Possible Power issue?? or something completely else?

So I am using a Uno connected with a GSM shield and a relay shield. I am using the gsm shield to turn on a relay on the relay shield. When I have it hooked up to my computer via usb cable it woks great and when I say turn on the relay turns on and stays on until I unplug the board or tell it to turn off.

Sadly when I connect it to a 12volt power supply instead of the usb cable it doesn't work so well. It will turn on and run but after a few minutes or a few hours something happens and the relay turns off. Why? I thought that it might be a power issue since I was only using a 840 mAmp power supply. With that I bought a 2 amp power supply. I still have the same problem! I even tried to supply the Uno and the GSM shield from different power supplies and I still have the problem. What is happening? Does anyone know?

Thanks in advance

Heat is happening.

When you use 12V to power the Arduino Uno, the voltage regulator on the Arduino Uno makes 5V out of it. Suppose you request 100mA from the 5V, then the voltage regulator dissipates 100mA * (12V-5V) = 0.7 Watt. If the voltage regulator gets hot, it reduces the output current and that will drop the voltage.

Put you finger on the voltage regulator, if you can not keep your finger on it, it is too hot.

That is why the ideal voltage for the Arduino Uno at the DC power jack is 7.5 Volt. It is high enough to make a nice 5.0V, and low enough te prevent a lot of wasted heat.

Voltage regulators cut out at much higher temperatures than that,150C or so, your finger will burn well before that.

Having said that hot components means reduced reliability, so its good to keep the temperatures finger-friendly.

Caltoa: Heat is happening.

When you use 12V to power the Arduino Uno, the voltage regulator on the Arduino Uno makes 5V out of it. Suppose you request 100mA from the 5V, then the voltage regulator dissipates 100mA * (12V-5V) = 0.7 Watt. If the voltage regulator gets hot, it reduces the output current and that will drop the voltage.

Put you finger on the voltage regulator, if you can not keep your finger on it, it is too hot.

That is why the ideal voltage for the Arduino Uno at the DC power jack is 7.5 Volt. It is high enough to make a nice 5.0V, and low enough te prevent a lot of wasted heat.

So you would say that it is common for the arduino to reset once it gets that hot? (you are right I tested it out and the voltage regulator is getting crazy hot!) If so that is true I will buy a 7.5 volt power supply right away and get that figured out. How many amp power supply should I be looking for? Is one amp enough? I tested how many amps were being drawn by the device and my really crappy 3 dollar voltmeter read like 100mA. does that sound right??

A relay coil consumes quite a lot of current. You need to consider how that current is being supplied. If you want to keep the relay on nearly all the time, it might be better to wire up the circuit so that the relay uses the "normally closed" contact, and you only apply power to the relay actuating coil when you want to turn the circuit OFF.

About 500mA should be enough for the Arduino board, one or two leds and a few sensors. So 1A power supply is good. If you use the same power supply to power other things, you need more ofcourse.

Do you use 12V relays ? A common way to deal with this is to use a DC-DC converter. Your power supply could be 12V and the DC-DC converter makes the 7.5V out of the 12V. Those DC-DC converters are very efficient and don't get hot.

jacob84401: So I am using a Uno connected with a GSM shield and a relay shield. I am using the gsm shield to turn on a relay on the relay shield. When I have it hooked up to my computer via usb cable it woks great and when I say turn on the relay turns on and stays on until I unplug the board or tell it to turn off.

Sadly when I connect it to a 12volt power supply instead of the usb cable it doesn't work so well.

That says it all. If you manage well supplying from a computer's USB port, those are typically rated 500 mA. Did you use the standard setup while testing, using Uno, GSM shield and relay shield, or did you leave anything out you would need while "in the field" ? I'd look for a power supply up to 1 A.