Miniature 100mH inductor

[luben111]

Hi,

I'm building a low frequency oscillator powered from 3V and it comes out that I need to use 100mH inductor (it's other story why I need so large 100mH inductor but it seems the only one possibility in my case). The only one problem is that I have strict limitations for the size - it should fit into chamber 4 x 4mm and 30mm long. The resistance of the coil should not exceed 450 Ohm.

The inductors I can find are 6-7mm in diameter minimum. I'll appreciate if you can bring me some information where to get such miniature 100mH inductors or how to build them.

One of the ideas I have is to use thin 1.5mm ferrite rod 25mm long where I can put a lot of turns of thin wire. On top of the coil I could place a thin layer of ferrite powder mixed with epoxy in order to close partially the magnetic field lines and hence to increase additionally the inductance.

Thank you.

Kind regards,
Luben

 

[kam1787]

could you use a gyrator?

**broken link removed**

 

[crutschow]

Why is a 100mH inductor the "only one possibility".

Exactly what type of oscillator are you trying to build?

 

[FvM]

Apart form possible design alternatives, there are sufficient small cores like this

If at all, 100 mH could be only achieved with an ungapped core, so you also have to check for required inductance stability.

An "open path" inductor like a rod core sounds even more unrealistic.

 

[luben111]

Hi,

Gyrator could not be used because I need the resonance/transformer property of the LC to increase the voltages from the low power supply (battery).

100mH is not the only one possibility - any inductance bigger than 100mH could do the job until it's small enough (max outside diameter is 3.6mm, length < 30mm).

The Epcos coil falls exactly in the specs I needed. Because the magnetic field lines are closed it should yield extremely high inductance compared to ferrite rod. Thank you FvM for the idea! There are two problems with the Epcos core - the price is too high and the space for the coil is too small.

>>An "open path" inductor like a rod core sounds even more unrealistic.

Actually at the moment I'm experimenting with such small "unrealistic" coil and it works perfectly well, the only one problem is that it's "home made" where I need something "on shelf", some standard product.

Thank you for the suggestions.
Luben

 

[FvM]

Actually at the moment I'm experimenting with such small "unrealistic" coil and it works perfectly well

That's good. Thanks for correcting me, I didn't actually calculate it.

 

[crutschow]

If you need to boost a voltage then just use a standard boost converter circuit. They can operate at a high switching frequency requiring inductances in the µH region not mH. What is the current you need, and the input and output voltages?

 

[luben111]

The target specs are:
- input voltage 3V
- output voltage 30V sine wave 14..16 KHz
- output current < 2uA
- power consumption from 3V to be less than 150 uA
- mechanical size - to fit into 3.6mm cylinder 25mm long

Really tough specs.


Using DC-DC is not a good option, I already explored it and the power consumption is too high. Seems that only LC resonance or piezo transformer can fulfill the specs. Piezzo transformers are too expensive so stays only the LC path.

Luben

 

[crutschow]

So what is the circuit that will generate this AC voltage?

 

[FvM]

So what is the circuit that will generate this AC voltage?

My question, too.

My guess for best efficiency: A 3V switcher, half- or full bridge, driving a resonant circuit with virtually no losses.
In my view it's a straightforward design, except for the size of the inductive element(s).

I agree with the conclusions in post #8 in so far that it may be the best or even the only feasible implementation under given conditions.