Hello guys, I made a project with sim900a and arduino for sending messages.
Everything works fine while I use short usb cable to power up arduino and sim900a(as power source I am using one of those fast chargers from ebay 5v 2.4A) but I need to use longer usb cable and when I use it than sim900a has not enought power and keeps restarting
Is there solution by adding capacitor between 5v so that I get some "help current" when needed?
Use a cable with thicker cores, so less resistance. Then you don't have this problem of voltage drops.
Are you sure it's really the SIM900A restarting, not the Arduino restarting and then doing a reset on the SIM900A?
wvmarle:
Use a cable with thicker cores, so less resistance. Then you don't have this problem of voltage drops.Are you sure it's really the SIM900A restarting, not the Arduino restarting and then doing a reset on the SIM900A?
Yes I am pretty sure that the module is restarting because when I power only module it keeps restarting as before
Will try different usb cable later and will tell you the results
Thanks a lot
Ok, I tried gold platet usb cable and still same problem happening...
I even tried to add capacitor in parallel between 5v and ground on the module but it still keeps restarting 
Which Arduino, and which SIM900.
Both come in many shapes and sizes.
How do you power both.
Powering the SIM900 through the unspecified Arduino might not be possible, because of a <=500mA polyfuse and mosfet between USB socket and 5volt pin. That, plus the internal resistance in the USB cable might drop so much that things reset.
Details matter. Post all of them.
Leo..
Riki_g:
I even tried to add capacitor in parallel between 5v and ground on the module but it still keeps restarting
What do you call a capacitor? 
1 mF would be a start.
No, 10mF (10,000uF) ultra-low ESR would be a good start. GSM modules take heavy spikes of current every 4.6ms and expect the supply voltage not to droop.
USB cables and connectors will not work reliably, far too much series resistance.
Don't try to power a GSM module via USB in the first place, use good thick wires direct to the supply, you want
0.05 ohms per power and ground wire maximum by my calculations.
GSM modules are considerably more current hungry than servos in fact.