Capacitor Discharge Ignition(CDI) Working Principle, Its Advantage and Disadvantage

What is a CDI System? A Capacitor Discharge Ignition 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 Igntion System
Capacitor Discharge Ignition System

Working principle 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 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.

CDI System
CDI System

Working 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 the timing mark.

Typical Setup of Capacitive Discharge Igntion
Typical Setup of Capacitive 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 the 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 our 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.

Advantages of CDI

  • 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 the 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 shorter spark duration (about 50-80 µs).
  • The fast voltage rising makes CDI systems unaffected to shunt resistance.

Disadvantages of CDI

  • 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 the topic 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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