How to calculate twisted coil resistance?

[Andrew Freeman]

How do you go about calculating the resistance of twisted coils?

I was thinking of ordering a spool of some pretty thin Kanthal, that way I can make coils with varying resistance fairly easily and without needing to order many different sizes of wire...

All of the calculators I have found online seem to calculate stats more geared towards data-transmission wires, etc. or ask for stats I simply do not understand.

Does this sound like a good idea? If I were to order some 36AWG Kanthal, and if I want lower resistance, I can simply twist a couple strands together.

I am looking for an economical way to be able to try many different setups and see what I like best, without needing to spend a ton of money on supplies...

 

[lctrc]

I don't think there is a reliable way of calculating it. Differences in the number of twists alone is enough to throw off any calculations. Best you can do is measure a length with your multimeter and go from there.

Or - find a vendor that sells all the gauges you're interested in and get 10 feet of each all at the same time to consolidate shipping. Each foot is enough for 2-3 coils, so that's 20-30 coils per 10'. Plenty for experimentation without breaking the bank.

 

[pdib]

One thing you can be sure of; twisted will get you a bit more resistance than just running wires parallel. (which, by the way, you can calculate with simple fractions . ... 2 wires = 1/2 . .. .3 wires = 1/3). The twisting nets you more wire length per finished length. (cause point A to point B . .. straight line vs squiggly line). I recently made a dual coil (dual because it was 2 wires running parallel, on one wick tho) twisted . 7 wraps on 1/16" ID. Normally, a single coil of this 27g wire (7 on 1/16) would net 0.7Ω. A dual would be 1/2, so 0.35Ω. It came out at 0.5Ω.

Here's 3 wires running parallel, no twisting, simple math. this triple coil will net 1/3 the resistance of a single coil of similar dimensions.




Since you are always going to net out higher than running wires parallel (if you twist them); it would be safe to assume the straight wire resistance, and then enjoy the surprise of how much (how little) higher you net out.

If you get into SUPER tight twists, and really tightly packed coils, you may run into "controlled shorting" and find your resistance lower than you thought, so be careful there.

like this ↓

 

[ScottP]

One thing you can be sure of; twisted will get you a bit more resistance than just running wires parallel. (which, by the way, you can calculate with simple fractions . ... 2 wires = 1/2 . .. .3 wires = 1/3). The twisting nets you more wire length per finished length. (cause point A to point B . .. straight line vs squiggly line).

This is 100% true but that does not mean you cannot calculate and predetermine the resistance. What you have to know is the end length that you want. Lets say you are planning on twisting 3 strands of Kanthal A-1 32AWG to make a single strand wire and then fashion a coil out of it.

Kanthal A1 32 AWG oms per inch = 1.15
Target resistance is 0.8 ohms

This means you will need to use 3 strands of EXACTLY 2.09 inches of wire twisted to reach the target goal. So what I would do is measure out out 3 strands of wire at 4.09 EXACTLY inches each. Then using a sharpie mark a place on each wire exactly 1 inch from each end. This premarks your target contact points. Now twist the wires normally as tight as you want. Now make the coil as normal and make sure you leave just enough wire coming off of each lead so that the black marks you made are going to be right at the contact posts. Whallah! Your final coil should be within normal tolerance of your target resistance.

EDIT: I used my Android app to determine wire length, but here is the math:
(Target Restiance x number of strands) / wire resistance per inch
(0.8 x 3) = 2.4 ohms
2.4 / 1.15 = 2.09 inches per strand