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Safe power supply/current limit for Arduino RGB LED tutorial
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Safe power supply/current limit for Arduino RGB LED tutorial

by bfinio on Tue Aug 27, 2013 10:23 am

Hi,

I am following your quite handy Arduino RGB LED tutorial, http://learn.adafruit.com/rgb-led-strips/usage and have a question about a power supply that is safe to use with this project without frying the Arduino.

I had a 12VDC power supply lying around, but quickly found out that it's only rated at 0.5A, which is not enough to power the LED strip I have (which draws about 1.2A at full-blast brightness). So, I just went ahead and ordered your 12V/5A and 12V/1A power supplies:

http://www.adafruit.com/products/352

http://www.adafruit.com/products/798

I'm in a hurry, wasn't sure which one I needed, and figure they'll come in handy anyway - I'm guessing that if I'm doing color fade and never using full brightness, then I can get away with the smaller supply.

Your tutorial states: "Connect a 9-12V power supply to the Arduino so that Vin supplies the high voltage to the LED strip. If you want, you can also just use a separate wire that connects to a power supply that provides about +12V. Make sure to connect the ground of that supply to the ground of the Arduino/MOSFETs! " (emphasis added)

My question is whether it's safe to do that with the bigger supply - can the GND pin on the Arduino UNO R3's power section sink that much current, if I do happen to have the strip at max brightness for each R, G and B? I have been googling around trying to find specs for this...so far I have only been able to find official information about current limits for the digital IO pins, and this thread http://forum.arduino.cc/index.php?topic=70445.0 which hints at a 1A current source limit for the Vin pin...but nothing about GND.

Just want to know if I can safely do this without frying my Arduino. If not, is it possible to run the LED strip off of a totally separate V+/ground bus from the external power supply? I'm guessing no - I actually tried to do that first and got some really weird results (with a smaller, 3-LED segment of the strip that my power supply could handle); so my assumption is that something about having the MOSFETs connected to the Arduino's PWM pins, but a separate ground from the Arduino, made them unhappy.

Thanks to anyone who can help!
bfinio
 
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Re: Safe power supply/current limit for Arduino RGB LED tutorial

by adafruit_support_rick on Tue Aug 27, 2013 10:42 am

GND isn't an issue for sinking current.
It wasn't exactly clear to me - you planning to use two power supplies, one for the Arduino and one for the LEDs? Or are you using a single power supply for both?

With two power supplies, you want to make sure that you have a common ground. If you use the barrel jack to power the Arduino, make sure the ground from the LED power supply is connected to one of the Arduino's GND pins.

If you are using a single power supply, don't connect it to the Arduino's barrel jack. Instead, wire the PW to the Arduino's Vin and GND in parallel with the LED strip.

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Re: Safe power supply/current limit for Arduino RGB LED tutorial

by bfinio on Tue Aug 27, 2013 10:58 am

Ideally I would like to use just one power supply for everything. I wasn't sure if I needed to use two and isolate the Arduino from the LED power due to the high current. According to your answer it looks like I can just use one.

What is the difference between connecting to the external supply to the Arduino's barrel jack vs. the Vin and GND pins? I thought those were just directly connected to each other, e.g. if you plug a 5V, 9V, 12V power supply into the barrel jack, you get that respective voltage available from the Vin pin?
bfinio
 
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Re: Safe power supply/current limit for Arduino RGB LED tutorial

by adafruit_support_bill on Tue Aug 27, 2013 11:03 am

The barrel jack is directly connected to the VIN and GND. I think Rick's point was that you should not run power to the jack then try to power your LEDs from the VIN pin - that little pin is not designed to handle the current.

Instead, run power and gnd directly to the LEDs - since they are the biggest current load. Then connect a wires from GND and VIN (or the barrel jack if you want to wire up a suitable plug).

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Re: Safe power supply/current limit for Arduino RGB LED tutorial

by bfinio on Tue Aug 27, 2013 11:12 am

That makes absolutely perfect sense now. Thank you guys for the quick response!
bfinio
 
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