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Created July 20, 2020
Last modified July 21, 2020
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The to-fro movement of a positive charge can switch off an n-channel JFET.


Prompted by :- Q&A Static electricity and "Static detector"

A to-fro movement of a positive charge near a detector plate will produce a pulse that is positive going only. How does this switch off an n-channel JFET which needs a negative gate voltage to cut off?

This simulation does 2 things

1] Models the pick up from the movement of a 500pC spherical charge by means of a varying voltage and fixed 0.5pF, notional "capacitor". In reality the "capacitor" varies as 1/d^2 because of the diverging electric field from the spherical charge. In normal, fixed value capacitors, with parallel plates, the capacitance varies as 1/d (d = spacing between plates).

2] Provides extra modelling around the input to the n-channel JFET. The gate-substrate diode dissipates positive charge on the rising edge input; when the input falls, the gate is pulled negative. The gate recovers to zero by reverse current from the gate-substrate diode and discharge through Rgs.

References: biology paper "Electrostatic Charge on Flying Hummingbirds and Its Potential Role in Pollination"; 2N3819 datasheet.


Thanks! I was planning to run a wire around the rim of the feeder to pick up the static. Just where they land to feed. If that doesn't work (ie not enough static) I will try to build a Faraday cage, a la biology paper you've quoted. Hope I don't have to because there may be familial objections to a huge cage hanging by the door! I will let you know how it goes. I appreciate the effort you've put into this isn't it fascinating how HBs do this? I have tried other sensor (IR) and also sound because their wings beat at 200Hz.

by stephenp
July 21, 2020

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