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October 13–19 | Wireless Power Tests

Test the receiver coil and LED together, focusing on brightness, placement, and the distance between the stone and tray. Check whether the light remains stable during repeated placement and monitor heat during extended operation.

Deliverable: One LED assembly that lights reliably within a clearly defined area of the tray.

Coil Testing and Resonance Tuning

The goal of this experiment is to develop a compact receiver coil that can power two LED filaments connected in series.

I tested a coil wound around a 6 mm diameter bobbin with 40 turns. With only the oscilloscope connected, the measured voltage reached approximately 7.38 Vpp at the best position and dropped to around 6 Vpp when the alignment changed. This suggests that coil positioning has a significant effect on the received voltage.

1. Electromagnetic Induction

The receiving coil converts a changing magnetic field into an electrical voltage:

v = −N × dΦ/dt

Here, N is the number of turns, and Φ is the magnetic flux through each turn. The induced voltage depends on the number of turns, the coil geometry, and its position relative to the transmitting coil.

Experiment Data:

Coil diameter: 6mm 8mm 10mm

Number of turns： 40 16,32

2. Resonance Tuning

Adding a capacitor creates an LC resonant circuit with the receiving coil. Its ideal resonant frequency is:

f₀ = 1 / (2π × √(L × C))

To calculate the required capacitance:

C = 1 / [(2π × f₀)² × L]

My preliminary inductance measurement was approximately 4.5 µH. Using an assumed operating frequency of 225 kHz:

L = 0.0000045 H
f₀ = 225,000 Hz
C ≈ 111 nF

This gives a starting point for testing. The inductance measurement occasionally fluctuated, and the operating frequency should be checked again before final tuning.

Two capacitors connected in parallel have a total capacitance of:

Ctotal = C₁ + C₂

For example:

100 nF + 10 nF = 110 nF

For this test, both capacitors are connected across the same two terminals of the receiving coil.

3. Understanding the Voltage Reading

The oscilloscope measures peak-to-peak voltage:

Vpp = Vmax − Vmin

For a sine wave centered around zero:

Vpeak = Vpp / 2
Vrms = Vpp / (2 × √2)

Therefore, 7.38 Vpp corresponds to approximately:

Vpeak = 3.69 V
Vrms = 2.61 V

This is an unloaded AC measurement. It does not mean the coil can provide 7.38 V DC or supply enough power for both LEDs.

4. Experimental Procedure

1. Test the two LED filaments using a current-limited DC supply to establish the voltage and current needed for the desired brightness.
2. Measure the receiving coil’s inductance with the transmitter switched off and the coil disconnected from other components. Record the measurement frequency.
3. Fix the transmitter voltage, coil spacing, and alignment so that comparisons are consistent.
4. Record the receiving coil’s voltage without a tuning capacitor or LED load.
5. Compare 100 nF and 110 nF tuning capacitors, then adjust the capacitance around the better result.
6. Connect the rectifier, current-limiting circuit, and two series-connected LEDs. Measure the loaded voltage and current, and observe the brightness.
7. If the output is insufficient, change the turn count, measure the new inductance, and repeat the tuning process.

For a reasonably steady DC output, the power delivered to the load can be estimated as:

P ≈ VDC × IDC

The final evaluation will focus on LED brightness, delivered power, temperature, and performance inside the stone enclosure. The highest unloaded voltage alone is not enough to determine the best coil design.