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Old 11-25-2005, 12:20 PM   #1
arneson
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Default Ferrites

I'm asking out of curriosity about small caps usually in an rf circuit that are mounted with small beads under the legs?
To me they look like ceramic but I assume it's ferrite. If the caps were flush there would be no exposed leg but they are put on there for some reason.
Anyone know more about this?
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Old 11-25-2005, 12:28 PM   #2
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Default Re: Ferrites

I've seen plastic or ceramic spacers used to raise a component off the PCB. OTOH, ferrite beads may used to slow the turn-on and turn-off of MOSFETs and/or rectifiers to suppress ringing that can cause EMI problems. Perhaps the bead on the cap lead is intended to make it an L-C filter. Ferrites would either be dark charcoal grey or painted.
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Old 11-25-2005, 12:42 PM   #3
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Default Re: Ferrites

Someone asked me the other day about a ferrite used to suppress line noise.
All the new systems I've installed have cable ferrites.
Sometimes at each end, sometimes two in a row, sometimes the cable loops twice thru the ring.
These are on Radar cables, power cables, antennas, transducers,vga video, they're everywhere, sometimes internal sometimes clamped on, sometimes molded on.
Does this block? absorb? dissapate?
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Old 11-25-2005, 02:28 PM   #4
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Default Re: Ferrites

I'm a bit vague on this arne. I think it's a bit of both: the inductance is a high impedance to high frequencies, but I've also heard that the ferrite also looks like a resistor to the high frequencies. It may be two ways of saying the same thing.
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Old 11-25-2005, 08:37 PM   #5
linuxguru
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Default Re: Ferrites

A ferrite ring is basically a low-value inductor. I've seen then on the legs of capacitors as well - it's either some kind of LC(R) pole-zero compensation filter to improve the stability of the control-loop, or a transient/ringing suppression circuit for reliability or EMI compliance. They're also sometimes found on the snubber sections next to the Schottky rectifiers or FETs.
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Old 11-25-2005, 09:24 PM   #6
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Default Re: Ferrites

The large ferrites are described as EMI compliance filters.
Thankfully most of them are split core and can be unsnapped, cable pulled thru small opening then, then clipped back on.
I usually carry some around and stuck one onto my DSL phone line,
Hey maybe it has slowed my connection.. or cleaned up the phone?
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Old 04-27-2006, 05:55 AM   #7
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Lightbulb Re: Ferrites

Quote:
Originally Posted by linuxguru
A ferrite ring is basically a low-value inductor. I've seen then on the legs of capacitors as well - it's either some kind of LC(R) pole-zero compensation filter to improve the stability of the control-loop, or a transient/ringing suppression circuit for reliability or EMI compliance. They're also sometimes found on the snubber sections next to the Schottky rectifiers or FETs.

like this?



they are on my silverstone 360W. the ferite is found in the capacitor legs, all secondary mosfets (5 chip), and output legs of big tranformer.

my tt 480w that more bulky/heavy not even has them.
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Old 04-27-2006, 06:36 AM   #8
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Default Re: Ferrites

It looks like those would absorbe radiated energy, but makes me wonder from which direction, the transformer to the MOV or from legs of MOV to anything near it?

My application is more like suppresion of spurious signals between system cables.
When I started putting together marine electronic devices there were a few problems that would occasionally interact between devices.
The worst was background noise in a vhf radio that could not be squelched.
Next was pulsing video screens, now that I see these large ferrites clamped onto the shielded multi conductor cables these issues are not as common.
The worst offender is often the sonar pulse, these can bang out 1000W RMS 200khz ping and then listen for a very faint return echo.
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