Difference between external Power source and Arduino - *~.'Magic*~°^-,

Hi Tinkers! I am facing an interesting problem with an external Powersource. The basic setup: Arduino controls up to 9(3 + 3 + 3) Tlc5940 in row - each fully equipped with leds.

I did a very simple animation - just light up each led alone with a little time difference. This works very well if the Tlcs and the Arduino are powered via USB.

Now the magic: To light up more Leds (more powerconsumption) i bought an external powersource, with 5V and 12V output. I power the Arduino with 12V and my Tlcs with 5V. If i run the same sketch described before with external power, the leds on the Tlc's later in the row are blinking unregularely, noisily. The ones on the first few Tlcs are doing everything as expected. The noise is getting worse the later the Tlc in the row.

I assume: Is this an effect of power decoupling? I didn't decouple anything - but why it works with USB power then? I noticed the arduino has some decoupling itselves http://itp.nyu.edu/physcomp/Tutorials/ArduinoBreadboard In step 2. / 3rd Picture 2 C's à 10uF are installed. Do they do the job on the Arduino? I also assume that my USB has a very stable power source - i guess there is maybe no need to decouple. Do i have to shield the PowerSource? I could imagine that the coils on the powersupply (it is openframe) are influencing the pwm on the bus between the Tlcs. I don't have an oscilloscope so i can't tell exactly which kind of signal loss i am facing with. I'am using shielded cables...but my circuits are unshielded...

Just fyi...i bought the PSA 25L-201 http://www.reichelt.de/index.html?;ACTION=7;LA=3;OPEN=0;INDEX=0;FILENAME=D400%252Fpsa25l.pdf;SID=29TSuWG38AAAIAAGvTfmM33d7834abf011b4e206157f613d43d8a

I am very exited about your suggestions and ideas!


First start decoupling your TLC's. If you don't erratic behaviour is to be expected.

The C's on the Arduino go with the crystal, without them the crystal won't work as expected.

Is this an effect of power decoupling?

It is the effect of lack of decoupling.

but why it works with USB power then?

There is some decoupling on the USB power supply and at the arduino, you just ran out of it when you started adding stuff.


3rd Picture 2 C's à 10uF are installed.

One of my least favorite sites, they are often wrong. You need 0.1uF ceramic on each chip as close to the chip as possible.

i guess there is maybe no need to decouple

You guess wrong.

Thank you guys! I'll check this out!