• Easy to build.
  • Advance curve slightly adjustable.
  • Need a working charging coil for the capacitor.
  • Don’t need a battery


AC-CDI Analog v2.2
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
2 pickups:
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)


Version 1.0:

Version 2.1:



– This ignition fitted a Scooter:

Analog CDI on a DR125:

Analog CDI on a XT400:


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.


14 Comments on "AC-CDI ANALOG"


  2. Hello,
    I’d be interrested to make on for my DR125, how do you connect it ? Some modifications to do ?
    Schematics :
    Thanks !

    • Hi,
      As you have only one pickup, connect it to GND and 36deg inputs.
      You pointed the connector [P-Br] on the schematic but it could be the other connector on the right too: [Bl-G] I don’t know !
      Check what wires goes to the pickup that look like that.
      Bear in mind that polarity is of the most importance to get the positive pulse in the first place.
      ie: connect GND to Bl and 36 to G
      or connect GND to G and 36 to Bl.
      A oscilloscope would be a BIG help to find what is what…

  3. Andy Van Herwynen | 13 March 2017 at 15 h 08 min | Reply

    Wow, this design and information may be exactly what I need for a CDI replacement on my Suzuki GN400. The GN400 has two trigger pickups, one at 10 degree BTDC and the other at 35 degree BDTC. The trigger pickups have single wire from each and ground at the mount plate. I like the idea the transition from the 10 BTDC to the 35 BTDC pickup is automatic and adjustable for the RPM.
    I have very good mechanical skills, I have done all my own work on my cycles for 35+ years. However, I have never built an electronic device. I do have a wiring schematic for my GN400 if that helps to determine how to best apply this CDI design.

    • As you described it, The Suzuki GN400 pickup wiring is exactly the same as Yamaha XT600 so the connection is straightforward.
      Connect the 10deg hot wire output on the 12deg input. The 35 on the 36 deg input and connect the common ground.
      But keep in mind that Suzuki pickups gives a NEGative pulse first !
      As my CDI use the POSitive pulse, this one come later (less then 10deg BTDC, less then 35deg BTDC) the advance will be lower than with the genuine CDI.

  4. Andy Van Herwynen | 13 March 2017 at 15 h 11 min | Reply

    Forgot to mention. I also have a CDI from a Suzuki SP500 that I thought may work as it is also an AC powered CDI with only one tripper pickup. However, it needs a trigger pickup with two wires, to ground at the CDI. The GN400 trigger pickup has only one wire, and it grounds at the pickup mount plate at the magneto. So I am not sure it would be adaptable to the ignition on the GN400.

    • The common wire can be grounded on pickup side or CDI side, that doesn’t matter.
      The key point is : Is there the same number of pickup? (10 & 35deg BTDC)

  5. i have a Hero honda cbz of 1999 model, which is 4 stroke single cylinder, 12 v ac (battery). the CDI is dead, No-where i am geting it. Please help me.

  6. bonjour Thierry
    sur la dernière image de cette page ou sont les 12 et 36 degrés?
    quelque chose m echappe (60-47=13 capteur A???)(80-87???? capteurB)
    cordialement, Yves

    • Bonjour Yves,
      Je n’ai jamais résolu ce mystere non plus !
      Comment 7° et 13° deviennent 12° et 36° ? Comme les capteurs analogiques reagissent avant que l’aimant soit pile en face, il y a forcement une difference de qq degres mais quand meme….
      Cdt Th

  7. Hi everyone.
    Original CDI circuit. Analog ignition timing.
    Yamaha YZ80-125 2T module dismantling result.

    Regards nyemi from hungary.

  8. Hi !
    Will this circuit works for honda cr125 1996?

    Thank you !!!

Leave a comment

Your email address will not be published.


9 + 4 =