Found Out Something Else Today

I bought a bag of 12 Amidon ferrite Beads, # FB-77-6301 so I could make some more inductors for my V boost circuits. I wound about ten turns of 24 gauge telephone wire on one, which is a toroidal sleeve .375" OD, .194" ID, and .41" long. I measured the inductance at 405 microhenrys, which is more than I expected.

I was reading the literature that comes with the beads, and it said that some mixes are conductive. I thought that ferrite was mostly glass, and was an insulator. So I measured one with my DMM and I found that the resistance was down in the hundreds of ohms, much _lower_ than I had expected. I had thought that I really didn't need to wind them with pvc insulated wire, but now I'm thinking that it's not such a bad idea after all. I have wound the toroids before and seen the insulation scraped off the enameled wire by the sharp edges of the totoid but I didn't think much about it. So it's possible that it could short out with the low resistance material of this particular toroid. That's something I never expected.

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Watson A.Name - "Watt Sun, Dar
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Beads are designed to be lossy. Seems a wee bit foolish to me to use them for inductor cores.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

It would definitely lower the "Q". You can wrap a thin strip of paper around the core first, or spray paint the core first, to protect the wire from the edges.

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Michael A. Terrell
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Michael A. Terrell

The high permeability ferrites are all quite conductive (semi conductive, actually), and this represents a resistive loss to any inductor would on them. Think of the core as a single turn load with a resistor. For one offs, I usually take the edges off the core with silicon carbide sandpaper. If I am going to be pulling a lot of turns through the hole, I may paint the ends of the core with a thin layer of epoxy to protect the wire.

By the way, if you are making energy storage inductors, these ungapped cores will saturate a only a few amp turns. I like to use the two piece cores made to be put on power cords as RFI suppression. I sandwich a layer of paper between the halves to make an energy storage gap. It is also easy to wind wire around the individual halves than it is to pull it through the hole.

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John Popelish
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John Popelish

On Sun, 25 Jan 2004 16:54:46 -0800, Watson A.Name - "Watt Sun, Dark Remover" Gave us:

If you had, the inductance would have been LOWER than expected, not higher. It is very likely that you made no such shorts. Mag wire insulation is in the 1500 V range.

Wind four or six of them at ten turns each to see what your tolerances are.

Typically, ferrites are in the 20% range. That is a considerable number.

Reply to
DarkMatter

On Sun, 25 Jan 2004 18:09:49 -0700, Jim Thompson Gave us:

Good for snubbers, but not much else.

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DarkMatter

In article , snipped-for-privacy@example.com mentioned...

Lossy at a few MHz and up. Much, much less lossy at a hundred kHz.

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Watson A.Name - "Watt Sun, Dar

Yeah, I got some 600 grit wet-or-dry sandpaper that i may use on it. I also started sanding down the other beads that I sawed in half with a ceramic tile saw. I used a round 60 grit sanding disk for that, but it's getting worn down on one side, where it's beginning to polish instead of sand. That side may make a good way to take the edges off the beads. Maybe I should save the dust and make an old fashioned coherer.. Yeah, right.

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Watson A.Name - "Watt Sun, Dar

Sounds interesting, but the two halves are kind of largish for my needs. That's why I picked these beads. I've been sawing the suppressor beads from keyboard cables in half, and they're not too big, about a half to 5/8 inches in diameter. But it's a hassle sawing them in half. As for the winding, I'm only able to get ten turns on them, and that only takes a minute or so to wind.

The guys on Candlepower Forum use a bead they buy from Digi-Key or Mouser, and it has an even smaller center hole, so they can only wind a few turns on it. It works okay.

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Watson A.Name - "Watt Sun, Dar

You shouldn't need to take measures to prevent saturation if you are using them as RFI suppression common mode choke. The differential mode currents will cancel and the core will not saturate. That is why you often find high permeability toroids in CM choke applications.

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    Boris Mohar
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Boris Mohar

Perhaps use them as Memory Cores? ;)

Reply to
Mark J.

in

A.Name - "Watt Sun, Dark

# FB-77-6301 so I could

circuits. I wound about ten

which is a toroidal sleeve

measured the inductance at 405

with the beads, and it said

that ferrite was mostly

one with my DMM and I

hundreds of ohms, much

that I really didn't need

now I'm thinking that it's

wound the toroids before and

enameled wire by the sharp edges

it. So it's possible that

material of this particular

bit foolish to me to use

lossy at a hundred kHz.

...Jim Thompson

Sometime ago I bought a "jar" of .08 dia memory cores, I can get 4 to 6 turns of #28 to #30 enameled wire on them. They work great to suppress RFI and since I got about 60,000 of them for $3 I will likely never run out. I have strung up to ten on a piece of #22 wire for a Power input filter too. (Don't have the exact value of the inductance or loss, but it worked!

Reply to
Roger Gt

That seems like a huge tradeoff, though -- you have a nice energy storage gap, but your inductance goes way down and now you need a ton of wire. I can certainly see doing this to get good linear inductance characteristics, though.

Most of the switch> >

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Reply to
Tim Wescott

Is there something I'm missing, too? Like, there are millions of gaps between the magnetic particles in the ferrite and the binder, which I believe is ceramic, and is not magnetic. If so, why should there need to be an additional air gap added?

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Watson A.Name "Watt Sun - the

The powdered metal cores have distributed gaps between high mu particles, but the ferrite is (ideally) a single crystal, solid material with no gaps. In order to get high permeability and low hysterisis the material must reach high flux with low magnetization (the definition of high mu). But ferrites do not have as high saturation flux as iron (2,000 to 3,000 gauss versus 10,000 to 20,000 gauss). The energy stored in an inductance is 1/2 * L * I^2. So using the core ungapped gives a high inductance for a low number of turns but a low I^2. By adding a tiny gap, you lower the L by say a factor of, say 25 but also raise the saturation magnetization by 25 (25 times as many amp turns to reach the same saturation flux). You increase the turns count by 5 to get the inductance back (since inductance goes as the square of the number of turns), but the saturation flux still supports 5 times the current. But now the stored energy has gone up by a factor of 25 (same L, 5 times the current, squared). All that extra energy is stored in that tiny gap. Of course, if you don't mind using a core volume with 25 times more volume, you can store it there.

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John Popelish
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John Popelish

Well, the reason I wanted to use a higher mu core material in the first place was to reduce the number of turns. That's so that the wire resistance would not lower the peak current. When you have only a volt and a half at a couple hundred mA to work with, you need all the reduction in resistance you can get.

I think one possible solution is to work with the lower inductance by raising the freq of oscillation. That allows the circuit to get by with less turns and less reasistance. I don't know how they do it, but the commercial V boost circuits use inductors smaller than the size of a pencil eraser, maybe about the size of a 5 mm LED but not as tall. And they're also surface mount, making them really squat, like the size of a

3 mm screw head. And apparently to get around the internal resistance, the inductance is lower, maybe 10 uH or less. The freq may be as high as .5 to 2 MHz in some of the Zetex, Maxim and LT chips.

For an example of a converter using a (not so) small ferrite bead with no air gap and just a few turns, see URL (thanks, Stepan)

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Reply to
Watson A.Name "Watt Sun - the

And you can do that by just using more core volume than the absolute minimum. But it is hard to get low losses this way, because of the higher volts per turn applied to the core resistance. But for very low power operations, having a core several times the smallest possible may still be plenty small.

(snip)

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John Popelish
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John Popelish

You say "absolute minimum" and "smallest possible" but I have no way of knowing what these are. I have used some 180 uH chokes that are smallish, maybe 6 mm diameter by 7 mm long, and they worked, but their resistance was too high, maybe 1.5 ohms. So I got some 100 uH chokes from Mouser, # 580-22R104, which are about 8 mm diameter by 10 mm long. They have a resistance less than a half ohm, and can handle a lot of current, so I have no problem driving three white LEDs to super brightness and sucking 200 or more mA from a single AA cell.

The guys on Candlepower Forum use a smallish core, and enough current to drive a 1 Watt Luxeon Star LED to full brightness. One of their small converters is called the "Ill Pill"

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Watson A.Name - "Watt Sun, Dar

In news: snipped-for-privacy@news.dslextreme.com (Watson A.Name - Watt Sun, Dark Remover):

How long do those AA batteries last at 200mA/cell???

Reply to
Mark J.

"Mark J." wrote in news:SY2dnUmJ3c0tBYXd4p2dnA@buckeye- express.com:

I know it's OT, but since you brought up the thought about current per cell...

I was given one of those little tiny digital cameras, about 2" square and about 3/4" thick. I forget the name. Anyway, the single AAA battery never would last long enough to even fill the memory with images, so I checked it out. It was drawing a whopping 850ma from a single AAA battery! No wonder it was given away. I will concede that there MAY have been something wrong with it, however it took quite good pictures and worked well, with the exception of battery life. It was supposedly the smallest digital camera in the world, at the time of its release and I don't mean a video camera, rather a digital, single image camera that you could download images from, to a computer thru USB. No flash. res was 1024x768(probably interpolated)

buck

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buck rojerz

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