RF Harvesting and Cap Discharge

Hello Everyone,

I have a question about a basic RF Harvest circuit I am working on: The following is a link to my schematic. https://ibb.co/sJr89rj

I live relatively close to a transmission source, and as a result the circuit charges up very quickly. The caps will actually change to 9v in about one hour or so.

I want to use this to send pulses of energy into a rechargeable battery. I am not worried about overcharging at this time as the battery has an full charge safeguard. Conceptually I would like to send a series of small changes to the battery overnight and see where I end up at.

Normally this battery would take a 5v source for charging. What I would like to do is wait until the caps charge to 8-10v and pulse that energy into the battery. I am stuck on how to 'signal' a charge when the voltage reaches the threshold.

I have tried with a Zener diode 1N4738A with has a rb of 8.2v. I ttied putting the Zener with a LED completing the circuit. I assumed once the threshold of 8.2 is reached the LED would flash and the circuit would drop below 8.2v and repeat the process. However, when I do this the output voltage just drops to 2-3 volts and does not incrementally change.

When I pull the Zener the incremental charge starts to accumulate again. Any assistance would be greatly appreciated!! Thank you!

by wmikrut
November 14, 2021

Is a simple example.

Use D1 as a Schottky diode (low threshold voltage).

Use an NMOS, M1, with a Vgs(th) of 8 Volt.

You may have to add a small extra resistor to limit the initial current when the battery is totally discharged. Consider, in the simulation, the difference of voltage between the two named nodes to get the voltage in the battery. The source V1 simulates a voltage which increases, then drops, and repeat.

by vanderghast
November 15, 2021

Wow, thank you very much!
I will definitely give this a try!!

by wmikrut
November 15, 2021

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