Tests of CDI Ignition Modules

[74Sprint]

could you elaborate on what you mean by "6 time constants" this makes no sense to me, particularly in this context- in the CD ignition I designed, the capacitor is charged from a bridge from a power supply that can source a fixed amount of current without saturating the transformer (and the voltage on a transformer with constant current is just the current times time). It is discharged into an ignition coil through an SCR, and the back voltage at the end of the cycle turns off the SCR and is recovered by a diode, as I recall. These are very simple circuits, parts values are non-critical. I found that setting the operating voltage (e.g. the voltage you have on the cap) to a little above the back EMF across the points when they open is sufficient for a hot spark - that is generally around 200V. Maybe you are referring to the non-electronic time constants such as the burn front propagation in the mixture?

Sure I can.

"Maybe you are referring to the non-electronic time constants such as the burn front propagation in the mixture?"

Nope it has nothing to do with propagation.
Since all capacitors and coils(inductors) have resistance either ESR or wire resistance you have RC and RL time constants. It is the time it takes to charge/discharge the capacitor or inductor. If you put a resistor in front of a capacitor or inductor you automatically slow down the charging. These charge times or time constants can be calculated and are well know in electronics. Sometimes it is there and we use it without knowing that we are, a good example is the 555 timer. Now don't get me wrong but, yes we can make a simple circuit work by bogging something together. The circuit may or may not work the way we want it but, it still works. So if one puts something together and it works that good, if it doesn't then we're left with why not. The time constants can be used to get something to work or fine tune a circuit.

I now use a charging circuit that starts with a 1,100 volts to the capacitor. Need to get as much charge as I can in that T1.

The time constants:
The time constants are a fixed percentage based on measured times.
The series resistance can be either or both the internal resistance or external.

RC Charging
The time constant, τ is found using the formula T = R x C in seconds.
For example - The time constant τ is given as: T = R x C = 47k x 1000uF = 47 Secs

Time	RC Value	Percentage of Maximum	Percentage of Maximum
Constant		Voltage	Current
0 time constant	0	0.0%	100%
0.5 time constant	0.5T = 0.5RC	39.30%	60.70%
0.7 time constant	0.7T = 0.7RC	50.30%	49.70%
1.0 time constant	1T = 1RC	63.20%	36.80%
2.0 time constants	2T = 2RC	86.50%	13.50%
3.0 time constants	3T = 3RC	95.00%	5.00%
4.0 time constants	4T = 4RC	98.20%	1.80%
5.0 time constants	5T = 5RC	99.30%	0.70%

Remember current leads voltage by 90 degrees in a capacitive circuit.

Series resistance ESR	0.5​	Input Voltage	12​	Capacitor uF	1000​
	Voltage	Current		Time
T0	0.000​	6.000​		0.00500​
T1	7.584​	2.208​		0.00316​
T2	10.380​	0.810​		0.00433​
T3	11.400​	0.300​		0.00475​
T4	11.784​	0.108​		0.00491​
T5	11.916​	0.042​		0.00497​

RL coil charging:
The percentage of charge for an inductor are the same as capacitance but the charge formula is different. Voltage leads current by 90 degrees.

The time constant, τ is found using the formula T = L/ R in seconds.
Therefore the time constant τ (0) is given as: T = L ÷ R = 47uh ÷ 2 ohms = 0.0000235 Secs
RL Charging Table
Time Constant	LR Value	Percentage of Maximum
		Current	Voltage
T0	L/R	0	100
0.5 time constant	0.5T = 0.5L/R	39.30%	60.70%
0.7 time constant	0.7T = 0.7L/R	50.30%	49.70%
1.0 time constant	1T = 1L/R	63.20%	36.80%	T1
2.0 time constants	2T = 2L/R	86.50%	13.50%	T2
3.0 time constants	3T = 3L/R	95.00%	5.00%	T3
4.0 time constants	4T = 4L/R	98.20%	1.80%	T4
5.0 time constants	5T = 5L/R	99.30%	0.70%	T5
Series resistance = 20	​	Input Voltage =12	​	Inductor = 100uh	​
	The internal Voltage Drop of the Inductor	Current in Amps		Time in seconds
T0	12.000​	0.000​		0.000005000​
T1	4.416​	0.379​		0.000003160​
T2	1.620​	0.519​		0.004325000​
T3	0.600​	0.570​		0.004750000​
T4	0.216​	0.589​		0.004910000​
T5	0.084​	0.596​		0.004965000​
​ The internal voltage drop of the inductor is not the voltage measured across the inductor but, it is a measure of the resistance to the change in current flow (Reactance). This along with frequency is a good approximation measure of internal heat build up through XL impedance.

I have attached my Excel file in zip format if anyone wants to check it out.

Normally people/hobbyists don't bother with these calculations and why should they, capacitors and inductors/coils are generally inexpensive and just keep plugging in something until it works. I've learned that for low speed engines <5,000 RPM a 2.2uf works good. For RPM =>10,000 or multi-cylinder engines a 0.47uf works better.

Also here are some links if one wants to learn more. Ok to best. Some include formulas for calculating the energy stored in a capacitor or coil/inductor.
-https://www.allaboutcircuits.com/tools/resistor-capacitor-time-constant-calculator/
-https://www.digikey.ca/en/resources/conversion-calculators/conversion-calculator-time-constant
-RL circuit - Wikipedia
-23.1: RL Circuits
Best
-https://www.electronics-tutorials.ws/inductor/lr-circuits.html

If there are anymore questions I'll try to answer them.

Cheers
Ray

 

[74Sprint]

Ray,

Thans for your comments. You have a lot of insight into the complicated world of ignition and your experience far exceeds my own.

My target when I started this project was just to try both inductive and CDI systems one can easily buy and use. My experience and training was as an engineer in an avionics company, where I did a lot of electro-magnetic types of things, including switching power supplies, lightning protection, and interference between systems. I have designed and bult some decent model engine magnetos but am starting essentially from zero knowledge about battery type ignition systems. Your insights are very helpful.

I wrote earlier that I'm struck by how little of the initial energy in a CDI system actually shows up as heat in the spark. As you say, the CDI coil is a key element in this. A coil is a simple little device with just a core and some wires. How hard could it be? The answer is, really hard! Most equivalent circuits representations of transformers are single-frequency models. An ignition pulse is anything but that. Its a broad spectrum package of energy from dc to light. The standard transformer "T" model doesn't really take into account some frequency-dependent effects such as interwiring capacitance, skin effect resistance changes, and even radiation losses. (Try to make a radio work next to an unshielded ignition system.)

As a secondary goal I hope to pursue this along the way, but I'm going to move on right now to test a number of different coils using a Sage-Gedde driver to see how they do. At least I'll have some actual measurements of the differences between the two systems.

I started looking into ignitions when I was 13 and racing my dirt bike in school boy class. I out weighed everybody else and was looking for more performance outside the rules. LOL

Me, I started my career off as a heavy duty mechanic working on gas and diesel trucks. Later I went to cars mostly also I worked on a lot of racecars and raced my own. I switched careers and joined the military and became a RADAR Technician. I learned everything from tubes to mainframe computers. High voltage, high current, and high frequency stuff used on ASR, IFF, SSR, ILS, TACAN, radio links, computers, telecom and everything down to the component level. After the military I went back to school and now I'm a computer analyst/programmer with a small business.

Trying to filter out ignition noise is a PITA. Here's a neat one, when I lived on base you couldn't record music on a tape recorder and get a clean recording because every time the ASR RADAR swung by you'd pick up and record a big 'zzzzz'. Yup trying to match a transformer/ignition coil to a system/driver is hard, the frequency range is just too great. Everything has to be a compromise. But yah, use 1 ignition box now and see what different coils can offer.

Ray

 

[74Sprint]

it seems to me, that any voltage above some margin over the breakdown voltage across the plug gap and the operating pressure is just wasted. If I was trying to make a miniaturized spark system, that is where I'd start - use a full size ignition system with a storage scope with a HV probe, and see what voltage you are seeing - the peak you see will probably be just before breakdown of the gap, and if you add 50% to that you should have a relable spark. that would let you set the turns ratio in the coil and probably avoid hte #40 wire issue discussed upthread. A lot of automotive ignitions run in the 2-3KV area, so a 200V primary only needs a 10:1 ratio to give you 2KV at the output - that's not really how inductive ignitions work, I know, but that is a way to make things a lot smaller and then use surface mount parts to create the desired primary voltage

I agree with the starting point, that's how/what I started out at using automotive stuff. I then looked at camera flash circuits for one and Xeon tube flash circuits and neon light stuff even. Most automotive and even small engine coils usually are 1:100 turns ratio, at least the ones I looked at. Also a lot of them (small engines) have a starting and idle output of between 100-120 volts and top out @225 volts. Mind you this is pulsed AC that is rectified and I usually don't check that DC value. As far as any energy above the strike voltage being wasted, it really isn't. Because when the ionization takes place the collapsing field in the secondary still carries energy (current) and that has to go somewhere. So why not use it to burn the mixture? If you short out the output of a mag or the secondary of coil you'll only read the voltage of the secondary winding resistance, so very little voltage but, the current can be extremely high. Doug Doucette a local top alcohol racer here in Winnipeg asked me if there was anyway stop burning up his kill switch (50 amp switch) on his mag system which shorted out the mag output. We ended up using 6 high current SCRs in parallel until he went with an electronic mag system (MSD Grid). This is also why some CDI ignitions die if you try to run them without a sparkplug, that energy has to go somewhere. I agree that there should be a high limit on the energy created because, generally the higher the energy the higher the cost but, I found that a little extra doesn't hurt. I had some small custom CDI ignition coils made that are rated for 30,000 volts output and so far they have been ok. They're cheap and small and work from 100v to 1,100v depending on the engine. I have a box full, I guess I should start selling them.

Cheers
Ray

 

[DKGrimm]

it seems to me, that any voltage above some margin over the breakdown voltage across the plug gap and the operating pressure is just wasted. If I was trying to make a miniaturized spark system, that is where I'd start - use a full size ignition system with a storage scope with a HV probe, and see what voltage you are seeing - the peak you see will probably be just before breakdown of the gap, and if you add 50% to that you should have a relable spark. that would let you set the turns ratio in the coil and probably avoid hte #40 wire issue discussed upthread. A lot of automotive ignitions run in the 2-3KV area, so a 200V primary only needs a 10:1 ratio to give you 2KV at the output - that's not really how inductive ignitions work, I know, but that is a way to make things a lot smaller and then use surface mount parts to create the desired primary voltage

Thanks, Bill.

One thing about using minimum voltage to strike an arc: it will take longer for the arc to form. I have no numbers and no idea if that is significant.

 

[Zeb]

(Try to make a radio work next to an unshielded ignition system.)

My go-to troubleshooting steps were:
1. P-Lead shield grounding. 2. Megger 3. Ohm ranger expeditions

Another shelved design is the shower of sparks vibrator circuits. The circuit had its own set of points. Might be interesting to see it brought back to lengthen the spark window, but that would be another project entirely.

 

[william_b_noble]

Sure I can.

Nope it has nothing to do with propagation.
Since all capacitors and coils(inductors) have resistance either ESR or wire resistance you have RC and RL time constants. It is the time it takes to charge/discharge the capacitor or inductor. If you put a resistor in front of a capacitor or inductor you automatically slow down the charging. These charge times or time constants can be calculated and are well know in electronics. Sometimes it is there and we use it without knowing that we are, a good example is the 555 timer. Now don't get me wrong but, yes we can make a simple circuit work by bogging something together. The circuit may or may not work the way we want it but, it still works. So if one puts something together and it works that good, if it doesn't then we're left with why not. The time constants can be used to get something to work or fine tune a circuit.

I now use a charging circuit that starts with a 1,100 volts to the capacitor. Need to get as much charge as I can in that T1.

The time constants:
The time constants are a fixed percentage based on measured times.
The series resistance can be either or both the internal resistance or external.

RC Charging
The time constant, τ is found using the formula T = R x C in seconds.
For example - The time constant τ is given as: T = R x C = 47k x 1000uF = 47 Secs

Time	RC Value	Percentage of Maximum	Percentage of Maximum
Constant		Voltage	Current
0 time constant	0	0.0%	100%
0.5 time constant	0.5T = 0.5RC	39.30%	60.70%
0.7 time constant	0.7T = 0.7RC	50.30%	49.70%
1.0 time constant	1T = 1RC	63.20%	36.80%
2.0 time constants	2T = 2RC	86.50%	13.50%
3.0 time constants	3T = 3RC	95.00%	5.00%
4.0 time constants	4T = 4RC	98.20%	1.80%
5.0 time constants	5T = 5RC	99.30%	0.70%

Remember current leads voltage by 90 degrees in a capacitive circuit.

Series resistance ESR	0.5​	Input Voltage	12​	Capacitor uF	1000​
	Voltage	Current		Time
T0	0.000​	6.000​		0.00500​
T1	7.584​	2.208​		0.00316​
T2	10.380​	0.810​		0.00433​
T3	11.400​	0.300​		0.00475​
T4	11.784​	0.108​		0.00491​
T5	11.916​	0.042​		0.00497​

RL coil charging:
The percentage of charge for an inductor are the same as capacitance but the charge formula is different. Voltage leads current by 90 degrees.

The time constant, τ is found using the formula T = L/ R in seconds.
Therefore the time constant τ (0) is given as: T = L ÷ R = 47uh ÷ 2 ohms = 0.0000235 Secs
RL Charging Table
Time Constant	LR Value	Percentage of Maximum
		Current	Voltage
T0	L/R	0	100
0.5 time constant	0.5T = 0.5L/R	39.30%	60.70%
0.7 time constant	0.7T = 0.7L/R	50.30%	49.70%
1.0 time constant	1T = 1L/R	63.20%	36.80%	T1
2.0 time constants	2T = 2L/R	86.50%	13.50%	T2
3.0 time constants	3T = 3L/R	95.00%	5.00%	T3
4.0 time constants	4T = 4L/R	98.20%	1.80%	T4
5.0 time constants	5T = 5L/R	99.30%	0.70%	T5
Series resistance = 20	​	Input Voltage =12	​	Inductor = 100uh	​
	The internal Voltage Drop of the Inductor	Current in Amps		Time in seconds
T0	12.000​	0.000​		0.000005000​
T1	4.416​	0.379​		0.000003160​
T2	1.620​	0.519​		0.004325000​
T3	0.600​	0.570​		0.004750000​
T4	0.216​	0.589​		0.004910000​
T5	0.084​	0.596​		0.004965000​
​ The internal voltage drop of the inductor is not the voltage measured across the inductor but, it is a measure of the resistance to the change in current flow (Reactance). This along with frequency is a good approximation measure of internal heat build up through XL impedance.

I have attached my Excel file in zip format if anyone wants to check it out.

Normally people/hobbyists don't bother with these calculations and why should they, capacitors and inductors/coils are generally inexpensive and just keep plugging in something until it works. I've learned that for low speed engines <5,000 RPM a 2.2uf works good. For RPM =>10,000 or multi-cylinder engines a 0.47uf works better.

Also here are some links if one wants to learn more. Ok to best. Some include formulas for calculating the energy stored in a capacitor or coil/inductor.
-https://www.allaboutcircuits.com/tools/resistor-capacitor-time-constant-calculator/
-https://www.digikey.ca/en/resources/conversion-calculators/conversion-calculator-time-constant
-RL circuit - Wikipedia
-23.1: RL Circuits
Best
-https://www.electronics-tutorials.ws/inductor/lr-circuits.html

If there are anymore questions I'll try to answer them.

Cheers
Ray

I guess we are just using the terminolgy differently. The voltage across a cap is the integral of 1/C integral of I t, across an iductor it's L di/dt, when you add in resitance and you solve the differential equation you get an exponential which is readily expressed in the laplace domain --- of course you can also put your circuit into a schematic capture program but as you note it's easier to start out close to what you need. it's tedious using lumped element models but the computer helps a lot.

 

[74Sprint]

Here I want to answer the questions of how much circuitry do I need? How much power? How much spark gap do I need? Does combustion chamber design matter? Why does the coil ring? Does the coil need to ring? Is there a cheap off-the shelf design?

So how much circuitry do I need for a CDI? Well like anything it depends on your needs. If you have a low speed/RPM single cylinder low compression < 10:1 then you can get away with a simple design like this one.

This is a simple design that's been around for quite along time @40 years. You'll find variations of it all over the internet. Some work with as little as 3 volts, this one is recommended to use 12 volts. Q1 & Q2, R1 & R2, plus the transformer form a Royer oscillator and step-up the voltage. It uses a FS24-500-C2 transformer but, to me this one is a bit expensive. It is a step-down transformer of 230/115 to 24/12 volts. The primary and secondary are both center-tap coils and here they are hooking the transformer up backwards from it's original design. Pins 7 & 8 and 6 & 5 are normally our 12 volt outputs but, we are going to use them as our 12 volt inputs. This will give us 230 volts output between 1 & 4. This circuit actually puts out quite a bit of power but, you could use a smaller less powerful transformer. Because it uses the TIP35C NPN Bipolar Transistors which have a gain of only 25 you could change R1 & R2 to something like 125 to 560 ohm to decrease the current flow in the transformer you choose. Here is a transformer that could work 7508170311, it's only 5 watts but, in this circuit it will always be charging the capacitor.
H1 is the 12 volt + connection.
C1 is a filtering capacitor.
D1 through D4 form a bridge rectifier, use diodes rated above 250v, no need for high current ones like >1 amp.
R4 is a bleed-off resistor for the charge cap when the ignition is shut off.
C2 is the charge capacitor, I would use 400v Polyester/metal film of 0.47uf to 2.2uf.
Q3 is the SCR that discharges the charge cap, I like using the BT151
R3 provides the gate voltage to the SCR.

As for complex, well that's up to you. I've seen some that go as high as $1,200 or more, it all depends on what you want to spend. In my collection I have systems that range from $20 to $600 and coils from $6.00 to $200.
On a special note; top racers running like up to 60lbs. of boost are using MSD magneto ignitions. Magnetos don't generate voltage only current, their coils generate the voltage.

So how much power do you need?
Basically this comes down to 3 things. Type of fuel used, compression ratio, and combustion chamber design. For fuel it is the octane rating. The higher the octane rating the more spark energy that is required to ignite the fuel. Rule of thumb is not to use any higher octane than what the engine wants and not what you think you need. Engine Masters Does the Fuel You Choose Matter?

The greater the compression ratio the greater the voltage needed to jump the sparkplug gap. Both air and fuels have a dielectric strength (resistance). The more the molecules are compressed the higher the resistance between the sparkplug terminals/electrodes. This is where the chamber design and the sparkplug gap come into play. From testing I have found that hemi-spherical chambers cause the squish area to be more compressed than wedge heads, especial if the use of dome pistons are involved. So more spark voltage is required to jump the gap. Problem is that you have to run a smaller plug gap with the hemi head or the voltage will be so high and because the plug is closer to the grounded metal around it, I was getting spark coming through and around the plug boot to the plug tube. You could easily see the white arc tracks. So I ended up have to decrease the gap and use dielectric grease to stop it from arcing across. It was either that or get a weaker CDI.

How much spark gap do I need?
As much as you can get to reliably run the engine. The more gap you can run the more plasma you can create and the more fuel you will get burning, less chance of a misfire. Too large or too small of a gap and you risk a misfire. So what is the right gap? well that's something you have to play with to get it right. On my racecar I use 0.052" on moist low air pressure days and 0.045" on dry high air pressure days. Took about 70 1/4 mile passes to figure this out.

Why does the coil ring?
Because of the way most ignition coils are wound, secondary winding on top of the primary, when the primary field is expanding it goes through and outside of the secondary winding. When that field collapses those magnetic lines of flux collapse through the secondary generating the HV but, those lines of flux also collapse through the primary windings. Also current always wants to keep flowing in the same direction it was flowing in. This flow is what causes that spark you see when you pull a plug out of the wall or points getting arced. Now when the flux lines go back through the primary it self regenerates a magnetic field of reduced strength because of resistance and impedance through the secondary again, the ringing effect. So that ringing goes on until the energy is spent. The voltage required to generate this effect is caused by the opposing magnetic field lines.

Does the coil need to ring?
Actually it does need to ring. I did some testing trying to get rid of the negative part of the pulse to reduce part damage/wear. I tried several different snubber/damping circuits but, the only thing that happened was either a weakened spark or no spark at all. How the ignition coil is wound has a lot to do with the ringing effect.

My custom coil is shown below and you can see the windings are stacked on a central iron core and not on top of each other and the ringing is reduced.

Is there a cheap off-the shelf design?
The best COTS (common off the shelve) is the one that Bluejets showed on his thread using a 4 wire DC CDI.
Model engine CDI easy and cheap

Cheers
Ray

 

[74Sprint]

I guess we are just using the terminolgy differently. The voltage across a cap is the integral of 1/C integral of I t, across an iductor it's L di/dt, when you add in resitance and you solve the differential equation you get an exponential which is readily expressed in the laplace domain --- of course you can also put your circuit into a schematic capture program but as you note it's easier to start out close to what you need. it's tedious using lumped element models but the computer helps a lot.

It seems that way. I tried editing SPICE model programs but, found it to be PITA. So long as I get a good starting point I'm happy.

Ray

 

[petertha]

So how much power do you need?
Basically this comes down to 3 things. Type of fuel used, compression ratio, and combustion chamber design.
Ray

Is there a rule of thumb to estimate ignition power consumption for battery sizing on circuits like these? For example a single cylinder at X rpm consumes Y amp-hour at 12v, therefore a N multi-cylinder engine would consume N times? I've seen what might be smaller motorcycle? batteries on stationary model engine setups, maybe because they match the requirement & not too heavy to lug around. Maybe more to the point, if it was intended as an airborne system where weight was a premium, what is the minimum battery you could get away with.

 

[peterl95124]

back to the original question, why do CDI seem to produce shorter spark duration than LDI,
are we putting the same amount of energy into the cap in CDI as into the inductor in LDI ?
are we using comparable switching transistors (I'm using IGBT version of Sage-Geddy, while
the CDI schematic above shows SCR), are their behavior during AC ring down comparable ?
do the CDI and LDI capacitor-and-primary LC circuits have similar time constants ?
are we using coils whose primaries are properly matched to the CDI circuits ?

as a do-it-yourselfer I prefer one coil and one transistor LDI over two coils and three transistors CDI,
and until someone can explain why CDI would be better will stick with simplicity (it seems like you
should be able to run a CDI at a higher intermediate voltage to get faster charge time and run at
higher RPM with multi-cylinder, but I don't see people advocating that, which is strange to me)

 

[74Sprint]

Is there a rule of thumb to estimate ignition power consumption for battery sizing on circuits like these? For example a single cylinder at X rpm consumes Y amp-hour at 12v, therefore a N multi-cylinder engine would consume N times? I've seen what might be smaller motorcycle? batteries on stationary model engine setups, maybe because they match the requirement & not too heavy to lug around. Maybe more to the point, if it was intended as an airborne system where weight was a premium, what is the minimum battery you could get away with.

Good questions. With the projects I've been involved with the minimum battery I've seen used the most is 4.8 volts (4xAA) and a current consumption of 0.5 amps for a single cylinder. Of course multiple that by the number of cylinders. On my basic Sparky ignition it is 4.5 volts and 0.4 amps but, this only works for something like a single cylinder hit-and-miss engine. For single cylinder engine running at < 7,500 it works best with at least 6 volts and 0.5 amps. CDIs are popular with airborne stuff because with a CDI you can use a much smaller ignition coil. As far as how much power? most studies I have found has to do with only an air gap setup. I do have a research paper somewhere that has a air/fuel mixture and compression versus voltage required, when I find it I'll post it. Also most DC CDIs are of the constant charge type which taxes the battery. On my Sparky-1 ignition I use a feedback hic-up circuit so that at low speeds/RPM once the charge cap is charged it stop charging the cap until the voltage on the cap goes below a certain level, saves battery power. With a smallish LiPo 11.2v 2,000 maH battery it will run for just over 12 hours at 1,000 RPM. You can get transformers that are 230 : 2.5 volt AC so theoretically 2.5 VAC in and 230 VAC out. The biggest problem with CDIs that I have found is the ignition coil, next to that is the charging circuit.

Ray

 

[74Sprint]

back to the original question, why do CDI seem to produce shorter spark duration than LDI,
are we putting the same amount of energy into the cap in CDI as into the inductor in LDI ?
are we using comparable switching transistors (I'm using IGBT version of Sage-Geddy, while
the CDI schematic above shows SCR), are their behavior during AC ring down comparable ?
do the CDI and LDI capacitor-and-primary LC circuits have similar time constants ?
are we using coils whose primaries are properly matched to the CDI circuits ?

as a do-it-yourselfer I prefer one coil and one transistor LDI over two coils and three transistors CDI,
and until someone can explain why CDI would be better will stick with simplicity (it seems like you
should be able to run a CDI at a higher intermediate voltage to get faster charge time and run at
higher RPM with multi-cylinder, but I don't see people advocating that, which is strange to me)

-"why do CDI seem to produce shorter spark duration than LDI?"
The whole idea behind a CDI is to produce a higher voltage spark without having to build a huge coil. The automotive CDI coils are about half the size of the LDI coils, in pre computer EMUs era. Capacitors are known more as a voltage device than for current and that's the way they operate. If we think just energy wise, we have more voltage than current, so we need a coil we lower resistance but, we would also like more inductance. Problem is we can't have both. Transformers work by converting current to voltage and visa-versa, step-up (less current) or step-down (increase current). Because we have and need more voltage than current we can use finer wire on both the primary and secondary windings. Because of this we don't have as much inductance. This allows the magnetic fields to build faster and collapse faster and we get a shorter spark.

-"are we putting the same amount of energy into the cap in CDI as into the inductor in LDI ?"
If we look at just energy, yes we can. I say can because we can also increase the LDI energy with more dwell and a higher input voltage. With the LDI we will reach a point of coil saturation though. Because watts = I x E, it doesn't really matter matter if we increase voltage or current so long as we match the energy output. Some of todays racecars running GM style LS ignition systems (an LDI) are using 16-16.5 volt battery and charging systems to get more power out. My most powerful ignition so far IS an LDI ignition. When it charges the coil it draws 48 amps and does multiple sparks of increasing voltage to 1,000 volts. It puts out a >90,000 volt 1 amp spark. I took a poke from it once and it damn near killed me.

-"are we using comparable switching transistor?"
No and the reason being is the higher CDI voltage. If we use what I consider a low voltage CDI =<250 volts then we can use the same. In the old days we didn't have cheap transistors that could handle 250 volts but, we did have SCRs. Today we have GAN and SIC transistors, IGBTs, and MOSFETS that can handle as high as 1,200 volts. This is thanks to electric cars. So now we can say yes. To get a good spark we need to switch off the current flow as fast as possible. The faster the magnetic lines of flux collapse the better the spark. It's just like a generator, the faster you turn it the more voltage it makes.

-"are their behavior during AC ring down comparable ?"
No, the CDI coils have less inductance and ring down faster, even if you use an inductive coil but, that's because we have less current flow from the capacitor. How the ignition coil is built has an effect on the ring down also. Series stacked has less than parallel stacked windings.

-"do the CDI and LDI capacitor-and-primary LC circuits have similar time constants ?"
The time constant percentages are the same it is just the values and formulae that are different.

	% of Max Charge Up	% of Charge Remaining Discharge
1.0 time constant	63.20%	36.80%
2.0 time constants	86.50%	13.50%
3.0 time constants	95.00%	5.00%
4.0 time constants	98.20%	1.80%
5.0 time constants	99.30%	0.70%

-"I prefer one coil and one transistor LDI over two coils and three transistors CDI"
For a low or normal compression engine you can't beat simplicity. But for a lot of people who don't build engines it seems that they just have to have a CDI. LOL

-"(it seems like you should be able to run a CDI at a higher intermediate voltage to get faster charge time and run at higher RPM with multi-cylinder, but I don't see people advocating that, which is strange to me)"
Yes you can and yes I do. Most CDIs use @230v and I use @1,100v. If we use that T1 of each we get; 230 x 63.2% = 145.36v and with mine 1,100 x 63.2 = 695.2v. Now most CDIs quit sparking @100v and because as RPM goes up or more cylinders there is less time to charge the cap, the 230v one will run out of steam first. MSD ignitions are between 450 - 600 volts to the primary. People don't advocate that because i guess they don't know about the time constants, I don't know, I can't answer for them. Oh MSD rates their ignitions as high as 690mj and how they get that is because they have multiple sparks at idle (only) and they add up 5 sparks per firing to get the 690mj. Now a days pretty much everybody does that now. I prefer watts per strike instead of the milli-joule thing and spark duration.

Ok I think I've highjacked Don's thread long enough, let's see what kind of results he gets by using the same ignition with different coils.
Sorry Don, I apologize.

Ray

 

[DKGrimm]

Ok I think I've highjacked Don's thread long enough, let's see what kind of results he gets by using the same ignition with different coils.
Sorry Don, I apologize.

Always grateful for more advice, Ray. I will admit it takes a lot of time to read and digest everything you have to say and to figure out how it applies. I'm trying to keep up. We've had interruptions due to weather, Covid and other medical stuff, and of course the Christmas holiday season is filled with activities. I tried out my skills at making two-sided circuit boards and am happy with that experiment, so now I have a good Sage-Gedde driver board and a table full of parts. With luck I'll have a driver and a test setup yet this week.

You mentioned that an inductive ignition coil is usually around twice as big as one for CDI. If that's the linear dimension, that means that the inductive coil has eight times (the cube of the linear dimension) the volume of materials. In my work on miniaturizing magnetos, I found that the energy one of those could deliver was directly proportional to the combined volume of iron and copper in the magnetic circuit. I'm kind of expecting to find something similar for pulse transformers of both kinds.

Don

 

[william_b_noble]

Here I want to answer the questions of how much circuitry do I need? How much power? How much spark gap do I need? Does combustion chamber design matter? Why does the coil ring? Does the coil need to ring? Is there a cheap off-the shelf design?

So how much circuitry do I need for a CDI? Well like anything it depends on your needs. If you have a low speed/RPM single cylinder low compression < 10:1 then you can get away with a simple design like this one.
View attachment 143198
This is a simple design that's been around for quite along time @40 years. You'll find variations of it all over the internet. Some work with as little as 3 volts, this one is recommended to use 12 volts. Q1 & Q2, R1 & R2, plus the transformer form a Royer oscillator and step-up the voltage. It uses a FS24-500-C2 transformer but, to me this one is a bit expensive. It is a step-down transformer of 230/115 to 24/12 volts. The primary and secondary are both center-tap coils and here they are hooking the transformer up backwards from it's original design. Pins 7 & 8 and 6 & 5 are normally our 12 volt outputs but, we are going to use them as our 12 volt inputs. This will give us 230 volts output between 1 & 4. This circuit actually puts out quite a bit of power but, you could use a smaller less powerful transformer. Because it uses the TIP35C NPN Bipolar Transistors which have a gain of only 25 you could change R1 & R2 to something like 125 to 560 ohm to decrease the current flow in the transformer you choose. Here is a transformer that could work 7508170311, it's only 5 watts but, in this circuit it will always be charging the capacitor.
H1 is the 12 volt + connection.
C1 is a filtering capacitor.
D1 through D4 form a bridge rectifier, use diodes rated above 250v, no need for high current ones like >1 amp.
R4 is a bleed-off resistor for the charge cap when the ignition is shut off.
C2 is the charge capacitor, I would use 400v Polyester/metal film of 0.47uf to 2.2uf.
Q3 is the SCR that discharges the charge cap, I like using the BT151
R3 provides the gate voltage to the SCR.

As for complex, well that's up to you. I've seen some that go as high as $1,200 or more, it all depends on what you want to spend. In my collection I have systems that range from $20 to $600 and coils from $6.00 to $200.
On a special note; top racers running like up to 60lbs. of boost are using MSD magneto ignitions. Magnetos don't generate voltage only current, their coils generate the voltage.

So how much power do you need?
Basically this comes down to 3 things. Type of fuel used, compression ratio, and combustion chamber design. For fuel it is the octane rating. The higher the octane rating the more spark energy that is required to ignite the fuel. Rule of thumb is not to use any higher octane than what the engine wants and not what you think you need. Engine Masters Does the Fuel You Choose Matter?

The greater the compression ratio the greater the voltage needed to jump the sparkplug gap. Both air and fuels have a dielectric strength (resistance). The more the molecules are compressed the higher the resistance between the sparkplug terminals/electrodes. This is where the chamber design and the sparkplug gap come into play. From testing I have found that hemi-spherical chambers cause the squish area to be more compressed than wedge heads, especial if the use of dome pistons are involved. So more spark voltage is required to jump the gap. Problem is that you have to run a smaller plug gap with the hemi head or the voltage will be so high and because the plug is closer to the grounded metal around it, I was getting spark coming through and around the plug boot to the plug tube. You could easily see the white arc tracks. So I ended up have to decrease the gap and use dielectric grease to stop it from arcing across. It was either that or get a weaker CDI.

How much spark gap do I need?
As much as you can get to reliably run the engine. The more gap you can run the more plasma you can create and the more fuel you will get burning, less chance of a misfire. Too large or too small of a gap and you risk a misfire. So what is the right gap? well that's something you have to play with to get it right. On my racecar I use 0.052" on moist low air pressure days and 0.045" on dry high air pressure days. Took about 70 1/4 mile passes to figure this out.

Why does the coil ring?
Because of the way most ignition coils are wound, secondary winding on top of the primary, when the primary field is expanding it goes through and outside of the secondary winding. When that field collapses those magnetic lines of flux collapse through the secondary generating the HV but, those lines of flux also collapse through the primary windings. Also current always wants to keep flowing in the same direction it was flowing in. This flow is what causes that spark you see when you pull a plug out of the wall or points getting arced. Now when the flux lines go back through the primary it self regenerates a magnetic field of reduced strength because of resistance and impedance through the secondary again, the ringing effect. So that ringing goes on until the energy is spent. The voltage required to generate this effect is caused by the opposing magnetic field lines.

Does the coil need to ring?
Actually it does need to ring. I did some testing trying to get rid of the negative part of the pulse to reduce part damage/wear. I tried several different snubber/damping circuits but, the only thing that happened was either a weakened spark or no spark at all. How the ignition coil is wound has a lot to do with the ringing effect.
View attachment 143202

My custom coil is shown below and you can see the windings are stacked on a central iron core and not on top of each other and the ringing is reduced.
View attachment 143199

Is there a cheap off-the shelf design?
The best COTS (common off the shelve) is the one that Bluejets showed on his thread using a 4 wire DC CDI.
Model engine CDI easy and cheap

Cheers
Ray

 

[william_b_noble]

an observation about that schematic - it is very close to the Delta Mk 10 schematic from the early 70s (late 60s?). Delta used germanium transistors for low voltage drop, in my design I used silicon, today's solution would be MOSFETS. since whatever you use is driven hard into saturation or hard off, the actual dissipation is fairly small so I'm not sure it matters. Delta used a torroidial transformer, today wiht higher frequency parts, a ferrite E-core would probably be fine if not better//

 

[dsage]

A possibly useful module for ignition systems.
I ordered one and it arrived pretty quickly. I haven't checked how fast it responds to triggering the input. Could be too slow to respond but if it's repeatable, then an appropriate advance in the timing might make up for it.
ALSO the input might have to be triggered with something like a reed switch or points not a hall sensor because the trigger connection is part of a high voltage circuit apparently. See the reverse engineering video.
Dual output so maybe it could be used for a two cylinder engine as well. Bonus

Edit: It does have a self repetition rate. Too slow for a muti-spark type ignition. If you get the dwell right at low rpm it would not appear anyway. May be hackable.

Food for thought anyway on cheap and small modules possibly ready to use. The price is certainly right even if you harvest it for parts.

Ebay Listing here:
https://www.ebay.ca/itm/165758942941
Reverse engineering of the module by Big Clive Youtube channel:

 

[dsage]

I did a bit of testing. It's pretty slow at responding to a trigger signal. Too slow I think.
The input trigger can't be a reed switch - at least not the ones I have here - not without some sort of snubber across the contacts because there is quite a spark even when you manually touch the trigger wire to ground. The one reed I tried it welded the contacts closed. A suitable transistor would probably work instead.

Seems like it might have all the makings to be hacked into something useful though. A medium voltage power supply of a couple hundred volts and a HV output transformer for 12Kv. Hack it by coupling the two pieces together with a capacitor and an SCR and you might have a CDI for cheap.
The spark output is pretty substantial.

I'll have to investigate some more.

 

[dsage]

I went back and looked at the video again. My module operates differently from his. Mine puts out individual sparks with spaces between them repetitively while it is activated. His produces a more or less a continuous arc while activated. Interesting. His would be much better as an ignition system being more like a multi-spark operation.
Either module contains all the guts necessary to hack it into something useful though.

 

[RetNM]

It seems that you have one made for Gas Appliances. Our gas stove has a delay between sparks as well as our Gas Furnaces. The Furnaces have a shorter delay though.

 

[dsage]

Big Clive didn't give a link to the exact one he has there. It seems mine is slightly different inside. But it looks exactly the same outside. I just found it based on appearance etc.
Makes no difference really. All the guts are there to rebuild it into a CDI - I figure.
I'll reverse engineer mine later and see what's required.