works perfectly in its place.

    • Hi Abu-Hafss,

      I referred you the above circuit to indicate how the low current 220V input from an ordinary trafo works perfectly to produce the required 20kv sparks at the output of the CDI coil

      I am afraid removing the diode capacitor network will not help, it's required for the proper functioning of the design, may be because it operates by charging and releasing the capacitor stored energy only when the capacitor charge reaches some optimal point.

      The think the CDI coil or any such trafo responds best when bursts of high voltage are dumped in their primary winding instead of a continuous frequency.

    • Hi Swagatam

      It seems to be quite difficult in getting the 555 boost circuit to work as HV supplier to a CDI. I have spent many hours but, not successful.

      Presently, since I need the HV supply only for bench test unit for CDIs, I am considering to use a transformer.

      I have a 220V-13V 100mA small transformer. Will it be okay, to use it with a 555 astable oscillator at 50-60Hz + TIP31C? Or do I need to have a bigger transformer?

    • Hi Abu-Hafss,

      A high frequency input will not work with a CDI coil, you will have to use the converter in a burst mode or a pulsed mode, and it should match the input winding voltage rating of the CDI.

      A 100mA transformer will also work but again it needs to be operated in a pulsed mode that's why the diode/capacitor stage becomes so crucial.

    • Hi Swagatam

      1) If we use the diode+capacitor before the CDI, the HV would turn to 600VDC whereas the CDI needs an AC voltage to operate.

      2) As far as the 100mA transformer is concerned, I simply followed your circuit


      only replaced the darlington with TIP31C. I could get only 130V which, if fed into the CDI results nothing.

      3) As per your above statement if diode & capacitor are crucial then how your enhanced CDI worked without them?

      4) I also tried this


      replacing 2N3055 x 2 with TIP31C x 2 and 12-0-12 200mA transformer.

      The voltage at primary is 24VAC but the voltage at the secondary is less than a volt!

    • Hi Swagatam

      I think you got confused with my long message.

      Please see the attached picture:


      1) With HV Supply 1, I could get 600V. The diode and capacitor has to be removed, only HV AC has to be fed into the CD unit. But the set-up did not worked as I reported on my post of Feb. 7th.

      2) I tried HV Supply 2 and 3, got only 130VAC but again the set-up did not worked. This set-up is based on your article of enhanced CDI for 2-wheelers.

      3) The HV Supply 4 is based on your article Simplest Inverter with 70% efficiency but this also not worked, I could get less than a volt.

      The High Voltage Capacitor you are referring is already included in the CDI unit. I am not talking about the CDI unit (it is untouched). I have been talking about various set-ups for HVAC supply which has to fed into the CDI unit. I hope now everything is clear. Now I await your valued comments.

    • Hi Swagatam

      Today, I re-started the project from beginning.

      Firs,t I checked the HV supply in a bike, I found it to be 125VAC.

      Second, I checked the CDI unit. It was working fine.

      I rebuilt the circuit same as yours, only changed the RC components to fixed values to get 51.50% duty cycle and 57.75 Hz frequency. The output was 136V.

      When I connect these 136V to the CDI unit, they dropped to 40V. But I could not get any sparks at the spark plug 🙁

    • 21 swg looks OK to me.
      check the current through the coil and the BJT, this magnitude will actually decide the watts pushed into the ignition coil and will influence the sparks.

      what is iron powder, do you mean ferrite?….ferrite core is a must for this application. iron core will only convert current to heat.

    • Hi Abu-Hafss,

      Do not use direct 220V, it can damage the diodes and the scr, you will need to isolate it using two step down trafos connected back to back

    • Hi Swagatam

      Recently I physically made the 555 boost circuit with an output of smooth 600V.
      The BJT network was removed. The Coil is super cool but the mosfet gets little warm.

      However, when I feed the HV into the CDI………………..Nothing Happens.

      1) If the triggering circuit is separated, the LED at the output shows perfect triggering. But when the output is connected to the CDI unit, the LED goes off.

      2) If manual triggering is done with 9V battery/power supply, a faint single spark could be heard at the spark plug but not visible.

      3) One point which might be or not important. The CDI unit is having 400V capacitor whereas, the boost circuit is producing 610V. Could this be the reason?

    • Hi Abu-Hfs,

      What is the gauge of the wire that you have used? It should be selected appropriately for getting the required amount of current.

      And the current from the power supply should also be rated high enough for the CDI coil to respond.

      even 400V is high enough for acquiring 20000 volts from the CDI coil, so it's fine

  40. Hello Dung,

    It's quite easy, after making the above explained CDI circuit, you will just have to integrate the inputs to the indicated voltage outputs of the alternator.
    Next, the C4 bottom link will need to connected with the free end of the primary of your ignition coil.

    The other terminal of the primary coil will not require any attention as it would be originally connected to the ground of the vehicle.

    That's all, this would conclude the installing of the explained CDI circuit

  41. I have a need for an ignition system for a 6 cyl 2-cycle engine. 2 cylinders fire at a time, two times per revolution. i.e. that's 12 sparks per 360 degrees of crank rotation. I am quite gifted mechanically but alas, electrickery has me by the shorts. Can any reader offer assistance? Thanks, M8 (Mate)

  42. hall effect/ transistor can be used for triggering the SCR, however it's more simply done directly through the AC received from the alternator 12V coil, as depicted across the diagram output terminals.

  43. Good day Swagatam Majumdar! I have a 2 stroke Yamaha L2GF 110 cc 1970's model that uses points. Can i convert it to cdi and use your circuit? Thanks..

  44. Hey just surfing around the net looking at info on CDI design and implementation on the hobbyist level.
    I have a 46 cc retrofit kit for a bicycle and it has given me 4 years reliable use even though it is very cheap engine that gets hard used.
    I have had an alcohol still for a couple of years and have recently began running it on the head of the still 98% methanol 2% hydro-sol.
    It has a magneto coil, I want a little more spark. I understand that i will need to implement a better cdi but also wonder if i can rewind the magneto coil or change the magnet provided there is enough room

    • yes making the magneto stronger would hep generating stronger sparks, so you can either modify the winding or simply procure a mobike magneto and replace it with the existing one,

  45. Hi Swagatam

    What is the function of the diode connected across the primary of the ignition coil?
    With cathode grounded, won't it obstruct the capacitor's energy from passing on to the primary of the ignition coil ???

    • Hi Abu-Hafss,

      simulating the capacitor discharge pattern could be little difficult, however i am assuming the diode to be reversed biased, because practically the circuit is working perfectly so it means everything is right in the circuit.

    • the above circuit was designed by 'Bajaj" engineers, so the diode does work in someway to protect the longevity of the unit….because conditions in automobile electrical AC are too unpredictable, which show its impact in course of time and not immediately.

  46. Hi sir… I have a honda dio 50cc 2 stroke scooter? Would this work instead of the stock cdi which has 5 pins and limiter?

    • Hi Ryan,

      Yes the above circuit is a standard CDI circuit suitable for all 2 and 3 wheeler, basically it's an Indian, Bajaj 2E Rickshaw CDI circuit.

  47. Hey Swagatam Majumdar,

    I have a Indian make yamaha Rx 135 (2stroke) and a TVS apache RTR 160.
    These bikes have CDi's from the company itself.
    Is it possible to enhance the performance of these bikes or a matter of any small cc bikes?
    (P.S. I have read a lot about Performance CDi's and want to make one on my own. Please Guide me as this is very interesting topic for me)


    Chinmay Bisht

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