The following is a circuit that wirelessly transfers power via inductive coupling. The circuit is supposed to power a 12 V 0.15 A DC fan and a typical smartphone, which requires 5 V DC across it. The loads are connected in parallel to each other, as seen in the circuit schematic(though there is no capacitor at the output of the voltage regulator). As you can see in the picture below, it powers both just fine, but the phone only charges for a few seconds. And it can only charge both the fan and the smartphone only when the transmitter and receiver coils are very close to each other. The transmitter coil has 9 turns and the receiver coil has 39 turns.
The voltage induced in the receiver coil hand an RMS voltage of 28.3 V as shown in its waveform. It also had some ringing, which I assume is because the PCB terminal block acts as a capacitor. Anyways, I assumed that the voltage would remain 28.3 V because the DC fan and the phone are both connected in parallel so the the voltage at the receiver coil shouldn't change, or that's what I thought.
So I then measured the voltage at the receiver coil when both loads were added, and, as you can see in the waveform below, two things changed: the ringing is now gone and the RMS voltage decreases to 13.0 V.
Can someone please explain why the receiver coil voltage decreased even when the loads were added in parallel?
I am sorry for the long post, but I just wanted to make sure every little detail is given.
Edit: Thank you, everyone, for the answer regarding the decrease in receiver coil voltage. I fully understood it.