Did you check if SCR is actually extinguished after discharge? An unsuitable ignition coil might prevent SCR turn-off. I also wonder if the freewheeling diode, which is uncommon in this circuit might cause problems.
A SCR staying on will kill the inductor by 12V short circuit because there's no means to disconnect the current.
FvM, yes this could well be the reason why, if i hold the Gate of the SCR at 0V there is no problem, if i bring it up to 5V (with 500ohm resistor) quickly and then back to 0V i can generate a spark, but then i have to be very quick to turn the power off to the system or the resistor (OPTION 1) starts smoking, it seems to me like the SCR does not turn "off" again.
Hi,
Ok. if your description is correct, then the ignition circuit doesn´t use back EMF to generate high voltage.
I don´t know whether this circuit and function is correct or not.
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The SCR stays conductive as long as there is current flow in one direction.
If you use a resistive load then the SCR won´t release anymore, once it is triggered.
But wiht the inductive load... it may push the energy from the capacitanc into the inductance then the current may become zero.
For the circuit to work properly you need to release the gate trigger before new positive current can flow through the SCR.
--> try to lower the trigger pulse from 1ms to about 10us.
...Still unsure if I understand the function properly.
Klaus
Thanks for the Power calcuations Klaus, i will link you to the original article for the CDI ignition
HERE. As Brian has pointed out below, what is all the trigger circuit conditioning, this is for the Trigger pulse that was usually from a coil that could generate [some] voltage (dependent on engine RPM). I must add i had built a prototype of the CDI circuit a few years ago (with the rigger circuit conditioning) and tested it with a transformer (connected to the mains at about 130V) and triggering it with the 5V microcontroller and the circuit worked fine. i cannot remember exactly what the pulse length to the SCR trigger was however. but it was most definitely only 5V.
i have tried 350nS positive 5V pulse to the SCR (on the current circuit) and this still results in inductor/Resistor melting.
I'm not sure that design will work at all.
As observed, 'option 2' will fry the components because the SCR will remain conductive once triggered and basically connect L1 across 12V through D2.
'Option 1' will limit the current, preventing damage but also reduce the available current to charge the capacitors although maybe not enough to be sure the SCR stops conducting.
I also can't see what all the components in the gate of the SCR do. It looks more like a transistor based circuit with an SCR substituted for an NPN BJT.
Brian.
Discussed above thanks Brian. As in the below diagram in the blue box, allot of the trigger circuit conditioning could be removed if the trigger was coming from a digital source and not a trigger coil.
I didn't notice the long trigger pulse. You are right, it's likely to keep the SCR on even with proper LC load.
It's quite easy to start with low pulse frequency (e.g. 1 Hz) and fail safe current limiting resistor. Provided the trigger pulse and circuit operation are O.K., you still have the problem that a defective ignition coil can prevent SCR from turning off. I think there should be a disconnect feature, at least a fuse.
FvM where in the circuit would you speficy the position of the fuse?
i think the most likley case is that the SCR is not being turned off. but im unsure why this would be the case. As mentioned before (a few years ago) i did have the CDI circuit working with the Transformer attached to the mains as the HV source, i would not like to take this as an example that the SCR in this circuit should behave the same simply because i have no circuit diagram for this circuit and it was so long ago i cannot remember the specifics, but this is the reason i have used this CDI circuit again in this current circuit, because i had proved that it did work (all those years ago). The difference now is that i have changed the High Voltage generator. Could this contribute to the SCR not turning off?
from Wiki the Modes of Operation for an SCR are:
1: Forward blocking mode (off state)
2: Forward conduction mode (on state)
3: Reverse blocking mode (off state)
Forward conduction mode:
SCR can be brought from blocking mode to conduction mode in two ways: either by increasing the voltage across anode to cathode beyond breakover voltage or by applying positive pulse at gate. Once SCR starts conducting, no more gate voltage is required to maintain it in the on state. There are two ways to turn it off: 1. Reduce the current through it below a minimum value called the holding current and 2. With the gate turned off, short out the anode and cathode momentarily with a push-button switch or transistor across the junction