External Power Problem

Great Forum - Much Read - Thanks All. I have issue with External power to following set up: UNO R3 SIM900 Shield TFT Screen Touch Shield V2.0b It's a Phone and all works fine when USB connected to UNO.

Will not work with external power connected to 2.5mm Plug. I have tried just about everything I can think of: 1. 9V Battery 2. 9V from Mains Adaptor 3. 12 V from Mains - UNO gets warm 4. 5V regulated from all the above 5. 3.7V 2000Mah Battery Direct - Brand New fully charged - giving 4.56V checked

What happens is: 1. UNO fires up 2. SIM900 fires up 3. Screen fires up but 'dimmer' than USB powered 4. When SIM900 gets connection to phone network - screen reboots

It is like there is just not enough power AMPS getting through - hence Dim screen when any external power is connected. I have tried several UNOs and all same result.

Being a Phone I really need a Battery supply of some sort connected to either the 2.5mm conn or VIN/GND.

Any suggestions welcome. Thanks

The plug in power feeds a regulator to give 5v. If you put in less than 7v, the regulator does not have enough "overhead" (voltage above the output voltage) so cannot output the correct voltage.

The 9v battery may not have been able to supply the current required.

Are you absolutely sure you have the correct polarity?


Ok, you have to consider that when SIM900 module send data requires about 2A during the sending, so if your power supply canĀ“t provide that current, the system can be inestable. That might be the reason because your screen resets.


Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png or pdf?

Tom.... :)

GSM modules require lots of current when transmitting, certainly more than the on-board regulator of the Uno can possibly handle. Several amps typically are needed - note that a multimeter reading will be utterly misleading here as the duty cycle is about 12% on transmit so a multimeter reading of 250mA could mean 2.2A.

Typically a GSM unit would be directly powered from a 3.7V lithium battery, which is capable of high currents without problem.

Thank you all for contributions.

Can't provide a circuit as there are no circuits - this is 'shield stacking'.

I undertsand the GSM needs 2 Amps when 'connected to network'.

The ideas here provide food for thought and some testing. It appears that I need 2 power supplies and that is worrying in a 'Stack of Shields'.

The GSM shield is in the middle of the stack so takes power FROM UNO and gives power to SCREEN.

Spliting the 'Stack' is of course possible but kind of defeats the object of keeping the system 'compact'.

I'll do some experimenting and report findings back.

Thanks again

Hi, the 900 shield, do you have the V1 or V2 seed unit? V1 has a power socket for external power, for some stupid reason they have taken it off the V2 version.

Power Circuitry ChangeReplace the original LDO circuitry with DC-DC circuitry. Heat dissipation gets lower and efficiency gets higher up to 80%+. Meanwhile, the EXT_PWR jack on V1.0 was removed. V2.0 can draw current directly from Arduino now without additional 5V adapter.

Hmmmmm.... Tom....... :)

Thanks Tom, Yes V2.0b - which as you say has the independent power removed. Anyway a lot of fiddling and I have solved the problem. 2 X 9v PP3 type batteries in parralell did the trick. Although 1 Brand new PP3 worked - not for very long!

I was hoping to use Rechargeable so will look into that although I was hoping 2 X 3.7V 2000mah in parrallel would do it...

At least it is working - now have to jam all this in a box!

Thanks for all assistance and comments.