Inductor design/construction

[drf]

I am currently making some super frugal MTM's for my theatre (wanted something different) but due to cost I am going down the old path of passive xo's. Whilst playing around with attenuation, BSC, notching and zobeling I decided I would do that once the unit was together etc etc etc, you know the rest (or at least can see some reason for it).

Now, all that aside, I did a quick search and realised that no-one has made there own inductors. So I am now wondering if:

Anyone does?
Anyone is interested?
Does it actually save any money or allow for a more customised inductor?

For me in Australia the answer is yes to all questions, so While I am designing these latest XO's I thought I'd find out if anyone wants a quick tutorial in inductor design/constructon?

Oh, and on the topic of the super frugals, I will post pics and stuff after I get the cabs made.

[BoomieMCT]

Please do. I've only unwound inductors to make custom ones.

I noticed Radio Shack sells insulated wire and lalena.com has an inductor calculator. I was never sure how to get a nice stacked wrap in making my own.

[Anthony]

One of the FRD tools has an inductor calculator in it as well. It tells you inner diameter, outer diameter, height, and gauge for an inductor of specific size and DCR.

I think they call it the Crossover Design Calculator.xls or something similar (it's on one of the many pages)

[mrstampe]

I'm intrigued! It's going to the next level of DIY. If your going to go as far as we do with this hobby, you might as well make your own XO components!!

[Anthony]

well, the inductor is probably the ONLY one you can build yourself

[drf]

Anthony wrote:

well, the inductor is probably the ONLY one you can build yourself

True, but air core inductors are just the begining. Once you understand the mathes and can turn a half decent inductor the knowledge will allow you to do custom chokes, PSU filters, output transformers and line level transformers.

I'll start with some mathes and some pics soon.

[drf]

O.K here we go. This first bit is for those who have no idea at all. I have tried to explain what an inductor does and how to caculate the size required. If any of the more knowledgable peeps out there spot a flaw or think something can be worded better (including speelin mistakes) don't hesitate to let me know asap.

The next post to come (assuming it doesn't go all pear shaped) will be about the physical attributes of the inductor and how to calculate height, depth, width, No. turns and wire length. The post after that will encompass how to physically make it, making a bobbin, winding the copper and finnishing off.

------------------------------------------------


How do inductors work?

The very basics, when current flows through a coil a magnetic flux is generated
proportionate to the current flow. When the current changes direction inside the coil
the magnetic field must also change. How easy it is for this current to alter the
already established magnetic field depends on the ammount of inductance and speed of
current change. The more inductance you have the harder it is to alter the current flow. If you have a very small inductor (say 300 micro Henries) it will start to oppose current that changes
at or faster than 4.24Khz. What this means is that it will choke any current that
alternates above 4240Hz because (to put it in laymens terms) that speed of change
is too fast to alter the magnetic flux. However currents that alternate slower,
that is any below 4240Hz will have sufficient time alter the flux and thus don't
get opposed by the inductor. Another analogy that can be given for the inductor is water, the faster you try to change directions in water the harder it becomes.

I normally have a lot of trouble with communication so if the above doesn't make sense
I will include a few links that might help explain it better.

Wiki
Howstuffworks although it isn't all that crash hot.


Now the maths:

lets start with how I worked out that a 300 mH inductor would filter at 4240Hz,

L = (0.159*Z)/F

or

F = (0.159*Z)/L

If L = Henries
F = Frequency
Z = impedance of circuit (in our case the speakerdriver) in ohms
then

(0.159*8)/0.0003 = 4240 Hz

Another example:

Assuming we have a speaker with an impedance of 6 Ohms and we want to cross at 3500Hz.

then: (0.159*6)/3500 = .000272 Henries or 272 micro henries.

thats all for now.


And if this takes off maybe we could work on an XO design tutorial and sticky it for all to learn

[mrstampe]

Here's a nice online tool for turning your own inductors:

Air Core Inductor Calculator

It includes a tutorial, and coil parameters by multiple AWGs based on the inductor value you select:
DCR, inductor height/radius, # of turns, estimated power handling, and wire length

[drf]

Well. I have made 4 inductors and taken a few pics but I am having trouble wording the post so it is easy for people to follow.

So I think I'll post the pics and a basic description then others can clarify or change it to make sense.

mrstampe has already posted the calculator I used, One thing I like about this calculator is that you can sort through the results to find the best solution for your needs. For most people it will be power rating, but I am assuming that some people may have an excess stock of a particular wire gauge so will use that as there governing factor.

Having said that my system amp is only 40Watts RMS and the drivers are only rated to 30watts peak each, so I can go with the smallest inductor from the results (50watts).

The first thing you need is a coil winding lathe:

lathe.jpg

As you can see you don't need to buy a motorized contraption with digital readout, five minutes and $5 is enough. If you look close enough you'll see I made the handle out of two speaker cutouts.

After looking up the dimensions from the inductor calculator, the first thing I needed was a former or bobbin. This is not a permanent part of the inductor so it can be made from anything. The closest thing I had to the correct dimension was a peice of towel rail. To bring the former up to the right diametre I wraped it in paper

bobbin.jpg

tempbobbin.jpg

To ensure the height of the coil was maintained, I cut a hole in some plastic the same diametre as the formerand slid this on. It is held in place with some electrical tape and a acouple of dobs of hot melt glue if neccessary. Then I cut another one and fixed it to the former (remember this has to come of to get the coil of) nice and snug. This will give you a channel to wind the copper in allowing for a nice firm coil. Now you can start winding the enameled copper on, Remember where each turn starts so you can keep count. When winding the copper try no to wind to tight or you will not get it of the former but don't wind it loose or you will alter the inductance too much.

winding.jpg

Once you have reached the desired number of turns, carefully remove the outer plastic and slide the coil off the former keeping it together. For this coil I have tied it together with cable ties. It is important that you do not use any ferrous metal on or near the inductor. After you have the coil a quick check with the tape measure to ensuree you haven't booboo'd and your good for electrical testing.
finished.jpg


A very quick test, I have calculated the size of this inductor based on there being a 4 ohm load, but when testing I had an eight ohm load which has skewed the results slightly. As you can see on the graph below the measured drop from 3000Hz isn't far off from the calculated.

frinduct.jpg

And the last test, I put them into my super frugal's. Results where extremely good, I didn't blow up $60 worth of tweeter or $120 worth of drivers and my amp didn't become a molten pile of semiconductors and plastic.

sf.jpg

[mrstampe]

Awesome job on the coils and your jig! BTW, didn't mean to steal your thunder by posting the calc link. As I price the larger air coil inductors in the $20-35 and higher range and consider the prospect of buying numerous inductors for several upcoming speaker builds, this starts to make sense for the adventurous soul.

I wonder if adding a crank handle to the wheel of your coil winding lathe could make the process a bit easier and serve as an index to help keep track of the turn count. And if you were going to do more than just a couple inductors, I bet a turn counter could be added to the rig. After that, just slap a DRF Electronics Inc label on and start selling those puppies online!

Once again, great job!

[drf]

Awesome job on the coils and your jig! BTW, didn't mean to steal your thunder by posting the calc link.

Thank you, It's cool about the thunder thing, As far as I am concerned the more people who post links and opinions the better it is for everyone.

As I price the larger air coil inductors in the $20-35 and higher range and consider the prospect of buying numerous inductors for several upcoming speaker builds, this starts to make sense for the adventurous soul.

It cost me less to build the coil winder and build the 4 inductors needed to make 2 2nd order XO's than it would have cost me to buy 2 1st order crossovers pre-built. So I get the advantage of the xo's being custom and cheaper.

I wonder if adding a crank handle to the wheel of your coil winding lathe could make the process a bit easier and serve as an index to help keep track of the turn count. And if you were going to do more than just a couple inductors, I bet a turn counter could be added to the rig.

I should have shown the this pic earlier then:
handle.jpg

I already have plans (schematics) for a turn counter but didn't want to overcomplicate this thread. maybe if people get enthusiastic about it I'll do a turn counter mod thread or something.

[BoomieMCT]

Out of curiousity, do you have any premade inductors you could measure to compare the homebuilts too?

[drf]

Not at the moment, and to be honest I didn't even think about comparing I just thought testing would tell all. If I can I will try and pick one up this week and do some electrical tests side by side as well as in the XO.

[drf]

Well I still havn't got one of the shelf for comparison, unfortunately getting to the shops is very low on my current list of priorities. But just to let people know I still intend to do the comparisons I just need to sort out a few issues first.

[Anthony]

Take your time. Family first -- inductors can wait.

Best of luck.

[Larryfff]

I think you may want to check your inductances with an LCR meter. MCM has a basic Tenma LCR meter on sale for $39.95. I just got one.

[drf]

Well I finally got around to taking measurements of one of the test inductors I made and it measured at 240uH. Thats 11% tolerance on the 272uH I needed. Hopefully I will find enough time to post some more detail. And some photos. I will also try to make a direct (measured and tested) comparison to some off-the shelf inductors.

But I may need to make some more as I am rather happy with the ones I made earlier inside the speakers and don't want to disturb them.

Cheers

[Jason Schultz]

I've tried unwinding inductors from Jaycar to get desired value. what a nightmare when you find you have unwound too much and have to wind it back on, or worse still have to rejoin the wire!! I would welcome this DIY as long as it is fairly accurate.

[drf]

The ones I have done using the above method produce inductors with a 10% tolerance (11% for my first attempt). We can get the tolerances tighter by simply cutting the copper longer and throwing on a few more turns (or a few less) as needed before finishing off. However one question that remains is how important is it for the inductor to be precise? Using the test inductor I made, assuming the driver impedance is 4 ohms then the crossover freq' will be 2338Hz for the 272uH inductor, however the actual inductor is 240uH therefore the freq' will actually be 2650Hz. Is this difference enough to upset your design? I some cases it will and in others it will probably make no perceptable difference. Some people reccomend a 5% tolerance maximum, this is quite achievable in diy without much effort. To be quite honest if you want more accuracy than this you should probably be looking at a fully active system .

[Jason Schultz]

DRF
I wasn't having a go at you or your home made inductors. I think its great. And you are probably right about not needing 100% accuracy. My comment comes about because: after i adjust an inductor or make one myself I have no equipment to measure its inductance value. I have to trust in the value that i guestimated and then inert it into a crossover and measure the measure frequency response. primitive i know. but its what you do when you have no measrement equipment. I plan on purchasing some multimeters and that wt3 woofertester in future.

[drf]

I didn't think you were having ago. I just want to be as thorough as I can. Especially when things can get very complicated very quickly in this field.


Although it is hard, you can get away without having an inductance meter. but it requires a bit of patience and a lot of testing/retesting until you are happy with the results.

[drf]

Well I finally got hold of an off-the-shelf inductor, I grabbed a couple of .22mH but I needed .6mH so it isn't exactely a fair comparison but good enough for what I have done. Both off-the-shelf inductors measured to there rated henries, however the dcr was rated at .3ohms (a little too hopefull I think), they actually measured at .8 ohms. I require a 0.6mH for some speakers I am currently working on, so I decided I would spend a litle more time on these inductors (6 minutes instead of five) and see if the results are better or worse. This is the inductor all wound up on the meter:

measure.jpg

As you can see 0.59mH. Not bad considering the nearest off-the-shelf is either 0.56 or 0.82.
The dcr was 1.7ohms, which is also within cooee of the expected. The other advantages I had with this was that not only is it closer to what I actually need but it was half the price and can handle more power. The ots inductor was only rated to 100watts yet the DIY should be able to run at 180watts.

One last shot:

coil 018.jpg