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Chip ferrite bead need free wheeling diode

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raman00084

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I am using smd chip ferrite bead 60 ohms in micro controller input for reducing emi noise my doubt is weather i must use free wheeling diode? Will there be any back emf from chip ferrite bead that may spoil my micro kindly help me


regards
kalyan
 

raman00084

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do ferrate bead has inductor windings? what they contain how they are different from inductor
 

betwixt

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A Ferrite bead is normally a small (~3mm) tube of ferrite material with a wire passing through it's hole. I think the ones you have are ferrite cored inductors for EMC reduction.

All ferrite beads are inductors but their purpose is to block signals while letting DC current flow though as easily as possible. It follows that the resistance, which would block DC, should be as low as possible. Usually the resistance is < 1 Ohm which is why I asked what kind you are using.

Brian.
 

raman00084

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it is 50 ohms blm but if i measure in multimeter i am getting around .6 ohms only

- - - Updated - - -

should i must use a free wheeling diode if i use blm ( ferrite beads smd 1206) ?

i am using this in series of pulse generator ic that in turn connected to micro controller to count the pulses
 

betwixt

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0.6 Ohms is good and what I might expect. 50 Ohms seems quite wrong.

Can you post your schematic so I can be sure but from what you are saying I don't think a diode is necessary or advisable.

Brian.
 

FvM

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Ferrite beads are specified by all major manufacturers with their impedance at 100 MHz. The specification doesn't tell which part of the impedance is resistive or reactive, or what's the useable frequency range of a specific part. You need to look at the impedance versus frequency curve for an exact picture, but the ohms specification gives a first idea of the ferrite bead impedance level.

A 60 ohms ferrite bead connected to a µC input without additional filter means (e.g. capacitor to ground) has about no noise supression effect. Ferrite bead are lossy inductors, if the 60 ohms bead would be mostly reactive, it has only 100 nH inductance.

But apart from the rather low inductance, there's no reason why an input series choke would need a freewheeling diode. Sketch an equivalent circuit and consider against which overvoltage scenario the diode should protect?
 

SunnySkyguy

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ferrite beads tend to be low Q and used more to reduce radiated noise transients or RF noise.

Better way is to use 100 Ohm CAT5 type transmission lines from 50~100 Ohm CMOS LOGIC ( e.g. ALVC types) and termination R @ V/2 or equivalent Thevenin circuit with Pullup/dn if you want best speed and noise immunity.

THen use large CM choke around cable to raise CM impedance to CM noise. ... or flat ferrite bar loop over FPC cable.

What H field or Amps dI/dt or E field and dv/dt with capacitive coupling are you concerned about? Magnetic loads nearby? or large Current pulses?
 

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