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Help w JFET static detector

Discussion in 'General Electronics Discussion' started by Trent, Apr 17, 2010.

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  1. Trent

    Trent

    16
    0
    Apr 17, 2010
    Hello, this is my first post here.
    I built several versions of JFET static charge detectors and I'm seeing some differences I can't explain so I'm looking for help. I'll state some facts and observations and thoughts here and I'd appreciate comments about if I'm right or wrong and what's going on. 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 replacing the FET with a new one.

    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 small models mentioned consisted simply of a 9v battery, the nJFET, and an LED. The antenna was just the FET gate lead sticking up in the air. I gradually added everything else. BTW, the unit is powered by a small plug-in pwr supply that gives 9 VDC at 200 mA. It uses only 2 to 6 mA. I just didn't draw the step-down transformer for the pwr supply.
     

    Attached Files:

    Last edited: Apr 20, 2010
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,849
    Jan 21, 2010
    I've always thought that "Static sensitive" was a warning and not a feature to be exploited!

    Please tell me you're not rectifying the mains to power that circuit!
     
  3. Trent

    Trent

    16
    0
    Apr 17, 2010
    *steve*
    The unit is powered by a small plug-in power supply that puts out 8 - 9 VDC at 200mA. It only uses about 2-6mA of current. The step-down transformer is not shown for the power supply. I just drew it as shown to indicate that I was using a power supply vs a bunch of batteries.
     
    Last edited: Apr 18, 2010
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,849
    Jan 21, 2010
    Is there anything else you've left off?

    Is there any filtering of the rectified AC used to power this?

    Can you provide a detailed explanation of the various switched and jacks?

    Or perhaps there's no problem with that?

    Is the problem that you're getting the reverse of the charge that you're expecting?

    A jfet will conduct base charge away in one direction, so I can't see how you could get much of an opposite charge (limited to 0.6V)

    I can also imagine the jfet being damaged by excessive voltage at the gate.
     
  5. Trent

    Trent

    16
    0
    Apr 17, 2010
    I think I have all the pertinent info included.
    I don't know if there is any filtering on the power supply, but it does not matter. The heart of the unit is a Wheatstone Bridge. As long as there is enough voltage and current, it matters little if the voltage varies. Also, the bridge automatically filters out any common mode interference such as ripple. The bridge consists of the two 50K pots, the 5K pot set for 1100 ohms, and the detector JFET. The bridge has a "positive" voltage drop across the meter terminals when the detector FET has a positive charge on the gate and is conducting. The bridge has a "negative" voltage drop when the FET gate has a negative charge and is turned off. That's when the other FET and LED conduct.
    One jack is a chassis ground. Another (Vfc) has a neon tube connected between the circuit (which floats) and the chassis. If lightning is extremely close, the detector FET will be burned out and the high voltage on the antenna can come in through the drain and source wires in the cable and could charge the circuit. At 70 volts, the neon tube will conduct and short the circuit to ground. If I see it light up, it means get out of there fast. Other jacks are for the antenna cable and to plug in a DVM.
    I put in a switch that would connect, open or short the detector FET because I didn't know if I would need to zero the unit with the FET in the circuit or not. I also tried to connect a dummy load to try and equalize the zero points with and without the FET in the circuit, but that just made things worse for some reason. As it is, I just zero it with the FET in the circuit. These are not a problem.
    The problem is that I'm getting an output on the meter that is the opposite of what I expected. I bring a negatively charged piece of plastic near the antenna and I get a voltage drop indicating a positive gate voltage when the same circuit had turned the red LED off and the yellow LED on when in the breadboard stage which indicates a negative gate voltage. The gate is good to 25 volts. Gate current is 10 mA, reverse current is 2 uA. Drain-source current is about 6.6mA now. The most voltage I've measured on the gate (with this circuit) has been 0.1 volts during fair weather. I have not had any thunderstorms here yet to test it with.
    The FET conducts (or not) in one direction. The positive and negative voltage drops are across the Wheatstone Bridge.
    Yes, the FET can be damaged by excessive gate voltage. I've tried using zener and other diodes as a means to drain excess voltage away from the antenna before it reaches 25 volts, but anything I connect to the antenna reduces the sensitivity to the point of uselessness. It is very sensitive to capacitance.
    I added a schematic with the bridge sections highlighted.

    More info:
    The unit correctly detects a positive atmospheric charge during the day, which drops at night. But I get the opposite response when I try to test it with charged objects.
    Also, bringing a negatively charged object near the gate should turn the red LED and FET off. It will come back on when the charge is removed. Positive charges brought close don't exactly cause the opposite effect. The red LED will get brighter as the FET conducts more, but the LED and FET stop conducting when the positive charge is moved away as if the FET was now sensing the CHANGE in the electric field rather than polarity of the static field. The field is changing in a negative direction from positive to zero but the LED and FET go off, not just to zero voltage operation. Nobody who has written about this type of circuit has had an explanation for this. I can add links to the original articles I saw. They will explain more than I have here about the basic operation of the FET detector.
    http://amasci.com/emotor/chargdet.html
    http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p050.shtml
     
    Last edited: Apr 18, 2010
  6. Trent

    Trent

    16
    0
    Apr 17, 2010
    Possible explanation for observed responses

    This series of events can account for the meter responses I've seen and it seems possible, though I don't know how probable it is...

    As the antenna is approached with the test charged object, the cable betwen antenna and house is passed. At times I'm closer to the cable than the antenna.
    A positive charge can be induced on a section of cable shield.
    The charged shield would, in turn, induce negative charges on the drain-source wires within.
    If the drain and source are negative, they can induce a positive charge on the gate.
    Parts of the antenna are positively and negatively charged and these charges do not conflict with the charges coming from the shield.
    The result is that when the negative test charge is brought close, the gate ends up positive relative to the drain-source.
    When detecting atmospheric charges, this does not happen because the cable and antenna are at equal distances and the antenna is much larger so its charge dominates any charges from the cable.
    See attached diagram.
     

    Attached Files:

  7. Trent

    Trent

    16
    0
    Apr 17, 2010
    Another explanation

    I just realized that after rubbing the piece of plastic to generate a negative test charge, I may have retained some residual positive charge. The antenna could be reacting to my approach more than to the relatively smaller piece of plastic. Another possibility.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,849
    Jan 21, 2010
    Very likely. Is your circuit grounded? Or can it build up it's own charge?

    It sounds like the latter. I suspected this earlier, but I think you've confirmed it.
     
  9. Trent

    Trent

    16
    0
    Apr 17, 2010
    The circuit is floating right now. Offhand I forget my reasons for doing this. It only connects to ground if it develops enough charge for the neon light to breakdown (70V), then the circuit connects to the grounded chassis through the bulb. I can connect a DVM into the Vfc jack in place of the neon bulb to monitor circuit to ground voltage. I'll do this.
    Thanks.
     
  10. wbeaty

    wbeaty

    2
    1
    Apr 30, 2010
    Essentially these circuits are capacitors. If you hold a negatively charged object near the FET's gate wire, the FET switches off, but if you hold the charged object away from the gate but near the rest of the circuit (near the battery, ) the FET switches on.

    This becomes clear if you connect a long floating alligator cliplead to the FET gate, and connect a second floating cliplead to any point on the circuit. The device then acts like a "DC dipole antenna."

    Don't forget that you can ground the gate lead and then use the rest of the floating FET circuit as the antenna. The circuit will then think that all charge polarities are backwards. When held in your hand, try lifting the circuit by it's gate lead, then use the rest of the device as then antenna.

    Or, you could build the circuit into a plastic box with adhesive foil covering the left and right halves. Connect the circuit to one foil half, connect the gate lead to the other. Now the circuit will behave forward or backward depending on which foil his held in your hand, and which foil is used as the antenna.
     
  11. Trent

    Trent

    16
    0
    Apr 17, 2010
    Thanks, Wbeaty, for reminding me about the dipole properties of the circuit. I guess mine's a big dipole because I have the main part of the circuit inside a grounded, metal box on my desk and a 50 foot long shielded cable connects it to the antenna out on my balcony.

    When I test the unit, I go out on the balcony with a negatively charged piece of plastic. There is a wall and metal framed balcony doors between me and the unit inside. I confirmed that the antenna is sensing the presence of the approaching charged object and not the cable leading back inside.

    Nothing I've done has changed or explained why I get positive responses from the antenna when the test charge is negative.

    Also, the FET connected to the antenna has never turned off since I haven't been able to get the antenna negative enough. I figure once the gate gets negative enough for the FET on-resistance to go above 1100 ohms, the red LED will go off and the yellow LED will come on fully. At the moment, the on-resistance of the FET is about 180 ohms according to measurements I made of circuit voltages and currents.

    The smaller, handheld units I built first do react correctly and do act like dipoles. I just want the big unit to sense approaching thunderclouds.

    I am also picking up noise from somewhere. I've managed to filter out some and going to battery power reduced the noise a lot, but it's still there. Could it be "resistor noise"? I don't know if the amplification is enough for that. Right now it's about 0.3 mV peak to peak. I can't tell the frequency. The only strange thing about battery power is that the baseline drifts. The batteries are brand new alkaline 9 volt, 3 batteries in parallel.

    I don't mind living with the problem, but I'd like to understand what is going on.

    Might get a thunderstorm this weekend.

    Thanks
     
  12. Trent

    Trent

    16
    0
    Apr 17, 2010
    Just a follow-up post here to give closure. I figured out why a negative test charge gives a positive movement on the analog meter .....

    As I recall, when current in the analog meter flows from neg terminal to pos terminal, the meter moves to the right (pos) as it would if it was an ordinary 0-100 meter and current flowed the correct direction.

    In this circuit, current flows from the neg to pos terminals when the FET is LESS conducting and resistance is greater than Rx = 1365 ohms.

    This moves the meter to the right (pos) even though it is a negative charge on the FET which is causing the decreased FET conductivity.

    Problem explained.
     
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