This is achieved by having a stepped or shaped coil core that has a larger gapĪt its leading edge compared to the trailing edge – see Fig.3.Īt low speeds the coil voltage required for triggering isĭeveloped at the trailing edge of the magnet but as revs increase, the leadingĮdge of the magnet is able to induce more voltage in the coil and so firing Some engines do incorporate RPM advance using a special triggerĬoil and magnetic core design that advances the firing edge with increasing RPM. So it provides a fixed timing from the trigger coil – most common with small Note that the CDI Module does not incorporate RPM advance and Much quicker than the time between sparks, even in a high-revving engine. If C1 is 1mF, then it will charge in about 350ms –
#Ted williams outboard motor magneto capacitor capacitor generator#
Typically, the generator coil delivers about 1A in charging theĬapacitor up to about 350V. The ignition coil develops a reverse current flow via diode D2 to partially Once the spark plug is extinguished, the collapsing field of Resulting current through the ignition coil causes a high voltage to beĭeveloped across the secondary winding of the ignition coil, to fire the spark A trigger pulse (from the trigger coil) then turns on the SCR and this quickly discharges C1 by allowing current to flow back through the coil primary.Ĭ1 is discharged when the SCR is subsequently triggered,Īllowing current to flow back through the ignition coil primary. Initially, the generator coil charges C1 to a high voltage (via diode D1). The current path is shown in red asįig.2: how the CDI module works. Positive voltage from the generator then charges C1 via diode D1 and the primary Initially, the SCR is off and capacitor C1 is discharged. After triggering, the SCR switches off when the current It then conducts andīehaves like a diode. Trigger voltage is applied between its gate and cathode. It is normally a high impedance until a small The ignition coil, a capacitor (C1) and a Silicon Controlled Rectifier (SCR). High-voltage supply from the generator to prevent ignition.įig.2 shows how CDI works. The CDI module), while the trigger coil provides the signal to dump theĬapacitor’s high voltage charge into the ignition coil. The generator (magneto) coil provides the high voltage to charge a capacitor (in The generator (magneto) coil provides a high voltage to charge a capacitor in the CDI module, while the trigger coil provides the timing signal to dump the capacitor's high voltage charge into the ignition coil.įig.1 shows the connections required for a typical CDI module. You are unwilling to fork out lots of hard cash for a genuine replacementįig.1: how a typical CDI module is connected. This CDI Module uses cheap and readily available parts, it may be worth a try if While some tests can be performed to check for suitability, weĬannot guarantee that the module will work for every engine. Polarity for voltage generation and are therefore unsuitable for our module. In a similar way but there are variations in design that use the opposite Provide the high-voltage and the firing point. The CDI Module described here may be used to replace a failedįactory unit for an engine that incorporates a generator and trigger coil to Ignition coil, it is most likely to be the CDI module itself and then you willįind that the replacement can be very expensive. While the failure can be within the flywheel generating coils or the The one drawback is that CDI systems don’t last forever – theyĬan fail. Points which were inevitably subject to wear and required periodic cleaning, Not only does the CDI deliver higher spark energy but it also dispenses with the Used to drive lights and ancillaries but this is used independently of theĬDI is a great improvement on the old magneto ignition systems. The high-voltage capacitor is charged directlyįrom a generator located on the flywheel. Those days, a great many small petrol engines use a Capacitorĭischarge Ignition (CDI) module. When you see the circuit components involved. It is even harder to justify the prices charged It can be a real shock to front up to your local dealer and find Module for motorbikes, outboards and other small petrol motors. Readers have been asking us for years to design a drop-in CDI
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