# Why am I getting odd inductances from home made inductor?

This question is related my my previous question: What to do about very hot planar inductor?

What I'm trying to do

I am trying to make a planar inductor (made from the tracks of the PCB, and surrounded by a 2 part a ferrite core). According to the datasheet of the ferrite core, the AL value is 1700nH, which means that with 12 turns around the core, I should get 12x12x1.7 = 244uH.

The problem

However, when I measure the inductance on my LC meter, it reads only 1.8uH. What's stranger is that if I create an inductor with the same core, but using stranded wire, and only 10 turns, I get 46uH!

I measured the resistance of the PCB tracks, and it's 0.25R, just as it should be, so I don't think there's a short in there.

My questions

What's going on? Is the AL value not sufficient for calculating inductance? How could it be that fewer turns gives a much higher inductance? Is the stranded wire massively better than PCB trace?

• @Samuel they look OK to me! Nov 20 '15 at 19:10
• the slightest gap between mating faces of the core will collapse the inductance value. Are you sure the faces are meeting, and not being fouled by the PCB? Nov 20 '15 at 20:22
• Yeah they look cool and exactly how I did it on a regular transformer. Shorted turn is my guess. Nov 20 '15 at 20:23
• If the inside of the hole is plated, there's your shorted turn. Get the scalpel out! Doesn't matter if it's electrically isolated, it's still tightly coupled via the core. So you're measuring the leakage inductance at 1.8 uH. Nov 20 '15 at 20:58
• I think @BrianDrummond nailed it. Whether internal holes are plated or not seems to be a bit of a crap shoot. Some use copper on either side of the edges as a marker for 'should be plated', but you have a definite gap there. All you need to do is to break the plating at one spot so it isn't a complete turn. Nov 20 '15 at 21:09

Brian has nailed it. Good shout. @Brian please take this picture and make your own answer. I'll delete this answer as soon as: -