Simple Capacitive Discharge Ignition (CDI) Circuit

Think of a tiny lighter for engines!

A CDI circuit is an electronic system that packs a punch.

It stores electricity like a balloon full of air, then releases it all at once to create a hot spark that ignites the fuel in things like motorcycles, lawnmowers and even some older cars.

Circuit Working:

Parts List:

This post brings into a simple and versatile way to create a Capacitive Discharge Ignition CDI system using a regular ignition coil and a solid state SCR circuit.

The ignition system is crucial for any car because it is responsible for igniting the fuel mixture which gets the engine running.

In the past, this job was done by a circuit breaker.

Today, most vehicles use a more dependable and efficient electronic ignition system called capacitive discharge Ignition CDI.

Here is a breakdown of how a CDI unit functions:

The CDI system receives two voltage inputs.

One is a high voltage 100V to 200V AC coming from the alternator and the other is a low voltage pulse 10V to 12V AC from a pickup coil.

The high voltage gets converted to DC and used to charge a high voltage capacitor.

The low voltage pulse triggers an SCR silicon controlled rectifier that discharges the stored voltage in the capacitor into the primary winding of the ignition coil.

The ignition coil steps up this discharged voltage to thousands of volts sending it to the spark plug to create the spark that ignites the engine.

Taking a Closer Look at CDI Circuit Operations:

As the name suggests the ignition system creates the spark needed to ignite the fuel mixture and start the engine.

This spark is produced by high voltage electricity jumping across a gap.

We need transformers to generate high voltages.

The alternator in most vehicles does not provide enough voltage on its own.

The ignition coil, a familiar component in cars is specially designed to increase the incoming voltage significantly.

The alternators voltage might not have enough current for the ignition coil to function properly.

The CDI unit acts like a storage container, accumulating and releasing the alternators power in quick bursts to create a powerful, high current output.

This revised version clarifies terminology, shortens explanations and improves the flow of information.

It also removes unnecessary conversational phrases and focuses on a more technical explanation.

Formulas:

In order to produce a high voltage spark, a CDI circuit basically stores electrical energy in a capacitor and quickly discharges it through the ignition coils primary winding.

Here are a few basic equations that may shed light:

Capacitor Charging:

Capacitor voltage (Vc) over time (t):

V_c(t) = V_s * (1 – _exp(-t / (R * C)))

where,

• The supply voltage is represented by Vs.
• The charging resistance is denoted by R.
• C stands for capacitance and
• exp for exponential function

Capacitor Energy Storage:

Energy stored in a capacitor E:

E = 0.5 * C * V²

where,

• V is the capacitor voltage and
• C is the capacitance.

Inductor Voltage and Current:

Voltage across an inductor Vl:

V_l = L * (d_i / d_t)

where,

• Di/dt is the rate of change of current.
• L is the inductance.

Note:

With adherence to these recommendations and comprehension of the fundamental formula involved, you will be able to design and construct a CDI circuit that meets your particular needs while experimenting.

In light of your design limitations and performance objectives, change component values and configurations as necessary.

How to Build:

To build a Simple Capacitive Discharge Ignition CDI Circuit follow the below mentioned steps:

• Connect the cathode of SCR to ground.
• Connect a diode 1N4007 between ground and capacitor 105 400V.
• Connect two diodes 1N4007 in series between the ground and the anode of the SCR.
• Connect a 100Ω resistor between gate and ground of SCR.
• Connect one end of 56Ω resistor to SCR gate and other end to high voltage AC from alternator.
• Connect one end of capacitor 105 400V to the anode of SCR and other end to the primary of the ignition coil.

Conclusion:

A Capacitive Discharge Ignition CDI circuit is an advanced electronic system that creates a powerful spark for igniting engines.

It offers a hotter spark, improved efficiency and easier starting compared to traditional systems.

However, its complexity and sensitivity to damage make it a better fit for specific applications like motorcycles and small engines.

References:

Capacitor discharge ignition

Low-Cost Automotive Capacitive Discharge Ignition (CDI) Coil for Low Frequency Ozone Generator