WIRELESS POWER

This project is a simple way of demonstrating inductive coupling. Many charging mats made for use with cell phones and other small electronics use a similar principle.

What we essentially have is an air core transformer. A convential transformer has iron laminations that efficiently couple the magnetic field between the primary winding and the secondary winding. However, in this case the lack of an iron core means that we must tune both the transmit and receive coils to resonate in order to create adequate power transfer.

The transmitter circuit is a simple MOSFET multivibrator. The charge – discharge cycle of the two 300pF capacitors causes the two MOSFET’s to alternately turn on and off. Normally a current limiting resistor is used between V+ and the MOSFET drain. In this application the resistor is replaced with one of the coil windings on each side of the center tap. The center tap is connected in series with a 10 ohm resistor and a 1 amp PTC fuse to the power source. The resistor and PTC fuse prevent excessive current flow should one of the MOSFETs fail. The 400pF capacitor and 10M resistor help with oscillator start-up when power is applied.

The receiver is simply a coil with a capacitor and an LED connected in parallel.

Most efficient coupling seems to occur when both coils are parallel with one another on the same vertical axis. With a supply voltage of 18 VDC, the LED would begin to light when held about 7 inches above the transmit coil. Coupling also occurs when the two coils are placed side by side on the same plane as shown in the photograph. The circuit will work with a supply voltage down to 4 VDC, with a corresponding decrease in coupling distance. A supply voltage of 10 VDC or greater seems to work best.

My prototype with a supply voltage of 12 VDC and a horizontal (plane) center to center distance between receiver coil and transmitter coil of 3 inches, I measured a receiver output of 8 VAC peak to peak at a frequency of 4.76 Mhz.

It was interesting to note that I got the same results with the coils stacked vertically on center also at a distance of three inches. At a distance of one inch vertically, the output was 10 VAC peak to peak with a slight increase in frequency to 4.8 Mhz.

The probe used was rated at 100 Mhz, 13pF and 10 meg and may have some loading effect on the results.

It is quite possible that there are modifications to this circuit that will make it work more efficiently. One thing to try is a transmit coil made with 18 or 16 ga wire and also with a different number of turns. Maybe two or four turns instead of ten or a larger diameter.

Also, you can experiment further by using four Schottky diodes in a bridge configuration with a filter capacitor to create a DC source to power a MCU or other electronics.

TRANSMITTER AND RECEIVER IN OPERATION



RECEIVER OUTPUT