SIM900 random shutdowns

Question

I have a SIM900 GPRS/GSM Shield and an Arduino Mega. Now for the past week I have been using USB as power to send and receive GPRS data with no issue. Today the SIM900 module is continually randomly turning off, not even turning of at specific stages of communication.

I thought this could be power related, so have external power (5V 5A UBEC) into the 5V pin of the Mega, which I believe should cover the voltage and current requirements of the SIM900 shield. However this has not solved the problem.

Any ideas on what could be causing the random shutdowns?

EDIT: Is it safe for me to wire power up like this:

enter image description here

• Code Gorilla: Are the mega and the PSU sharing a common ground?

Answer 1

Cool, an UBEC is indeed a smart choice for powering a SIM900. I could get away with a small switching power supply and just putting in a big capacitor (since the sim900 only draws high current in very short peaks).

But, the limiting factor in this case would be the 6AA batteries. Your UBEC may be able to supply 5A's but those AA's may not.

Also, look up "transient response" (related) according to the related article, you can have an 20A power supply, but still have the voltage drop out when the sim900 starts to transmit. This is because your supply needs to be able to deliver the required 2A in a very short period of time, so basically if your supply can't go from 100ma to 2A fast enough, it'll dip.

I suggest finding a more optimal source as AA's and adding a "big ass capacitor" close to the sim900's input.

Answer 2

Pumping 5Amps into the Mega isn't going to make a difference to the current the SIM900 is receiving (needs>2A) it will still get 500mA (?).

You need to feed the power directly into the SIM900 from the PSU, because when it transmits it needs a nuclear reactor to power it. Don't forget the common ground.

• Calco: Ok, i'm not massively clued up on the power side of things. I have this UBEC. How can I safely power the Arduino AND the SIM900 with this?
• Calco: Is it safe for me to wire power up as shown in the main post edit?
• Calco: Hold on, even though I am plugging into the 5v pin, then current is still limited 500mA?
• Code Gorilla: Looking at you diagram (could see it yesterday). Does the "Turnigy" have two separate outputs? You description seems to say that the SIM900 is powered from the Mega, but you picture seems to say the wire splits and goes separtly to each. Rereading the description it looks like you have flat batteries, "worked last week but has stopped working", have you tried changing the bat's? (Sorry to ask a dumb question, I'm sure you have, but...)
• Avamander: You could always try adding a large electrolytic capacitor 10000uF to the power rail (- to GND and + to +5V), it might be that SIM900 resets when it just browns out due to transmission power usage.