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how to choose the best FET to detect weak e-field?

Discussion in 'Electronic Design' started by catty wu, Oct 16, 2012.

  1. catty wu

    catty wu Guest

    I'm working on a school project to detect the weak e-field(<1000V/m).
    I want to use the FET. But I just don't know what kind of FETshould I
    choose. Can anyone help me? Thanks a lot!
     
  2. Guest

    Yes, that, and bootstrap it. An electrometer fet, like the pn4117a
    might be good.

    http://www.fairchildsemi.com/ds/MM/MMBF4117.pdf

    An r.f. FET might be better; this was the best I noticed.
     
  3. Guest

    The 2n7002 is the SMT part.

    It's a very neat part, but the input capacitance is an order of
    magnitude higher than a small-geometry JFET. The input's effectively
    capacitively coupled to the world, so Cin attenuates the input voltage
    accordingly.

    ___
    \ /
    *
    | C(ant) |--
    '---||--+--->|
    | |--
    ---
    --- Cin
    |
    ===


    James
     
  4. -what everyone else said-.

    But what are you trying to measure? 1000V/m is not that small. I've
    been dreaming about making an electric field mill to measure the
    Earth's electric field (~100V/m). And when I last pushed the numbers
    around I thought I might be able to do it with a nice FET input
    opamp. If you have a largish collector plate (maybe 10 - 100 cm^2)
    you can get 10^-11 or 10^-10 coulombs of charge. If you're doing
    something like that you might try searching under electric field
    mill. I know there are circuits posted on the web.

    George H.

    (As a side question, say I've got a 10cm diameter plate pointing up
    into the sky, what's its capacitance?)
     
  5. Guest

    High Cin gives a modicum of overvoltage protection too. Cin matters
    less as a d.c. measurement rather than a.c., I s'pose--there are some
    neat VLF circuits that use a voltage probe antenna, bootstrap a FET's
    capacitance to nothingness with transformers, then go to town.

    I made an e-field demo for some kids with Radio Shack parts. With an
    MPF102, a 4" wire antenna to the gate, and an LED in the drain, you
    could turn the LED full on and off by scuffing your feet from a good 5
    meters away. Low Cin was key, obviously.
     
  6. Thanks Jim, the web has circuits too. If I ever get around to
    actually building one, I'll be sure to ask here.

    George H.
     
  7. Jamie

    Jamie Guest

    Yes they are, I just wish the last batch I got were better performing
    like all the prior batches I got in the past..

    THe last batch it seems the body diode starts producing a forward of
    around 1.8v when it gets down to ~ 1 degree C. they are used in a
    sensing circuit that gets cold. We have a cluster of these circuits
    spread out to form an array of sensors so that each area can be monitored.

    I've found that not only does the body diode forward increases more
    than they should, the Vgs(th) does too, that gets up around 3.5

    All other batches prior to this were just fine in this temperature
    operation and the body diode forward never exceeded 1 volt.

    So what I have done is to add an external diode instead of using the
    body diode and the Vgs(th) increasing I can live with. But that body
    diode plays an important part of the circuit, I was just trying to save
    on component count..

    Jamie
     
  8. Jamie

    Jamie Guest

    they maybe cheap but with my older eyes, they are becoming harder to
    work with.

    I have a 47x boom scope, although I don't use that sitting, am now
    thinking of getting a head set USB scope. The hands still work and
    I find it easy these days for PCB houses to do the boards.

    Jamie
     
  9. Artemus

    Artemus Guest

    IIRC the research that led to LLP was done by Phil Krider's group in
    the UA Physics dept back in the '70's. Any patents will likely have his
    name on them.
    Art
     
  10. Guest

  11. John S

    John S Guest

    Seems to me that it would be easy to blow the gate.
     
  12. John S

    John S Guest

    That is curious to me. I would like to understand why there are so many
    warnings about floating gates if there is not much worry about that.

    Can you help me with that?
     
  13. Tim Williams

    Tim Williams Guest

    The instant you connect gate to drain (a mechanical circuit closes in
    under 1ns, by the way, so it does make a reasonable impulse), current
    flows into the gate capacitance. Some will flow out of the drain's
    (assuming the transistor was off first), as well as the source (which in
    this example is a harmless resistor and LED). As the gate charges, drain
    voltage falls. Within a few ns, charge is equalized and Vg = Vd ==
    Vgs(th).

    There might be some hazard running such a circuit from a supply over 40V,
    where the drain charge may be sufficient to cause dangerous gate
    transients before the entire gate charges (a process limited by gate
    spreading resistance). Note I say this with some confidence while
    recognizing Vgs(max) is only 20V (or is it 30V on the 2N7002?).

    What you don't want to do is touch the gate after scuffing your feet on
    the carpet. Which John never recommended, and I certainly don't either!

    As for general purposes, uncommitted pins are generally a hazard, so you
    don't want to leave things floating. Sometimes this has real consequences
    (floating CMOS inputs are constrained by the input diodes, but otherwise
    free inbetween; around the midpoint, the input stage goes class A and
    supply current increases dramatically). Other times, it doesn't so much
    (John's FET-resistor-LED will stay where it was more or less, and drift is
    simply part of the experiment). But general advice is to avoid undefined
    voltages anywhere, be they in circuit, on components (like unused gates)
    or PCBs (I hate it when copper pours add those damn floating patches
    without checking for vias to them!).

    Tim
     
  14. "Cheeeese Gromet"

    (Shaking hands)

    George H.
     
  15. Ouch, I'm never exactly right.
    (Grin :^)
    My daughter is taking some science class (8th grade)
    She told me her teacher was interested in weather science,
    and I said we should build a field mill.
    So we've recently been looking at web pages.
    I've been to the second link.
    If I can carve out some time to work on it, maybe I can hit you up for
    ideas on how to improve the front end.

    George H.
     
  16. Tim Williams

    Tim Williams Guest

    That's quite fine -- generalities are meant to have specific exceptions.
    :)

    Would be more accurate to say: "avoid unintended open circuits", since in
    low-leakage stuff, like this circuit, the signal of interest is
    more-or-less indistinguishable from an "open circuit". But it's intended,
    which makes it all okay. ;-)

    Tim
     
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