Who can let me know what is the different about below question.
I have connected 9 LED in series, but I need to add a resistor to
control the current.
As I know the resistor should be add after the LED cathod,
that mean on negative on my power supply.
But I saw some circuit, they add at the Anode side, before LED Anode.
In a series circuit, the current is the same at every junction so it makes no difference if you connect it at one side or the other. In fact the resistor can go at any point in the chain of LEDs without making any difference to operation.