Also read this thread because I think the GY6 coils might make for scaled size. Model engine CDI easy and cheap

Ray

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- Thread starter DKGrimm
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Also read this thread because I think the GY6 coils might make for scaled size. Model engine CDI easy and cheap

Ray

Thank you for the reply. One of my hobbies is ham radio and I have recently been going down the rabbit hole of magnetic circuits. There is a transformer called a fly back transformer that stores energy in a gap in the ferromagnetic coil used to boost the voltage with a smaller size transformer. Perhaps that could be employed. The little devils can reach 30,000 volts or more. However I do not consider myself an expert so not in a position to make good technical arguments on how to use them. What I can do is toy with a couple of ideas on making a coil to work with a capacitor. If I find something that fits in the 2 cubic inch range I will let you know. Keep up the very interesting work. I will continue to watch the thread.Along and with the help and prodding of John Vietti I have been trying to do something along the lines you laid out in your message for well over ten years now. It's a long story, but to give a brief peek at the bottom line, the minimum volume of a magneto is directly related to how much energy you need to store in it to create the spark. John has made the smallest magneto I have seen running an engine, with somewhere near 2 cubic inches (I'm guessing at that). His engine can be temperamental, but it really does run.

The thing that kills you is that the volume shrinks as the cube of the linear dimensions when you scale down a particular shape. Not to put too pessimistic a face on it, I think the answer for miniaturization might be a tiny rotary generator charging a capacitor in an accessory box outside the rotary "magneto". Electronic wizardry would play a role here, but things like semiconductor switches eat their share of the stored energy, so there are always compromises.

What I'm interested in on the side is finding some way to more efficiently transfer stored energy from a capacitor into an active spark. That's going a lot slower than I would like, but maybe I can free up more time to work on that in the next month or two.

HMEL

This stuff is challenging to everyone. The only way to really understand it is to live with it for a while and try out a bunch of things. Forums like this one are a major source of information (some good, some bad). Interesting thing about flyback transformers: The coil in a Kettering-type ignition system is itself a flyback transformer.Thank you for the reply. One of my hobbies is ham radio and I have recently been going down the rabbit hole of magnetic circuits. There is a transformer called a fly back transformer that stores energy in a gap in the ferromagnetic coil used to boost the voltage with a smaller size transformer. Perhaps that could be employed. The little devils can reach 30,000 volts or more. However I do not consider myself an expert so not in a position to make good technical arguments on how to use them. What I can do is toy with a couple of ideas on making a coil to work with a capacitor. If I find something that fits in the 2 cubic inch range I will let you know. Keep up the very interesting work. I will continue to watch the thread.

HMEL

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it seems no one is staying on the topic of why CDI produces short (er?) sparks, so I'll

add my $0.06 of off topicicallityness-expialidocious.....

an air gap in a magnetic circuit is like a resistor in an electric circuit, and just like a

resistor doesn't store energy in an electric circuit so to the function of an air gap in

a magnetic circuit is not to store energy, its there to make the magnetic field more

linear with the primary current so that the flyback circuitry is easier. and a flyback

transformer is one that includes extra windings that are used as feedback to the

oscillator that powers the primary (hence its not really appropriate to use that term

on a standard ignition coil).

a good example of doing the impossible is Barry Hares' magnetos for his 1/5'th scale

RR Merlin, they are in a StrictlyIC article which I have, and I have to admit that's

where I draw the line, my Merlin uses external (moped) coils with Sage/Geddy circuits

and external (too large for scale) distributors all hidden behind the firewall. I have

empty shells of magnetos on the engine for looks. I'll probably do something similar

for my De Havilland Cirrus.

if/when I build a large(ish) hit-and-miss engine I'd like to use a lawn-mower style magneto,

magnets in the rim of the flywheel and a coil in the wood box that the engine sits on.

three NIB magnets in a row with alternating polarity should produce a whopping magnetic

charge to the coil (I hope !!!)

PS, the rabbit hole I went down to learn magnetics was designing this dynamo from scratch

to output 12V at 1200RPM, which is does (but that's too fast for the Stuart No 9, so I only

actually run it at around 3 to 6 V)

add my $0.06 of off topicicallityness-expialidocious.....

an air gap in a magnetic circuit is like a resistor in an electric circuit, and just like a

resistor doesn't store energy in an electric circuit so to the function of an air gap in

a magnetic circuit is not to store energy, its there to make the magnetic field more

linear with the primary current so that the flyback circuitry is easier. and a flyback

transformer is one that includes extra windings that are used as feedback to the

oscillator that powers the primary (hence its not really appropriate to use that term

on a standard ignition coil).

a good example of doing the impossible is Barry Hares' magnetos for his 1/5'th scale

RR Merlin, they are in a StrictlyIC article which I have, and I have to admit that's

where I draw the line, my Merlin uses external (moped) coils with Sage/Geddy circuits

and external (too large for scale) distributors all hidden behind the firewall. I have

empty shells of magnetos on the engine for looks. I'll probably do something similar

for my De Havilland Cirrus.

if/when I build a large(ish) hit-and-miss engine I'd like to use a lawn-mower style magneto,

magnets in the rim of the flywheel and a coil in the wood box that the engine sits on.

three NIB magnets in a row with alternating polarity should produce a whopping magnetic

charge to the coil (I hope !!!)

PS, the rabbit hole I went down to learn magnetics was designing this dynamo from scratch

to output 12V at 1200RPM, which is does (but that's too fast for the Stuart No 9, so I only

actually run it at around 3 to 6 V)

I got tripped up on this air gap thing at work back in the late '60s. We were designing choke coils for filtering rectified power when I first learned a that the air gap is where most of the energy is actually stored. A choke coil with dc current running in it will saturate the iron and kill the inductance. Chokes have air gaps to prevent that.an air gap in a magnetic circuit is like a resistor in an electric circuit, and just like a

resistor doesn't store energy in an electric circuit so to the function of an air gap in

a magnetic circuit is not to store energy, its there to make the magnetic field more

linear with the primary current so that the flyback circuitry is easier. and a flyback

transformer is one that includes extra windings that are used as feedback to the

oscillator that powers the primary (hence its not really appropriate to use that term

on a standard ignition coil).

This was what was so hard for me to wrap my head around: take a coil with an iron core and an air gap and look at a slice of cross section of the core and another of the air gap. The flux in the air gap has to be the same as the flux in the iron because flux flows in closed loops. The energy stored in either slice is (1/2) x (flux in that section) x (ampere turns across that section). Air has to have a lot more ampere turns to drive flux than iron does, so there will be a lot more energy stored in the section of air. A lot of chokes, ignition coils, flyback transformers, etc. don't even have iron wrapped around them; they just use a linear core through the center.

Not that this matters to anyone who is using a coil for some particular purpose. Mostly they just are lookinig for a certain amount of inductance, and they could care less about the theoretical niceties of how they work.

I can imagine the challenges you faced designing this. It's kind of like trying to hold a bathtub full of rubber duckies under water.PS, the rabbit hole I went down to learn magnetics was designing this dynamo from scratch

to output 12V at 1200RPM, which is does (but that's too fast for the Stuart No 9, so I only

actually run it at around 3 to 6 V)

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I got tripped up on this air gap thing at work back in the late '60s. We were designing choke coils for filtering rectified power when I first learned a that the air gap is where most of the energy is actually stored. A choke coil with dc current running in it will saturate the iron and kill the inductance. Chokes have air gaps to prevent that.

This was what was so hard for me to wrap my head around: take a coil with an iron core and an air gap and look at a slice of cross section of the core and another of the air gap. The flux in the air gap has to be the same as the flux in the iron because flux flows in closed loops. The energy stored in either slice is (1/2) x (flux in that section) x (ampere turns across that section). Air has to have a lot more ampere turns to drive flux than iron does, so there will be a lot more energy stored in the section of air. A lot of chokes, ignition coils, flyback transformers, etc. don't even have iron wrapped around them; they just use a linear core through the center.

Not that this matters to anyone who is using a coil for some particular purpose. Mostly they just are lookinig for a certain amount of inductance, and they could care less about the theoretical niceties of how they work.

DK, I believe you have your analogies mixed up, the "energy" of the magnetic flux is not divided in any way, as you say the lines of flux form closed loops, but it is the number of such closed loops that determine the energy in the field, not what the lines of those loops flow through be it iron or air. It takes more energy to create field lines in air than iron, but that doesn't mean the energy is stored in the air. the air is to a magnetic field like a resistor is to an electric current, the resistor doesn't store energy, it just requires more

energy to get the same current as without the resistor, similarly air doesn't store energy in a magnetic field it just requires more energy to get the same amount of flux flowing through it.

here's one way to remember this

resistance --- electrical resistance, requires more energy to move a current

reluctance --- magnetic resistance, requires more energy to create a field in it (the lines of flux are like a current)

the reluctance of air is higher than iron, that's all. Yes, an inductor with an air gap has higher inductance

than the same without the air gap, but that doesn't mean the energy is in the air gap. who ever told you

that 1/2 the energy is in the iron and 1/2 is in the air was very, very confused.

the law for determining the reluctance of a series magnetic circuit is the same as for a series electrical circuit,

1 / ( 1/R1 + 1/R2 + 1/R3 + ....), IE you add the conductivities which are the reciprocals of the resistances,

then take the reciprocal of that sum to get back to resistance, ditto for reluctance.

it's pretty hard to get 1/2 and 1/2 out of this formula !,

in most magnetic circuits with any air gap anywhere that's the part that dominates the reluctance, usually by a factor of 100 to 1 or more.

HTH, sorry for being pedantic...

Sorry, but I'm going to have to stick with what I said on this one. I tried to look up some simple references to cite here but the textbooks that deal with fields are pretty hard to digest in a paragraph or two. One confusing part of this is you can talk about the total MMF -- apmpere-turns -- for the whole device, but you can also talk about the H-field intensity -- ampere turns per inch -- of a differential element within the flux field. It's easy to get a headache trying to keep all this straight.DK, I believe you have your analogies mixed up, the "energy" of the magnetic flux is not divided in any way, as you say the lines of flux form closed loops, but it is the number of such closed loops that determine the energy in the field, not what the lines of those loops flow through be it iron or air. It takes more energy to create field lines in air than iron, but that doesn't mean the energy is stored in the air. the air is to a magnetic field like a resistor is to an electric current, the resistor doesn't store energy, it just requires more

energy to get the same current as without the resistor, similarly air doesn't store energy in a magnetic field it just requires more energy to get the same amount of flux flowing through it.

here's one way to remember this

resistance --- electrical resistance, requires more energy to move a current

reluctance --- magnetic resistance, requires more energy to create a field in it (the lines of flux are like a current)

the reluctance of air is higher than iron, that's all. Yes, an inductor with an air gap has higher inductance

than the same without the air gap, but that doesn't mean the energy is in the air gap. who ever told you

that 1/2 the energy is in the iron and 1/2 is in the air was very, very confused.

the law for determining the reluctance of a series magnetic circuit is the same as for a series electrical circuit,

1 / ( 1/R1 + 1/R2 + 1/R3 + ....), IE you add the conductivities which are the reciprocals of the resistances,

then take the reciprocal of that sum to get back to resistance, ditto for reluctance.

it's pretty hard to get 1/2 and 1/2 out of this formula !,

in most magnetic circuits with any air gap anywhere that's the part that dominates the reluctance, usually by a factor of 100 to 1 or more.

HTH, sorry for being pedantic...

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To convert it you need to remove the SIDAC and insert a 150volt zener in it's place. (I used two 75v zeners in series). That will limit the HV production to 150v (which, for safety) is where the SIDAC used to operate.

Caution: without something to limit the output it will produce more than 900volts and self destruct so be careful). (Been there done that).

Across the zener(s) I put an SCR that, when triggered, shorts out the sidac150volts (like the sidac used to do).

I triggered the SCR with a 555 1ms pulse generator to test the maximum repetition rate.

The HV supply has a pretty long recovery time so the the best it would do is about 66hz which is about 8,000 rpm for a single cylinder engine. Firing a regular car plug with 44thou gap. Not bad for a couple of bucks and operating on two AA batteries (3v) and pretty small.

You can divide that by performance by 2,4,6,8 rpm for other multi-cylinder engines. (Not very good for 8 cylinders).

Some minor additional circuitry would be needed to trigger from points or a hall sensor.

I'll do some more testing but I think that's the limit for this module. Your results may vary.

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The inductance of the coil will decrease when you insert an air gap. The reason for the gap is to reduce the magnetic flux in the core to make it more linear and also avoid saturation of the core. This is important in many situations.DK, I believe you have your analogies mixed up, the "energy" of the magnetic flux is not divided in any way, as you say the lines of flux form closed loops, but it is the number of such closed loops that determine the energy in the field, not what the lines of those loops flow through be it iron or air. It takes more energy to create field lines in air than iron, but that doesn't mean the energy is stored in the air. the air is to a magnetic field like a resistor is to an electric current, the resistor doesn't store energy, it just requires more

energy to get the same current as without the resistor, similarly air doesn't store energy in a magnetic field it just requires more energy to get the same amount of flux flowing through it.

here's one way to remember this

resistance --- electrical resistance, requires more energy to move a current

reluctance --- magnetic resistance, requires more energy to create a field in it (the lines of flux are like a current)

the reluctance of air is higher than iron, that's all. Yes, an inductor with an air gap has higher inductance

than the same without the air gap, but that doesn't mean the energy is in the air gap. who ever told you

that 1/2 the energy is in the iron and 1/2 is in the air was very, very confused.

the law for determining the reluctance of a series magnetic circuit is the same as for a series electrical circuit,

1 / ( 1/R1 + 1/R2 + 1/R3 + ....), IE you add the conductivities which are the reciprocals of the resistances,

then take the reciprocal of that sum to get back to resistance, ditto for reluctance.

it's pretty hard to get 1/2 and 1/2 out of this formula !,

in most magnetic circuits with any air gap anywhere that's the part that dominates the reluctance, usually by a factor of 100 to 1 or more.

HTH, sorry for being pedantic...

A good example from the really old days in scool when we learned to vind audio transformers for impedance matching between a high impedance PL84 pentode to a 4 ohm speaker.

Yes, I agree with that. (Been there myself.) This discussion of how the energy is stored in an ignition coil are quite different from just using it as a linear transformer. The details of how a magnetic field stores energy is only important if you are actually designing the coil or transformer (or magneto) itself. Field theory is really complex. The good ones I have known were mostly EE PhDs who specialized in the subject. I don't consider myself to be a qualified lecturer on the subject. What I'm trying to do is to find what works and what doesn't and a little bit of why.The inductance of the coil will decrease when you insert an air gap. The reason for the gap is to reduce the magnetic flux in the core to make it more linear and also avoid saturation of the core. This is important in many situations.

A good example from the really old days in scool when we learned to vind audio transformers for impedance matching between a high impedance PL84 pentode to a 4 ohm speaker.

I'm no expert on ignition coils but my thoughts are as follows.

The total energy stored in a coil is

E = 1/2*L*i^2.

This is the limiting amount of energy available.

If you increase the Inductance, you will increase the time i takes to charge the coil and limit the RPM.

T=L/R

If you decrease the resistance, you will Draw more current.

On the old Points car ignitions a high.power coil could be very hot if the engine was not running.

The transfer of the energy from the primary to the secondary and spark plug is of course the complicated part

This will depend on the coupling between primary and secondary and also on the resonance caracteristics of the two vindings.

The resonans of both will control this tranfer and the length of time the spark lasts.

The old points-condensor-coil systems in cars did nor work well with a bad condenser.

Just my thoughts!

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Sorry, but I'm going to have to stick with what I said on this one. I tried to look up some simple references to cite here but the textbooks that deal with fields are pretty hard to digest in a paragraph or two. One confusing part of this is you can talk about the total MMF -- apmpere-turns -- for the whole device, but you can also talk about the H-field intensity -- ampere turns per inch -- of a differential element within the flux field. It's easy to get a headache trying to keep all this straight.

DK, the analogy between electrical current flowing in an electrical circuit with resistance and a magnetic field "flowing" in a magnetic circuit with reluctance is technically accurate, but most every text book fails by

not talking about these equivalences:

electrical.....................magnetic

EMF(volts)...................MMF(amp-turns)

resistance(ohms)........reluctance(henries)

conductance()............permeability() --- reciprocal of resistance/reluctance

current(amps).............flux(webers)

density(A/mm^2)........field(gauss)

these equivalences are really simple, take the time to get acquainted with them,

especially the difference between flux (total-over-entire-cross-section-area) and

field (per-unit-cross-section-area), there's no headache here !!!

then, the notion that 1/2 the energy is stored in the gap doesn't fly because it utterly

fails when you try to make the analogy with electrical current, the gap is a magnetic

resistance, IE reluctance, and resistors just don't store energy.

PS, the Oersted, amp-turns-per-inch, is for a long solenoid, the longer the solenoid

the more amp-turns you have, but also the longer the field lines have to travel through

the reluctance of air, so the field strength is independent of the coil-plus-core length,

its a very confusing unit because it combines two concepts, the strength of the coil

(ampere-turns) with the reluctance of the medium in the coil, with the implicit assumption

that the length of both varies identically. Or at least for me its confusing and distracting

because there's no good analogy in electrical circuits. OK, here's one, if you stack up

batteries in series you can measure volts-per-meter(of battery stack), and ask what is

the current if you short them out, which because the batteries all also have the same

resistance and hence resistance-per-meter, turns out to be a constant, same current

no matter how many meters of batteries you have.

Peter, you may be interested in the table I put together for a working paper on magnetos about 15 years ago. That working paper and some others are available on my website www.dkgsite.com. This stuff is scattered all over in textbooks and I found it useful to have it all in one place for easy reference.DK, the analogy between electrical current flowing in an electrical circuit with resistance and a magnetic field "flowing" in a magnetic circuit with reluctance is technically accurate, but most every text book fails by

not talking about these equivalences:

electrical.....................magnetic

EMF(volts)...................MMF(amp-turns)

resistance(ohms)........reluctance(henries)

conductance()............permeability() --- reciprocal of resistance/reluctance

current(amps).............flux(webers)

density(A/mm^2)........field(gauss)

these equivalences are really simple, take the time to get acquainted with them,

especially the difference between flux (total-over-entire-cross-section-area) and

field (per-unit-cross-section-area), there's no headache here !!!

then, the notion that 1/2 the energy is stored in the gap doesn't fly because it utterly

fails when you try to make the analogy with electrical current, the gap is a magnetic

resistance, IE reluctance, and resistors just don't store energy.

PS, the Oersted, amp-turns-per-inch, is for a long solenoid, the longer the solenoid

the more amp-turns you have, but also the longer the field lines have to travel through

the reluctance of air, so the field strength is independent of the coil-plus-core length,

its a very confusing unit because it combines two concepts, the strength of the coil

(ampere-turns) with the reluctance of the medium in the coil, with the implicit assumption

that the length of both varies identically. Or at least for me its confusing and distracting

because there's no good analogy in electrical circuits. OK, here's one, if you stack up

batteries in series you can measure volts-per-meter(of battery stack), and ask what is

the current if you short them out, which because the batteries all also have the same

resistance and hence resistance-per-meter, turns out to be a constant, same current

no matter how many meters of batteries you have.

Last edited:

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I owe you at least an explanation of the difficulties I see with your analogies. Reluctance and MMF are often used as in your analogy to measure an entire magnetic circuit as the summation of all flux in the circuit and all mmf applied everywhere around the loop. However, the concepts of reluctance and MMF are also applicable to a magnetic field in any small element of space within the overall magnetic circuit. Those vary all over the map depending on where in the field you measure them (like inside or outside a solenoid, in iron or in air, etc.). If you want to determine the energy stored in any little element of a magnetic field you must look at the flux and the MMF OF THAT ELEMENT and use the equation for stored energy; W= [flux x mmf] / 2.DK, the analogy between electrical current flowing in an electrical circuit with resistance and a magnetic field "flowing" in a magnetic circuit with reluctance is technically accurate, but most every text book fails by

not talking about these equivalences:

electrical.....................magnetic

EMF(volts)...................MMF(amp-turns)

resistance(ohms)........reluctance(henries)

conductance()............permeability() --- reciprocal of resistance/reluctance

current(amps).............flux(webers)

density(A/mm^2)........field(gauss)

these equivalences are really simple, take the time to get acquainted with them,

especially the difference between flux (total-over-entire-cross-section-area) and

field (per-unit-cross-section-area), there's no headache here !!!

then, the notion that 1/2 the energy is stored in the gap doesn't fly because it utterly

fails when you try to make the analogy with electrical current, the gap is a magnetic

resistance, IE reluctance, and resistors just don't store energy.

PS, the Oersted, amp-turns-per-inch, is for a long solenoid, the longer the solenoid

the more amp-turns you have, but also the longer the field lines have to travel through

the reluctance of air, so the field strength is independent of the coil-plus-core length,

its a very confusing unit because it combines two concepts, the strength of the coil

(ampere-turns) with the reluctance of the medium in the coil, with the implicit assumption

that the length of both varies identically. Or at least for me its confusing and distracting

because there's no good analogy in electrical circuits. OK, here's one, if you stack up

batteries in series you can measure volts-per-meter(of battery stack), and ask what is

the current if you short them out, which because the batteries all also have the same

resistance and hence resistance-per-meter, turns out to be a constant, same current.

no matter how many meters of batteries you have.

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I owe you at least an explanation of the difficulties I see with your analogies. Reluctance and MMF are often used as in your analogy to measure an entire magnetic circuit as the summation of all flux in the circuit and all mmf applied everywhere around the loop. However, the concepts of reluctance and MMF are also applicable to a magnetic field in any small element of space within the overall magnetic circuit. Those vary all over the map depending on where in the field you measure them (like inside or outside a solenoid, in iron or in air, etc.). If you want to determine the energy stored in any little element of a magnetic field you must look at the flux and the MMF OF THAT ELEMENT and use the equation for stored energy; W= [flux x mmf] / 2.

DK, ok, now we're getting somewhere, your education had Maxwell's Equations in differential form rather than the original not differential form of Faraday's Law, Ampere's Law, Gausses Law, etc.

Maxwell's equations are great for deriving that electro-magnetic waves are self-perpetuating and travel at the speed of light, but they aren't practical for basic transformer design. What, for example is the MMF over a small element of space not inside the coil, eg the other legs of the transformer core and the air gap if its not inside the coil. This isn't a productive way to look at the problem of transformer design, and is probably why it makes your head ache thinking about it.

Instead, it has to be understood that the magnetic lines of flux always form loops (they are continuous and closed, IE they have no starting or ending points) and for all practical purposes all those lines of flux flow through both the iron core and the air gap. In particular the iron core and the air gap are like a series circuit, so we can compute their reluctances separately and the combine them with 1/ (1/R1 + 1/R2) for a total reluctance, and similarly for total energy as a combination of individual energies.

for the Edison Bipolar Dynamo I designed, the iron core had a reluctance of about 423, or 1-over-that is 2.36 milli-Henries, and the air-gap (between the stator and the rotor) had a reluctance of about 161,000, or 1-over-that is 6.21 micro-Henries.

so we have two inductors in series, one with 2.36 milli-Henries and the other with 6.21 micro-Henries, and I dare say that the energy stored in each isn't going to be 50:50, it will be more like 300:1 IE the ratio of inductances.

I can understand your frustration, Maxwell's Equations aren't useful for this type of problem, and I fault our education system, I tried to figure out transformer and more specifically generator/dynamo design by referring to my college physics text books and that didn't work out so well for me either.

HTH,

let me know if you still disagree,

Peter.

You are free to use your own theories in your own way. The whole question of how energy is stored in a magnetic field is an academic sidetrack. I'm going to move on. Some day maybe we can meet and have more discussion on this very interesting topic.

Don

You might want to consider theory around a fly-back transformer. When a core becomes saturated the magnetic field is distorted around the gap in a ferromagnetic material. This distortion will collapse yielding an additional energy. You will find a flyback transformer on CRTs. I just removed one last night to experiment with. An extra winding is put around the secondary to feed back this energy into the primary. This will boost the voltage output over a normally wound transformer. I wanted the ferromagnetic material more then I wanted the winding and circuits. I went down this rabbit hole because i am working on a magnetic loop antenna and wanted to know more about magnetic circuits. and ran across fly back transformer that used this property. It is a very real phenomena.

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