- Easy to build.
- Advance curve slightly adjustable.
- Need a working charging coil for the capacitor.
- Don’t need a battery
|To make the PCB||PCB.pdf||Parts list|
|To build the CDI||Schematic.pdf||components.jpg||wiring.gif|
Printed Circuit Boards and Ready to use CDI are available for sale !
This circuit replace the genuine CDI.
The charging coil produces a alternative voltage around 50Vac at idle to 200Vac at high RPM.
Output from Charging coil:
This tension is rectify by the bridge B1, and loads C1 to a average voltage of 270Vcc.
Tension at the SCR anode (at 1700rpm):
Both pickups gives AC pulses from ±10v (at idle) until ±20v (at WOT)
The 12° pickup (white/green) is used for idle and low RPM while 36° pickup (white/red) is used for high RPM.
(The green wire is the common of the both sensors)
At startup and idle, positive pulses coming from the 12° pickup goes through D1-R2 then trigger the SCR T1.
T1 discharge the energy stored in the capacitor C1 into the primary of the ignition coil, which delivers high voltage to the spark plug.
At higher RPM, the positive pulses coming from 36° pickup goes through D2, R3, R2 and trigger the SCR.
There is a interesting video describing how a CDI works.
This CDI doesn’t have seamless advance, it jump directly from 12° to 36° BTDC.
The moment where the CDI jump from Low advance (12°) to Full advance (36°) is adjustable by resistor R3
– The threshold is adjustable from 2000 to 5000rpm
Start tuning with R3 at the maximum value (totally screw) which is equivalent to 5000RPM then slowly unscrew it to lower the resistance, therefore the RPM where it switch.
If you are unable to use the full range of R3, that mean your pickup provide a voltage less then 30volts and you cannot use 100% of R3 rotation.
In this case increase R3 sensitivity by solding the gap of the jumper SJ1.
You wants R5 resistor be on parallel with R3
|This ignition need either
|or only one pickup|
|1st pickup at 36° BTDC:
and 2nd pickup at 12° BTDC:
|Positive than negative:
|But not negative than positive!
If the pickup is not internally tied to ground inside the stator, you can reverse the polarity (NP to PN) by crossing the wires.
If the bike has only one pickup, connect it to the 36º input and leave the 12º input unconnected
With this short, we get this equivalent circuit:
D1 suppress the negative wave.
D2 allows the positive wave to pass.
D3 kills the negative peaks of tension
produced when the thyristor cut off
Tested good on:
- Yamaha YZ125 (1987)
- Yamaha XT125 (1982)
- Yamaha XT400 (5y6)
- Yamaha XT550
- Yamaha SRX600
- Yamaha DTMX125
- Aprilia Climber 280R (1994)
– This ignition fitted a Scooter:
Analog CDI on a DR125:
Never run the engine if the exit of the ignition is disconnected from the coil or if the ground is disconnected (PAD1 or PAD2 not connected) otherwise the tension rises over 1000volts and destroy the SCR.
Never run the engine without any spark plug: due to the high secondary voltage, it is possible to damage the internal insulation of the secondary coil.
This simple unit is design for XT, which have 2 pulsars: one for idle, one for middle/high rpm.
KLR and some others bikes have only one pickup for middle/high rpm range and there are some electronics inside their black box to delay the pulse at idle.
With this unit, the spark will appear too soon for the KLR. Therefore you’ll have kick back when kick start and (if it’s starts!), the engine will bang a lot at idle (this can damage the connecting rod)
I disclaim all responsibility !!
If it’s ok, well done! you made a success of your first electronic ignition.
You can use it to verify that your problem is a faulty CDI and as a spare unit.
The pulse from second sensor (sensor A and magnet A on the left drawing) is early 12 ° before TDC.
(Reminder: the TDC is finds when the sign like a H engraved on the rotor is view by the small opening on the left crankcase.) The advance is sufficient to start the motor straight.