Favourite crappy toroids?

Probably the said shielded inductors could also fit your needs. Way more to choose from...

Thanks, Fred, that's a good candidate.

Cheers

Phil Hobbs

is

Should

he

t -

Ahh, A common mode choke. That explains the two extra leads :^) I wondered what those were for. I'm using the 100uH version single ended. All the inductacne is in one lead. About 60 m Ohms of resistance at 100 Hz, measured on a LCR meter.

George H.

We stock a 4-winding ISDN transformer, 1:1:2:2, cute little surface-mount toroid, 30 mH on the "1" windings I think. All the windings in series would be 300 mH! The Q has got to be terrible at audio frequencies.

Another way to kill Q is to hang a resistor across one winding of a transformer. That has different effects from a series or a parallel resistor. Or a resistor across one winding of an autotransformer.

John

Guys, careful with common mode chokes. Their current rating only applies if the very same current that flows through one wire flows _back_ through the other. IOW, the magnetic sum of currents through all windings (some have more than two) must be zero or at least very close to zero. Otherwise the core will saturate and the inductance collapses.

The Talema ISDN 1:1:2:2 measures 0.8 and 1.6 ohms on the windings. The "2" windings measure 40 mH on my Extech RLC meter, which I trust some fraction of the distance I could throw it. L meters are like economists, never agree on anything. An L/R measurement with a pulse generator and a scope comes out around 20 mH.

This is a tiny toroid, so its magnetic pickup area should be small.

John

Must be spot welded ;-)

-- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

Understood. That's why I didn't pick one of those. I could order some and measure them, but that doesn't fit the project time line.

Cheers

Phil Hobbs

Thanks.

I'm a bit worried about the saturation current, though, with all the windings adding flux--do you have any data for those?

A parallel resistor turns it into a one-pole rolloff, though, as Vladimir intimated.

Cheers

Phil Hobbs

ng is

=A0Should

at a

and

the

ohm

, so

ext -

Hi Joerg, Thanks for the 'heads up'. I just spent a few minutes bashing one of these 'common mode chokes' apart. The leads are buried in some black epoxy stuff. (I'd measured the leads, 100uH between two, 50 nH between the others, but seeing is believing.) This thing has fours leads, two of which wrap around the toroid. The other two are just a short piece of wire that makes no turns around the toroid. I have no idea why they call this a common mode choke. It's a single inductor with an extra piece of wire buried in the epoxy. At least now I can sleep tonight.

George H.

In a perfect world, if one turn on a core gets 30 mH, then 6 total turns gives 36 times that or about 1.08H.

John S

Oops, right.

John

That's not a common mode choke. Make sure the other line doesn't go through the center of the toroid because then you'd have a current sense transformer which would also not be suitable at all as an inductor.

If you have a crappy enough xfrmer, then you have damping at LF which is isolated at high frequency with the xfrmer leakage inductance. With a good crappy one and a turn ratio far enough from unity it can be somewhat high...

Not with leakage inductance!

John

True. At that point, though, I might as well use a 10-uH inductor and an 0.1 ohm resistor.

Cheers

Phil Hobbs

hing is

=A0Should

s at a

to

r and

ed the

n ohm

ke, so

d.

text -

=A0I

es

Yeah, well I didn't name it! Here's the data sheet... looks like an inductor

George H.

is

Yes, those are real inductors, those are good. I wouldn't use them in vibration-prone gear though, like on vehicles or aircraft, because the tallness combined with the weight can really stress the solder joints.