I have a hard time understanding what the problem in this circuit is, can someone help please?

I have a Moped with its capacitor discharge ignition box. It works fine, the spark exist, but I want to make my own capacitor discharge ignition circuit because I like electricity; electronics is my hobby so I did some research and I found circuit diagrams and explanations and it seemed to me that I understood them, but when I created the circuit it didn't work; no spark. I tried anything.

This is the diagram below:

enter image description here

  • 2
    \$\begingroup\$ "when i created the circuit it didn't work no spark, i tryed anything this is the diagram below" - your circuit has no component values or part numbers. How can you build a working circuit when you don't know what parts to use? \$\endgroup\$ Dec 5, 2020 at 7:09

1 Answer 1


It may seem confusing, but in the spark circuit, we have the spark when we try to turn off (cut the current) the primary winding. By cutting off the current from the primary winding, the transformer creates a back EMF with a very high voltage in the secondary winding and that creates the spark.

So if we want to check the spark, just connect a dc voltage to the primary winding and then disconnect it. Just by disconnecting the dc from primary, you will see the spark. Take caution. If you don't put a reverse-biased diode in parallel to the primary winding, you will see a spark there too; and if you are touching the wires while disconnecting, you make get an electric shock too!

Now let's get to your circuit. The circuit uses the positive voltage on the 'Excuter coil' to charge the primary winding. Mostly just when the voltage is at its peak, the 'pulse rotor' coil will trigger the SCR. By triggering the SCR, the voltage around the primary winding will drop and the back EMF will create a spark in output. When the 'Executor coil' creates a negative voltage, SCR will reset and you can repeat the procedure.

  • \$\begingroup\$ yes i understand what you mean, but what baffles me is that my Moped electric system with its CDI box work fine with spark, but when i connect the cdi circuit that i made exactly like on diagram there are nothing and what baffles me more are is all the components of the circuit is fine also the most of CDI circuit who i have seen on the net they are the same \$\endgroup\$
    – user270262
    Dec 5, 2020 at 19:26
  • \$\begingroup\$ Is the 'Excitor Coil' synchronized with 'Trigger Coil'? The 'Trigger' needs to create a positive pulse exactly when 'Excitor' created the peak positive voltage. Maybe one of the coils has the wrong polarity. You can check the circuit by replacing the coils with a dc voltage. Connect the dc voltage between the ground and the terminal of 'Excitor'. Then put a positive pulse on the 'Trigger' terminal. If the circuit is right, you should see a spark. If you see the spark, it's definitely the polarity and/or synchronization of coils. \$\endgroup\$
    – Saadat
    Dec 7, 2020 at 10:52
  • \$\begingroup\$ no the exciter coil is not synchronized with trigger coil i am sure about it, also i have a CDI box work fine so if the exciter coil is synchronized with trigger coil the CDI box dont work too, i am really confused \$\endgroup\$
    – user270262
    Dec 10, 2020 at 21:04
  • \$\begingroup\$ I think that in the absence of a battery, the exciter coil just creates an AC voltage, that is then rectified by D1 to charge the capacitor. This may be just a magneto type arrangement, where a magnet on the flywheel moves pass a coil to generate a voltage. \$\endgroup\$
    – SteveSh
    Mar 17 at 19:26

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