I've connected an 8-ohm speaker in series with coil. When I apply 5 volts / 500 mA to the combination (coil and speaker), I heard a very high frequency sound (something like whistling).

The coil is not air-cored (I think the core is made of ferrite or iron) and it has about 50 turns.

I would like to know why this circuit oscillates or produce this sound?

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    \$\begingroup\$ Did you try moving the speaker away from the coil? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 10 '14 at 4:47
  • \$\begingroup\$ @IgnacioVazquez-Abrams yes, I applied the 5 volt to the speaker only and the sound disappeared. \$\endgroup\$ – Michael George Oct 10 '14 at 4:55
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    \$\begingroup\$ That's not what I asked. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 10 '14 at 5:01
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    \$\begingroup\$ What kind of power adapter are you using? I suspect you hear the PSU ripple rather than your circuit oscillating. \$\endgroup\$ – jippie Oct 10 '14 at 5:09
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    \$\begingroup\$ Connecting the speaker directly on the PSU would draw 5V/8ohm=625mA which will will turn the PSU in overcurrent protection, hence you don't hear the whistle. If the coil adds anything near 2 ohms or more, the overcurrent protection won't kick in and you may hear the switching of the PSU. The only way to know for sure is to check with an oscilloscope, probably use a power resistor as a load. \$\endgroup\$ – jippie Oct 10 '14 at 5:23

Modern cell phone chargers operate a very high frequency- well above audible even for small dogs.

However, what I suspect is happening is that your inductor is perhaps 1mH and your adapter is perhaps capable of 600mA before overcurrent protection kicks in.

So, when you connect a fixed voltage to an inductor in series with a resistor, the current increases with time constant L/R towards Vin/R = 0.65A (allowing a bit of resistance for the inductor- and the DCR of the speaker won't be exactly 8 ohms).

This time constant (based on my guess as to the inductance) is of the order of 125usec, so if the overcurrent protection kicks out then restarts, the frequency of restarting should be in the several kHz range.

You don't hear the sound when the speaker is connected directly across the power supply because the restart frequency is above the audible range.

To confirm this, look at the voltage and current waveforms with an oscilloscope.

  • \$\begingroup\$ "well above audible", that's what I thought, but I do hear my cell phone charger buzzing (that's high-pitchy, granted, but still...). What would that be? Might it be some oscillation of the output voltage control loop?.. \$\endgroup\$ – Nicolas D Oct 10 '14 at 12:38
  • \$\begingroup\$ @NicolasD maybe a beat frequency between the mains ripple and the PSU switching frequency exciting a mechanical resonance. Sometimes you can hear the switching frequency itself on power-down- kind of a death rattle. \$\endgroup\$ – Spehro Pefhany Oct 10 '14 at 22:37

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