Electronic – arduino – Connecting arduino pro mini (3.3v) to wall wart via dc jack

arduino

So I've built this circuit on my breadboard with an arduino uno and largely have it working.
http://makezine.com/projects/make-34/the-dryer-messenger/

Now I'm trying to miniaturize it by using a pro mini; however, I keep blowing them up. I've confirmed that the soldering is fine and there are no shorts on the board since I've powered it up fine with a FTDI friend.

I have 9.2 DCV coming from the wall wart, when I connect the positive to the raw pin and the negative to the ground pin then I get a red flashing light on the arduino, and the wall wart also flashes red. If I leave it on for about 10 seconds, then it blows up and I see sparks.

A picture of the setup: breadboard with arduino pro mini

The arduino pro mini : http://www.sparkfun.com/tutorials/244

The arduino pro mini schematic : http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Dev/Arduino/Boards/Arduino-Pro-Mini-v13.pdf

The dc plug:
http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/18742.pdf

The voltage regulator stage of the arduino pro mini board
voltage regulator stage

Any idea as to what I'm doing wrong?

Best Answer

I think you're killing the tantalum capacitor across the input. In fact, it looks like it's bulged up in your photo.

enter image description here

Although the capacitor may be rated at 16V (I don't actually see a rating anywhere, but I'm inferring it from the size and input voltage spec) seasoned engineers will never use tantalum capacitors in this sort of situation. The input current needs to be limited by several ohms and/or the rated voltage should be something like three times the operating voltage. Your wall wart can probably supply several amperes.

enter image description here

enter image description here Years ago, this information was anecdotal, but today you can find it buried deep within the manuals for such parts. They bury it, because it means you very seldom should use tantalum capacitors, and almost never across power supply rails unless the current is limited to much less than an ampere.

Many have been burned by this (perhaps literally). Often failures take place long after product has been shipped, and can cause localized charring, meaning (if your standards are high, as in aerospace or medical) the entire board must be scrapped.

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