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wireless charging coil size Answered

So I bought one of these to play with:

The transmitter board always gets really hot and smells faintly of something burning.  Is this normal?  Should I use some type of heat sink?

Anyway, I discovered that you can add another coil of the same-ish size to the transmitter, and both coils will work.  However, if I remove the coil and add a single coil twice the size, it does not work.  I'm curious as to why.  What determines how large the coil can be? 


You need to create a coil with around 30 MicroHenry of inductance.
Its a complicated piece of maths, but its affected by the turns^2, by the area^2 and the average coil diameter^2

If its getting hot, I'd be inclined to turn the supply volts down.

Hot is not right, yes, you need a heat sink.

The coils work by matching up magnetic fields - if the coils are too far off in diameter, the receiver won't "catch" the full field of the transmitter.

I'm wanting to do something like this (multiple little coil receivers in a larger transmitter coil):

I can put multiple tiny receiver coils in my small transmitter coil and it works, but I'm failing to scale it up a bit. How do I know if the problem is my coils not matching frequency or if my setup just can't handle the larger size?

Sorry, that's one for somebody else to answer.