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.
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.
CIRCUIT DESIGN:
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.
TRANSFORMER SPECIFICATIONS:
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.
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.
Sir what are the first circuit capacitor voltage values.can you send parts list pls
PADMAHARSHA, The capacitor voltage values that are not specified in the diagram are all 50 V rated. Resistors are all 1/4Watt 1% MFR type.
Hi ,
Does motorcycle has a alternator as well. Please let me know.
Is thta not part of CDI circuit.
Thanks
Susanta’
Yes, all vehicles have an alternator. A voltage of around 100 to 200 V AC is fed to the CDI for generating the sparks
Hi
I made this circuit
The first problem for me was the number of turns of the transformer. When I reduced the number of primary turns to 10 and the number of feedback turns to 2 , the circuit was fixed.
Now it works properly but at low speed
When the rpm goes up to ~3000, the spark is very low
Can you tell me my problem?
(I changed the number of turns because the transistor was saturation)
Thank you
Hi,
I am glad you could make this circuit successfully.
At high speeds it seems the current entering the CDI coil cannot reach optimal value. I guess the C6 should be increased a bit, which might solve the problem, but I am not sure I am only guessing.
Try increasing the value of C6 from 1.5uF to 3 uF/600V and check the response
Hi Swagatam, Joop here from Holland. I made two dc cdi ignition and it Works 100% up to 350hz with a sparkdistance of 20mm with a quality Wilma mkp 1uf 630Volt condenser and a good coil. It draws 7-8 Watt at 14Volt With for 75 Volt zeners and a very small transformer.
Now my question: Can I use 18 or even 20Volt? I need 480hz. My bike is a triple cilinder, and it has one cdi with a strange distributor wich connects the ground off the 3 coils in turns to ground.
I tryed it for a short time and it gives a stronger spark up to 530hz.
Thank You for schering you knowledge.
Regards Joop
Hi Joop,
You can use 18 to 20V, but make sure to increase the gate resistor value of the SCR to around 470 ohms 1/2 watt. The frequency of the sparks will depend on the RPM of the bike wheel. The wheel rotation will be converted to pulses from the pickup coil and sent to the CDI for triggering the ignition coil. As your bike moves faster the sparks will become faster and vice versa.
I would like to try construct the circuit for a half-bridge for a DC to DC inverter . I have a hope that for a large part of my desired application, you did a commendable job as if you are an alien from space. I shall try to make a minor modification or none at all.Thankfor your effort in arranging that the converter must operate with the fire semiconductor there are other areas that deserves technical appreciation, also I leave that to other. Thank you Homemade.
Alfred,
Malawi.
Thank you Alfred, glad you found the post helpful! Surely, you can construct it for your inverter project. Hope it works for you.
All the best to you!
Hi Swagatam,
I made this circuit but i’m not satisfied with final results because i’m facing these issues
Can you please help me, how can i overcome these issues?
Hamza.
Hi Hamza, try increasing the primary turns to 30 or 40 turns and check if that reduces the current consumption or not?
The transformer is the crucial part in this design, if it is not built correctly or the winding polarity is not correct, there could be problems. Make sure to put an paper gap between the E-core edges.
Hi, i been looking gor a good dc cdi drawing and think i found on, yours.
But it is for 4cyl car with distributor. This mean 2 ingnitions/rew. Do this construction manage to reload so fast. It Will be 8000r/min order 16000 unloads/min.
Tanken in advance.
Eric.
Hi, the above CDI circuits are for two wheeler and 3 wheeler application, not for 4 wheeler and is not suitable for 8000 RPM speeds.
Would it be possible to modificate it for 4 cyl with 8000 rpm? What is the week part, transformer or the ingnition capasitor?
Or other?
regards,
Eric
Sorry, I have no idea what type of CDI is required for operating a 4 cycle engine, so not sure how the above circuit can be modified?
The capacitor, SCR and the transformer are the 3 important parts of the circuit.
I have a 12VDC Car battery Source only. I am trying to make a CDI for my single sparkplug engine. I cannot find a way to make 200-300VDC without a very expensive transformer. Is there a cheap and simple way? Could I just buy an inverter from amazon 12vdc to European 220vac then a diode bridge to make DC? Please advise.
You can try the first concept from the following article. The transformer is an ordinary 0-12V/220V step down transformer connected the opposite way round for stepping up the 12V from the 555 oscillator into 220V AC for the ignition coil..
https://www.homemade-circuits.com/make-this-enhanced-capacitive-discharge/
What would a spark advance and rev limiter look like?
How high can it rev?
Do you etch your own circuit boards, can you recommend someone?
I do not know much about spark advance and rev limit.
I don’t etch PCBs I build them on strip boards through manually soldering the connections
regarding this circuit i have checked the trigger pulse with a occilloscope and got a reading of .8 v . do you think it will be high enough to opperate your circuit trigger ?
0.8V may not be enough for triggering the above explained circuits. Because D6 will itself drop about 0.6, and the remaining 0.2V will not be enough to trigger ON the Q2….a minimum of 2 V could be required for triggering Q2 and the SCR effectively…
Sir
What are the 3 zener diode specs for dc cdi
Padmaharsha, The zener diodes decide the voltage level that can be stored in the capacitor C6, which in turn decides how strong the CDI output voltage and the spark will be.
Be careful if you build this circuit. Capacitor C6 will hold it charge 400V even after power
has been removed. If you touch the back of the circuit board the capacitor will discharge giving you a mighty shock. The circuit packs a mighty punch. Treat with respect.
When running correctly the circuit will pull about 200ma at 12V and give out 400V.
Thanks Swagatam for posting this circuit.
Thanks for your feedback Alan, I hope the users will find it useful!
Hello
What are the necessary changes for this circuit to operate on 6V? In the primary winding of the transformer I put 11 turns but unfortunately I can not get more than 29v per output.
Thanks
Hello, The coil winding determines the output voltage so you may keep experimenting with the number of turns until the right output voltage is achieved. The idea was contributed by another author therefore I can’t suggest much on this.
Hi
What needs to be changed to run this off 48 volts?
Thank You!
Hi, nothing needs to be changed, you can use the circuit as is…however, the signal from the pick up must be 12V, and the CDI coil must be rated at 48V