Tuned Antennas Lighting Up LEDs Video

[hulk69]

Hey guys

I came accross this video:
https://www.youtube.com/shorts/aSSqrddG6B0?feature=share

What puzzled me is why on earth would he put a schottky diode in parrallel to make the LED light up???

Would not a capacitor be a better idea?

Thanks

[Andy Chee]

The diode is to allow reverse current to flow in the opposite direction.  If you remove the diode, the LED will block the reverse voltage, but will be destroyed because the reverse voltage became too high.

[hulk69]

After thinking a lot about this i do not think it has anything to do with protection because of the very minimal power involved but more about the voltage/current characteristic of a fast diode.

An LED would need a lot more current to get to a certain voltage.
The schottky diode can start conducting @100µA -> 0.4V which would be able to drive an LED very dimly

So my theory is that the diode is there to increase the voltage at very low current

[hulk69]

looking at it again makes no sense because the diodes are reversed...

[KE5FX]

The LED has a fair bit of junction capacitance, probably 50-100 pF, and at subthreshold voltages it will appear capacitive in both directions, preventing any DC voltage from building up (unless/until you hit the diode with a lot more power than it receives here.) 

By shorting out the negative half-cycles (neglecting Vf), the Schottky forces the LED junction capacitance to charge only on the positive half-cycles, allowing enough voltage to build up to turn the LED on.