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Problem understanding CDI circuit

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Futterama

Member
Hi forum,

I found a CDI ignition project on RC Universe forum, and I'm having trouble understanding an oscillation part of the circuit, perhaps you can tell me how it works?

The part of the circuit marked with red should oscillate and by using L1, L2 and L3 it should generate a few hundred volts.

Can you explain how it works and perhaps tell me if there is a name for this kind of oscillation circuit (like "astabil multivibrator" is a term for a specific kind of circuit).

Look at the attached schematic.

Thanks.

Regards,
Futterama
 

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Diver300

Well-Known Member
Most Helpful Member
It works a bit like a "Joule Thief".

Current flows through R5, L3 and R6 to turn on T2.

T2 puts a positive voltage on L2, which results in a positive voltage on the other windings of the transformer.

(By positive, I mean that the dot ends of the windings are more positive.)

The voltage on L3 turns on the T2 more, and it also charges C7. Turning on T2 more drives L2 more etc so that nearly the full 6V is applied to T2. C7 will end up charged to about 2V.

After a short time, the transformer will saturate. When that happens the voltage generated on L3 falls. Because C7 is charged, there is no voltage left to turn on T2, so it starts to turn off. As soon as the T2 starts to turn off, the voltage on L2 falls, so the voltage on L3 falls and the T2 turns completely off.

As the current stops in L3, a negative voltage will appear on the transformer, and the voltage on T1 will charge C6 until the magnetic field falls to zero.

Then the negative voltage on L3 falls to zero. C7 discharges through R5 and the whole cycle starts again.

There are a couple of things about the circuit that don't seem quite right.
C7 is charged positive and negative so it shouldn't be a polarised capacitor. The same applies to C3

Also there should be voltage limiter for the voltage on C6. The voltage will be a lot larger than 6 V multiplied by the turns ratio of L1:L3 because the energy in the transformer has nowhere to go except into C6, and the oscillator will continue to oscillate all the time. There could be very big voltages if the engine stops.

This sort of CDI ignition isn't used much any more. Modern ignition systems use coils with very small primary resistance. The computers decide when to turn on the coil current so that is done just before the spark is needed. The spark happens when the coil current is turned off. The time that the coil is on is what controls the current. The ignition coil is used to store the spark energy so there is no need for a big capacitor or another transformer.

You've got a PIC16F628 in there that could do the timing and the whole thing would be a lot simpler. You do need a high voltage transistor and a small capacitor to limit the peak voltage.

Really old ignition systems had high resistance primary coils, and that resistance limited the current. It is a very simple system but a lot of heat was generated in the coil at low engine speeds so it had to be quite big to get rid of the heat.
 

jpanhalt

Well-Known Member
Most Helpful Member
This sort of CDI ignition isn't used much any more.
You've got a PIC16F628 in there that could do the timing and the whole thing would be a lot simpler. You do need a high voltage transistor and a small capacitor to limit the peak voltage.
Actually, the OP has been on RC Universe, and I believe this type of ignition (actually the NPN circuit and a variation produced in China) is by far the most common type used for models (cars, boats, and airplanes).

I agree about current engineering practices for cars.

John
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi forum,

I found a CDI ignition project on RC Universe forum, and I'm having trouble understanding an oscillation part of the circuit, perhaps you can tell me how it works?

The part of the circuit marked with red should oscillate and by using L1, L2 and L3 it should generate a few hundred volts.

Can you explain how it works and perhaps tell me if there is a name for this kind of oscillation circuit (like "astabil multivibrator" is a term for a specific kind of circuit).

Look at the attached schematic.

Thanks.

Regards,
Futterama
hi,
Its a simple 'blocking oscillator' circuit.

Look here for a description:
Blocking oscillator
 

Futterama

Member
Diver300, thanks for the explanation, but did you swap L2 and L3 here:

Current flows through R5, L3 and R6 to turn on T2.
T2 puts a positive voltage on L2
I'm wondering if it would be more efficient with a square AC input on only one coil winding (eliminating L2). The circuit is built with simplicity, low cost, low part count, easily accessible parts in mind but I don't mind making it a bit more complex if it gives me any advantages. Also, I think it would be nice to only have 1 primary winding.

Regarding polarized capacitors, I can see your point, but somehow the author of this circuit made it work with electrolytic capacitors. I guess it would be best to avoid polarized capacitors for C7 and C3.

Regarding the voltage limiter on C6, again your are right, if the engine stops, the PIC will no longer give pulses to T1 and L1 will keep charging C6.
So how would you make the voltage limiter? If another oscillating circuit was used, the PIC could stop the oscillator if the engine stops and I guess this would solve the problem?

As jpanhalt says, this type of ignition is widely used in models which is what I would use it for (Zenoah G260RC 2-stroke 1-cylinder gasoline engine).

Regarding the CDI method by discharging a capacitor into the coil generates the spark, this should save energy as there is no need for sending a large current into a low resistance coil. But I can see the advantage of letting the collapse of the magnetic field in the spark coil, generate the spark. But the CDI should work fine for model hobby purposes. Perhaps there is an issue about the high RPM for a model engine (20.000 RPM) that makes the CDI more useful compared to the low resistance coil method.
 

Futterama

Member
Note: If I was to built a oscillating circuit for the transformator, I would use a small 6-pin PIC and a dual MOSFET driver with protection diodes for driving the coil. For me, that is simple ;-)
 
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