Hello, this is my first post here. I recently built several models of a JFET static detector. All work, but the last version is giving some readings I can't explain. I'll post some facts, thoughts and observations here and I'd appreciate comments on whether I'm right or wrong and what might be happening. This is my first electronics project in many years.
According to the articles that accompany this type of static detector, the antenna is supposed to have the same polarity as the electric field, i.e. a negative object makes the unit respond as if the gate is charged negatively. This is seen on smaller models I made and for the larger model while it was on the breadboard. However, in the final set-up, a negatively charged object brought near the antenna causes a positive spike. First of all, nothing appears to be wired backwards, and I have tried changing the JFET.
Positive charges would be drawn onto the antenna by the nearby negative object. This is how normal electrostatic induction works, as I am familiar with it. But then, what was happening in the small models which gave the opposite response?
Could it have something to do with the small models having a wire as an antenna and the large model having a large (17'' diam) metal bowl?
It can't have to do with the distance between the antenna and the FET gate, or the presence of the cable between the antenna and indoor unit.
In the small models, antenna and FET gate are extremely close, a fraction of an inch. In the large model, There is only a few inches distance, and the FET is under the antenna bowl so is shielded by it. The 50' cable has a grounded shield and contains wires for the FET source and drain, not the gate.
With normal induction, if a negative object approached the antenna, positive charges would accumulate on the antenna, attracted to the negative charge nearby. Negative charges on the antenna would be repelled to the farthest point, in this case, to the gate. Therefore, the gate ends up with the same charge as the object which approached. So, how would the gate be charged oppositely in my final setup?
Thanks.
Additional Info:
The "smaller models" I mentioned consisted simply of a 9V battery, one nJFET, and one LED. The antenna on these was just the FET gate wire sticking up in the air. Bit by bit I added everything else to this basic tiny circuit. BTW, the unit is powered by a small plug-in power supply that puts out ~9 VDC at 200 mA. The unit only uses 2 to 6 mA. I did not draw the transformer for the pwr supply.
According to the articles that accompany this type of static detector, the antenna is supposed to have the same polarity as the electric field, i.e. a negative object makes the unit respond as if the gate is charged negatively. This is seen on smaller models I made and for the larger model while it was on the breadboard. However, in the final set-up, a negatively charged object brought near the antenna causes a positive spike. First of all, nothing appears to be wired backwards, and I have tried changing the JFET.
Positive charges would be drawn onto the antenna by the nearby negative object. This is how normal electrostatic induction works, as I am familiar with it. But then, what was happening in the small models which gave the opposite response?
Could it have something to do with the small models having a wire as an antenna and the large model having a large (17'' diam) metal bowl?
It can't have to do with the distance between the antenna and the FET gate, or the presence of the cable between the antenna and indoor unit.
In the small models, antenna and FET gate are extremely close, a fraction of an inch. In the large model, There is only a few inches distance, and the FET is under the antenna bowl so is shielded by it. The 50' cable has a grounded shield and contains wires for the FET source and drain, not the gate.
With normal induction, if a negative object approached the antenna, positive charges would accumulate on the antenna, attracted to the negative charge nearby. Negative charges on the antenna would be repelled to the farthest point, in this case, to the gate. Therefore, the gate ends up with the same charge as the object which approached. So, how would the gate be charged oppositely in my final setup?
Thanks.
Additional Info:
The "smaller models" I mentioned consisted simply of a 9V battery, one nJFET, and one LED. The antenna on these was just the FET gate wire sticking up in the air. Bit by bit I added everything else to this basic tiny circuit. BTW, the unit is powered by a small plug-in power supply that puts out ~9 VDC at 200 mA. The unit only uses 2 to 6 mA. I did not draw the transformer for the pwr supply.
Attachments
Last edited: