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Arduino resets periodically while being powered via an external power supply. Answered

I am using an 5V Arduino Mini (https://www.sparkfun.com/products/11113). While the Arduino is plugged into the 5v USB power coming from my computer it runs as expected (the lights flash in the right order and with the right timing, it accepts code etc.), but when it is powered through a regulated power supply it will turn on for a few seconds, then restart, run for a few more seconds, restart and repeat this indefinitely. I am programming it via an FTDI programmer (https://www.sparkfun.com/products/9716), and using two Polymer Lithium Ion Batteries (https://www.sparkfun.com/products/8483) wired in series to get it up to the necessary voltage. I have tried disabling auto reset via bridging 120 ohms equivalent resistance between RST and VCC and connecting a 10 micro-farad capacitor to RST and GND, but that did not solve the problem. I am fairly certain there are no problems with the code, because it runs perfectly when plugged into the computer, and I get the problem when running any code.
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Measure the output voltage from the supply, Ideally monitor it with a scope, and report back.

I'm getting 7.8 volts from the battery and 5.01 volts out of the regulator.

Don't just take a single measurement. Keep the meter on the output and monitor it for about as long as your Arduino lasts before resetting. Chances are the regulated supply is cycling on and off at some point causing the Arduino to reset.

I watched it for a couple of cycles and it displayed 5.1 while off, when it turned on it dropped to some value usually around 3.9, then restarted, and then jumped back up to 5.1 until it turns back on.

Your supply is "drooping out" and your uP does not like that is why it resets.
Put some big filter capacitors 1000uF at 15VDC across the uP or
Just get a GOOD stable power supply.

I'll get a good power supply. Would this work well?

Not unless you want to supply a transformer, rectifier etc as well. I'd look for a wallwart. What was the rating of the supply you were trying ? Because you need a bigger one, but probably not much. I'll guess the LEDs are pulling around an amp.

After reading the technical details I found the LED's pull about 1.5 Amps when they are all on full brightness. The info for the strip is found here.
I'm using the regulator built into the micro, part number MIC50205 (datasheet here). From what I can gather, the datasheet say's the regulator can provide 320 mA typically and 500 mA max. So... I can see the problem now clear as day.
I think the power supply will work for me. It's well within the realm of safety, and will fit well within my project.

Using the onboard regulator for big loads is a no-no ! That 3A supply is a good one, but you could run the board off a 6-9V supply with no problem, only feed the LEDs OFF the board.

Adafruit makes good products.

If your design uses five volts at less then three amps give it a try.

As Mpilch says, monitor it through your reset cycle. What else is the arduino connected to when the PSU is connected ? Don't "disable" the reset - you'll stop the processor starting up properly, and in some cases it can destroy the bootloader.

I got the same problem.

What are you DOING with the arduino ? Is it connected to a load ? What PSU are you using ?

I'm running 25 RGB leds on a strip. Saying that tells me that it's probably drawing way too much current from the little regulator I have. Iceng said to get a better, stable power supply, which is what I'll be doing.

Exactly what's happening. Your PSU is pushed too far.