On 06/11/2010 09:29, David wrote:
> At least one eagle eyed person spotted that the inductor (which should
> be a ferrite bead) in my USB to serial breakout had no value.  Now I
> have read the datasheet, it says:
>
> "The value of the Ferrite Bead depends on the total current drawn by the
> application."
>
> Is there a rule of thumb for picking a correct value?  The maximum power
> draw will be 500mA.  There are plenty to choose from, but after a little
> Googling I'm still unsure what value would be correct to act as an EMI
> filter on the power line.
>
> David
And on any RFI you wish to filter.
More critical on a switch regulator powered rail.
Decide first on Inductance
1uH to 100uH are common power rail filters.
I use 100uH often as there is RF

Look act catalogue.
100uH that's just RF and no DC can be very small
100mA part needs more bulk so that the core doesn't saturate, maybe 1/3W=20
resistor size
500mA bigger.

Choose a ferrite type (or dust iron) for an Application is one thing,
deciding on the bulk to avoid saturation (DC current x turns) is difficult.
Mostly people choose catalogue parts.
--=20
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