The following post describes a simple yet enhanced 12V capacitive discharge ignition system 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.
The Circuit Concept
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
CDI PCB Circuit
All resistors are 1/4w unless stated
1K - 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