[sdiy] Choice of ferrite bead inductor

[Simon Brouwer]

What I usually do is select parts like this on the Farnell site (if 
Farnell has it this is some indication that it is not a hard-to-get part).
On http://uk.farnell.com/, search for "ferrite bead", check the boxes 
for "only RoHS Compliant products", "Exclude extended range items" and 
"In stock items only", click on the Refine button, choose "EMC/RFI 
Suppression", "Ferrite Beads". Select Current Rating from 200 to 500 mA, 
and click on the up arrow in the List Price column to sort according to 
price.

This gets you to 
http://uk.farnell.com/jsp/search/browse.jsp?N=601+551+401+500006+1002389+217387+259729+217417+259735+217463&Ns=PRICE_PLS_006_PRICE1|0&Ntk=gensearch_001&Ntt=ferrite+bead&Ntx=

Then choose the cheapest type with the characteristics you like.

For example, I looked at 
http://uk.farnell.com/fair-rite/25060310270/ferrite-bead-1000ohm-0-1-0603/dp/1463451

This is a really cheap part, can handle 300 mA and has an impedance of 
1000 ohm at 100 MHz. However, you should always check the manufacturer's 
data. In this case I think Farnell made a mistake because according to 
http://sigma.octopart.com/834/datasheet/Fair-Rite-2506031027Y0.pdf this 
part can only handle 100 mA.

Farnell's info on this one: 
http://uk.farnell.com/fair-rite/25080560170/emi-sheild-bead-smd/dp/1651722 
is also incorrect (there is even a spelling mistake in the URL...), but 
it does handle 200 mA and has an impedance of 600 ohm at 100 MHz.

The impedance chart for this component is figure 23a on page 60 in 
http://sigma.octopart.com/834/datasheet/Fair-Rite-2506031027Y0.pdf

In agreement with Eric I would use ceramic capacitors of 100nF in 
parallel with the electrolytics.

By the way, nowadays relatively large capacity ceramics are getting 
cheaper than electrolytics, for power decoupling these are really a good 
choice (much less so in the audio path though). For example:
http://uk.farnell.com/taiyo-yuden/tmk316f106zl-t/capacitor-1206-10uf-25v5v/dp/1463372

However, you should add a small ceramic in parallel with these as well, 
as they will have a fairly low resonance frequency.

Seb Francis schreef:
> Hi,
>
> I need to choose a suitable ferrite bead inductor for power supply 
> isolation but am a bit baffled by the myriad of different inductors 
> out there.
>
> Essentially I want to isolated power spikes caused by digital 
> switching, etc. from sensitive analog parts of the circuit.  I will 
> use a ferrite bead followed by a 10uF capacitor.  The 3 places I'm 
> using this arrangement is:
> - Where the +15V and -15V power comes onto the board.
> - To create a separate clean 3.3V from the dirty 3.3V that powers the 
> uProc, SRAM, etc.
>
> Don't need a huge current capacity - 200mA should do it.  Needs to be 
> SMD package, something like 0805, or bigger if necessary.
>
> Could someone give me a quick pointer to a suitable inductor, or 
> perhaps give me a rule of thumb by which I could select one myself?
>
> Thanks,
> Seb
>
>
>
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>


-- 
Vriendelijke groet, 
Simon Brouwer. 

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