Source for or how to make variable inductors in the mH range

[dr pepper]

I need some fairly high value rf inductors, in the mH range.
You used to be able to get pot cores with adjustable centres, but I see now they are not so common.
Is there a source in the uk for coils formers with adjustable slugs?

 

[BobW]

Most of the variable ones that I see nowadays, have such a narrow adjustment range, they're not much good for anything that I would use them for.

 

[Les Jones]

Hi dr pepper,
Is it just a small number of pot cores you require or is it for a production project ? What sort of inductance range do you require and what frequency range ? (Could it be for 81 KHz ?) I have a couple of pot cores if I can find them.

Les.

 

[chemelec]

Hi dr pepper,
Is it just a small number of pot cores you require or is it for a production project ? What sort of inductance range do you require and what frequency range ? (Could it be for 81 KHz ?) I have a couple of pot cores if I can find them.

Les.

Pot Cores Don't make Adjustable Coils.
It needs to be like a Tube to wind the coil on, and with a moveable ferrite rod inside it.

 

[JimB]

Pot Cores Don't make Adjustable Coils.

Not completely true.

Most pot cores are fixed.
Some, such as the old Mullard "Vinkor" type do have an adjusting slug, which moves across a gap in the centre limb of the core.

JimB

 

[chemelec]

Hi JimB, I have Been in Electronics for over 50 Years, Never seen that.
Must be a Pretty Rare Item.

 

[ericgibbs]

hi drp,
Are you asking about the RM series of adjustable ferrite pot cores.?
Page 6/11
E

 

[JimB]

Must be a Pretty Rare Item.

Yes, Mullard disappeared many years ago, like many other British companies.

The Vinkor series of inductors were used in many items of electronic/radio equipment.

In the top row on the left , there are two different sizes of the Mullard Vinkor in their metal screening/mounting assembly. At the right is an unscreened core, not sure who made it but it was sold by RS Components, this one also has an adjustable core.

In the bottom row, a Vinkor unit split into its parts.
Left to right, the metal body/screen, the bobbin lower half of core and terminal panel, upper half of the core, body end and mounting boss, adjusting core, ferrite sleeve on a plastic core.
The plastic core of the adjuster is screwed onto a threaded spigot which can just be seen in the bottom of the base/terminal panel.

JimB

 

[chemelec]

I Remember the Mullard Company, Just never seen that type of adjustable core before.

But it is easy Enough to make and Adjustable Core, if a person has a Lathe and some Ferrite Rods.
Can't easily do a Screw type, But can make a Friction Type of retention to the Ferrite in the hole.

I have Lots of Rods and have done this before.

 

[BobW]

Would one of these be any use?
https://www.coilcraft.com/pdfs/slot10.pdf
The biggest one just barely gets into the mH range

 

[Nigel Goodwin]

Yes, Mullard disappeared many years ago, like many other British companies.

Mullard became part of Philips a VERY long time ago, and for many years they still traded as Mullard - but eventually became renamed Philips Components.

 

[Nigel Goodwin]

I've been trying to reme?ber the famous old coil manufacturer all day? - it just came to me

Denco - and a quick google found this nice PDF with lot's of examples:

https://www.g4dmp.co.uk/dencocoil.pdf

And this:

 

[Les Jones]

The pot cores I have are the Mullard type.

Les.

 

[Mikebits]

Did you say millihenries? That sounds more like audio than RF. I don't think you would get much tuning range for a millihenries coil.

 

[chemelec]

Did you say millihenries? That sounds more like audio than RF. I don't think you would get much tuning range for a millihenries coil.

I Agree, You will need a Large Ferrite Rod.

 

[dr pepper]

Nope not 81kc, but it is Vlf.
I've used Rm series before I didnt know you could get adjustable ones.
I'd been thinking about a smps type trans with a squishable spacer and nut/bolt clamp, but thats a pain.
One of those coilcraft ones would do it, I can adjust the circuit a bit to work with 1.1mH.

 

[Les Jones]

I have found these variable inductors. The largest they have is 1 mH which is adjustable by +/- 10%

Les.

 

[BobW]

The ones I linked to in post #10 have an adjustment range of 440 to 1143 mH. Not sure how readily available they are though.

 

[alec_t]

Could you use a fixed value inductor in series with a lower value tunable one? That might give you more choice of types/sources.

 

[dr pepper]

I can get coilcraft ones if I wait for overseas postage, mouser have no stock of those adjustable ones.

I sussed it, dumped the circuit altogether and redesigned with an op amp and caps, no silly value inductors.