If you need to transfer power across an air gap to a sealed battery enclosure, a DIY inductive wireless charger circuit is the most practical bench solution. The direct answer for a robust, tunable 3.7V Li-ion charging system is a 555-timer-driven MOSFET transmitter operating at ~68kHz, paired with a Schottky-rectified TP4056 receiver. Unlike simple self-oscillating "Joule Thief" designs that drift off-resonance as the coils move, a hard-driven 555 topology locks your switching frequency, keeping your power transfer efficiency stable.

Safety Callout: This project involves lithium-ion cells and unisolated inductive spikes. Always use a TP4056 module equipped with a DW01/FS8205 protection IC to prevent overcharge and short-circuit fires. Never leave a custom inductive charger unattended during initial burn-in testing.

The 555-Driven Wireless Charger Circuit Topology

The system is split into two isolated halves: the Transmitter (TX) and the Receiver (RX). We use a fixed-frequency astable multivibrator rather than a self-oscillating feedback loop. In air-core inductive transfer, a self-oscillating circuit's frequency wanders wildly (from 100kHz to over 300kHz) depending on coil proximity and parasitic capacitance, destroying your LC resonance. The NE555 locks the gate drive frequency, allowing you to precisely tune the receiver tank.

Node Labels and Signal Path

  • VCC_12V: Main 12V DC supply for the TX oscillator and MOSFET drain.
  • GND_SYS: Common ground for the 12V supply, 555 timer, and MOSFET source.
  • CLK_OUT: Pin 3 of the NE555, outputting a 68kHz square wave.
  • GATE_DRV: The junction between the gate resistor and the MOSFET gate.
  • DRAIN_SW: The MOSFET drain, switching the low side of the TX coil.
  • COIL_TX / COIL_RX: The transmitter and receiver inductive loops.
  • RECT_DC: The smoothed DC output from the RX Schottky bridge.
  • BAT_POS / BAT_NEG: The regulated 4.2V charge lines to the Li-ion cell.

By hard-switching the DRAIN_SW node to ground at 68kHz, we force an alternating current through COIL_TX. This collapsing magnetic field induces an AC voltage in COIL_RX, which is immediately rectified at RECT_DC and fed into the TP4056 linear charge controller.

Component Selection and Design Walkthrough

Here are the exact component values to achieve a 68kHz switching frequency and a reliable 5V/1A receiver output capability. You can source these from any standard electronics supplier like DigiKey or Mouser for under $15 total.

Transmitter (TX) Bill of Materials

  • U1: NE555P (Through-hole DIP-8)
  • Q1: IRFZ44N N-Channel MOSFET (Low Rds(on) of 17mΩ minimizes conduction losses)
  • R1 (Timing): 1kΩ
  • R2 (Timing): 10kΩ
  • C1 (Timing): 1nF (C0G/NP0 ceramic for thermal stability)
  • R_GATE: 100Ω (limits gate inrush current)
  • R_PULL: 10kΩ (pulls gate to GND_SYS to prevent floating turn-on)
  • L1 (COIL_TX): 15 turns of 24 AWG magnet wire, wound on a 40mm diameter form.

Frequency Math: Using the standard astable formula f = 1.44 / ((R1 + 2*R2) * C1), we get 1.44 / ((1000 + 20000) * 1e-9) = 68,571 Hz. This sits comfortably below the NE555's reliable maximum switching threshold while providing a high enough dV/dt for efficient inductive coupling.

Receiver (RX) Bill of Materials

  • L2 (COIL_RX): Identical to L1 (15 turns, 24 AWG, 40mm).
  • D1-D4: 1N5819 Schottky Diodes (Do not use 1N4007; their slow reverse recovery time will short the RX tank at 68kHz).
  • C_RX_BULK: 10µF / 25V electrolytic capacitor.
  • U2: TP4056 breakout board (Must include DW01 and FS8205 MOSFET protection ICs).

Behavior Matrix and Failure Extremes

Understanding how this wireless charger circuit reacts to component drift or catastrophic failure is critical for debugging. The table below maps element changes to system behavior, including the contrast between series and parallel RX tuning.

Element Changed Modification / Fault Effect on TX Current Effect on RX Output System State / Failure Mode
Air Gap (Coil Distance) Increased from 5mm to 20mm Drops slightly (less reflected load) Voltage sags below 4.5V Coupling coefficient (k) drops; TP4056 enters under-voltage lockout.
C1 (Timing Cap) Drifts from 1nF to 1.5nF Unchanged Severe drop TX frequency drops to ~45kHz, detuning the RX tank. Efficiency tanks.
COIL_TX Short Circuit (turn-to-turn) Spikes to >5A Drops to zero Inductance collapses. IRFZ44N experiences thermal runaway and melts.
COIL_RX Open Circuit (wire break) Drops to idle (~40mA) Zero TX runs unloaded. Safe for TX, but RX is dead.
RX Tuning Topology Series Cap vs Parallel Cap N/A Varies by load Parallel: High voltage at no-load, drops under heavy load. Series: Low voltage at no-load, peaks at matched load. Choose parallel for TP4056 to ensure startup voltage.

Step-by-Step Breadboard Testing Protocol

Do not wire the entire system and apply power at once. Inductive kickback can easily destroy the 555 timer if the MOSFET gate is floating. Follow this exact bench sequence:

  1. Validate the Clock: Wire only the NE555 astable network (U1, R1, R2, C1). Power it with 5V (not 12V yet). Probe CLK_OUT (Pin 3) with an oscilloscope or a multimeter with a frequency function. Verify you read ~68kHz with a ~50% duty cycle.
  2. Add the Gate Drive: Connect R_GATE and R_PULL. Wire the IRFZ44N gate. Keep the MOSFET drain disconnected. Verify the square wave at GATE_DRV has fast rise/fall times (<50ns) and no ringing.
  3. Dummy Load Test: Instead of the TX coil, connect an LED in series with a 470Ω resistor between 12V and the MOSFET drain. Apply 12V to VCC_12V. The LED should light up dimly (it's blinking at 68kHz, but your eye integrates it). This confirms the MOSFET is switching the low side correctly.
  4. Install TX Coil and Measure Idle Current: Remove the dummy load. Solder COIL_TX between 12V and the drain. Put your multimeter in series with the 12V supply. Idle current should be between 30mA and 80mA. If it's >500mA, your coil is shorted or your MOSFET is blowing through.
  5. Test Inductive Coupling: Bring COIL_RX within 5mm of COIL_TX. Probe the raw AC voltage across COIL_RX with your oscilloscope. You should see a high-frequency sine wave riding on the switching harmonics, peaking around 8V-12V AC.
  6. Rectify and Regulate: Connect the 1N5819 bridge and bulk cap to the RX coil. Verify RECT_DC reads between 5V and 8V DC. Finally, wire the TP4056 IN+ and IN- to RECT_DC and GND, and connect your 18650 cell to BAT_POS and BAT_NEG. The TP4056 red LED should illuminate, indicating constant-current charging has begun.

Wireless Charger Circuit FAQ

Can I use this wireless charger circuit for a 12V lead-acid battery?

Yes, but you must change the receiver regulation stage. The TP4056 is strictly a 4.2V Li-ion charge controller. To charge a 12V lead-acid or LiFePO4 pack (which requires ~14.4V or 14.6V respectively), you need to increase the number of turns on COIL_RX to generate a higher raw AC voltage (aim for ~20V AC). After the Schottky bridge, replace the TP4056 with a buck-converter module equipped with CC/CV (Constant Current / Constant Voltage) adjustment, such as an LM2596-based CC/CV board, dialed to your specific battery chemistry's absorption voltage.

Why does my wireless charger circuit overheat when the receiver is removed?

When the receiver is removed, the transmitter coil acts as an unloaded inductor. While the DC resistance of 24 AWG wire is low, the MOSFET is still switching 68kHz. Without the receiver to absorb the magnetic energy (reflected load), the energy rings back into the transmitter tank, causing high-voltage spikes on the DRAIN_SW node. These spikes force the IRFZ44N into avalanche breakdown, generating massive heat. To fix this, add a 15V Zener diode and a fast signal diode (like a 1N4148) in series across the TX coil as a clamping snubber to absorb the inductive kickback.

How do I calculate the resonant frequency for the transmitter and receiver coils?

Inductive power transfer relies on inductive reactance and resonance. To maximize efficiency, the RX tank should resonate at the same frequency as the TX switching frequency (68kHz). The formula is f = 1 / (2π√(LC)).

Measure your wound COIL_RX with an LCR meter (a typical 15-turn 40mm coil is roughly 5µH). To tune it to 68kHz, you need a parallel tuning capacitor: C = 1 / ((2π * 68000)^2 * 5e-6), which equals roughly 1.08µF. Solder a 1µF film capacitor in parallel with your RX coil before the bridge rectifier. This creates a parallel LC tank that dramatically increases the induced voltage at exactly 68kHz, allowing you to increase the air gap between the coils from 5mm up to 15mm without losing charge capability. For deeper insights into coil tuning, the Wireless Power Consortium (WPC) publishes extensive documentation on Qi-standard resonant tuning networks.