DC/DC converter giving acoustical noise 3

[SolarTrap]

Hello All,

I am using a DC/DC converter from 24V to 5V for my Arduino project. The converter is producing a high pitch sound that I would like to silence. I am using a TI MC34063AD in the Step-Down configuration on page 8 but coil 330mH instead of 220mH.

Any Suggestions?

Thanks
Markus

 

[MacGyverS2000]

Ah.. Well for starter's, not having a resistor from the FET's gate to ground is a rookie mistake. There is absolutely no guarantee that the output pin of the processor won't leak some small current even when 'turned off'. You're always supposed to include a resistor to ground to 'bleed' this leakage current away.

I can't tell what processor you have in there, but if it's a GPIO that FET is connected to, it's likely the GPIO line has an internal pull-up/-down that you can enable by flipping a register bit.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[SolarTrap]

It is an ATmega2560 and is has a pull-up mode - but I dont see how this would help because it would turn on the buzzer.

BTW: I have just another problem with my PCB and I started another thread:

http://www.eng-tips.com/viewthread.cfm?qid=368902

Thanks for all your help so far!

 

[MacGyverS2000]

In our next design, make the high keep the buzzer off... one less component to put on the board.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[MacGyverS2000]

I just thought of something... if you truly think radiated EMI is what is causing your problem, swap out the coil for a shielded version. A few pennies more, but it likely means not having to redesign the entire board for one simple component.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

MArkus; Even if the resistor isn't fixing the immediate problem 'good design' requires it so don't leave it out of any rev you do.

Have you tried a 200 ohm resistor across the piezo to swamp the magnetic leakage?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Oh, and Dan has an excellent point about using a sheilded coil instead of that positively un-sheilded inductor you have.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[SolarTrap]

I tried even to shorten the contacts of the buzzer but that has no audible effect. It seems this is entirely caused by radiated EMI.
In the next rev I will have to pick a different coil and a new location for the buzzer.

Thanks to everyone looking into this!

 

[MacGyverS2000]

Before you worry about the next design, you need to fix the current one. Don't assume a shielded can or a moved buzzer will fix the issue, only to find it's something else. Swapping out the inductor for a shielded version is a very easy test. Putting the buzzer and control circuitry on longer leads is also pretty easy in the grand scheme of things.

Try before you buy...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[SolarTrap]

I have a spare buzzer and just holding it next to the coil and the cap makes sound. Unfortunately the coil is SMD and I don't know how to get it off the board.

 

[iop995]

Try to cover coil (or coil + capacitor even) by a piece of copper or aluminum.
Buzzer is too close to switching converter.
Maybe more easy, as sugested, is to move buzzer away by twisted wires of minim 10cm.

 

[OperaHouse]

Had a thought. Some PWM outputs on the Arduino can never be fully turned off. It is a little glitch in the timer software. I found in in some notes about using the PWM.

 

[VEBill]

OP says he can simply hold another beeper in his hand near the circuit, unconnected, and it will emit sound.

Perhaps use a less-amazingly-sensitive beeper.

 

[MacGyverS2000]

Some PWM outputs on the Arduino can never be fully turned off. It is a little glitch in the timer software. I found in in some notes about using the PWM.

Yeah, it's a PITA when you want to drive a motor but still keep absolute positioning without a feedback mechanism. But I don't think that's the issue here...

Dan - Owner

http://www.Hi-TecDesigns.com