DIY Inductors

High power and fidelity from an easy to build cabinet.
Post Reply
Message
Author
tallevan
Posts: 15
Joined: Sun Oct 20, 2019 12:15 pm
Location: London, England

DIY Inductors

#1 Post by tallevan » Thu Oct 24, 2019 9:50 am

I have a condundrum.

I'm sourcing crossover parts for an Omni12 Tallboy 2x10.

In the plans, Bill specifies a solid-core 1.1mH inductor for the woofer low pass filter:
"The coil in the woofer LP filter passes a high current signal, so it requires high power handling with minimal DCR. Therefore the woofer coil must be a 16 gauge or more wire on a solid core."

I realise a solid core would use less wire, thus have lower DCR compared to an air cored inductor with the same wire thickness.

My question is, could these requirements be met by an air cored inductor with a thicker wire gauge?

I ask because:
1) I happen to have a bag of coil bobbins, a half-kilo of 1.5mm enamalled wire (a little better than 15 ga) and a can of potting varnish lying around
2) I'm having a hard time finding an inductor fitting Bill's specs at a reasonable cost.

Blue Aran has an air-cored inductor at 1.0 mH, 1.25mm wire thickness (slightly thinner than 16ga) for £7.57. Not solid-core, but the other values are pretty close. Close enough?

I calculate I could wind my own air-core inductor at exactly 1.1mH with 1.5mm wire to get a resistance of .23 Ohms for about half of the Blue Aran model.

Any advice?

tallevan
Posts: 15
Joined: Sun Oct 20, 2019 12:15 pm
Location: London, England

Re: DIY Inductors

#2 Post by tallevan » Thu Oct 24, 2019 10:14 am

I found an iron-cored inductor at exactly 1.1mH @ 0.18 Ohms, but the wire gauge at 1.2mm (between 16ga and 17ga) is a little below what Bill specifies (16 gauge or more).

Would that do the job?

The question stands: would an air-cored inductor, 14.5 gauge, 0.23 Ohm substitute for the solid-cored inductor Bill specifies?

Whew..

User avatar
Bill Fitzmaurice
Site Admin
Posts: 26904
Joined: Tue May 02, 2006 5:59 pm

Re: DIY Inductors

#3 Post by Bill Fitzmaurice » Thu Oct 24, 2019 10:50 am

You can use air core if the DCR is sufficiently low, but it will probably cost a lot more than a cored inductor. If you can't find a cored inductor that's within the +/-10% value tolerance you're looking in the wrong places. I mostly use Jantzen, made in Denmark, which is a lot closer to you than to me.

tallevan
Posts: 15
Joined: Sun Oct 20, 2019 12:15 pm
Location: London, England

Re: DIY Inductors

#4 Post by tallevan » Thu Oct 24, 2019 11:30 am

You'd think Denmark's proximity would make it quicker, cheaper and easier, wouldn't you?

HiFi Collective here in the UK has a great selection of Jantzen inductors and lots of other excellent things. Now that I looked.
They have an inductor within spec, 1.1mH, solid core, 15 gauge wire, probably around 0.1 Ohms or so at a guess.
18 days from order to dispatch..

Thanks, Bill.

tallevan
Posts: 15
Joined: Sun Oct 20, 2019 12:15 pm
Location: London, England

Re: DIY Inductors

#5 Post by tallevan » Mon Oct 28, 2019 7:57 am

In the Omni12 Tallboy instructions, Bill says (regarding the Bandpass for Midrange Driver)
"The 5mH coil shunts to ground and doesn’t need either low DCR or high current capacity, so here use the smallest available gauge."

Jantzen makes a 5mH air-cored inductor out of 29 AWG wire. That's the smallest available gauge they make at that value.

Just double-checking, is that too thin?

User avatar
Bill Fitzmaurice
Site Admin
Posts: 26904
Joined: Tue May 02, 2006 5:59 pm

Re: DIY Inductors

#6 Post by Bill Fitzmaurice » Mon Oct 28, 2019 12:17 pm

It is too thin, the DCR is a whopping 10.1 ohms. I'm surprised that they make one of that size from such small wire. I wouldn't go lighter than 24 ga. Over here it's hard to find anyone stocking smaller than 20 ga. As noted in the plans at that high a value a solid core will probably cost less than air core.

tallevan
Posts: 15
Joined: Sun Oct 20, 2019 12:15 pm
Location: London, England

Re: DIY Inductors

#7 Post by tallevan » Thu Nov 07, 2019 1:15 pm

Jantzen has a very comprehensive selection of inductors, and seem very good quality but their 18 day + lead time puts me off.
They wind them to order. Perhaps if I planned further ahead.

I've wound a couple of small inductors for a switching power supply in the past, so I've had a go at winding the air core inductors for my Omni 12 Tallboy and Jack 10 builds.

I'll study up another time on ferrite or iron core inductors another time, but for air core inductors the calculations are straightforward. There are also several sites that host calculators, including the excellent Tesla Institute: http://tesla-institute.com/%21app/sim/acic.php

As I mentioned previously, I have a couple pounds of enamelled wire in three gauges and a tin of potting varnish lying around. A bag of bobbins of various sizes cost around 20 pence each from Blue Aran.

Here is a picture of me winding one with a drill clamped to the bench and the spool of wire sitting on a slightly upturned Allan key also clamped to the bench.

It really doesn’t take long. I paused after each layer to brush a little varnish on, but you could probably forgo that.

A dab of hot melt glue keeps the final wind in place. I wrapped the coil up with electrical tape, but you could probably give that a miss.

I priced up the cost of the wire and bobbins, it comes in well under half the price of the commercial equivalent. You get the exact value you want, and the satisfaction of being looked at like some sort of crazy prepper.
Attachments
IMG_20191107_103029056.jpg
IMG_20191031_144012846.jpg
IMG_20191031_143907296.jpg

User avatar
Tom Smit
Posts: 6794
Joined: Sat Jan 05, 2008 1:24 pm
Location: Sarnia, Ont. Canada

Re: DIY Inductors

#8 Post by Tom Smit » Thu Nov 07, 2019 11:00 pm

Thanks for sharing.
TomS

Post Reply