Help identifying/selecting a ferrite bead

Hoping someone might be able to give some advice - I have a circuit board for a mains-powered LED light, with battery back-up. It looks like moisture has got in and one of the ferrite beads FB1 has blown up. See attached photo. Resistance across FB1 is about 4.5k, whereas across FB2 its about 2Ohms. There are no identifying numbers on FB1 or FB2. FB1/2 are adjacent BR1 and the mains connector, so I suppose they are there for noise suppression. Any idea what “value” of FB1 I should look for in a replacement?

Jaycar and Altronics don’t seem to do these surface mount FB’s.

Hi Steven,

Just to start, we aren’t qualified to advise on LV (240V) applications, so I hope you have the relevant certifications or are getting someone who does. Assuming we’re in the ELV domain:

Since ferrite beads and similar inductors vary in impedance with frequency, they can be hard to measure, I found an instructable on it, but I’ve never done it myself:

As for where to get them, Digi-Key has a whole category, you’ve just got to find one that behaves the same as yours on the freq/impedance graph:

Personally, if this device isn’t too old I’d see if you can pursue it as a warranty claim if the device was insufficiently weather sealed for purpose. This sort of repair isn’t done much because of the tiny unmarked components that are giving you trouble.

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Hi Steven

Personally I have never seen a surface mount ferrite bead. That does not mean they don’t exist.

I have used hundreds of beads in the past and they are usually what they say, a piece of ferrite with a hole in it to be slipped over a piece of wire. Some are quite large, that is what is under that blob you sometimes see on plug pack and other power supply leads. They are used to provide a high impedance to and suppress interfering radio frequencies.

Being just a piece of wire with a bit of ferrite on it I would expect something close to zero ohms, not 4k5Ω. But it could be a resistor inside a ferrite. You would need a circuit or some other info to establish this I think. “FB?” and a photo are not much help in this case.
Cheers Bob

Just had a look at Element 14 and they have heaps of surface mount ferrite beads. BUT the maximum DC resistance quoted is 78Ω so how accurate is the 4k5 you measured. The 2Ω you quote would be more like it.
These vary greatly in Dc resistance, current capability, RF impedance etc so you really will have to know what you are looking for I think. Physical size is another variable.

By the way these beads would not be there to suppress RF interference getting INTO the unit. More likely to stop said interference getting OUT. Ever tried listening to an AM radio while some of these LEDs are switched on.


Thanks Bob - I think the DC 4k5 is that high because the component FB1 is damaged. It should be very low like FB2. It looks to me like the selection is based on getting a certain impedance at high frequency - Element14 datasheets give figures like xxOhm at 100MHz, and a current rating. Good point about stopping the RF from getting out - I haven’t tried the AM radio trick. It is looking like the “too hard” basket so the board is heading for the tip :slight_smile:

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Thanks James - my initial intent was to just buy a new board and re-use all the other hardware, but the supplier said it is cheaper to supply a complete unit than spares - too much trouble for them. They have supplied a complete new fitting, free of charge, but I was trying to save this one from landfill and keep it as a spare.

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Hi Steven

That xxohm figure is the impedance at the specified frequency. It has absolutely nothing to do with the DC resistance. The 2 are completely different although they do come together when defining the “Q” factor of RF inductors. That is an entirely different subject.
Cheers Bob