What is a Capacitor Discharge Ignition (CDI) & Its Working

At present, many things have been changed because of technology. The researchers invented the CDI (Capacitive Discharge Ignition) system for SI (Spark Ignition) Engine using electronic ignition & contact point ignition. This system includes a pulse control circuit, spark plug, pulse generation circuit, main charge & discharge capacitor coil, etc. There are different types of ignition systems where different classic ignition systems are developed to use in different applications. These ignition systems are developed using two groups like CDI (Capacitor Discharge Ignition) systems as well as IDI (Inductive Discharge Ignition) systems.

What is a Capacitor Discharge Ignition System?

The short form of the Capacitor discharge ignition is CDI which is also known as thyristor ignition. It is one kind of automotive electronic ignition system, used in motorcycles, outboard motors, chainsaws, lawnmowers, turbine-powered aircraft, small engines, etc. It was mainly developed to conquer the long charging times which are connected through high inductance coils employed for IDI (inductive discharge ignition) systems to make the ignition system more appropriate for high engine speeds. The CDI utilizes capacitor discharge current toward the coil for firing the spark plugs.

Capacitor Discharge Igntion System
Capacitor Discharge Ignition System

A Capacitor Discharge Ignition or CDI is an electronic ignition device that stores an electrical charge and then discharges it through an ignition coil in order to produce a powerful spark from the spark plugs in a petrol engine. Here the ignition is provided by the capacitor charge. The capacitor simply charges and discharges within a fraction of time making it possible to create sparks CDIs are commonly found on motorbikes and scooters.

Capacitor Discharge Ignition Module

The typical CDI module includes different circuits like charging & triggering, a mini transformer & the main capacitor. The system voltage can be increased from 250V to 600V through a power supply in this module. After that, the flow of electric current will be there toward the charging circuit so that capacitor can be charged.

The rectifier within the charging circuit can avoid the discharge of the capacitor before the ignition moment. Once the triggering circuit gets the triggering signal, then this circuit will stop the working of the charging circuit & allows the capacitor for discharging its o/p fast toward the ignition coil of low inductance.
In Capacitor discharge ignition, the coil works like a pulse transformer rather than an energy storage medium because it does within an inductive system. The o/p of the voltage toward the spark plugs is extremely reliant on the CDI design.

The insulation capacities of voltages will exceed the existing ignition components which can cause components failure. Most of the CDI systems are designed to provide extremely high o/p voltages however this is not constantly helpful. Once there is no signal for triggering then the charging circuit can be re-connected for charging the capacitor.

Working Principle of a CDI System

A capacitor discharge ignition works by passing an electrical current over a capacitor. This type of ignition builds up a charge quickly. A CDI ignition starts by generating a charge and storing it up before sending it out to the spark plug in order to ignite the engine.

This power passes through a capacitor and is transferred to an ignition coil that helps boost the power by acting as a transformer and allowing the energy to pass through it instead of catching any of it.

The CDI ignition systems, therefore, allow the engine to keep running as long as there is a charge in the power source. The block diagram of CDI shown below.

Construction of Capacitor Discharge Ignition

A Capacitor Discharge Ignition consists of several parts and is integrated with the ignition system of a vehicle. The foremost parts of a CDI include the stator, charging coil, hall sensor, flywheel, and timing mark.

Typical Setup of Capacitor Discharge Ignition
Typical Setup of Capacitor Discharge Ignition

Flywheel and Stator

The flywheel is a large horseshoe permanent magnet rolled into a circle that turns-ON the crankshaft. The Stator is the plate holding all of the electrical coils of wire, which is used to power ON the ignition coil, bike’s lights, and battery charging circuits.

Charging Coil

The charging coil is one coil in the stator, which is used to produce 6 volts to charge the capacitor C1. Based on the flywheel’s movement the single pulsed power is produced and is supplied to the sparking plug by the charging coil to ensure the maximum spark.

Hall Sensor

The Hall Sensor measures the Hall effect, the instantaneous point where the flywheel’s magnet changes from a north to a south pole. When the pole change occurs, the device sends a single, tiny pulse to the CDI box which triggers it to dump the energy from the charging capacitor into the high voltage transformer.

Timing Mark

The timing mark is an arbitrary alignment point shared by the engine case and stator plate. It indicates the point at which the top of the piston’s travel is equivalent to the trigger point on the flywheel and stator.

By rotating the stator plate left and right, you effectively change the trigger point of the CDI, thus advancing or retarding your timing, respectively. As the flywheel turns fast, the charging coil produces an AC current from +6V to -6V.

The CDI box has a collection of semiconductor rectifiers that connected to G1 on the box allows only the positive pulse to enter the capacitor (C1). While the wave entering into the CDI, the rectifier allows only the positive wave.

Trigger Circuit

The trigger circuit is a switch, probably using a Transistor, Thyristor, or SCR. This triggered by a pulse from the Hall Sensor on the stator. They only allow current from one side of the circuit until they are triggered.

Once Capacitor C1 is fully charged, the circuit can be triggered again. This is why there is timing involved with the motor. If the capacitor and stator coil were perfect, they would charge instantaneously and we can trigger them as fast as we wish. However, they require a fraction of a second to full charge.

If the circuit triggers too fast, then the spark from the spark plug will be enormously weak. Certainly, with the higher accelerating motors, we may have the triggering faster than the capacitor full charge, which will affect performance. Whenever the capacitor is discharged, then the switch turns itself off, and the capacitor charges again.

The trigger pulse from the Hall sensor feeds into the gate latch and allows all the stored charge to rush through the primary side of the high-voltage transformer. The transformer has a common ground between the primary and secondary windings, known as an auto step-up transformer.

Therefore, as if we increase the windings on the secondary side, you will multiply the voltage. Since a spark plug needs a good 30,000 volts to sparks, there must be many thousands of wraps of wire around the high voltage or secondary side.

When the gate opens and dumps all the current into the primary side, it saturates the low-voltage side of the transformer and sets up a short but immensely magnetic field. As the field reduces gradually, a large current in the primary windings forces the secondary windings to produce extremely high voltage.

However, the voltage is now so high that it can arc through the air, so rather than being absorbed or retained by the transformer, the charge travels up the plug wire and jumps the plug gap.

When we want to shut down the motor engine, we have two switches the key switch or the kill switch. The switches ground out the charging circuit so the entire charging pulse is sent to the ground. Since the CDI can no longer charge, it will cease to provide the spark and the engine will slow to a stop.

Different Types of CDI

CDI modules are classified into two types which are discussed below.

AC-CDI Module

The electrical source of this module gets only from the AC generated through the alternator. This is the basic CDI system used in small engines. So, not all the ignition systems which have small engines are not CDI. Some of the engines use magneto ignition namely older Briggs as well as Stratton. The whole ignition system, points & coils are below the magnetized flywheel.

Another type of ignition system that is most frequently used in small motorcycles in the year 1960 – 70 known as Energy Transfer. A strong DC current pulse can be generated by a coil below the flywheel because the flywheel magnet goes over it.

These DC current supplies throughout a wire toward an ignition coil placed at the external of the engine. Sometimes, the points were below the flywheel for engines with two-stroke & usually on the camshaft for 4-stroke engines.

This explosion system works like all types of Kettering systems where the opening points activate the collapse of the magnetic field within the ignition coil and generates a high voltage signal to flow throughout the spark plug wire toward the spark plug. The waveform output of the coil is examined through an oscilloscope whenever the engine was turned, then it appears like AC. As the coil’s charge-time communicates with a complete revolution of the crank, the coil actually ‘sees’ simply DC current for the external ignition coil’s charging.

Some types of electronic ignition systems will exist so these are not capacitor discharge ignition. These types of systems utilize a transistor for switching the charging current toward the coil ON & OFF at suitable times. This removes the trouble of burned as well as worn points to provide a hotter spark due to the quick voltage raise as well as collapse time within the ignition coil.

DC-CDI Module

This kind of module works with the battery & thus an extra DC/AC inverter circuit is used within the capacitor discharge ignition module to increase the voltage from 2V DC – 400/600 V DC to make the CDI module somewhat larger. But, vehicles that utilize DC-CDI type systems will have more accurate ignition timing, as well as the engine, can be activated more simply once it gets cold.

Which is the best CDI?

There is no best capacitor discharge system as compared with the other however each type is best in various conditions. The DC-CDI type system mainly works fine in regions wherever there are very cold temperatures as well as exact during ignition. On the other hand, the AC-CDI is simpler & not often runs into trouble because it is lesser & handy.

The capacitor discharge system is insensible toward shunt resistance & can blaze several sparks immediately and thus great to utilize in a variety of applications without any delay once this system is activated.

How does Ignition System Work in Vehicles?

In vehicles, there are different types of ignition systems used like contact breaker, breaker less, and capacitor discharge ignition.

The contact-breaker ignition system is used to activate the spark. This kind of ignition system is used in an earlier generation of vehicles.

The breaker-less is also known as contactless ignition. In this type, the designers utilize an optical pickup otherwise electronic transistor like a switching device. In modern cars, this kind of ignition system is used.

The third type is the capacitor discharge ignition. In this technology, the capacitor suddenly discharges the energy which is stored in it using a coil. This system has the capacity to generate the spark in fewer conditions wherever the usual ignition may not function. This kind of ignition will assist in conforming to the regulations of emission control. Because of the many pros it provides, it is used in current automobiles as well as motorcycles.

Whenever you switch the key to activate the engine in the vehicle, then the ignition system will transmit high voltage toward the spark plug in the cylinders of an engine. Because that energy arcs at the underneath of the plug across the gap, a flame-front will ignite the mixture of air or fuel. The ignition system in the car can be divided into two separate electrical circuits like the primary & secondary. Once the ignition key is activated, then a flow of current with less voltage from the battery can supply throughout the primary windings in the ignition coil, throughout the breaker points as well as reverse to the battery.

How do I Test my CDI Ignition?

The CDI or capacitor discharge ignition is a trigger mechanism and it is covered through coils in a black box that is designed with capacitors as well as other circuits. In addition, it is an electrical ignition system, used in outboard motors, motorcycles, lawnmowers & chainsaws. It overcomes the long charging times, frequently linked through inductance coils.

A millimeter is used to access as well as test the CDI box status. Checking the CDI working status is very important whether it is good or faulty. As it controls sparks plugs & fuel injectors, so it is accountable to make your vehicle work properly. There are many reasons to become CDI faulty like faulty charging system & aging.

When CDI is faulty and connected to the ignition, then the vehicle may get into trouble because capacitor discharge ignition is accountable for storing spark power over the spark plug within your vehicle. So identifying of CDI is not easy because the faulty symptoms are visible on your system box might direct to a different way. So CDI fails to cause a spark when it is faulty so a faulty CDI can cause rough running, misfires & ignition troubles & stall the motor.

So these are the main CDI faults, so we have to be extra careful about the trouble affecting your CDI box. Once your fuel pump is defective otherwise the spark plugs & coil pack are defective, then we can face a similar types of defective symptoms. So, a millimeter is essential to diagnose these faults.

Advantages of CDI

The advantages of CDI include the following.

  • The major advantage of CDI is that the capacitor can be fully charged in a very short time (typically 1ms). So the CDI is suited to an application where insufficient dwell time is available.
  • The capacitor discharge ignition system has a short transient response, a fast voltage rise (between 3 to 10 kV/ µs) compared to inductive systems (300 to 500 V/ µs), and a shorter spark duration (about 50-80 µs).
  • The fast voltage rising makes CDI systems unaffected to shunt resistance.

Disadvantages of CDI

The disadvantages of CDI include the following.

  • The capacitor discharge ignition system generates huge electromagnetic noise and this is the main reason why CDIs are rarely used by automobile manufacturers.
  • The short spark duration is not good for lighting relatively lean mixtures as used at low power levels. To solve this problem many CDI ignitions release multiple sparks at low engine speeds.

I hope you have clearly understood an overview of Capacitor Discharge Ignition(CDI) Working Principle, It’s Advantage, and Disadvantage. If you have any queries on this topic or on any Electronic and Electrical projects leave the comments below. Here is a question for you What is the role of the Hall sensor in the CDI System?


  1. Does the cdi advance/retard the spark when accelerating /decelerating?

    1. Tarun Agarwal says:

      Hi Allister
      If the circuit triggers too fast, then the spark from the spark plug will be enormously weak. Certainly, with the higher accelerating motors, we may have the triggering faster than the capacitor full charge, which will affect performance. Whenever the capacitor is discharged, then the switch turns itself off and the capacitor charges again.

    2. Tarun Agarwal says:

      Hi Allister
      In CDI, a capacitor charged to a high-voltage is switched across the primary of the ignition coil for generating a spark impulse.

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