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DIY giga-ohm resistors???

M

mike

Jan 1, 1970
0
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
 
M

mike

Jan 1, 1970
0
You don't kill an electrometer sensitivity with a resistor. Use a
capacitor.
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.
mike
 
M

mike

Jan 1, 1970
0
90V strike voltage is NO "protection"...
Even the 65V operating voltage is NO "protection"...

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.
 
S

Spehro Pefhany

Jan 1, 1970
0
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.
mike

Digikey stocks 10G 5% resistors for less than $4. HVF1206T1008JE, and
others.

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

http://www.partsconnexion.com/terminal_connex.html

(ceramic base).


Best regards,
Spehro Pefhany
 
B

Bernhard Kuemel

Jan 1, 1970
0
Anybody experienced with DIY BIG resistors?

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.

Bernhard
 
S

Sjouke Burry

Jan 1, 1970
0
mike said:
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.
mike

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.
 
S

Spehro Pefhany

Jan 1, 1970
0
If the gate oxide breaks down at 15V (typical) youall aint gott no
perrtektion.

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.
STM makes a number of FETs with integrated back-to-back zeners for
protection.
Added leakage can be in the nanoamp or less region at 25C.

A nA can be huge. According to measurements made by skilled
practitioners (eg. Win Hill) actual leakage of a 2N700x is a few fA.
 
M

mike

Jan 1, 1970
0
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.

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.

Ready...AIM...
 
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.

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.

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.
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.

Ready...AIM...

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
circuit?
 
K

Ken S. Tucker

Jan 1, 1970
0
Bill said:
I once did this using india ink on painted acrylic. The ink is a
carbon resistor, and you can dilute it to produce large values.

One problem was with the junction from ink to electrodes. Better use
conductive silver ink as terminals, then run the india ink up over the
silver.

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.
Ken
 
J

Jamie

Jan 1, 1970
0
John said:
Hey, Phil, do you know anything about the reverse beta of a BFT25?

Imagine a linear ramp made from a current source and a cap; say 10 mA
and 20 pF, for a few ns to 2.5 volts. Now I want to switch in bigger
caps for slower ranges, like 200 pF and 2 nF. I figured I could use
BFT25s to do that. But when I discharge the ramp, current flows
backwards into the BFT25s.

isrc
|
v
|
| diode
+----------+---------+---ak----reset
| | |
| | |
20pF 200p 2n
| | |
| | |
| | |
gnd c c
e e
| |
gnd gnd

If there's any decent amount of reverse beta, I don't need a huge
amount of base current to get a decent reset time.

(I'll probably add a weak pullup to the transistor collectors to bias
them up when they're off.)

I have some very rough data that suggests a reverse beta around 5
maybe, but it's not solid.
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?


Jamie
 
J

Jamie

Jan 1, 1970
0
John said:
Yes, that would absorb most of the discharge current. Tiny schottkies
wouldn't add much capacitance. We have some nice diodes in stock,
SMS7621, that are only about 0.25 pF.


You could remove the




The diode capacitances are nonlinear on voltage, so that would bend my
nice linear ramp. That's the reason to pull up the "off" transistor
collectors, to reduce the nonlinearity of their c-e capacitance.
Yes, you are so correct there.:)

Jamie
 
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