wireless charging coil size

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.
Kiteman3 years ago
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.
aliasjanedoe (author)  Kiteman3 years ago
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.