Electricity wireless transmission is the movement of electrical energy from a source to a load without physical conductors, relying on time-varying electromagnetic fields to bridge an air gap. In a real circuit, this technology replaces physical contact resistance with mutual inductance and a coupling coefficient, forcing you to manage reactive power, tune LC resonance, and account for spatial misalignment. Makers commonly confuse near-field non-radiative transfer (inductive coupling, which your phone uses) with far-field radiative transfer (beaming microwaves or lasers), which are entirely different physical regimes with distinct safety and efficiency profiles.
The Core Mechanism: Near-Field vs. Far-Field
To design a wireless power link, you must first select your physical regime. Near-field techniques rely on non-radiative magnetic or electric fields. Think of it as an air-core transformer: an alternating current in the transmitter (Tx) coil generates a magnetic field, which induces a voltage in the receiver (Rx) coil. This only works efficiently when the distance between coils is a fraction of the wavelength (typically d < λ/2π). At 100 kHz, the wavelength is 3 kilometers, meaning near-field coupling is highly effective at distances of a few millimeters to a few centimeters.
The Math That Matters: Coupling and Thermal Reality
The defining parameter in near-field electricity wireless transmission is the coupling coefficient (k), which ranges from 0 (no coupling) to 1 (perfect coupling). Misalignment, air gaps, and foreign objects degrade k, directly tanking your system efficiency and generating heat.
Let us run a worked numeric example using a standard 15W Qi smartphone charger (9V at 1.67A output).
- Perfect Alignment (k = 0.6): The system operates at roughly 75% end-to-end efficiency. To deliver 15W to the phone battery, the Tx coil must draw 20W from the wall. The system dissipates 5W as heat, which is easily managed by passive thermal spreading.
- Severe Misalignment (k = 0.3): The coupling drops, and efficiency plummets to 40%. To maintain the 15W output, the Tx must now pull 37.5W from the wall. The system must dissipate 22.5W as heat.
This 22.5W thermal load is exactly why wireless chargers aggressively throttle charging speeds or shut down entirely when a phone is placed off-center. When sizing your thermal vias and copper pours on the Rx PCB, always design for the worst-case k drop, not the nominal aligned state.
Where You Meet This in Practice
You will encounter near-field electricity wireless transmission in three primary bench and jobsite scenarios:
- Consumer Electronics (Qi Standard): Operating at 100-205 kHz, delivering 5W to 15W. The Wireless Power Consortium strictly dictates the communication protocol (backscatter modulation) used to negotiate power levels.
- Electric Vehicles (SAE J2954): Operating at 85 kHz, delivering 3.6 kW to 11 kW across a 100mm to 250mm ground clearance gap. These systems use massive ferrite shielding and Litz wire to handle the high currents and prevent eddy current heating in the vehicle chassis.
- Medical Implants: Operating at higher frequencies (up to 13.56 MHz) to shrink coil sizes for pacemakers and neurostimulators, where hermetic sealing makes physical charging ports a failure liability.
Decision Tree: Choosing Your Wireless Transmission Topology
Do not guess your topology. Use this decision path to select the correct architecture and silicon for your next PCB spin.
| Condition (Distance & Power) | Topology | Concrete IC Pick (Tx / Rx) |
|---|---|---|
| Gap < 8mm, Power 5W - 15W | Tightly Coupled Inductive (Qi) | Texas Instruments bq501210 (Tx) / bq51013B (Rx) |
| Gap 10mm - 40mm, Power 1W - 5W | Magnetic Resonant (Loosely Coupled) | Analog Devices LTC4120 (Tx/Rx configurable) |
| Gap > 1 meter, Power < 100mW | Far-Field RF Energy Harvesting | Powercast P2110B (Rx Harvester) |
Frequently Asked Questions
Can I use standard magnet wire for my Tx coil?
For low-power (< 5W) prototypes, yes. For anything above 10W or operating above 100 kHz, you must use Litz wire. Solid magnet wire suffers from the skin effect and proximity effect at high frequencies, causing the AC resistance to spike and the coil to overheat. A typical 15W Qi coil uses 175 strands of 44 AWG Litz wire.
How do I shield the back of my transmitter coil?
You must place a ferrite shield behind the Tx coil to direct the magnetic flux toward the receiver and prevent eddy currents in nearby metal (like a steel desk or aluminum enclosure). Use a sintered ferrite sheet with a permeability (μ) of at least 1000, rated for your specific operating frequency.
Is wireless power transmission safe for pacemakers?
Stray magnetic fields from high-power (15W+) Qi chargers can induce voltages in unshielded medical leads. The Texas Instruments wireless power guidelines and medical device manufacturers explicitly warn against placing active implants directly on the center of a charging coil. Always implement Foreign Object Detection (FOD) in your Tx firmware to shut down the field if anomalous Q-factor drops are detected.
Wireless power is not magic; it is just an air-core transformer with strict geometric constraints. Calculate your worst-case coupling coefficient, select the correct resonant topology from the decision tree, and use C0G capacitors to keep your tuning stable.
