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I have a seemingly simple problem.

I am developing a system that harvests energy from a piezo-electric vibration source. The harvesting circuit works well and I am happy with that, but I also want to use the vibration source as a sensor to measure vibrations, and the harvester is interfering with that because it draws a current from the high-impedance source, distorting the signal from the vibration.

I thought to include a switch before the harvester circuit so that I can switch it off from a microcontroller temporarily, disconnecting the harvester from the source. The concept is depicted in the diagram.

Harvester Switch concept

The question is now, how to implement this.

The difficulty lies in:

  • the unknown voltage level at the source input
  • the relatively low voltage at the output of the MCU (1.8 V)
  • the requirement that the switch must be closed when the MCU is off, as the harvester needs to be able to cold-start the system

I came up with a somewhat complicated solution, using a depletion mode JFET, that cuts off the voltage if the gate-source voltage is positive. This still has the drawback that, as I see it, the JFETs have a fairly high on-resistance if VGS is zero (around 200 Ω, which is very considerable for the harvesting application).

Furthermore, I need two additional transistors to achieve the highest possible Vgs, when I want to switch the JFET off.

enter image description here

Am I missing some easier solution here? It seems like a not-so-uncommon problem to me, but I couldn't really find something fitting here. Are there ICs that you know of that would serve the purpose?

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  • \$\begingroup\$ Please explain why you can't use the sensor output whilst it is connected to the harvester. \$\endgroup\$
    – Andy aka
    Nov 7, 2022 at 10:40
  • \$\begingroup\$ Hi Andy, I edited the question. Problem is, that the harvester draws current from the piezoelectric disk, which is a high-impedance voltage source. This distorts the voltage over the piezo, and it can not be used as an accurate vibration sensor anymore. \$\endgroup\$
    – PeterO
    Nov 7, 2022 at 11:44

1 Answer 1

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after quite some searching, I found this excellent IC (FSA515) from OnSemi. It is a depletion mode switch, conducting with low Ron (<1Ohm), when no supply voltage is applied and isolating when there is a supply voltage.

Didn't know that existed, but that definitely solves my problem.

I would still be interested in other approaches if you have some.

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