Ferrite Bead

Try the gizmo available over at

alculator.htm

Looks like a good tool, but I know nothing about my ferrite bead except that its:

1. Works on a circuit 19v up to 4.7a
2. 2.5 turns
3. 10mm length
4. 6mm dia

Thats not enough to work out the resistance is it?

You'll need to know what ferite material is used. This may give you some ideas.

G=B2

Thanks for that link - reading it now. Since I first made the post I have noticed two things:

1. Almost all the ferrite beads with 2.5 turns that I have seen on component websites seem to have spec sheets that show freq response from 1Mhz through to 1000Mhz with corresponding resistance vales typically in the range 100-700Ohms.

I assume therefore that if this ferrite choke is anything like the ones I have found, it is not so much for suppressing mains hum but more likely high freq rf interference and the like? (whats more there is another choke on the dc side of the ac adapter for the laptop which looks more geared to mains hum)

1. If you know the: Length Diameter Turns Wire size of a given bead, and the above can be measured, can you calculate the Impedance @ a Frequency (F)?

I realise not knowing the exact ferrite material may require some generalizations in any equation related to the above.

Its prob not ideal, but I am beginning to think I will get away with any wide band ferrite bead in a similar packaging may do the trick?

-Al

Why do you care? It sounds like a simple ferrite line filter, which are very non-critical in value.

at

You mean inductance, surely? If you want to know the resistance, simply measure it with your multimeter.

Ah. If you just want to replace a broken one, any similar-looking one should do the trick.

ISTR that those things are spec'ed in terms of "equivalent resistance at certain frequencies" (which probably makes them easy to specify). They're just a few inches of (probably) #20 wire.

Isaac

You do realize that its purpose is to keep nasty stuff *in* the box, not the other way around?

Any choke that was effective at mains frequencies would be a *whole lot* larger.

Isaac

Actually, if it's broken, you an probably just forget about it, unless you see evidence that the box is causing interference to other gear nearby.

Isaac

Shove 25Mhz or 100MHz into the existing ferrite bead and measure it's impedance. Just a signal generator, the bead, and a 1Kohm load. Measure the voltage out of the generator, and the voltage across the load, using an RF volts guesser, or an oscilloscope. The rest is calculating the divider ratio between the bead impedance and the 1Kohm load.

Measuring the resistance such a short piece of wire isn't as trivial as using a common multimeter.

One way to accurately measure small resistance values is with a milliohm meter, digital models typically have 3 digits to the right of the decimal. The connections to a milliohm meter need to be secure, not just merely touching a couple of probe tips to the ends of a wire.

-- Cheers, WB .............