Gapped toroid - Page 1
Forum section: electronics / beginners

author: Mark
author_rank: Frequent Contributor
timestamp: November 25, 2020, 12:40:15 pm
content: If I wanted to make a certain inductance value using a toroid with a gap, BUT could not guarantee (or could not achieve) the small gap required, could I use a slot in the toroid instead?In other words, say I need a gap of 0.5mm but the narrowest blade I have is 1mm, can I make a 1mm slot that is not all the way through the ferrite? (so that some ferrite bridges the gap?).Any adverse consequences? The application is a coupled inductor for a 20W power supply.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 25, 2020, 12:58:34 pm
content: A 1 mm cut half way through is different from a 0.5 mm gap. At low fields the remaining area will still carry most of the field, but will saturate at some point than the inductance would sink to a value lower than with a full 0.5 mm gap.The alternative would be cutting all the way through and add spacers of 2x0.25 mm.For lower AL values there are powder cores, that also have higher saturation. If it needs to ferrite there are ready made 2 part cores that can add spacers.

author: Berni
author_rank: Super Contributor
timestamp: November 25, 2020, 01:02:45 pm
content: The field will want to focus itself into the uncut part of the gap and saturate the ferrite in that area. At high powers that area might also get hot.As suggested just get a core design that is split in half to let you shim it to any gap you want, or alternatively cut this core in half and shim it in 2 spots.

author: jpanhalt
author_rank: Super Contributor
timestamp: November 25, 2020, 02:56:23 pm
content: The carborundum cut-off wheels I have used to machine ferrite are hobbyist level from Dremel and are 0.63 mm thick. Works fine in a handheld tool. In a more rigid set up, one might get better results and be able to use thinner wheels. Depending on how deep a cut you need, you might be able to reduce the thickness of that wheel a little with a shaping stone.Thinner resin and diamond wheels, down to 0.004", are available:http://www.diacut.com/1a8-thinwheel-resin-bond/

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 25, 2020, 03:47:42 pm
content: Why use a toroid?The advantage of the toroid is that it has practically no leakage flow and therefore ensures better coupling of the windings.If you create an air gap, you create a large leakage flow and you lose all the advantages of a toroid.Instead, use an EI magnetic circuit for example.

author: jmelson
author_rank: Super Contributor
timestamp: November 25, 2020, 04:12:49 pm
content: Quote from: akimpowerscr on November 25, 2020, 03:47:42 pmRight, this is what pot cores are made for. They can be relatively easily gapped by grinding down the center post, and are WAY easier to wind.Jon

author: Mark
author_rank: Frequent Contributor
timestamp: November 25, 2020, 05:22:53 pm
content: Everyone, thank you for your help. I will look at using a pot core instead.Height is limited, so maybe that is why the horizontally mounted toroid was used in the first place. I'm trying to find a replacement for a custom part.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 25, 2020, 05:46:54 pm
content: You could also choose toroid with distributed air gaps :https://www.mag-inc.com/Products/Selecting-a-Distributed-Air-Gap-Powder-Core-for-Fl

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 25, 2020, 06:28:59 pm
content: Cut through both sides, lap the faces so they fit flush, and shim the gap.Avoid winding wire near the gaps: the losses near the gap are huge.Even better for compact inductors might be a square core. I've seen these in CMCs and inductors before, though don't know how available they are generally. The relevant shapes are C, R, U and the like.Tim

author: BravoV
author_rank: Super Contributor
timestamp: November 26, 2020, 03:09:35 pm
content: If its one off, crack the core, I did similar thing in the past with sandwiched between two hard woods, and clamped half of the toroid in the vise, and then using a small hammer to knock off the other half.Luckilyits broken in two pieces without any splinters nor broke into smaller chunks, and used high temp epoxy to glue it back in one piece, the gap created by cured epoxy should be < 0.5 mm, not very sure though.It was to increase the saturation and it worked pretty good in my case.