# How to build an inductor?

1. Mar 25, 2006

### juming

Hi there :)

I need to create an inductor of 1.35mH .... but I would like it to have a reasonably large area... maybe a coil with a diameter of 20cm? And if there's any formulae or program that can do calculations for a PCB trace inductor?

(its to be used as an antenna in an RFID project)

Greg

2. Mar 25, 2006

### Ouabache

There are standard concepts for designing an inductor. You probably want to start with those first and get a good feel for the parameters involved. What have you learned so far?

3. Mar 26, 2006

4. Apr 3, 2006

### juming

Attached is an MSpaint illustration I made of basically what I'm looking for. It's for an RFID reader. It needs to have an inductance of 1.35mH. What equations would I use for this?

Standard inductor equations wont work as the diameter of the loop is changing every time, so what would I do? I threw in the 45degree corners to reduce reflections.
So basically my variables are:
> width and length of the outside "loop"
> length of the "loop"
> width of the microstrip track
> distance between the respective "loops"

.... and that's as far as I can seem to get?
I found a really complex PHD thesis explaining how to build PCB trace transformers... with a tiny section on inductors in the beginning, but that was to calculate the inductance of full loops, not spirals.

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5. Apr 3, 2006

### Ouabache

You may want to consult with a good electromagnetics text (e.g. Electromagnetics by J.D.Kraus) and derive inductance for your geometry from first principles.

You can then compare your equation to what others have done, but confine your web/library search to keywords like "microinductors" and "planar inductor".

6. Apr 6, 2006

### juming

after much research & calculation, it seems to me that planar inductors can only be used for inductances in the range of nH to uH, and not mH.

For a 1.35 or 1.5mH inductor i would have to use a ferrite core? or can i use an air-core inductor?
(for a frequency of 125kHz)

7. Apr 6, 2006

### Staff: Mentor

An RFID reader at 125kHz is kind of an oxymoron. 125MHz or higher would be more typical for an RFID application. (I forget where the FCC usually puts RFID applications....)

If you just want to make your own 1.5mH inductor, then yes, wind some turns on a ferrite of some shape (slug, toroid, etc.). You get the specs for the ferrite material from the manufacturer (like TDK or Magnetics Inc.), and figure out how many turns to use from that.

If you want to make an RFID pickup, then first of all you shouldn't be running at 125kHz. That's not RF, it's more like audio. What exactly is the system, and what is this component supposed to do?

8. Apr 12, 2006

### juming

RE: berkeman

the RFID tag i'm using is the TK5530 by Atmel (they bought out Temic) and the reader IC is the U2270B.

The 125kHz low frequency RFID system is usually used for passive tags & small reading distances (8 to 30cm)

Features
• Carrier Frequency fosc 100 kHz - 150 kHz
• Typical Data Rate up to 5 kbaud at 125 kHz
• Suitable for Manchester and Bi-phase Modulation

9. Apr 12, 2006

### Staff: Mentor

Does Atmel or Temic have an application note on what the best 125kHz inductor structure looks like? They should have something that will help to guide you....

10. Apr 12, 2006

### Ouabache

From fig 9.1 in the TK5530 data sheet they give values for the external components to hook up to the reader (U2270B). It suggests a ferrite core 1.35mH inductor in conjunction with a 1.2nF capacitor, however other LC combinations should work as well (following the resonant frequency formula below Fig 9.1) Greg, I would go with aquiring the external components. Wrapping your own ferrite core inductor is a tedious task. If you go that route, you will probably want to measure its value using an LCR meter.

Last edited: Apr 12, 2006
11. Jul 27, 2010

### khaira

since this thread is about how to build an inductor....i really need some help in building an inductor for my hobby kit project. the parameter they've given me as below.

L1___________Primary (Connected to Q2 Collector): 100 turns
Secondary: 10 turns
Wire diameter: O.2mm. enameled
Plastic former with ferrite core. Outer diameter: 4mm.

where should i start???