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I am curious about a resonant LC circuit. Assuming a perfect inductor and capacitor, would it be possible to tap off the energy in the oscillations to use it as a battery? Literally, it would be an AC battery. With a bridge rectifier and filter cap would this act as a DC battery?

(This is just theoretical stuff - I doubt you could get useful energy from an LC circuit as it would dissipate too quickly due to losses in the inductor and capacitor, plus to store any reasonable amount of energy would probably require a massive inductor and capacitor.)

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  • \$\begingroup\$ Can you post an example circuit? \$\endgroup\$ – Kevin Vermeer Nov 2 '10 at 0:02
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    \$\begingroup\$ i once cracked open a power supply from a scrapped control system that used a bunch of 24VAC relays. The main transformer had three windings. A 120V primary, and a 24V secondary, which you'd expect. In addition, there was another secondary that was connected to ONLY a large 3uF oil filled capacitor. The o.c. voltage of that winding was 600V. I'm wondering if that wasn't something exactly like you're suggesting. \$\endgroup\$ – JustJeff Nov 2 '10 at 0:21
  • \$\begingroup\$ en.wikipedia.org/wiki/Resonant_energy_transfer \$\endgroup\$ – endolith Nov 2 '10 at 4:10
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    \$\begingroup\$ of course .. maybe that was a power factor correction thing of some kind. \$\endgroup\$ – JustJeff Nov 2 '10 at 11:20
  • \$\begingroup\$ It could be a "ferroresonant voltage stabiliser" thing. It was not a transformer 100%, but some hybrid with air gap for saturation. \$\endgroup\$ – user924 Nov 4 '10 at 2:11
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This actually exists in RF transmitters like walkie-talkies. People call them "LC tanks." You're right that the amount of energy that can be stored is quite small. The reason people use them is to store energy that can be withdrawn as oscillations at a certain frequency.

You might Google "Colpitts oscillator" or "Hartley oscillator" for more information.

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  • \$\begingroup\$ +1 I didn't know this. I don't think this would really be considered a battery in any since though, just a frequency generator. \$\endgroup\$ – Kellenjb Nov 2 '10 at 3:13
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    \$\begingroup\$ A tank circuit is normally used to generate a frequency, and not really as a battery, but it is used as a power source. \$\endgroup\$ – Kortuk Nov 2 '10 at 3:53
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    \$\begingroup\$ @Kellenjb True-- it's like a battery with a capacity of 1 microwatt-hour, which is arguably not a battery. \$\endgroup\$ – pingswept Nov 2 '10 at 5:08
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    \$\begingroup\$ A class C amplifier with a high-Q LC 'tank' draws power from the DC supply for 50% or less of the cycle, so the rest of the time the output power comes out of the energy stored in the LC tank. So, kind of a battery. On the other hand, it's a battery that gets a charge every cycle. I think I'd want something to source the AC for more than a few cycles without needing a charge before calling it a battery. \$\endgroup\$ – JustJeff Nov 3 '10 at 2:29
  • \$\begingroup\$ Agreed it's not really a battery. A better analogy would be that an LC tank is to AC what a capacitor is to DC. ("Battery" usually means storing energy as chemical potential energy, unlike a capacitor or even an ultracapacitor. But it's clear the OP just meant any kind of energy storage component.) \$\endgroup\$ – Peter Cordes Dec 26 '17 at 0:47
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You aren't talking about a battery, but an RF receiver. When you add the antenna and ground connections, you are receiving energy that is broadcast by a transmitter. The resonant frequency of your tank should match the frequency of the transmitter. If you're going to harvest energy, I guess you would target 60/50 Hz, unless you have a stronger source nearby.

Nikola Tesla started to build such a transmitter for communications and experiments on power distribution, but his financing was coming from people who had a vested interest in the power generation systems of the day. When he mentioned the possibility of broadcasting power that anyone could pick up for free, then his funding dried up. So, it took another forty years to lay a transatlantic cable to make reliable communications with England happen.

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