Choosing a ferrite bead

John Popelish wrote in news:toydnZerBIwyY1nenZ2dnUVZ snipped-for-privacy@adelphia.com:

I do have a scope - but the annoying thing will be that everything will be printed already - so if it doesn't work I would probabaly need to make a new board. I guess that's OK though. It'll just be another delay.

The board with the GPS module is already laid out (though I still have about a week before I need to send it off to be printed). The motor driver boards are going to be interesting to say the least - they'll be about 1.5x4cm long and control 3 motors each, along with having a load of sensors and other nonsense on them. Feedback on them would definitely be awesome. I don't think I'll be finishing them for another month or so though.

-Mike

Michael Noone wrote: (snip)

No. It will be effective up to higher frequencies if you can run a ground trace under the inductor, between the pads, with very short connections to the ground layer (like a few vias under the inductor). This will reduce the capacitance that bridges across the inductor pads.

That layout will fit the whole series (and several other series) of different values in that case size. But there is nothing I have seen that comes close to the Sumida CDRH127/LD (low drop) as far as resistance in that case size (to preserve as much of the 6 volts as possible for the GPS unit).

On Wed, 11 Jan 2006 10:03:13 -0800, Paul Mathews top-posted:

Would it make sense to use a wirewound resistor in this app?

Thanks, Rich

I'd say, no, you want as much continuous metal as possible for a "ground plane". You might want to look at the layout, and kind of trace what the current paths would look like, and how they might interact, But I wouldn't even begin to know how to fix it if there's some kind of situation.

In fact, I was wondering about mounting the choke (heck, a bead with leads is pretty much a choke, right? :-) ) on the ground plane side - would you cut a little notch for it and go right to a via? I also like John's idea of a bridge under it.

Cheers! Rich

how about

ugh, wrap probs, or something like this

martin

John Popelish wrote in news: snipped-for-privacy@adelphia.com:

I finally was able to fit it onto the top of my board. I decided that putting it on the bottom would be a constant annoyance as it is a good deal taller than all the other components on the bottom. After a fierce battle and many a moved trace I was able to fit it along with the two capacitors on the top of the board right next to the connector for the GPS module. Very glad to have that done! One question though: should this ground trace be just a single trace? Reason is I also have a ground plane on the top of the board in that area to provide some shielding for another component. So right now it has the full ground plane going between its pads, not just a thin trace. I could change that if that would be better though.

Have to finish up a couple last details on this board and then I can finally send it out to be made! (been working on this one 8.5x10cm board since September or October). Anyways, thanks for the help. I'll report back on how well it works when I get that far.

-Mike

Well, it has capacitance to the ground plane, yes, of course; but I'd presume that the power line would be in series with the inductor, and the capacitance to ground is in parallel with the whole thing, kind of like a distributed pi/T filter. :-)

Thanks! Rich

On Wed, 11 Jan 2006 21:40:41 GMT in sci.electronics.design, Rich Grise wrote,

Yes, it is exactly a choke. However, if it is laying on the ground plane then it is a capacitor, right?

That is fine, as long as this trace has a low inductance path to the ground layer. But I'll bet you have that, just by virtue of the two capacitor ground connections.

Can you show me the layout for comments before it goes out?

John Popelish wrote in news:hKmdnUmWYdQoUVjenZ2dnUVZ snipped-for-privacy@adelphia.com:

It should have that - but I think I'll give it some vias just to be sure.

I would love to have somebody look at the layout. It is my first 'serious' board - all my other layouts have been much smaller boards that had a small fraction of the components that this board has (I think it's now at about 110 parts). Are you familiar with Cadsoft's Eagle? I could post the raw Eagle files. Otherwise I could just post the files as jpegs, though they'll have to be fairly high resolution (due to the complexity of the board).

Thanks,

-Mike

Hm. I'm not an rf type, but I distinctly remember my lead engineer in the 70's, who was an rf type, telling me that ferrite beads were predominantly lossy, so acted more to increase trace resistance than like chokes. That's why he could get about as much effect from tacking one on top of the trace as he could cutting the trace and running it (a connecting wire, actually) through the bead, as I wanted to.

Am I misremembering, or are things different now, or...?

John Perry

The effect is certainly reduced when the core does not wrap all the way around a conductor, but there is measurable and, I guess useful effect in having the material only on one side of flat circuits. Steward now sells thin, flat squares and disks of ferrite with pressure sensitive adhesive, to be placed over some flat circuit elements.

I haven't used them for this purpose, but someone must be.

(snip)

I used to have Eagle loaded, but I deleted it for lack of use.

You are welcome to email graphics of the layout and the schematic, but please, use GIF or PNG encoding, instead of JPG. It keeps all the edges sharp and encodes only 256 or fewer colors per pixel, which is fine for drawings. I have a 5 meg inbox.

If you put the files up on a web page or something, I guess could reload Eagle but I hate downloading and installing software in a box that is working well.

John Popelish wrote in news: snipped-for-privacy@adelphia.com:

Hi John - I have just sent you an E-mail with both PNGs of the schematic and layout as well as the actual Eagle files. I'm very interested to here someone else's take on the boards as they represent my first attempt at making a fairly sophisticated board. Thanks,

-Mike

Got it. I hope to devote some time to it this evening. I'm sure I'll have questions. I'll email them.