Why do capacitors whine?
Mitchell P Patrie
More specifically, why do capacitors whine when being charged
from a DC source, e.g. the flash capacitor in a camera? The changing
pitch suggests that the resonant frequency of the charging circuit is
changing; why does this change?
Curious,
Mitchell Patrie (*mechanical* engineer)
pat...@cae.wisc.edu
bi...@mix.com
> More specifically, why do capacitors whine when being charged
> from a DC source, e.g. the flash capacitor in a camera? The changing
> pitch suggests that the resonant frequency of the charging circuit is
> changing; why does this change?
I think you're hearing the dc-dc converter - it starts out under a heavy
load and as the cap charges the load on the supply is reduced, thus so is
its chattering.
Billy Y..
Todd Freeman
> More specifically, why do capacitors whine when being charged
> from a DC source, e.g. the flash capacitor in a camera? The changing
> pitch suggests that the resonant frequency of the charging circuit is
> changing; why does this change?
It doesn't know the words?
:)
My 1.75 Yen
Todd
Tom Bruhns
: More specifically, why do capacitors whine when being charged
: from a DC source, e.g. the flash capacitor in a camera? The changing
: pitch suggests that the resonant frequency of the charging circuit is
: changing; why does this change?
More likely it's a magnetic core which is radiating most of the sound. A
small oscillator cirucit steps the battery voltage up to the couple hundred
needed to flash the lamp. The oscillator is generally "free-running" which
means its frequency depends, among other things, on the load it's driving.
The large capacitor starts out looking about like a short circuit, stepping
up in voltage a little with each cycle of the oscillator. The transformer
in the oscillator probably magnetically "charges" on one part of the cycle
and delivers that charge on the other part of the cycle to the capacitor;
it delivers it faster if the cap has a higher voltage, so that half of the
cycle shortens as the cap charges, and that causes the pitch to rise as the
cap charges. -- At least that's enough of the story to get started with.
mark_g._forbes
>More specifically, why do capacitors whine when being charged
>from a DC source, e.g. the flash capacitor in a camera? The changing
>pitch suggests that the resonant frequency of the charging circuit is
>changing; why does this change?
They don't. The inductors in the charging oscillator circuit whine. It's
an oscillator with a large step-up transformer that charges the flash
capacitor to many hundreds of volts from a 6-9 volt DC source. Depending
on the specific flash unit, it uses anything from a fairly simple
inductively-coupled oscillator to some fairly exotic multi-tapped
transformers. The wire in the coils vibrates a tiny bit with the
transient magnetic fields, and that causes the noise.
Reminds me of my first days as an audio-visual tech. First day on the
job, my boss handed me a professional-grade photoflash unit and asked
me to fix it. I worked there five years; never did get it to work right.
He didn't either....but it was a great way to observe how a new prospect
would tackle the troubleshooting task. Apparently I attacked it the
right way.
It *is* true that some capacitors will make noise. Monolithic ceramic
caps are somewhat piezoelectric, so impressing AC voltages across them
will cause them to sing. You can sometimes hear this if you stick your
ear inside a PC and fire up Windows. Or anything else that thrashes
memory a lot. Conversely, whacking on MLCs will cause small voltages
to appear across them. This gets embarrassing when you're trying to
do low-level AC signal acquisition on a platform that's getting
vigorously thumped. Like, for example, a certain laser-sensing control
system for a road grader blade I worked on a few years ago.......
--
for...@csos.orst.edu
Mark G. Forbes
"Never ascribe to malice that which can be blamed on the engineer."
le...@fndcd.fnal.gov
>
>> More specifically, why do capacitors whine when being charged
>> from a DC source, e.g. the flash capacitor in a camera? The changing
>> pitch suggests that the resonant frequency of the charging circuit is
>> changing; why does this change?
That's not the capacitor making the noise, it's the transformer used in the
boost circuit. The pitch changes as the capacitor charges and presents less
of a load to the circuit.
--
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Nick Gray
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>From: pat...@cae.wisc.edu (Mitchell P Patrie)
>Newsgroups: sci.electronics
>Subject: Why do capacitors whine?
>Date: 23 Jan 1995 15:08:22 GMT
>Organization: College of Engineering, Univ. of Wisconsin-Madison
>Lines: 14
>Distribution: world
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>Originator: pat...@sun-3.cae.wisc.edu
>Curious,
It is not the capacitor that is whining. The 1.5 to 6 Volts of the batteries
is not enough to fire the flash tube, so there is a voltage multiplier circuit
that is used to charge up a capacitor to a voltage high enough to fire that
tube. The circuit uses an oscillator to do this. You are hearing the
vibrations of the inductor they use in the oscillator. Some circuits do not
use an inductor, but have a transcucer to put out the sound so that you know
that the charging circuit is functioning properly.
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| Nick Gray National Semiconductor Corporation |
| Opinions expressed are those of the author and do not necessarily reflect |
| reflect the thinking or opinions of National Semiconductor Corporation. |
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R A Skeen
> from a DC source, e.g. the flash capacitor in a camera? The changing
> pitch suggests that the resonant frequency of the charging circuit is
> changing; why does this change?
In most camera-flash circuits, an oscillator drives a small step-up
transformer. The output from the transformer is rectified and stored in
the capacitor. I think the noise you can hear is actually coming from the
transformer. The frequency increases as the capacitor charges up, beacuse
the load on the oscilator reduces.
--
Cheers-bye,
Richard
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Richard Skeen, St. Aidans College, University of Durham
"TV is called a medium because it is neither rare nor well done."
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