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# Powering Circuit with electromagnetic waves

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#### zemanekj

##### New Member
Can you power a circuit with electromagnetic waves, as opposed to a physical connection like with a wire?

Like a electric tooth brush with charger.

Or a contactless cell phone charger. It's really inefficient though, even at practically zero distance compared to wires, so you don't want to use it if you don't have to.

Yes, but it is harder than you might think. In the 60's several people (military contractors) experimented with powering things with a directed microwave beam. Expensive, inefficient, and dangerous. To deliver any useable amount of power, the beam field strength is so high it is physically dangerous to block.

Wireless charging for phones uses an air-core transformer, with the primary driven by the charger electronics and the secondary connected to the phone's internal battery charge manager. The coils are tuned to resonate at the same freq so efficiency doesn't totally suck. Still, small changes in the gap between the two coils has a large impact on recharge time.

ak

Can you power a circuit with electromagnetic waves, as opposed to a physical connection like with a wire?
yes, but it takes huge amounts of power to get any useful results. before WWII, the idea of using a beam of microwaves to transmit power was investigated. to transfer 100 watts of power at a range of 100 meters would require about 5GW (yes, that's Gigawatts). you have to contend with the inverse square law for far-field coupling, and inverse cube law for near-field coupling. far-field refers to distances where the signal is received using an antenna, and there are no direct inductive or capacitive effects involved.... the wave is travelling through space and encounters an antenna. near-field refers to coupling through local capacitive and induction effects in addition to the propagation of the electromagnetic wave. this is how "wireless" phone chargers work, you have a pancake coil on the charger that couples with a coil attached to the phone, and the two coils act as a transformer. since near-field coupling has a cube law characteristic, small increases in distance between primary or secondary result in huge losses. if a charger secondary is receiving 1 watt when it's 1mm from the primary, moving it apart another millimeter will reduce the received power to 1/8 watt.

In another life, I designed a through-the-skin inductive coupler big enough to power a human artificial heart (~20W). It was not practical because the heat produced by both the transmit and receive coils would have damaged the tissue between the coils...

In another life, I designed a through-the-skin inductive coupler big enough to power a human artificial heart (~20W). It was not practical because the heat produced by both the transmit and receive coils would have damaged the tissue between the coils...
Sounds like an assassination device. Too bad you have to open up the person first anyways to install it...and then need to be nearby to use it.

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