Wirelessly transmitting electricity is the transfer of electrical energy across an air gap using oscillating magnetic fields between coupled coils, without physical conductors. When you design a circuit to do this, you trade the certainty of a direct galvanic copper connection for the variables of magnetic coupling coefficients, spatial alignment tolerances, and high-frequency AC switching losses. Makers often confuse near-field inductive power transfer with far-field radiative beaming (like microwaves or lasers) or assume it works like wireless data protocols (Wi-Fi/Bluetooth), but true near-field power relies strictly on non-radiative magnetic resonance or tight inductive coupling.
The Physics of the Air Gap: How Inductive Coupling Works
At its core, wireless power transfer (WPT) is an application of Faraday’s Law of Induction. An alternating current in the transmitter (TX) coil generates a time-varying magnetic field. When this field intersects the receiver (RX) coil, it induces an electromotive force (EMF), driving current into the load. The efficiency of this transfer is governed by the coupling coefficient (k), a dimensionless number between 0 and 1 representing the fraction of magnetic flux from the TX coil that successfully links the RX coil.
Think of it like two water wheels placed close together in a narrow stream; the kinetic flow from the first wheel pushes the second wheel, but if you move them apart or misalign them, the water disperses into the surrounding stream and the second wheel stalls. In electrical terms, as the air gap increases, k drops rapidly, and leakage inductance spikes.
To overcome poor coupling at larger gaps, modern systems use resonant inductive coupling. By adding capacitors to both the TX and RX coils to form LC tanks tuned to the exact same resonant frequency (typically 100 kHz to 300 kHz for consumer gear), the system creates a high-Q (quality factor) resonance. This allows energy to oscillate back and forth between the magnetic and electric fields, effectively 'tunneling' across the gap even when k is as low as 0.2.
Worked Numeric Example: Sizing a 5W Resonant Link
Let’s design a basic 5W resonant wireless link operating at 100 kHz, assuming we are building a custom TX/RX pair for a bench project.
- Define the Coils: Assume we have wound two identical planar spiral coils using Litz wire, each measuring 5 µH of inductance.
- Calculate the Resonant Capacitor: We need to tune both LC tanks to exactly 100 kHz. Using the resonant frequency formula f = 1 / (2π√LC), we solve for C:
C = 1 / ((2π × 100,000)² × 5 × 10⁻⁶)
C ≈ 506 nF - Select the Dielectric: You cannot use standard X7R ceramic capacitors here. At 100 kHz, X7R dielectrics suffer from severe internal heating and massive capacitance derating under AC bias. You must use NP0/C0G (Class I) ceramics or polypropylene film capacitors. A parallel combination of a 470 nF and a 33 nF NP0 capacitor will get you to 503 nF, which is well within the 1% tolerance needed for resonance.
- Calculate Tank Current: To deliver 5W to the load across a 10mm gap (assume 60% end-to-end efficiency), the primary side must process about 8.3W. While the DC input draw at 12V is only ~700 mA, the high Q-factor of the resonant tank multiplies the circulating AC current. Peak tank current can easily reach 2.5 A to 3.5 A.
At 100 kHz, the skin depth in copper is roughly 0.2 mm. If you wind your coils with standard solid-core 20 AWG magnet wire, the center of the wire carries zero current, resulting in massive I²R heating. Always use Litz wire (e.g., 660-strand 46 AWG) for WPT coils to ensure the full cross-sectional area conducts the high-frequency AC.
Where You Meet This in Practice
You interact with near-field wireless power constantly, though the engineering details are usually hidden inside molded plastic enclosures.
- Consumer Electronics (Qi Standard): The Wireless Power Consortium (WPC) Qi standard dominates smartphones. It uses a multi-coil array in the TX pad and communicates power needs via backscatter modulation (the RX coil slightly alters its load, which the TX coil senses as a change in voltage/current).
- Industrial AGVs: Automated Guided Vehicles in warehouses use heavy-duty inductive pick-ups that slide along a powered track buried in the floor, transferring kilowatts of power continuously without exposed contacts.
- Maker Modules: If you don't want to wind your own coils and build H-bridge inverters, you can buy off-the-shelf TX/RX pairs. Modules like the Seeed Studio 5W Wireless Power Supply or generic DFRobot 12V inductive bridges typically cost between $15 and $25. They operate around 110 kHz and include the necessary gate drivers and rectification on-board.
Real-World Scenario Walkthrough: Building a 12V IoT Sensor Charger
The Setup: I needed to power an outdoor ESP32-based weather station housed inside a sealed, waterproof ABS enclosure. Drilling a hole for a USB cable would compromise the IP67 rating, so I opted to wirelessly transmit electricity through the 4mm plastic wall using a generic 10W, 12V-to-12V inductive module pair.
The Numbers: The TX coil was driven by a 12V DC wall adapter. The RX coil output 12V AC, which was rectified on-board to DC. The ESP32 circuit, including a BME280 sensor and a 3.3V LDO, drew a baseline current of 150 mA (1.8W). The 4mm plastic wall yielded a coupling coefficient of roughly k = 0.65.
The Outcome: On the bench, the ESP32 booted perfectly, read the sensor data, and printed it to the serial monitor. I sealed the enclosure and mounted it outside.
What Went Wrong: The moment the ESP32 attempted to connect to the Wi-Fi network, it crashed and entered a continuous reboot loop. Connecting an oscilloscope to the RX DC output revealed the issue: Wi-Fi transmission causes a transient current spike of roughly 350 mA for about 50 ms. The wireless link’s analog control loop—which adjusts the TX duty cycle based on RX feedback—had a response time of roughly 200 ms. This lag meant the TX coil couldn't ramp up power fast enough. The RX DC bus sagged from 12V down to 4.2V, triggering the ESP32’s internal brownout detector (BOD) and forcing a reset.
The Fix: The wireless link was perfectly capable of supplying the *average* power, but it lacked the *transient* response. I added a 470 µF low-ESR polymer capacitor in parallel with a 0.47F 5.5V supercapacitor (stepped down via a buck converter) on the RX DC bus. This local energy reservoir rode out the 50 ms Wi-Fi burst without relying on the slow magnetic feedback loop, resulting in rock-solid operation.
Frequently Asked Questions
Does metal near the coil cause problems?
Yes. Any conductive metal (like an aluminum enclosure or copper ground plane) placed near the alternating magnetic field will develop eddy currents. These currents dissipate energy as heat and effectively act as a shorted secondary coil, drastically lowering your system's Q-factor and efficiency. If you must mount coils near metal, use ferrite shielding sheets (typically manganese-zinc or nickel-zinc ferrite) to direct the magnetic flux away from the metal.
Can I use a standard multimeter to measure the high-frequency AC in the tank?
No. Standard digital multimeters are designed for 50/60 Hz mains or DC. At 100 kHz, the input filtering and ADC sampling of a standard DMM will yield wildly inaccurate readings, often displaying zero or random noise. You must use a true-RMS meter rated for high frequency (like a Fluke 87V, which is accurate up to roughly 20 kHz, though still marginal for 100 kHz) or, ideally, measure the voltage directly using an oscilloscope with a 10x high-voltage probe.
How does the receiver tell the transmitter to stop charging?
In standardized systems like Qi, the receiver uses backscatter communication. The RX microcontroller switches a small MOSFET across its coil, slightly altering the impedance of the RX tank. This change reflects back to the TX coil as a tiny fluctuation in voltage or current, which the TX controller demodulates as digital data packets (e.g., 'charge at 5W', 'battery full, terminate').
