Schematics > Automotive > Capacitor Discharge Ignition Circuit (CDI)
Capacitor Discharge Ignition Circuit (CDI)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.
Capacitor Discharge Ignition Circuit
Title: Capacitor Discharge Ignition Circuit (CDI)
Source: unknown
Published on: 2007-07-21
Reads: 1284
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