AC vs DC fired or pulsed CDI IGNITION SYSTEMS

Hey MO! What’s all this I hear about AC and DC fired or pulsed CDI’S? – Harried in Henderson


Ok first the short answer. – Here is the readers digest of how a CDI works

In all cases there is a magnet that passes a magnetic coil and induces a pulse in that coil which then is wired to the CDI and causes the small induced pulse to fire an  electronic device (there are more than 1 type but this all happens inside the CDI)  Which discharges the larger pulse stored in a capacitor.  This larger output pulse is wired to the coil where the large pulse is able to make the coil function as a voltage increasing transformer thereby producing the very high voltage necessary to create a spark across the gap of the spark plug.

Now for the 2 main type of CDI used in the 49 cc scooter market,  AC and DC  Both function  essentially the same.  With the AC unit, alternating plus and minus varying voltages are used to charge the capacitor described in the paragraph above.  The DC unit takes 12 volts more or less DC from the battery and that voltage is used to charge the capacitor described in the paragraph above.

Pros and Cons:

AC unit allows the engine to run without the need for a battery or charging system of any kind.

DC unit was created to allow higher RPM ranges due to the instantaneous availability of the Charging current.  Where the AC one has lots of complex and difficult to explain things like coil flux density and residual decay etc.

The wiring is different also usually but not always, the ac ones have a rectifier or diode ahead of the CDI and the DC ones just connect directly to the battery.

The way to tell for sure is to turn on the key and see if 12v is present at the CDI assuming all else is correct.

Some units that are DC include:

Kymco Agility 50, Diamo Velocity, Vento Triton, Schwinn, Tomos, and other high end Chinese, Korean, and Taiwanese scooters.

One easy way to tell is the size of your CDI unit. The AC fired CDI will be much smaller than the DC fired CDI, as shown in this picture. Also, you have two plugs on the CDI, one with four, the other two. If the two wire plug actually has only one wire in it, it’s a DC. If it has two wires in it, it is an AC fired. The number of wires is only a loose guide. The size of the box, along with this guide is a better determination. An AC box will be smaller (see pic) and will measure about 1 1/2 by 2 1/2 approximately, the DC box will be much larger as it has a DC-DC converter in it. Such words as AC or DC pulsed or AC or DC triggered are the same thing as the AC or DC fired units.  One quick warning on performance units, with the advanced angle of our CDI’s your engine may actually run worse if it’s not running right or is carbureted too lean. I’d recommend installing a larger main jet to take full advantage of these units.Here is  somewhat simplified checklist to determine type:

Headlights turn on with key, DC.  Headlights turn on with engine,AC
Engine does not run without batter, DC. Engine runs without battery, AC
CDI box is pretty large, DC. CDI box is smaller, AC (note: our new digital DC units are smaller)
There is one wire on the two part plug, DC. There is two wires on the two part plug, DC.

Hope this helps. MO!

Here is a standard ignition circuit for a chinese scooter. Note that the red wire from the stator to the CDI will not be on a DC fired system. The kill switch wire will instead be the positive power source from the igntion key switch.