Capacitor Discharge Ignition

Description: The CDI ignition circuit produces a spark from an ignition coil by discharging a capacitor across the primary of the coil. A 2uF capacitor is charged to about 340 volts and the discharge is controlled by an SCR. A Schmitt trigger oscillator (74C14) and MOSFET (IRF510) are used to drive the low voltage side of a small (120/12 volt) power transformer and a voltage doubler arrangement is used on the high voltage side to increase the capacitor voltage to about 340 volts. More: A similar Schmitt trigger oscillator is used to trigger the SCR about 4 times per second. The power supply is gated off during the discharge time so that the SCR will stop conducting and return to its blocking state. The diode connected from the 3904 to pin 9 of the 74C14 causes the power supply oscillator to stop during discharge time. The circuit draws only about 200 milliamps from a 12 volt source and delivers almost twice the normal energy of a conventional ignition circuit. High voltage from the coil is about 10KV using a 3/8 inch spark gap at normal air temperature and pressure. Spark rate can be increased to possibly 10 Hertz without losing much spark intensity, but is limited by the low frequency power transformer and duty cycle of the oscillator. For faster spark rates, a higher frequency and lower impedance supply would be required. Note that the ignition coil is not grounded and presents a shock hazard on all of its terminals. Use CAUTION when operating the circuit. An alternate method of connecting the coil is to ground the (-) terminal and relocate the capacitor between the cathode of the rectifier diode and the positive coil terminal. The SCR is then placed between ground and the +340 volt side of the capacitor. This reduces the shock hazard and is the usual configuration in automotive applications.

The CDI (Capacitor Discharge Ignition) circuit operates by utilizing a charged capacitor to produce high-voltage sparks necessary for ignition in internal combustion engines. The primary component of this circuit is a 2uF capacitor that is charged to approximately 340 volts. The discharge of this capacitor is managed by a silicon-controlled rectifier (SCR), which allows for precise control over the timing and intensity of the discharge.

At the low voltage side, a Schmitt trigger oscillator, specifically the 74C14 IC, generates a square wave signal that drives a MOSFET (IRF510). This MOSFET functions as a switch to control power to a small transformer that steps down the 120V AC to 12V. The transformer is connected to a voltage doubler circuit on the high voltage side, which effectively doubles the voltage across the capacitor, enabling it to reach the desired 340 volts.

The SCR is triggered by a secondary Schmitt trigger oscillator, which operates at a frequency of about 4 Hz, allowing for the discharge of the capacitor to produce a spark. During the discharge cycle, the power supply to the circuit is disabled, ensuring that the SCR ceases conduction and returns to its non-conductive state.

The circuit draws approximately 200 milliamps from a 12V source, achieving an energy output that is nearly double that of traditional ignition systems. The high voltage output from the ignition coil can reach around 10kV, sufficient to create a spark across a 3/8 inch gap under normal atmospheric conditions. It is noted that the spark rate can be increased to about 10 Hz without significant loss of spark intensity; however, this is constrained by the transformer’s low frequency response and the oscillator's duty cycle. For increased spark rates, a power supply with higher frequency capabilities and lower impedance would be necessary.

Safety considerations are paramount when operating this circuit, as the ignition coil is not grounded, posing a shock hazard. Caution is advised when handling the circuit. An alternative configuration involves grounding the negative terminal of the ignition coil and relocating the capacitor to connect between the cathode of the rectifier diode and the positive terminal of the coil. This change positions the SCR between ground and the positive side of the capacitor, effectively reducing shock hazards and aligning with common automotive practices.The CDI ignition circuit produces a spark from an ignition coil by discharging a capacitor across the primary of the coil. A 2uF capacitor is charged to about 340 volts and the discharge is controlled by an SCR. A Schmitt trigger oscillator (74C14) and MOSFET (IRF510) are used to drive the low voltage side of a small (120/12 volt) power transformer and a voltage doubler arrangement is used on the high voltage side to increase the capacitor voltage to about 340 volts.

A similar Schmitt trigger oscillator is used to trigger the SCR about 4 times per second. The power supply is gated off during the discharge time so that the SCR will stop conducting and return to it's blocking state. The diode connected from the 3904 to pin 9 of the 74C14 causes the power supply oscillator to stop during discharge time.

The circuit draws only about 200 milliamps from a 12 volt source and delivers almost twice the normal energy of a conventional ignition circuit. High voltage from the coil is about 10KV using a 3/8 inch spark gap at normal air temperature and pressure.

Spark rate can be increased to possibly 10 Hertz without losing much spark intensity, but is limited by the low frequency power transformer and duty cycle of the oscillator. For faster spark rates, a higher frequency and lower impedance supply would be required. Note that the ignition coil is not grounded and presents a shock hazard on all of it's terminals. Use CAUTION when operating the circuit. An alternate method of connecting the coil is to ground the (-) terminal and relocate the capacitor between the cathode of the rectifier diode and the positive coil terminal.

The SCR is then placed between ground and the +340 volt side of the capacitor. This reduces the shock hazard and is the usual configuration in automotive applications.