Aftermarket Coils

[trbig]

I found a set of aftermarket coils from Bob Jones' garage. I was told I need 3 ohm coils for the XJ, but these are 6 ohm. I was told you could use the 6 ohm.. more ohms won't hurt, but that it just couldn't be done the other way around... that you couldn't run 3 ohm if you needed 6.

Thoughts?

Tod

[skids]

Yes Todd, something sounds fishy there.

[DGXSER]

I am no electrical guru, but that sounds wrong to me. Then again, with all the parts bikes you have, whats a fried TCI or two?

[MacMcMacmac]

You can go up in ohms, but not down. Too little resistance can lead to excessive currents and burnt components. Higher ohms will reduce the spark energy a little.

[crazy steve]

If you're running a ballast resistor, you should disconnect that. As 'mac' said, more ohms is ok, it's less that fries stuff.

'78E original owner

[trbig]

No ballast resistors on the XJ.

Thanks guys. Ivan, sorry I questioned your wisdom..


Tod

[Ivan]

Ok, so lets look at this analytically. Lots of people don't understand the fikle beast that is the electron. That's understandable, since it isn't visible, and can't be seen like water flowing. Its more like wind, but with less evidence to its being.

So, we have always heard that the dangerous part of being electrocuted is not the voltage, its the current, or amperage. The amperage is like the volume of water in a river. water moving at 10 mph but only 2" deep won't knock you over, but a 10 mph riptide will drown even the best swimmer. So amperage is the total amount of electrons passing a load at any instant. "What's load?" you say...

Load is resistance. Measured in Ohms. It is what opposes current flow. It is like a dam in a river, or a restriction in a pipe. This is where the electron gets put to work. Useful loads are things that convert electricity into something else like light or motion. Unuseful loads are things that turn electricity into undesired byproducts. Loose connections create heat instead of letting the starter create motion. Either way, they oppose the flow of electrons. The only way to overcome a load is to have enough current coupled with enough potential. Wait...Potential?

Potential is voltage. Its the amount of work that is stored in the current. It is the speed of the river, or more precisely the water being held in the tower. A battery can have 1.5 volts, 12 volts, or a million volts and won't do squat until those electrons get to move. Just like water in a tower, it won't be of any use until someone in a house turns on a valve. Voltage is energy at the ready. There must be enough voltage to overcome the resistance of the load to create current. There. All three aspects in one sentence.

So, now we know what each is, but we need to understand the interaction between them all. Thankfully it is all in simple ratios. Via the equation V=IR, we know that voltage is directly proportional to the product of the current and the resistance. Right now, you're probably thinking, "Hey, that looks strangely similar to that other equation about work and force and mass." And, you'd be right. F=MA. According to the law of conservation of energy, it really is the same thing, just in a different form.

Now we need to understand what is going on with Tod's coils. Will it damage the TCI? To know that, we need to check his current. The TCI was designed to only handle a certain amount of current. All electronics are like that. It gets into different classes and other weird ISO definitions that we really don't need to know to fix this problem. So, we know that the bike runs 12V nominal, and the stock coils are 3 ohms. So, substituting in our equation, we fint that 12=I x 3. And that I = 4 amps. So, at 100% duty cycle, the coils will take 4 amps to energize. That's not going to be the case in operation, since they switch off and on all the time, but for our purpose it works great. We need to know if Tod's new coils will make less or more amperage in the circuit. So, since the system voltage is constant at 12V and his new coils are 6 ohms constant, the only thing that will change is the current, due to the change in resistance between coils. 12=I x 6. So, I=2. Less operating current. Now, since the current has not went up, but down, we know the TCI will be pleasantly able to keep its magic smoke inside where it will make all those solid state pieces work wonderfully.

So, the coils can be safely put in the system without fear of damage to the TCI. Wether or not they lead to a brighter and more usable spark depends on a lot more factors, however, I suspect they will.

[randy]

Thank you, Ivan for a very lucid, detailed yet simple explaination of Ohm's law.

Other than the normal care in making clean and tight connections, the only major factor that would cause these coils to work differently than the stock coils is the ratio between the primary and secondary windings.

The ignition coil is simply a transformer. Like the one you use to charge your cell phone is a step-down transformer, converting 120VAC to 6v or 12v or something. The ignition coil is a step-up transformer, converting 12V to thousands of volts. The ratio between the primary and secondary windings determines the output voltage. Higher ratio = higher voltage. Some coils' specs are published. The service data specs for the XS Standard coil are Pri- 1.5ohms, Sec- 15K ohms for a ratio of 1:10,000. Find a higher ratio and you'll have a hotter spark.

[davidsymons53]

Ivan that is a beautiful peace of prose you put together. You took a very complex concept and boiled it down to it most simple explanation.

I have copied it and save it for future reference.

Thanks, David

[cywelchjr]

Thank you, Ivan for a very lucid, detailed yet simple explaination of Ohm's law.

Other than the normal care in making clean and tight connections, the only major factor that would cause these coils to work differently than the stock coils is the ratio between the primary and secondary windings.

The ignition coil is simply a transformer. Like the one you use to charge your cell phone is a step-down transformer, converting 120VAC to 6v or 12v or something. The ignition coil is a step-up transformer, converting 12V to thousands of volts. The ratio between the primary and secondary windings determines the output voltage. Higher ratio = higher voltage. Some coils' specs are published. The service data specs for the XS Standard coil are Pri- 1.5ohms, Sec- 15K ohms for a ratio of 1:10,000. Find a higher ratio and you'll have a hotter spark.

While your right about the ratio being the key to how much the step up is, the resistance does not have a direct relation here. There will be some since a given length of wire will have a give resistance per foot, but that also depends on the size of the wire as well. So, I don't know for sure if the XS coils are 1:10,000 or not, as I would need to have the spec for how many turns are in each winding. You can at least know the ratio by knowing the input voltage vs the output, but even there you have to account for loss from the coupling. You won't get 10,000 volts out for 1 volt in, but something less than that because of the loss in converting the current/voltage to a magnetic field and back. In addition, under operation, when the input current changes there is a reverse voltage induced in the primary windings as well, which give you impedence, which is the effective ac resistance of the coil, which will vary somewhat with the frequency of the source (this is all intended to make it harder to understand of course).

[Ivan]

Cy, you forgot the parts about coil oscillation, voltage spike, and especially the 1.21 gigawatt flux capacitor.

[Cobia]

That's too much Ivan, you're going for the jugular now.......he, he!!
Thanks for the very eloquent, yet simple, explanation of the electron flow.
It really elucidated on a dark subject for me.

[crazy steve]

...the resistance does not have a direct relation here.... So, I don't know for sure if the XS coils are 1:10,000 or not, as I would need to have the spec for how many turns are in each winding...

The resistances won't really tell you anything, other than a rough idea as to how much current it will draw/deliver. The turns ratio is the important thing, and most 12v ignition coils have roughly 1:3000 ratios, with some more (notably hi-po versions), some less. A typical output voltage will be in the 20-25 KV range. The highest secondary voltage you'll generally see is 40KV, as it gets extremely hard to properly insulate everything above that. Generally, the higher the primary resistance, the 'colder' the spark. A 'bigger' turns ratio for higher voltage helps, but the amount of current delivered is what does the work.

And impedence is just one item that effects these; saturation, inrush current, and core losses are factors too. In most 'transformers', only a 10% loss is considered very good; I would expect most ignition coils are lucky to hit 80% efficiency, more likely closer to 70% or even less.

'78E original owner