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high pitch noise

I have an issue with several Leonardo and Micro boards.

On all of my boards, there is a sawtooth of about 690mV ptp @ 4kHz on the 5V supply when powered from Vin (external power). The frequency changes slightly when changing input voltage from 7V to 12V.

C12 is a capacitor on the Leonardo board that is making the acoustic noise, it is "singing" with the frequency of this sawtooth. It follows the voltage regulator NCP1117 for the 5V power supply generation.

According to the schematics of the Leonardo and micro, the voltage regulator used on these boards should be the NCP1117, but on the boards I have, there is no number printed on this component, so I suspect an alternative was used, with these annoying results...

For a project I have for generating audio signals, this noise was unacceptable, so I cut out the regulator and powered everything with an external LM7805. One should not power the module via USB then as long as the external power is connected!!!

Anyone having the same trouble with these boards? Other solutions?

(The Leonardo is a genuine Arduino board.)

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  • How are you generating audio signals? More specifically, what are you driving in the way of outputs? Are you driving a low impedance magnetic speaker directly? Your observed behavior strongly hints that you may be placing a high load on the regulator - normally, with only the micro as a load it shouldn't do that, unless you have a dirt cheap implementation made with substandard components.
    – Chris Stratton
    Sep 27, 2015 at 2:02

2 Answers 2

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Yes, I've had this problem on a number of boards. It seems there are some faulty units coming out of the factory. I've had them replaced by my supplier, but I end up being sent more faulty boards.

It makes any kind of accurate ADC reading impossible.

A quick way to work out if your board has this problem

  1. Place a jumper between 3.3v and A0
  2. Put an analogRead in a loop and println the result
  3. When powered by 5V you'll see a fairly constant value (around 670), varying by 1 or 2
  4. When powered by 12V, if you have a faulty board, you'll see varying values, deviating by 50 or more.

There's a couple of solutions/workarounds to the problem listed here: http://forum.arduino.cc/index.php?topic=235729.0.

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It does sound like the regulator is oscillating to me. That happens when the output capacitor is of the wrong type for the regulator - the ESR is wrong.

My Leonardo, when powered from 12V, is nice and clean. The regulator has 7CE50Z on it. I do notice, on the layout of the Leonardo, that the output capacitor is somewhat further away from the regulator than I like to put it - though that shouldn't cause that much of a problem. 800mV is way too much to be considered normal under any circumstances.

What is the power source you are using?

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    it's 12DC coming from a precision DC power supply (Thandar)
    – pascalm
    Jun 22, 2015 at 9:44
  • @pascalm It's unlikely to be that then. It's probably the factory has changed regulators and they don't now have the same ESR requirements. You can try adding a 100nF ceramic capacitor direct across the Vout / GND pins of the regulator - an 0805 would fit nicely.
    – Majenko
    Jun 22, 2015 at 9:45
  • just used an "ordinary" ceramic cap 104, gives no improvement :(
    – pascalm
    Jun 22, 2015 at 9:56
  • Next step is probably to change out the regulator - shouldn't be too hard with desolder braid, though hot air is now a very affordable and useful tool. What is the origin of these boards? If it is not from one of the claimants to the trademark, you probably should add "-like" in your title rather than attributing issues with some random work-a-like to the branded product.
    – Chris Stratton
    Jun 22, 2015 at 11:49
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    it's original Arduino! - in original box
    – pascalm
    Jun 22, 2015 at 12:39

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