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August 20, 2010

DC-CDI schematic (updated)

DC-CDI counterpart of AC-CDI is an ignition analog or digital that uses low voltage external power supply to work. the difference AC-CDI needs external High Voltage COIL no battery configuration whereas DC-CDI needs a battery without HV COIL.
DC-CDI schematic diagram

Looking at the picture on the left, it is a complete schematic diagram of a dc-cdi. Its a 4-pin system, comprising pick-up input, battery +12 volts in, Gnd, and Ignition coil out pins. There is no High Voltage  input pin like those of AC-CDI. If it will be differentiated with the AC-CDI here, you will notice theres a transformer diagram on this schematic.The circuit on the RED BOX comprises the internal High Voltage generator of a DC-CDI, where the 12 volts battery in will be converted to 200-400 volts depending on the design of the inverter. This design uses two transistor dc-ac converter with external on/off circuit to charge and discharge the capacitor BLUE BOX via SCR. the HV converter is controlled by the circuit on GRAY box, that as soon as the pulse conditioning circuit sense the pick-up coil will send triggering signal on both SCR and HV oscillator respectively to turn them on and off at the right time.

CDI or Capacitor Discharge Ignition, from the name itself, capacitor is very critical during its operation, must handle charge and discharge time as well as heat being generated by the whole circuit inside the block box, that is why manufacturer uses special capacitors specifically made for ignitions, and not just like those mylar capacitor found on some electronic circuits. They are rated 400 to 630 volts. ranging from .47uf up to 2.2uf. Value is also so critical in every applications and designs. In this circuit, they use 1uf / 400 volts with HV out of 200 volts will equals to 20 mj.

Capacitor Energy:
Energy stored in capacitor is a function of V and Capacity:  
E (joules)= V^2 * C * 1/2.
For example: for 1uF capacitor, charged at 200 volt, E = 0.02000 J = 20 mJ.
Values near 50 mJ are standard, values higher than 50 mJ are high power

Capacitor value may be critical because it should match with the power of the HV circuit to enable to charge itself at maximum rpm. If capacitor is too large it may not charge enough at high rpm. And if capacitor is too small, spark energy can be lower.


The circuit above with the help of sir Robert Long, carefully put some annotations to fully understand how it works, the necessary waveforms voltage check, and the graph of the advance,

Many thanks to  sir Long.

Annotation Courtesy of Mr. Robert Long


  1. great post... is it possible to handle more than 10000rpm??

    can i put link to your blog from mine?? thanks before...

  2. possible if at any how we can modify the HV that it can cope with the faster time for the capacitor to charge...

  3. Hi

    I assume you have worked on the multi-spark CDI circuit published in Australian, Silicon-Chip Magazine in 1997.

    The basic topology was to convert one pulse from the pickup coil into a burst of (4-8) pulses and then fed into the HV capacitor thru the mosfets.

    I was just wondering, if we simply use a 555 to convert a pulse into a burst and feed it to the simplest AC-CDI of Yamaha DT125 would not it work?

    1. for reliablity issue of the high voltage converter section of any dc-cdi, the moment the scr is trigger to dump the energy of the capacitor to the ignition coil, the HV oscillator must stop momentarily in accordance and or in sync with the scr gate triggering. IF the HV converter keeps on feeding HV to the capacitor the SCR might lock up, producing misfire. If you look at the schematic above, both the gate of the SCR and the HV oscillator is connected to pin 1 of the lm339 IC, by analyzing that circuit. when pin 1 is ready to trigger the gate, the HV converter driving circuit will be turned off momentarily.

      I had seen that multi-spark CDI articles many times, i also do found their group at yahoo, only few made it work, and they are having problems with misfire, the high voltage keeps on feeding high voltage even if the scr is already dumping the energy stored in the capacitor to the ignition coil.

      Try bench testing such cdi and you will see what i mean.

      try visiting this site

      All information are there. thank you

    2. Yeh, I have that point in my mind. The HV supply can be blocked momentarily with a triac using the inverted signal from the pick-up coil (i.e. the triac will conduct when there is no signal from the pick-up coil).

      Here, I am not talking about the actual multi-spark CDI but, the 555-based as mentioned earlier.