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Re: gEDA-user: buzzing board
can you capture a current trace, knowing that information will
provide us an ability to actually calculate the required capacitance.
Where i work we do not use tantalum caps because there down sides are
pretty big in terms of safety.
when you over work a tant, they get very hot.
>> It should. The other thing to check is ripple: if you're getting
>> enough to make a ceramic sing, the ripple current might be a problem
>> for Ta. Check the ripple voltage and frequency across the cap and
>> use the resistance and capacitance ratings to deduce a current.
>>
>
> Yeah, the ripple voltage is pretty high, about 0.5vpp at 4.7uF and
> 14mA average draw.
>
> Hence my other thought, which was a big electrolytic and a medium
> sized cermic/tant.
>
If you have that much ripple, you need more capacitance.
more of a reason to have a current trace.
a high resolution voltage trace would also give the volts per second
to deduce the current, knowing the ESR.
Steve
On Sep 19, 2007, at 10:30 AM, DJ Delorie wrote:
>
>> The PCB for my alarm clock buzzes.
>
> Having poked and prodded, and getting a piezo unit that did nothing, I
> finally figured it out (the trick was to use a nail, not a toothpick,
> to poke and prod - I was using that as a better sound conductor for
> the piezo).
>
> There are not one but TWO buzzing caps. If I replace both of them
> with electrolytics, the buzzing goes away. If I replace them both
> with two new ones of the same type/size, it still buzzes.
>
> The caps are 4.7uF 16v. They're decoupling/bypass for a +12v rail
> (well, one is, the other is VcomH, which seems to be 7v). They're the
> biggest 0805's with that voltage rating.
>
> I can fit a 1206 in that spot. If I replace those with 10uF 25v, do
> you think they'll not buzz?
>
> Another alternative is 22uF 16v. I don't know if the buzz is due to
> insufficient capacitance, or damage due to being too close to the
> voltage rating.
>
> Alternatively, I can stack two 10uFs on top of each other to make 20uF
> 25v, but that's ugly. 1210's won't fit.
>
> Note that adding a 47uF 35v electrolytic across the 4.7u reduces the
> noise greatly. In fact, a 4.7u electrolytic helps much more than a
> second 4.7u ceramic.
>
> If I replace just the +12v cap with a 22u elec, the +7 cap still
> buzzes. I suspect the elec alone can't react fast enough to get rid
> of the ripple.
>
>
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smime.p7s
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