Sim module (sim900, sim800L) power supply problem

Hi all! I have a problem feeding Arduino and Sim module, I've tried two modules:

Both need 5V and 1A. For my proyect I'm using a old PC power supply which is able to generate an output with 5V/18A, and I've connected this to sim module and arduino. In the program I'm sending next commands to sim module:

AT+CPIN = XXXX (Return ok) AT+CGATT=0 (Return ok) AT+CGATT=1 (Module Power off)

First I thank that there are not enough current, but there are 18A.

Can you help me? I want power the proyect with a battery and now I am not able to feed it form power supply :(