Spider Web Coil
DESCRIPTION
This instructable will teach you how to make your own spider web coil/inductor. If you have the right tools, the build is remarkably simple. If you don't have the recommended tools (such as a laser cutter), I'll briefly discuss some alternatives.
Tools needed:
There are a number of ways to do what I'm describing in this build, and I'll try to discuss alternatives as we go. However, if you want to build your coil exactly how I built mine, you'll need the following tools:
- LCR meter
- Laser cutter
- 2D vector program. I recommend Vectorworks (free), autocad (free for students), and solidworks (very expensive but my favorite).
- Wire snips
- Knife, razor, or other wire stripping tool
- Calipers
Background
Before we start, a little background on the spider web coil.
Spider web coils are neat type of inductor often used in hobby radios applications. The advantages of a spider web coil over a more traditional cylindrical inductor is that its capacitance (yes, inductors have capacitance albeit typically only a small amount) is much lower. In sensitive radio applications, it is often quite beneficial to minimize the amount of undesired capacitance in your inductors.
How does this design do this? First, you'll notice that there are an odd number of grooves and "petals" in the coil frame that you're about to make. Winding around this odd number of grooves insures one winding is not immediately physically adjacent to the winding that went before and after it. This isn't completely true as you'll notice that they come into close proximity at each of the grooves.
Why is separating the wires important? If you remember from your basic physics, capacitance is a feature between adjacent conductors, and capacitance falls off as the conductors are moved apart. In this geometry, capacitance probably falls off something akin to 1/r^2 with r being the distance between wires.
The first step to creating your spiderweb coil is to determine the geometry and number of windings of your coil needed to produce your desired inductance. Fortunately, [1] provides a set of equations that allows us to calculate this before actually building the coil. Let's discuss what the variables are before we dive into the numbers.
Wire diameter (in inches)
The wire diameter is the diameter of the copper portion of the wire you're going to wrap around the coil. I recommend 24 AWG wire if you don't have any particular design constraints. Note: If your wire has thick insulation (which I don't recommend), make sure you include that in your wire diameter. I'm using magnet wire which has a VERY thin layer of insulation.
I checked 3 locations for a reasonable diameter:
- Looking on wikipedia [2], we find the wire diameter to be 0.0201 inches.
- Looking at the details of my wire, we find the diameter to be 0.0221.
- I had already bought my wire and measuring it with calipers provided 0.020 inches.
Number of turns
This is the number of times you're going to wrap the wire around the coil in order to get the inductance you want.
Inside diameter (in inches)
Look at the schematic of the coil I provided above. The inside diameter is the circle in the middle, and in my case, I used 1.0 inches. I recommend using this value if you don't have any particular design constraints.
Outer diameter (in inches)
After you select an inner radius and calculate your number of turns, the outer diameter of your coil is equal to = (innerDiameter+2*wireDiameter*numberOfTurns). In my calculations, I calculated this to be 3.1 inches but in reality, it was closer to 3.5 inches. Therefore, I recommend taking whatever result you calculate as a "rough" approximation.
Inductance (in micro Henries (uH))
This is the inductance of your coil. Assuming you have an application in mind, you probably know what value you need. I wanted approximately 150 uH in my application.
Wire length (in feet)
Finally, the total amount of wire you'll need is the wire length. When you acquire your wire, make sure you have well over this amount. I say "well over" because this length is an approximation, and you may also mess up like I did on my first try.
Summary
In summary, you really are trying to calculate the number of turns, the outer diameter, and the total wire length from your desired inductance.
Now let's move on to the calculations. I've included a spreadsheet with two sections in it for calculating the number of turns. The first section is the easiest method, but the second method is provided for the curious.
Section 1:
The easy way to do your calculations in section 1 is to insert your wire diameter and inner diameter and then guess at the number of turns until you get the inductance you want. From the number of turns, you can calculate your outer diameter.
Section 2:
This method takes a bit longer, but I'm including it because it's a little more mathematically "correct". First, enter your wire diameter, inner diameter, and inductance. Next, take the equation A*x^4+B*x^3+C*x^2+D*x+E=0, and enter it into wolfram alpha [3]. Use the values for A, B, C, D, and E that are provided with your calculations. See my screenshot for the procedure. Mathematica will give you four values of x (which is your number of turns). Disregard any values that are negative and have imaginary components. In my example, I calculated x to be 53.
References:
[1] http://makearadio.com/coils/spider.php
[2] https://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes
Attachments
In the parts list below, I'm assuming you're using a laser cutter to create your coil frame. If you go with another method, you'll need to modify your parts list accordingly.
Acrylic sheets, 1/8" thick (0.118" actual). I recommend Ebay, Amazon, Inventables, and McMaster-Carr as suppliers. Clear looks the best to me, but you may have other preferences or design constraints.
- I used 24 awg magnet wire, but there are a range of options. Litz wire is another popular option but expensive.
- Whatever mounting hardware you'll need to mount the coil.
Now that you have your dimensions calculated and parts ordered, your next step is to CAD your frame in a 2D vector program. I recommend Vectorworks (free), Autocad (free for students), and Solidworks (very expensive but my favorite).
I have included the dxf and solidworks files I used in my build. It uses an inner diameter of 1in and an outer of 3.5in. You may not want these dimensions, but you are welcome to use it as a starting place in your design.
I'll leave the drawing of the file up to you. However, keep the following points in mind:
- Use the dimensions you calculated back in step 1.
- I recommend rounding up on the outer diameter just in case you have a discrepancy between your calculations and reality like I did.
- Make sure you round your corners on your grooves down at the inside diameter. See picture for details. I recommend doing this as it will minimize the stress riser there and subsequently reduce the likelihood of cracking. I didn't round my first build, and a petal broke off the first time I dropped it.
- Another thing to consider is how you want to mount the coil. In my design, I put a hole in the center of the coil large enough to mount a single m3 nylon spacer and nut to. Alternatively, I could have extended one or more of the "petals" of the coil mount and added mounting hardware for that as well. [1] has a good example of this.
- I added two additional holes on one of the "petals" near the outer radius. I placed those there as "tying off" points for the two loose ends of wire. That way, I can keep the wound wires on the coil under tension and reduce the likelihood that they pop off.
References:
[1] http://makearadio.com/coils/spider.php
Attachments
Drop your acrylic into your laser cutter and cut. If you don't have access to a laser cutter, look into joining your local makerspace as they may have one.
On my cut above, you'll notice a few brown burn marks on the acrylic. To avoid, leave on the protective coating on the acrylic until after cutting.
Alternatives to laser cutting
Before I made this design, I did some searching and came across a number of methods to make the coil frame. Some people cut it from cardboard or a thin plastic. I even found an example where someone cut it from a old CD [1]. If you decide to try one of these methods, I recommend printing the CAD model onto paper, cutting out the outline, and taping it onto whatever material you chose. You can then trace out the outline with a marker and cut with your tool of choice.
References:
5 More Images
The next step is to wind your coil. Start by hooking the loose end of your wire into one of your "tying off" points and then start wrapping. Direction doesn't matter. Keep a fair amount of tension on the wire as you wrap, and don't get too caught up if the winding isn't perfect. You can always straighten out the windings after you finish if you feel so inclined. When you're done, trim the last end and tightly wind through the last "tying off" point. I've included a number of pictures here to show the process.
You have two options as to determine when to stop winding:
- If you don't mind wasting a little bit of wire, my recommendation is to go ahead and wind up a fair bit more than you need based on your earlier calculations. In the next step, you'll check the inductance with an LCR meter and trim back the number of turns as needed. Keep in mind that the later turns (with their larger loops) will contribute more to the inductance than the inner windings.
- If you don't like the previous overguess and check method (method 1), you can also check inductance as you go. Hook up your LCR meter to the inductor lead at the tie off point and the other to the last wrap you just made. You'll need to scrape off the enamel or wire insulation in order to make contact with the wire. You'll have a permanent exposed bit of wire on the coil, but it won't matter as long as it doesn't touch another exposed wire.
I went with the first method on my first try and under estimated the number of turns. This caused me to waste that wire and need to rewrap. I managed it on the second try.
Lastly, we need to double check the inductance of the coil and trim as necessary. First, take a knife or razer blade and scrape off the enamel coating or other insulation on the ends of both wire leads. Maybe 1/2 inch on each lead.
Next, take the now exposed leads and attach them to your LCR meter. As you see, I ended up with 153 uH at 52 turns. As you'll note, my calculation estimate placed it at 53 turns.
If you don't have access to an LCR meter, there are other methods for measuring inductance. Here is a link to a few ideas [1]. You can also try googling for other ideas [2].
References:
Finally, install your new coil into whatever system you need it in. That's it!