Audible sound from P1 module


Does P1 module produce any audible sound once its connected to a 3v power supply? I have 8 board build with P1 module on it. 7 out of the 8 boards have audible sounds. It have a distorting sound and sometime it click like something is turn on and off when I put my ear next to the board. Other then the sound, everything seem to be working fine.

The input voltage is 24 VAC, converter to 5v and 3.3v supply. When I disconnect the P1 module from the circuit no sound was heard. I also tried putting a load on the 3v power supply with out the P1 module and no sound was heard. Once I connect the P1 module the sound came back.

Could the P1 module not solder on right even everything else works except the sound?

@sheng, can you share your schematics and/or a picture of your board?

Why are you going for 3V instead of the commonly expected 3.3V?

@peekay123, below is a picture of the back of the board. The top side contain the lcd, light sensor, and butons.

@ScruffR, sorry its a 3.3V supply. Will edit the above post.

@scheng, the noise is most likely created by one or more of the 5 mini relays you have on the board. You may have a code or hardware issue causing the problem. Can you share you schematics? Where are the relays powered from?

@peekay123, sorry for not able to share the schematics. But below is the power flow:

24VAC --| LM2596S | -- 5v -- |LM117| -- 3.3V 
                       |                  |
                     Relay            P1 module, LCD, LED, Light Sensor

The relay is getting the 5V power from the LM2596S and the P1 module is from the LM117.

Let me cut the relay power supply and see if it make a difference.

@peekay123, Could it be that the p1 module not solder on correctly or damage during reflow?

The relays are a really good thing to check but if that is not the source, I would look at the power circuitry in the bottom right hand corner of the board.

Sometimes inductors or capacitors in power circuits like to “sing” and you may be able to apply some kind of adhesive as a damper. Many times if you press down each component in turn with your finger, you can make the pitch change. Don’t press down on the P1 metal shield or you will reset the device.

Sound still appear even after disconnect the relay from the circuit.

@bko, when I listen to the sound, it sound like disc spinning. could it be inductors or capacitors sound?

You have to check. Inductors make a buzzing sound to my ear.

Try covering that part of the board with something that absorbs sound (your hand, a thick towel, anything) and see if the sound reduces.

Almost certainly this is the LM2596 starting and stopping oscillation as it overshoots the target voltage. This then causes excessive ripple on the output, and as you have lots of ceramics on the output rail, you will hear them “sing”.

Looking at your picture, you have selected too big an inductor for the LM2596 (150uH). The datasheet recommends 33uH for 5v output and max 15v input - see table 3 in the datasheet:

I’d try that…


@hfiennes, thanks for pointing it out. I was trying to use the 150uH inductor to limit the current to 1A and I guess its working the system to hard or out of spec. I have a 100uH lying around and when I replace the 150uH the “sing” is much lower. I don’t heard the disc spinning sound after the replacement. Since my max input is 24V-35V, I am going with 47uh or 68uH suggest on Figure 28 of the datasheet. Just have to wait for the part to arrive and try it out.

Thanks all for the help.

Sorry, I’d missed the 24VAC input comment. Yes, you need to go higher for that input range. I was also not sure (without looking at PCB cad) whether your DCDC layout was following the recommendations in the datasheet - layout is especially critical for stability of switchers. Often you’ll see the right DC voltage but the load transient response will be awful if the layout isn’t right.

You could also move to a higher frequency buck - 150kHz is pretty old-school these days. A while ago we did a design for 24VAC input and 0.5A 3v3 output using the LM25007 which is 800kHz. See

The big scare of automotive moving from 12v to 42v power distribution caused a lot of companies to make high voltage bucks which are actually also great for rectified 24VAC!

@hfiennes, I follow the recommendation layout on the datasheet even there was not an example for the smd version. I agree with you, I am using an old-school part. I am going to put a scope on the Vout once I received the new inductor part, just be sure.

Thank You for the information, I am sure it will come in handy in my future projects or upgrades. :slight_smile:

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