The following post describes a couple of simple yet enhanced 12V electronic capacitive discharge ignition systems which derives its operating voltage from the battery instead of the alternator for generating the igniting sparks.

Since it works independently from the alternator voltage, without depending on a pickup coil signal, it is able to function more efficiently and consistently, enabling a much smoother ride of the vehicle even at lower speeds.

Contact Breaker Vs CDI

A capacitive discharge ignition unit also called the CDI unit is the modern alternative for the age old contact breakers, which were quite crude with their functions and reliability.

The modern CDI is an electronic version of the contact breaker which uses sophisticated electronic parts for generating the required arching across the spark plug terminals.

The concept is not complicated at all, the section of the alternator provides the required 100 to 200V AC to the CDI circuit, where the voltage is intermittently stored and discharged by a high voltage capacitor through a few rectifying diodes.

These rapid bursts of high voltage discharges are dumped into the primary winding of an ignition coil where its appropriately stepped up to many thousands of volts for acquiring the required arcing, which ultimately functions as the igniting sparks across the connected spark plug contacts.

I have already discussed the basic electronic CDI circuit in one of my previous posts, though the circuit is extremely versatile, it depends and derives its operating voltage from the alternator. Since the alternator voltage depends on the engine speeds, the generated voltages tend to get affected with varying speeds.

At higher speeds it works fine, but at lower speeds, the alternator voltage also lowers, this results in an inconsistent sparking forcing the alternator and the engine to stutter.

This inconsistency ultimately affects the functioning of the CDI and the whole system starts getting hampered,  sometimes even causing the engine to halt.

The circuit of an enhanced capacitive discharge ignition circuit which is discussed here, eliminates the use of the alternator voltage for functioning, instead it utilizes the battery voltage for generating the required actions.


1) The first 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 dont 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... 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.


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.

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.


2) The whole concept for this electronic CDI can be understood by studying the shown circuit diagram below:

The diodes, the SCR and the associated components form a standard CDI circuit.

The high voltage of around 200V which needs to be fed to the above circuit is generated through an ordinary step down transformer connected the other way round.

The secondary winding of the transformer now becomes the primary and vice versa.

The low voltage primary winding is fed with high current pulsating DC generated by a standard IC555 circuit via a power transistor.

This pulsating voltage is stepped up to the required 200V and becomes the operating voltage for the attached CDI circuit.

The CDI circuit converts this 200V into bursts of high current for feeding the input winding of the ignition coil.

These rapid high current bursts are further amplified to many thousands of volts by the ignition coil and finally fed to the connected spark plug for the required arcing and the initiating the ignition of the vehicle.

As can be seen the input voltage is acquired from a 12V DC source which is actually the battery of the vehicle.

Due to this the generated sparks are very consistent without interruptions providing the vehicle a constant supply of the required ignition sparks irrespective of the vehicle situation.

The consistent sparking also makes the fuel consumption efficient, makes the engine less prone to wear and tear and enhances the overall mileage of the vehicle.

Use a 1K resistor at the base of TIP122...... 100 ohm is incorrectly shown

If you want the above circuit to be triggered by the alternator, the above design may be modified in the following way:

Use a 1K resistor at the base of TIP122...... 100 ohm is incorrectly shown, also remove the 1K resistor which goes to positive.

The above configuration may be further modified as shown in the following diagram, which appears to be the most appropriate way of implementing the proposed enhanced CDI circuit for all 2 and 3 wheelers.

Finalized Enhanced CDI Design with PWM Control

electronic CDI circuit with 12V battery operation

CDI PCB Circuit

CDI ignition PCB design

Parts List

All resistors are 1/4w unless stated

1K - 1
10K- 1
POT 10K - 1
100 Ohms 1/2 watt - 1
56 Ohms 1/2 watt - 1
Diodes 1N4007 - 10


1uF/25V - 1
0.01uF/50V Ceramic - 1
105/400V PPC - 1


IC 555 - 1

Mosfet IRF540 - 1
SCR - BT151

Transformer 0-12V/220V/1amp - 1

CDI ignition coil - 1

Video Clip showing the Test Result of the above shown electronic capacitive discharge circuit system

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