This invention relates to a dc power supply circuit for the control unit of a capacitive discharge ignition unit (CDI Unit) deriving input power from the excitor coil of an alternator, such as, the magneto of an i.e. engine which is driven by the engine shaft.
The primary function of the CDI Unit is to store adequate charge in the CDI capacitor and discharge the same through the ignition coil at an appropriate moment which is decided by the input from the pulser coil and the internal control unit.
According to this invention, the dc power supply circuit, for the control unit of a CDI Unit, deriving input power from an excitor coil of the alternator of an i.e. engine, from a single wire input during the positive wave of the excitor coil ac supply, comprises a transistor; a zener diode connected to the base of the transistor and a first capacitor connected to the emitter of the transistor, to form a switching circuit, such that whenever the sum of the voltages across the first capacitor and the base-emitter voltage equals the voltage across the zener diode, the transistor suddenly switches off, stopping the charging current flowing through the excitor coil, to produce an "inductive kick", thus boosting the input voltage itself and avoiding a drop in the voltage of the second capacitor, namely, the capacitor of the CDI Unit.
Reference will now be made to the accompanying drawings which illustrat

In Fig. 1 the known form of a single wire dc power supply circuit (solution-1)
In Fig. 2 the known form of a two wire dc power supply circuit currently
used
(solution - 2)
In Fig. 3 the proposed form of a single wire dc power supply circuit, by way of example, according to this invention (solution-3)
In Figs. 4 to 8 graphs which are referred to hereunder.
For proper working of the CDI Unit, a low voltage dc power supply and adequate charging of the CDI capacitor C2 is required.
This invention relates to the development of a dc power supply circuit for the control unit of the CDI Unit as applied to the ac type of CDI Unit deriving input power from a excitor coil used in two/three wheelers and general purpose petrol engines. The excitor coil is provided in the alternator (usually called flywheel magneto) driven by the engine shaft.
For simple control units which require low capacity dc power supply usually of the order of 5 mA, the solution is to derive the dc supply from the excitor coil ac wave input 1 during the positive half cycle as shown in Fig.l Solution 1 wherein diode Dl resistor Rl zener diode Zl and capacitor CI are used. The capacitor CI gets charged through diode Dl and resistor Rl

and powers the control unit. The value of the resistor Rl is high enough to minimise the loading of the excitor coil to get adequate charging voltage at C2. Typical wave form of such a circuit is shown in wave form 1 (Fig. 4).
For control units which require higher capacity dc power supply usually due to the use of a microcontroller or complex control circuits, the power supply requirement goes up to the order of 30 mA and if the circuit of Fig. 1 is used, the value of Rl needs to be reduced to draw higher current resulting in lowering of charging voltage of the CDI capacitor C2 thus affecting the primary function of CDI Unit - see wave form 2 Fig. 5 To overcome the above, the known solution is to derive the dc supply during the negative half cycle of the excitor ac wave as shown in Fig. 2, wherein the capacitor charging voltage is not affected. See waveform 3 in Fig.6
The present invention allows a higher capacity dc supply to be derived from the excitor coil using single wire (input 1 only) during positive wave of excitor coil ac supply, that is, the same wave which is used to charge the
CDI capacitor C2 without sacrificing the charging voltage of C2.
*
The power supply circuit comprises diode Dl resistors Rl and R2 , zener diode Zl transistor Tl and capacitor CI.
Initially the voltage across the capacitor C1 is zero and the said capacitor is made to charge through Tl. When the voltage of capacitor CI reaches a voltage VC1 where VC1 + VBE = VZ1, the transistor switches off abruptly stopping the charging current flowing through the excitor coil. This produces an

inductive kick which increases the excitor coil voltage at the input 1 (wave form 4 Fig. 7).
Thus the circuit while deriving the required power supply for the control unit also boosts the input voltage itself thus avoiding drop in C2 capacitor voltage.
The terms and expressions herein are of description and not of limitation, there being no intention of excluding any equivalents of the features illustrated and described, but it is understood that various other embodiments of this invention are possible without departing from the scope and ambit of this invention.

We Claim:
1. A dc power supply circuit, for the control unit of a CDI Unit,
deriving input power from an excitor coil of the alternator of an
i.e. engine, from a single wire input during the positive wave of
the excitor coil ac supply, comprising a transistor; a zener diode
connected to the base of the transistor and a first capacitor
connected to the emitter of the transistor, to form a switching
circuit, such that whenever the sum of the voltages across the
first capacitor and the base-emitter voltage equals the voltage
across the zener diode, the transistor suddenly switches off,
stopping the charging current flowing through the excitor coil,
to produce an "inductive kick", thus boosting the input voltage
itself and avoiding a drop in the voltage of the second"
capacitor, namely, the capaeitor of the CDI Unit.
A dc power supply circuit as claimed in Claim 1 wherein the
switching circuit comprises a diode connected to resistors in the
base and collector circuits of the transistor.
A dc power supply circuit, for the control unit of a CDI Unit,
deriving input power from an excitor coil of the alternator of
an i.e. engine, from a single wire input during the positive wave

of the excitor coil ac supply, substantially as herein described and illustrated by Figs. 3 to 7 of the accompanying drawings.