Tests of CDI Ignition Modules

[74Sprint]

Ray, you are confused, you said:
"A capacitor stores voltage! You can not step up voltage but, you can step up current using a transformer aka ignition coil."
no, a capacitor stores charge, and no, an ignition coil does not step up current it steps up voltage (and steps down current). and from there things just get worse...
Peter.

Yah Peter it was a late night. Yes I blew that one. And yes you can step-up both voltage and current but, inversely. Yes a capacitor stores a charge but, current is not the major side of the charge. It's just like an inductor leans more toward current. Certain capacitors that are good at storing a charge however usually have a rating Dv/Dt on how fast you can safely charge a storage capacitor. In fact some of the big capacitors we used on the locomotive alternators for noise suppression use to once and awhile puke their guts out if the traction motors got disconnected under full load and gen voltage spiked. The metal film charge capacitors that I use have a 300v/usec. Dv/Dt and 8mOhm ESR rating and they have been holding up.

I have attached a PDF file and in it, it explains the ringing and what causes it. I think it explains it better than I did.

Ray

 

[DKGrimm]

Yah Peter it was a late night. Yes I blew that one. And yes you can step-up both voltage and current but, inversely. Yes a capacitor stores a charge but, current is not the major side of the charge. It's just like an inductor leans more toward current. Certain capacitors that are good at storing a charge however usually have a rating Dv/Dt on how fast you can safely charge a storage capacitor. In fact some of the big capacitors we used on the locomotive alternators for noise suppression use to once and awhile puke their guts out if the traction motors got disconnected under full load and gen voltage spiked. The metal film charge capacitors that I use have a 300v/usec. Dv/Dt and 8mOhm ESR rating and they have been holding up.

I have attached a PDF file and in it, it explains the ringing and what causes it. I think it explains it better than I did.

Ray

RAY, that attachment appears to be a very comprehensive overview of coil type systems from a textbook. I had not seen it before. It's going to take some serious quiet time to absorb what it has to say. What is the source?

Don

 

[74Sprint]

RAY, that attachment appears to be a very comprehensive overview of coil type systems from a textbook. I had not seen it before. It's going to take some serious quiet time to absorb what it has to say. What is the source?

Don

To be honest I can't remember. If I do I'll let you know.

 

[indexcuttermill2]

Ray, you are confused, you said:
"A capacitor stores voltage! You can not step up voltage but, you can step up current using a transformer aka ignition coil."
no, a capacitor stores charge, and no, an ignition coil does not step up current it steps up voltage (and steps down current). and from there things just get worse...
Peter.

I've been following this thread with interest because I use both inductor and capacitor ignition systems. I usually watch these forums and don't chime in very often. The question of what a capacitor stores in really semantics. Capacitors sore voltage and they also store charge and they also store energy. With capacitors, all three, voltage, charge and energy are related. When pass current through a capacitor for period of time, it will have a voltage, and the voltage will remain (stored) for some time depending on leakage. A current passing through a capacitor will also put a charge on the capacitor, Q is usually used to signify charge, and the units of charge are coulombs. The charge put on a capacitor is dependent the current and time ie. Q=I xT. The relationship between charge and voltage is Q= C x V, where C is capacitance, or stated another way is V = Q /C. So in addition to voltage and charge being stored on a capacitor, you also have energy stored on a capacitor. The energy stored in a capacitor as has been frequently stated is E =1/2 x C x V^2. The energy stored can also be stated as E = 1/2 x Q^2 / C.
So in terms of what a capacitor stores, well all are correct, voltage, charge, energy.

 

[74Sprint]

The problem with all this is that this is a forum for marching engines and when it comes to ignitions how much information is too much? I know that for myself I sometimes go too far and give out information that is well, irrelevant and not required for the end purpose. Then there is how much ignition is too much? It's been my experience that CDI's are only needed for high compression engines (@>10:1) or high RPM ones (@>6,500 RPM). I totally believe in the 'KISS' system. After testing ignitions that range in price from $40 to $900, I find the LS1-3 ignitions (LDI) the best overall. As for small engines say up to 250cc, most of the Chinese ignitions are terrible. Anyone who needs or wants to make an ignition should go onto the internet and go to websites like RCGroups and see what the R/C people are using and saying. There is a lot more of them than us. I don't know why but a lot of people seem to want a CDI system and nothing else. If it's small size then it boils down to just the ignition coil. There are LDI coils out there that are small or one could even wind their own.

Finally, On my race car, I run a 455 CID Oldsmobile big block with 10.25:1 compression. I turn that sucker to 6,500 RPM even though everyone tells me you can't turn a BB Olds more than 5,500, yah right. I also use a stock GM 4 wire, coil-in-the-cap dizzy with copper core sparkplug wires. This is a stock 1970 engine, except cam, lifters, springs, intake, curve kit, carb, bearings, piston rings, oil pump, and headers. Everything else is from 1970, block, rods, pistons, and crank, I also run on 100 octane fuel. Do I need a CDI, nope.

So use the time constants to find the biggest capacitor that you can charge for the RPM you want to run, bigger is better. Then use the time constants to find an ignition coil to match and you'll be fine.

Ray

 

[DKGrimm]

The problem with all this is that this is a forum for marching engines and when it comes to ignitions how much information is too much? I know that for myself I sometimes go too far and give out information that is well, irrelevant and not required for the end purpose. Then there is how much ignition is too much? It's been my experience that CDI's are only needed for high compression engines (@>10:1) or high RPM ones (@>6,500 RPM). I totally believe in the 'KISS' system. After testing ignitions that range in price from $40 to $900, I find the LS1-3 ignitions (LDI) the best overall. As for small engines say up to 250cc, most of the Chinese ignitions are terrible. Anyone who needs or wants to make an ignition should go onto the internet and go to websites like RCGroups and see what the R/C people are using and saying. There is a lot more of them than us. I don't know why but a lot of people seem to want a CDI system and nothing else. If it's small size then it boils down to just the ignition coil. There are LDI coils out there that are small or one could even wind their own.

Finally, On my race car, I run a 455 CID Oldsmobile big block with 10.25:1 compression. I turn that sucker to 6,500 RPM even though everyone tells me you can't turn a BB Olds more than 5,500, yah right. I also use a stock GM 4 wire, coil-in-the-cap dizzy with copper core sparkplug wires. This is a stock 1970 engine, except cam, lifters, springs, intake, curve kit, carb, bearings, piston rings, oil pump, and headers. Everything else is from 1970, block, rods, pistons, and crank, I also run on 100 octane fuel. Do I need a CDI, nope.

So use the time constants to find the biggest capacitor that you can charge for the RPM you want to run, bigger is better. Then use the time constants to find an ignition coil to match and you'll be fine.

Ray

I dunnow, Ray. Seems to me the forum covers an incredible range of subjects. The uninteresting threads don't last long.

 

[Vietti]

Ray,

Do you have a link to a good ignition system for a model engine?? Googled LS1-3 and seems to be a system for autos.

 

[willray]

The question of what a capacitor stores in really semantics. Capacitors sore voltage and they also store charge and they also store energy...

I'm sorry, I'm sorry, I'm sorry - I should just shut up, I know better, but...

No, this is not just semantics. Capacitors store charge. This is physics. Voltage exists because a charge is stored and therefore there is a charge differential across the dielectric.

One can say "well, you can go 'round the circle any way you want, from any starting point with the math", and this is true, but ignoring the underlying physics is a road to eventual confusion. Saying "Capacitors store voltage" is a lot like saying that the gas tank in your car stores torque (modulo a unit of length).

Now I'll shut up...

 

[indexcuttermill2]

I'm sorry, I'm sorry, I'm sorry - I should just shut up, I know better, but...

No, this is not just semantics. Capacitors store charge. This is physics. Voltage exists because a charge is stored and therefore there is a charge differential across the dielectric.

One can say "well, you can go 'round the circle any way you want, from any starting point with the math", and this is true, but ignoring the underlying physics is a road to eventual confusion. Saying "Capacitors store voltage" is a lot like saying that the gas tank in your car stores torque (modulo a unit of length).

Now I'll shut up...

I didn't intend to open up a can of worms with only one post, and add to further clutter in this thread.
This is a rhetorical question, no need for a response but you are certainly at liberty to do so.
Are you saying emphatically that capacitor can store only a charge, and never a voltage?
Are you stating that capacitors store only charge, and never energy.
Think about what an inductor stores, and whether it stores energy or not.
These are questions hopefully to promote thought and not an argument. I will bow out from further posts to this thread.

 

[willray]

I didn't intend to open up a can of worms with only one post, and add to further clutter in this thread.
This is a rhetorical question, no need for a response but you are certainly at liberty to do so.
Are you saying emphatically that capacitor can store only a charge, and never a voltage?
Are you stating that capacitors store only charge, and never energy.
Think about what an inductor stores, and whether it stores energy or not.

Ok, so I'm not smart enough to stay shut up... I apologize for the digression, but hopefully some precision and accuracy in the use of terms will help some with understanding exactly what's going on in CDI and LDI systems.

Yes, I am saying emphatically that a capacitor stores charge, or more precisely a difference in charge. Voltage and energy are properties of the system due to the fact that there are separated charges that are stored.

This isn't just word games. What "the thing is", and what are "properties that are due to the thing" matter.

Let me move this into a realm where day-to-day physical intuition is helpful.

Imagine you have a rock. The rock stores mass. That's all the rock stores.

Now put the rock at the top of a hill. Assume gravity exists. The rock now "has potential energy". The more mass you store in the rock, the more potential energy it has.

Is the rock storing the potential energy? Is the hill storing the potential energy? What about the gravity?

Keep the rock exactly where it is, but bulldoze the valley below it full. Suddenly, no potential energy. We didn't change the rock at all, so it can't be the rock that's storing the potential energy.

Keep the hill exactly as it was but make the rock smaller. Suddenly, less potential energy. We didn't change the hill at all, so it can't be the hill storing the potential energy. We also didn't change gravity in either of these, so the energy isn't stored in the gravity.

The answer to all 3 questions is "no", the potential energy isn't stored in the rock, the hill, or in gravity.

The potential energy is instead a property of the combined system of the rock, hill and gravity.

If one misunderstands the underlying physics and just says "well, I can work out any value from the others, so they're interchangeable", one ends up in situations where it looks like one can create or destroy energy without touching the thing one claims contains the energy. This way, madness lies.

Like rocks store mass, capacitors store difference in charge. The closest I can currently get to physical-intuitioning the capacitor situation would be the following:

[edit - crap - Friday afternoon brain is really toast! pull, not push!]

Imagine after you've charged your capacitor, if you grab the two plates, and physically move them apart. Suddenly there is more stored energy, even though you haven't changed what you put in. The same charges/difference-in-charge continues to exist, but the energy changes (For the physicists, we're going to ignore the fact that I did work on the system when I separated the plates - I know the energy did not just appear...) The same _physical_ stuff is still stored, but the energy changes because the overall system changed. Move the plates back together, and the energy is back to where it started. Still haven't changed what's literally stored - an excess of charges on one plate and a depletion on the other - but the energy again changes. Ergo, the energy is a property of the complete system.

It's not a perfect analogy, but it's the best I can do between things on a Friday afternoon.

... my first inclination is to say that inductors store a magnetic field, and that it's the magnetic field that stores energy, but I'll need to ponder that a bit before I'm sufficiently confident to argue specifics.

 

[DKGrimm]

After a lifetime of electrical engineering, I am probably not sensitive enough to the need others have for analogies in talking about coils and capacitors and stuff like that. A friend said all you need to know about engineering is F=ma and you can't push a rope. For pure capacitors, Q=CV, I=C (dv/dt), and W = (CVsq)/2. If you think about it, that set of relationships will always be able to answer any question (except what happens if you move the plates apart).

 

[willray]

After a lifetime of electrical engineering, I am probably not sensitive enough to the need others have for analogies in talking about coils and capacitors and stuff like that. A friend said all you need to know about engineering is F=ma and you can't push a rope. For pure capacitors, Q=CV, I=C (dv/dt), and W = (CVsq)/2. If you think about it, that set of relationships will always be able to answer any question (except what happens if you move the plates apart).

Well, even the underlying physics can't tell you what happens when you move the plates apart if your brain is in sideways on a Friday afternoon!

 

[Richard Hed]

I'm sorry, I'm sorry, I'm sorry - I should just shut up, I know better, but...

No, this is not just semantics. Capacitors store charge. This is physics. Voltage exists because a charge is stored and therefore there is a charge differential across the dielectric.

One can say "well, you can go 'round the circle any way you want, from any starting point with the math", and this is true, but ignoring the underlying physics is a road to eventual confusion. Saying "Capacitors store voltage" is a lot like saying that the gas tank in your car stores torque (modulo a unit of length).

Now I'll shut up...

Finally I will put in my half cent worth. since electricity is such a difficult subject to some peeps, I will tell you how to thimpfk of electricity. It is EXACTLY like the water in a dam-river system. In fact, in some instances the same terms are used because the first discoverers of electricity thot that it was a fluid. ANd it IS, in a way.

If you have the Grand Coulee Dam in your back yard, which I do, my back yard being 90 miles away, you will notice that at the very top of the dam, where the water is one inch deep, there is very little pressure, but at the botom of the dam (me thimpfks it is about 600 feet), there is enough pressure to literally cut a body in half with a tiny leak. This pressure, whether high or low in electricity is called Voltage.

You also get a "flow" of electricity just as a river flows called, of all things, Current! Just as Johnson Creek (a tiny creek that dumps into the Deschutes River in the Soviet of Washington which is where Olympia beer got it's famous water), this trickle of water was about an inch deep and 18" wide, it had salmon in it when I was a kid (last week) but no longer. This trickle has flow or current but it is very small compared to the mighty Columbia--Hell, more water evaporates off the Columbia in a minute than ten or twenty of these tiny creeks could pass in a minute--wich also addresses "loss" in electric lines. Any way, flow is the AMOUNT of water over a dam (or past any point in a river), the same in electrics, only it is called Amps, after senor Ampere. In water flow it is cubic feet per minute or cubic meters/min. Please notice that if you have a dam a mile wide (like the Grand Coulee), you could have a huge amount of flow just being an inch deep, this is sort of how much of modern electronics works--that is, with LOW voltage but a lot of flow (amps) relatively speaking to what it accomplishes. In the case of the soviet of Washington, a lot of little canals are created (originally with very high pressure) to feed the mighty Columbia Basin Project started just after the war which feeds a lot of the nation.

In the case of capacitors, it is virtually exactly as a dam--not the dam itself but the reservoir behind it. A capacitor IS a reservoir of electric pressure (voltage) with a potential ability for flow (current) and naturally the result of volume when allowed to flow (amps). Depending how you release the capacitive reservoir, you get a large damaging spark (hot melted metal ligterally flies) or you get a small useful bit of electricity when blockt with a resistor. The resistor being equivalent to a water spout--whether the water spout is a garden hose or a gigantic spigot (don't know what they are called) that the mighty Grand Coulee uses. And just as a dam collects water from run-off in the winter and spring and then releases it for use in watering the local economy and generating electricity, so does a capacitor collect a charge after it has discharged and again it is how much resistance is added to the circuit which determines it rate of re-charging.

The way a transformer works is very interesting, however, I don't want to get into that. I just wish to say that with very low river flow, you can also build higher pressure than the river delivers itself--it's with a type of tube pump which allows the water to flow thru it, building up water momentum then it quickly closes a trap door--the momentum however, is used to push a small amount of water up a side pipe. My grand-father got his water that way off the Deschutes River for something like 50 years. Probably pattently illegal now, STEALING WATER!

There is only one type of action that electricity does, to my knowledge, that water cannot do and that is the action of an electric coil called a choke in which the electric pressure of each coil acts like a resistor on all the other coils--a pressure to slow down the electric flow.

 

[HMEL]

I've been using S/S CDI ignition modules for years. With Roy Sholl retiring a lot of people are wondering where to go for substitute CDI modules. At the same time there is quite a bit of enthusiasm in HMEM and other places for inductive coil systems and for the Sage-Gedde driver module for those. Following discussions with several people at the Black Hills engine show in September I decided to launch a project to study and compare both kinds of systems to see just what the advantages and disadvantages of each system may be. There's a long way to go before I get enough data to answer questions, but I started by looking at CDI modules. I begged, borrowed, bought -- but did not steal -- as many of those as I could lay my hands on to run the tests. I ended up with sixteen modules, four of which were broken, which gave me data on twelve.

The tests involved some highfalutin test equipment including a sampling oscilloscope with a 50 MHz bandwidth and 1 gigasamples/second sampling rate. I used data from that to compute the total energy that ended up heating the plasma of the spark itself. That energy is measured in millijoules, which I think everyone has at least heard of. (A joule is a watt-second.) One of the most interesting things I learned was that only a quarter or so of the total energy spit out by the module ended up in the spark itself. I have read suggestions on HMEM that a CDI pulse is too short to really give a hot spark. Right now, I think there may be some truth in that.

This is all preliminary stuff, but I summarized the work on CDI modules in a short report, attached below. I'm looking for critique on all this stuff. Let me know what you think.

Don

Don: I have for some time considered building a small magneto to obtain a hot spark. I would not be able to gear it to the crank but it would have to be modified with an electronic sensor and a coil. How small of a footprint would I have to make it for it to be practical. The issue such a design proposes is one of physical size to work. I recently have been playing with a tiny drone and never expected the motors to be so small. And it got me to thinking maybe it is doable.

 

[DKGrimm]

Don: I have for some time considered building a small magneto to obtain a hot spark. I would not be able to gear it to the crank but it would have to be modified with an electronic sensor and a coil. How small of a footprint would I have to make it for it to be practical. The issue such a design proposes is one of physical size to work. I recently have been playing with a tiny drone and never expected the motors to be so small. And it got me to thinking maybe it is doable.

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.

 

[DKGrimm]

Finally I will put in my half cent worth. since electricity is such a difficult subject to some peeps, I will tell you how to thimpfk of electricity. It is EXACTLY like the water in a dam-river system. In fact, in some instances the same terms are used because the first discoverers of electricity thot that it was a fluid. ANd it IS, in a way.

If you have the Grand Coulee Dam in your back yard, which I do, my back yard being 90 miles away, you will notice that at the very top of the dam, where the water is one inch deep, there is very little pressure, but at the botom of the dam (me thimpfks it is about 600 feet), there is enough pressure to literally cut a body in half with a tiny leak. This pressure, whether high or low in electricity is called Voltage.

You also get a "flow" of electricity just as a river flows called, of all things, Current! Just as Johnson Creek (a tiny creek that dumps into the Deschutes River in the Soviet of Washington which is where Olympia beer got it's famous water), this trickle of water was about an inch deep and 18" wide, it had salmon in it when I was a kid (last week) but no longer. This trickle has flow or current but it is very small compared to the mighty Columbia--Hell, more water evaporates off the Columbia in a minute than ten or twenty of these tiny creeks could pass in a minute--wich also addresses "loss" in electric lines. Any way, flow is the AMOUNT of water over a dam (or past any point in a river), the same in electrics, only it is called Amps, after senor Ampere. In water flow it is cubic feet per minute or cubic meters/min. Please notice that if you have a dam a mile wide (like the Grand Coulee), you could have a huge amount of flow just being an inch deep, this is sort of how much of modern electronics works--that is, with LOW voltage but a lot of flow (amps) relatively speaking to what it accomplishes. In the case of the soviet of Washington, a lot of little canals are created (originally with very high pressure) to feed the mighty Columbia Basin Project started just after the war which feeds a lot of the nation.

In the case of capacitors, it is virtually exactly as a dam--not the dam itself but the reservoir behind it. A capacitor IS a reservoir of electric pressure (voltage) with a potential ability for flow (current) and naturally the result of volume when allowed to flow (amps). Depending how you release the capacitive reservoir, you get a large damaging spark (hot melted metal ligterally flies) or you get a small useful bit of electricity when blockt with a resistor. The resistor being equivalent to a water spout--whether the water spout is a garden hose or a gigantic spigot (don't know what they are called) that the mighty Grand Coulee uses. And just as a dam collects water from run-off in the winter and spring and then releases it for use in watering the local economy and generating electricity, so does a capacitor collect a charge after it has discharged and again it is how much resistance is added to the circuit which determines it rate of re-charging.

The way a transformer works is very interesting, however, I don't want to get into that. I just wish to say that with very low river flow, you can also build higher pressure than the river delivers itself--it's with a type of tube pump which allows the water to flow thru it, building up water momentum then it quickly closes a trap door--the momentum however, is used to push a small amount of water up a side pipe. My grand-father got his water that way off the Deschutes River for something like 50 years. Probably pattently illegal now, STEALING WATER!

There is only one type of action that electricity does, to my knowledge, that water cannot do and that is the action of an electric coil called a choke in which the electric pressure of each coil acts like a resistor on all the other coils--a pressure to slow down the electric flow.

Your pressure:voltage and current:flow analogies were where I started decades ago, and it works pretty well. I think the dam:capacitor analogy is also very useful. I wouldn't give up completely on an analogy for an inductor. My own was to think of an inductor as a long pipe. If you neglect friction/electrical resistance you can imagine that the flow:current cannot be changed except by putting pressure:voltage between the ends of the pipe:inductor. These ideas have been helpful to me, but one has to be cautious about extrapolating the analogies into more complex situatons.

 

[74Sprint]

Ray,

Do you have a link to a good ignition system for a model engine?? Googled LS1-3 and seems to be a system for autos.

One to make or one to buy?

Ray

 

[74Sprint]

I think if we're going to be on the topic of capacitors we should mention for the layman that there are different types of capacitors that use different materials for different applications. Not all capacitors store a charge, some like ceramics and there are others (Mica and PTFE) that are best used to pass through a sine wave voltage (signal) such as a coupling/decoupling capacitor. These capacitors can store a charge but, they tend to have high leakage and self discharge quickly. Ceramic capacitors also are not polarity conscious, so you can't hook them up backwards. On the other hand there are storage capacitors, also generally called electrolytic capacitors (a bad thing) and these ARE polarity conscious and must have their leads hooked up to the correct polarity and are generally marked somehow. Storage capacitors are generally made from materials such as tantalum and metal foils that are separated by thin plastics and even paper. Generally all capacitors have a working AC and/or a working DC voltage rating that one needs to keep in mind and check that it is usable in your design.

As for CDI ignitions we use the ceramic cap ones to pass noise to ground (filtering) or to pass a signal onwards isolating 2 circuits. We use the storage capacitors for 2 reasons. One to absorb large voltage spikes that can appear on the supply lines and through the circuitry to pass that to ground. Two, we need to use the storage capacitors to store the some what high voltage charge that we are going to discharge through the coil, also generally 220 volts or lower. This is where the type of storage capacitor that you use matters. Since we are going to store an ever increasing charge the AC rating is not as important as the DC rating. We need a capacitor where the voltage level is no more than 75% of it's working DC voltage rating for safety reasons. I use non-polarity conscious Metallized Polypropylene (PP), or PEN, or PET, ones because of their high voltage ratings and are considered to be self-healing. If I charge to fast or over charge one (voltage) and it internally shorts it will self-heal and keep going, most of the time. Paper ones on the other hand, I have seen these ones spit their guts out or even catch on fire. I have also seen caps like tantalum and aluminum electrolytic ones short internally and yet from the outside they look fine, but no self-healing. On the aluminum electrolytic ones some times when they short will try to push the rubber end out and it shows a bulge. Sometimes all that's left is the 2 leads sticking out of the board.

There is a lot more science and physics behind all this and how much you want to learn is up to you. As with anything, which capacitor you use and for what purpose is up to you but, please do your research first. I haven't mentioned all the different types of capacitors so be aware of that and yes there really is a flux-capacitor, I've only seen one in the military though and it doesn't look like the one in the movie.

Cheers
Ray

 

[Vietti]

Ray,

Looking for suggestions of the best CDI ignition module for sale. Single spark and wasted spark two spark types for a Hoglet build.

 

[Richard Hed]

Your pressure:voltage and current:flow analogies were where I started decades ago, and it works pretty well. I think the dam:capacitor analogy is also very useful. I wouldn't give up completely on an analogy for an inductor. My own was to think of an inductor as a long pipe. If you neglect friction/electrical resistance you can imagine that the flow:current cannot be changed except by putting pressure:voltage between the ends of the pipe:inductor. These ideas have been helpful to me, but one has to be cautious about extrapolating the analogies into more complex situatons.

I always notice when I write something, I leave a lot out that would make it more clear to those who don't know what I am talking about. It's a problem I have always had--ASSuming that the listener (or reader) knows the references and doesn't need explainations. So re-reading what I wrote, I see glaring points of unclarity.

As for your idea of a tube, it is so novel to me that I will have to thimpfk about that for a while. Thanx for the mental food.