resistor-like inductor

[mdanh2002]

Hi,

Is there a proper term for resistor-like inductors and how different are they from the air-core inductors? I have seen them available in various values, from 0.1uH to 470uH. Many FM transmitters build instructions I have read mention that you should not use resistor-like inductors, but only air-core types. So far I only heard that these are not stable at high frequency, but any thing other than that?

Also, I build my LC Colpitts oscillator using this type of inductor and a trimmer capacitor. The frequency is not stable and decreases every minute the circuit is running. Is this related to the inductor properties?

 

[Sceadwian]

Do you mean ferrite cored inductors? Many cored inductors fair poorly in high frequency, but not all.

 

[mdanh2002]

Hi Sceadwian,

I mean this type of inductor, what is the proper name for it? I google but find various different names

**broken link removed**

 

[Sceadwian]

Those are just encapsulated ferrite core inductors, the core material isn't generally suited for high frequencies, at higher frequencies their effective inductance drops drastically.

 

[mdanh2002]

Thanks Sceadwian

Is there perhaps a threshold above which the frequency is considered high and unsuitable for these inductors?

 

[Sceadwian]

Other users here many have more experience with them, I have virtually none, just know what they are. I would hazzard a guess that anything above audio frequencies is probably going to drop the effectiveness and increase loss to the point where they're not useful.

 

[ronv]

The self resonant frequency of a cored 220 uh will be 3 - 5 mhz, while the same air core will be 800 900 mhz.

 

[unclejed613]

they are also called chokes.

 

[colin55]

Don't use a potted inductor for the coil in the oscillator section of an FM transmitter. These inductors have "low-Q" due to the low-frequency core material and the thin wire in the winding - and the winding is jumble-wound. The only core material to use for 27MHz to 150MHz is F29. It has a "Q" of 20, going down to 3 for 150MHz. Air has a constant value of 1.
This means a coil wound on ferrite can have fewer turns than an equivalent air-core and will be less "microphonic."

 

[The Electrician]

The self resonant frequency of a cored 220 uh will be 3 - 5 mhz, while the same air core will be 800 900 mhz.

When you say "...while the same air core...", do you mean that if the ferrite core in the inductor were replaced with a plastic or ceramic or other non-ferromagnetic material, while retaining the exact same winding (wire size, number of turns and geometry), the self resonant frequency would be 800-900 MHz?

 

[mdanh2002]

Thanks all for the info. I think I still just use air-core inductor inductor instead.