Calculating resistance in complex coils.

[obeythepeople]

I really want to post this in the proper section but I haven't reached my 5 posts yet so...

I'm looking for information on how to calculate resistance complex coils such as claptin wrapped coils or hybrid parallel coils. Unlike twisted coils or standard parallel builds which can be calculated with vapors tool box by selecting the quad coil option, hybrid coils can contain multiple alloys and/or gauges which make building to a specific resistance ( .3 for my raptor box to be specific) difficult.

I suppose I'm looking for a formula to take the guess work out of it. This question is really for advanced vapors, please don't comment unless you have a understanding about ohms law and wire resistances.

 

[Lova]

You have to measure any coil you put on a mod, so there's no point in trying to estimate a coil if it's not either single, twisted or parallel coil.

No formula will give you exact ohms regarding complex coils, they are just estimates, so the final build might be lower in ohms or higher than you thought it would be, so be safe and use either an ohm meter or a digital multimeter and test your builds

 

[SpanishSunday]

In the case of Clapton coils, the length of the cladding wire is so relatively big, and its gauge is so high (or its cross-section so low), that one could discard its contribution to the total resistance of the coil. I think that cladding wire functions more like a wick than like a heating resistor.

You may also use the Steam Engine to calculate resistance of twisted coils, if they are of the same material and the same gauge.

For a coil made of several wire types, its a bit more complicated, but there is a easy workaround.

Lets say you want to calculate the resistance of a twisted coil made of 26 and 28 AWG Kanthal A1.

Go to the calculator, and calculate the resistance of a twisted coil made of 26 AWG Kanthal A1, lets call this value R₁. The resistance of a single strand of this coil would be 2R₁ (twice the resistance of the twisted coil).

Then the same with 28 AWG Kanthal A1, and lets call this value R₂, and the resistance of a single strand of this coil would be 2R₂.

And now, you have to calculate the equivalent resistance value of two different resistors connected in parallel.

Calculate the inverses of 2R₁, and 2R₂, then add these inverse values. The inverse of that addition will be the total resistance of a twisted coil made with 26, and 28 AWG Kanthal A1.

Note that this method works if the gauges of the different wires are pretty similar, albeit the resistivity of the materials could be very different. If you connect in parallel wires of very different gauges, the geometry of the different helixes will be so vastly different to render this method wildly inaccurate.

 

[obeythepeople]

Lova, resistance is a fixed value dependant on material and length that is calculated using a mathematical formula thus there has to be a way to calculate the resistance, like SpanishSunday said. What can't be calculated is how the wire responds to these complex builds. For instance a Clapton coils resistance is Dependant on the lowist resistance wire used in the coil, adding Clapton warp to the coil will not lower or add to the resistance because electricity will always flow with the path of least resistance.

I am not a novice vaper, please don't treat me as such.

 

[classwife]

ummm...someone not had enough coffee today ?



Be respectful of others here please


Thank you !

 

[twgbonehead]

Lova, resistance is a fixed value dependant on material and length that is calculated using a mathematical formula thus there has to be a way to calculate the resistance, like SpanishSunday said. What can't be calculated is how the wire responds to these complex builds. For instance a Clapton coils resistance is Dependant on the lowist resistance wire used in the coil, adding Clapton warp to the coil will not lower or add to the resistance because electricity will always flow with the path of least resistance.

I am not a novice vaper, please don't treat me as such.

This is just not true. Adding a Clapton wrap to your coil WILL lower the resistance. It has less of an effect than the main wire, but it most certainly does have a significant effect.

And calculating it is extremely difficult because it depends not only on the lengths of wire and gauges, but also on how conductive the junction between the two wires is. If the wires were fired separately before doing the Clapton wind, there will be little or no contact between the 2 wires, if they were wound first there is likely to be a high degree of conductivity between the two. This also makes a difference.

I certainly don't see that you were being treated as a "novice vaper" by Lova, and in fact his answer was accurate, and your response was just wrong.

 

[BeerGolfClouds]

This is just not true. Adding a Clapton wrap to your coil WILL lower the resistance. It has less of an effect than the main wire, but it most certainly does have a significant effect.

And calculating it is extremely difficult because it depends not only on the lengths of wire and gauges, but also on how conductive the junction between the two wires is. If the wires were fired separately before doing the Clapton wind, there will be little or no contact between the 2 wires, if they were wound first there is likely to be a high degree of conductivity between the two. This also makes a difference.

I certainly don't see that you were being treated as a "novice vaper" by Lova, and in fact his answer was accurate, and your response was just wrong.

This.

You are not making direct electrical contact with the lower gauge wire using a Clapton coil. Your post contacts will not touch it at all, they will contact the spiral wire surrounding it. So in theory you'd have to account for that at a minimum. But that's not even the whole story...if you were relying on only the heat produced by the lower gauge wire to heat the spiral wire, the whole coil would be terribly flawed and inefficient.

 

[RayofLight62]

1. Calculate the resistance of each coil based on length and wire resistivity (Ohm second law). each wire has a specific resistivity' in ohm/cm.
2. Divide your setup in branches and knots. You can have many branches, but only two knots, connecting to the battery.
3. Coils in electrical series (the branches) are obtained by adding their ohm values. calculate the resistance at the two knots with 1/((1/R1)+(1/R2)+(1/R3)) where R1, R2, R3, Rn are the ohm value of each branch.
The theoretical value so calculated, if the resistivity is correct, can be close to reality within 1 percent. If you want an explanation of the formulas, google for I and II principle of Kirchoff and the Norton theorem.
There is a whole branch of technology dealing with heating coils specific to the design of incandescent light bulbs, get a book as there is very little on Internet. Also, if you dont' master the Joule laws, Ohms laws, Norton theorem and Kirchoff principles, you may not get far in building multiple coils. Also, in some design, air and thermal flows do not pass anymore between the windings, so check the technology books on air thermal flows before building them.

 

[SpanishSunday]

These guys sell pre-made Clapton wire in 4" lenghts:

28 gauge A1 Kanthal core with 32 gauge A1 Kanthal tightly wrapped around

4" of 28 AWG Kanthal A1 have a resistance of 1.757 Ohm @ 68°F. Lets call this R₁.

The length of 32 AWG wire needed to clad 4" of 28 AWG wire is 19.792" (calculated using this), which have a resistance of 21.61 Ohm @ 68°F. Lets call this R₂.

How much power is dissipated in each wire? Using the voltage squared over resistance expression, and a voltage of 4V:

P₁ = 9.1064 W
P₂ = 0.7405 W

So, P₂ is a bit less than 10% of P₁.

Finally, when two conducting bodies are in contact, the electrical contact resistance decreases when one applies more pressure, so if the bodies are merely touching then the resistance will be higher. And the formation of a layer of isolating aluminium oxide when heating Kanthal should also be considered.

 

[obeythepeople]

Yea I guess my response was a tad hostile, for this I apologize. Just reading "you have to measure any coil" and "there's no point" kind of sat wrong with me as I always have checked my builds. To me it just translated to build blind and hope for the best, this would be fine if I was building on a mech, but I'm not.

Anyway these posts have been helpfull and thank you guys for helping me understand this. I've waisted too much wire building blind only to find out I'm over amp limits of my batteries or to low of a ohm to fire.