Cause of High-Pitched Sound in Camera Flash Circuits

[brewnog]

What causes the increasingly high-pitched sound emitted by a capacitor charging circuit, such as that found in a camera flash?

 

[Danger]

What causes the increasingly high-pitched sound emitted by a capacitor charging circuit

I think that it's like that smelly smoke that Faust mentioned the factories installing in components to show when they aren't working properly. The manufactures of capacitors build noise into them so you can tell when they are working.

 

[dlgoff]

What causes the increasingly high-pitched sound emitted by a capacitor charging circuit, such as that found in a camera flash?

It's the electronic circuit. The high voltage is generated with a switching power supply, I believe.

 

[brewnog]

It's the electronic circuit. The high voltage is generated with a switching power supply, I believe.

But what's actually happening?

What's vibrating? Just anything metallic within the circuit which is being influenced by a changing magnetic field? Why does the pitch increase? Is this due to any change in switching frequency?

Excuse any ignorance, I've only taken a few EE modules.

 

[GENIERE]

...What's vibrating? Just anything metallic within the circuit which is being influenced by a changing magnetic field?

Best guess(s) is the ransformer, choke, and just maybe an electrolytic capacitor. These things are designed with the switching frequency above the audio range, but somehow a sub-harmonic may be generated.

Why does the pitch increase? Is this due to any change in switching frequency?

Are you sure it is a change in pitch? The sound volume may increase with load, but I would think the frequency would not.

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[Danger]

Are you sure it is a change in pitch? The sound volume may increase with load, but I would think the frequency would not.

Any one that I've experienced definitely got higher in frequency as well as volume. Some of them are downright painful just short of full charge.

 

[brewnog]

Are you sure it is a change in pitch? The sound volume may increase with load, but I would think the frequency would not.

It certainly does. I'd say it goes up about 8 octaves. I'm not sure whether it becomes inaudible due to high frequency, or just because it stops making a sound, but the frequency definitely increases.

 

[berkeman]

But what's actually happening?

What's vibrating? Just anything metallic within the circuit which is being influenced by a changing magnetic field? Why does the pitch increase? Is this due to any change in switching frequency?

Excuse any ignorance, I've only taken a few EE modules.

It's magnetostriction in the inductor that is the main energy storage elelment in the "boost" DC-DC converter. Magnetostriction is a mechanical vibration caused by the AC magnetic flux in the magnetic material. That's why most DC-DC converters are run above the range of human hearing. I don't know why camera flash chargers are run in the audible range -- kind of obnoxious. Even the 15,750 flyback transformers in TV sets can be bad sometimes, and the design of flybacks and their mechanical mounting methods usually take into account the need to avoid mechanical resonances at their swithching frequency.

 

[brewnog]

Thanks berkeman! I didn't realize that the camera flash sound and the high-pitched TV sound were related.

 

[dlgoff]

I was thinking the change in pitch might actually be because of less loading on the circuit when the capacitor get closer to full charge. Does that sound right berkeman?

Regards
Don

 

[berkeman]

I was thinking the change in pitch might actually be because of less loading on the circuit when the capacitor get closer to full charge. Does that sound right berkeman?

Regards
Don

There are different ways that DC-DC converters regulate their output voltage. Some do it with strict PWM and a constant frequency, and others change frequency as their pulse width narrows. This second type of DC-DC is usually frowned on by folks that have to worry about power supply noise in their circuits (like low-level analog circuits, or RF circuits, etc.), because the fundamental and harmonics walk all over the place as the power supply output loading changes.

But allowing the frequency to change as the pulse width changes can sometimes let you save a little money in the design of the DC-DC converter circuit, so if the rest of the circuitry in the device can tolerate it, the power supply switching frequency may chirp as the loading varies. Or as in this case, as the output boost voltage gets closer to the final desired voltage for the flash circuit, the (On-time) pulse width of the boost circuit is narrowing, and the (repetitive switching) frequency is going up.

BTW, you can often watch the PWM waveform of a DC-DC converter or isolated switching power supply by just holding an ungrounded oscilloscope probe near the main switching magnetic element, and picking up the drive voltage waveform capacitively. I haven't tried it with boost converters, but it works great for buck (downconverter) DC-DC circuits and for isolated flyback or feedforward supplies.

 

[GENIERE]

…But allowing the frequency to change as the pulse width changes can sometimes let you save a little money in the design of the DC-DC converter circuit, so if the rest of the circuitry in the device can tolerate it…

I wouldn’t have thought to do that. If the transformer had several secondary windings for different voltages, the several regulator stages would be chasing the most loaded winding. I suppose it is fine for a single output device.


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