AVR-Chat

Kathy Quinlan wrote:
> Robert Adsett wrote:
> 
>>>2) What is the purpose of the ferrite bead?
>>
>>They can be used for EMI, I've not used one myself.
> 
> OK how do we select these, or are they just a one size fits most ?
> 
> People keep telling me to use them in SMPS designs, but I am still not 
> sure what I need, also which leg has the bead ? (in some designs I have 
> seen them on all legs ant thought it was more support that need)

Beads typically have less than 5 turns on them. Often only a straight-through
single turn, so they don't have much AC impedance, and only have it at higher
frequencies. They are most effective in a pi configuration where you have the
series bead and two shunt capacitors. The idea is that the inductance of the
bead is much higher than the self inductance of the capacitors, so creating
much better attenuation at high frequencies. Make sure the DC current does not
saturate the bead.