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DIY giga-ohm resistors???

Discussion in 'Electronic Design' started by mike, Feb 29, 2012.

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  1. mike

    mike Guest

    I built a crude electrometer.
    Poke the gate of a jfet in the air and read the
    drain current.
    Uses a neon bulb as input protection and gate bias.

    Problem is that it's WAY too sensitive for what I want.

    100M resistor to ground kills the sensitivity.
    Looks like I need something on the order of a few Gigahoms.

    Anybody experienced with DIY BIG resistors?

    About an inch of pencil line on card stock is about right.
    But that's gonna be very unstable.

    Best I've come up with so far is sheet magnetic material
    used for stick-up business cards.

    It's dimensionally stable enough to bolt on a contact.
    Two parallel strips shunted by another strip
    makes a variable resistor. I expect it's gonna
    be unstable too, but at least it's adjustable.

    Better ideas?

    NO, I'm not gonna buy a resistor. I'll be done with
    this thing before tomorrow's mail.

    Thanks, mike
  2. mike

    mike Guest

    Thanks, excellent advice. Certainly cuts the gain.
    Works great for what it is.

    Next experiment is to build a field mill.
    Don't think I can stand the increased capacitance in a field mill.
    And need it to come off the peg back into the dynamic range of the
    amplifier...eventually. I think I need a resistor. But this is
    all new territory for me.

    Still want ideas on making BIG resistors.
  3. mike

    mike Guest

    OK, what would you suggest?

    Strike voltage is about 70V in the preferred direction.
    That limits the gate current to 4 microamps reverse.
    Suppose that's excessive?

    I bit the bullet and put back-back zeners at the gate.
    I can see the added leakage, but
    I wanted some leakage to ground anyway.
  4. Digikey stocks 10G 5% resistors for less than $4. HVF1206T1008JE, and

    Or, if you really feel the need to make something, maybe your pencil
    line on something like this:

    (ceramic base).

    Best regards,
    Spehro Pefhany
  5. Not much.

    Maybe distilled water.

    Or a gas gap exposed to ionizing radiation. Water vapor and a UV LED
    might work, but I don't know the required wavelength. Should be
    adjustable by the amount of ionizing radiation or the electrode
    distance. Pressure should be high enough to avoid avalanche break
    through. You could try to use a smoke detector that uses americium as
    source of ionizing radiation. Uranium is fairly easy to acquire, but has
    a low activity. There are professional radiation sources available, too.
    Betalights come to my mind. They contain radioactive tritium. You would
    have to break open the vial, though, to make use of the tritium, as the
    radiation won't penetrate the vial.

    A 1N4148 diode has 0.8 GOhm reverse resistance at 20 V and 25 degC. You
    can certainly find diodes with higher resistances. The resistance
    depends on voltage and temperature, though, and is DC only.

  6. Sjouke Burry

    Sjouke Burry Guest

    Take a 4-8 inch high transmitter tube, power up the
    cathode keating, and use a negative voltage on the grid
    to tune for the resistance you want.
    In a pinch an audio endstage tube will do.
  7. That's normal for an abs. max rating, but I wonder what the typical
    actual breakdown voltage is. For example, a Calogic 2N7002 has an abs
    max rating of +/-40V, I would guess that the typical breakdown is more
    like 100VDC, so a neon might be okay.
    A nA can be huge. According to measurements made by skilled
    practitioners (eg. Win Hill) actual leakage of a 2N700x is a few fA.
  8. mike

    mike Guest

    I started by asking about making big resistors.
    This has morphed into a party where every drunken cowboy is shooting his
    gun in all directions.

    Let's put a target on the wall and encourage everybody to AIM.

    The question has become, "what does it take to damage the gate of a JFET.
    (2n700x is a MOSFET)

    Neon lamp is across the input.
    10Meg series resistor to gate of 2N4416 JFET. Why a 2N4416?
    Simple, that's what I could find in the junk box, and it works.
    7K in the source, Meter in the Drain, 9V battery.

    Neon fires at about 70V (in the dark) if you stick it in the right
    direction. About 90V in the other direction
    Gate breakdown voltage spec is 30V.

    In the forward direction, I've got about 9 microamps of gate current.
    That shouldn't be harmful???

    In the reverse direction, I've got about 4 microamps of gate current
    at -30V.
    So, the basic question is, just that; "is 4 microamps at -30V on the
    gate harmful in this application?"
    I'm not worried about increasing the noise figure.
    I am worried about significant increases in gate leakage current at ~0V.

    For now, let's just ignore all the nasty parasitic and transient things
    that happen if you actually strike an arc at the input. That's a whole
    other bucket of worms.

  9. Guest

    Yee Haw! Hey if you want a STABLE high value resistor it ain't so
    simple. First you have to be very careful of the insulator. Glass (which
    includes ceramic) is OUT. The absolute best is polystyrene. For quick
    and dirty a pencil line on mylar drawing paper isn't too bad.

    In a case where I needed very high resistance with reasonable accuracy,
    we built a circuit board with hundreds of high value surface mount
    resistors in series. Handles voltage, low leakage (with teflon circuit
    board) and accurate. We only had one blow itself to kingdom come. Don't
    know why it failed.

    Yes, I've tried the neon "protection" thing. Basically worthless.
    Problem as noted above is transient response. And anyway, don't you know
    that the gates of FETs of ANY type make better fuses than ANY protection
  10. Guest

    Dark Emitting Axial Diodes?
  11. Good idea that India ink. Stir some into some plastercine to put
    in a pair of wires for a variable resistor.
    Stir into glue, wires at each end, let dry and measure.
  12. Jamie

    Jamie Guest

    Clamp diode of low pF from C to E on each one? You could remove the
    20pF and use what is in the diodes for that?

  13. Jamie

    Jamie Guest

    Yes, you are so correct there.:)

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