# How to choose the correct inductor for my application

I am planning on using induction to power some game pieces in a project I am building.

My plan is to use identical coils in board and piece, so they will be small (2" max diameter).

My problem is that I don't understand the technical terms. As such, I'm at a loss to identify which pieces meet my needs.

The base, ideally, will have a 5v supply. Some of the game pieces will draw upto 20mA and some will draw upto 200mA. The pieces are not interchangeable, so if it's better to use different value inductors for the different pieces to cut the overall power draw to the base, that is the plan.

But I can't work out from the data sheets what units will be able to supply those requirements.

It has been suggested to me to use a frequency of 125kHz.

So the supply values are 5v DC (current requirements to be determined) and oscillator frequency of 125kHz.

How do I interpret the data sheet specs:

Inductance (Henrys)

DCR typ/max (Ohms)

SRF typ (Hertz)

Isat (Amperes) @ 10/20/30%

Irms (Amperes) @ 20/40°C

With my supply data to work out what would be suitable.

EDIT

Forgot to mention, working distance between transmitting and receiving inductors will be max 15mm, probably less.

There will be the 3-4mm cardboard game board, and then a couple of mm in the plastics (probably 3D printed) for the game piece and the base.

2ND EDIT

Ok. It's evident that I might not have asked the right question, so I'll try and explain a bit better what I want to achieve, maybe that will help.

Theoretically what I want to do should be achievable. Whether it's feasible is another matter. I'm fully aware I can't just magic electricity or off thin air.

Imagine a chess board where the pieces are electric toothbrushes and the squares are the chargers, you should get a kind of picture of what I'm attempting to achieve.

I only need to power 32 squares.

Of these, 16 need to supply enough power via induction to a floating piece so that the internal electrics of the piece get 5v and 20mA (0.1W). Up to ALL 16 of these spaces could be occupied at the same time.

The other 16 spaces need to supply enough power via induction to a floating piece so that the internal electrics get 5v 200mA (1.0W). However only upto 4 of these spaces will ever be occupied at the same time.

This means that the total amount of power required by the pieces is 5v×((16×20mA)+(4×200mA)) = 5v×(320mA+800mA) = 5v×1120mA = 5.6W

The power required by each piece has been over estimated by about 30% to make sure I have plenty of headroom.

The base will have some small circuitry, plus the primary coils/inductors to power the pieces.

At the moment, if I decide to use physical contacts to power the pieces, including the base,I should be able to get away with a 5v 1.5A supply.

Obviously that is going to increase because induction is not a perfect medium for power transfer.

However, I plan to only have power supplied to occupied spaces so that I'm not wasting emery powering coils that are not powering pieces.

So I need to work out what I need to do to supply the 5v 20mA or 200mA to the required pieces. And whether or not it will be possible within a 2 inch square area.

The max distance between the coils would be 10-15mm based on the fact that the game board is 3-4mm thick glossy card plus the 3D printed plastics for the base and the pieces.

The reason I asked about inductors rather than transformers is because I saw an article using an RF choke to power an ATTiny and turn it into an RFID tag (http://hackaday.com/2009/06/27/avr-rfid-tag/). And researching that lead me to power inductors. Which I thought would do what I want and induce power.

I don't mind winding my own coils but would rather use off the shelf parts if available.

I also am open to using slugs of metal to aid the windings.

It has been suggested that I want to use an oscillation frequency of about 125kHz.

• This post is rather confusing. Why don't you give us your available voltage rail and the requirements? You might be better off getting a fully assembled switched mode power supply. Jan 9, 2016 at 20:45
• @SunnyBoyNY He doesn't want a DC/DC-converter. He wants to transfer power wirelessly. I could be wrong, but I don't think you'll find many pre-assembled gameboards with that.
– pipe
Jan 9, 2016 at 20:49
• pipe is quite correct. I wish to power movable game pieces via induction. Think of a chess board where the pieces are electric toothbrushes and each square are their chargers. @SunnyBoyNY I have 2 types of piece. Neither share the sabre spaces so can be configured differently if that is beneficial. 1 type of piece requires 5v/20mA (0.1W) supply. The other requires 5v/200mA (1.0W). Jan 9, 2016 at 21:20
• I have been looking at data sheets here and don't know how to select what I need. .. (Each space will have a transmitter and each piece a receiver, but I plan to only supply power to the spaces where a piece is placed, not every space all the time). - coilcraft.com/prod_pwr.cfm Jan 9, 2016 at 21:31
• You don't really want inductors. What you are trying to design is the primary and secondary coils of a transformer that is either an air core transformer or an iron core transformer with an air gap. This question seems to be related: electronics.stackexchange.com/questions/13334/…
– user80875
Jan 9, 2016 at 21:59