Regarding making your own extreme high-value resistors - any object that
has insulators and leads but with nothing connected inside will have
some high R that can be perhaps be measured, but won't be stable against
environment effects on the outer surfaces. There's not much point to
carbonizing
I'm guessing the application relates back to your leaf electrometer
project discussed earlier - trying to assess how the bias charge on the
capacitor holds up from leakage and use of the instrument. If this is
the case, then it's for a one-time use for design of the item, so
shouldn't be too fa
Yes Hendrik, same principle as the butterfly disk style, but mine use
cylinders - the field exposure is radial instead of axial. Ed
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There is another kind of static electric field meter that was commonly
used over the past few decades for monitoring charges/voltages in work
areas dealing with sensitive semiconductors. It has a small motor
spinning a hollow brass cylinder that has a radial hole or slot that
alternately shield
Here's a simplistic view that may be sufficient. Some energy (in the
form of charge redistribution, which includes current flow) has to come
from the capacitor, and some from the input signal, to do the work
needed to push the leaf against gravity. When the input signal is
removed, some of the
Curiosity forced me to look around a little more for info on the 640.
The first link includes some info on the 640 with pictures. The
vibrating capacitor thing looks like a sort of vacuum tube. I thought
maybe it was custom made by or for Keithley, but it seems to actually
have been an off-shel
I looked at that link that Brooke put up about Bohnenberger's
Electroscope. I don't know what your specific arrangement needs to be,
but it appears you need a plus and a minus HV wrt ground in the most
general form. If so, then this would mean having to split the voltage of
a single cap, or hav
Another thing I noticed in these instruments - the highest R value used
is E12, even though decades higher would have been appropriate in
certain ranges. It shows that was about the practical limit for somewhat
decent precision and cost. Filling in the desired higher ranges had to
be done by ad
Oops - forgot to mention a detail about microwave oven caps. Sometimes
they have built-in bleeder resistors, which would of course spoil this
kind of application. Ed
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Yup - Keithley 640 - that must be the one. This stirred my memory
somewhat, and I just located info on the model 642 also, which was
apparently newer. The 642 went to (or back to) ultra-low bias MOSFETs,
while keeping sapphire insulation and a separate input head. The MOSFETs
need all kinds of
For static bias, look up "electret" for ideas on some other possible
options.
I would recommend against your option 2 capacitor - that's a dangerous
amount of energy to store in something that may be fooled around with
experimentally. Also, even though it's a lot of C, being electrolytic,
the
Oops - I think I didn't send this message properly yesterday - here goes
again. Ed
Yes, David, unless you go to very extreme measures, you won't see real R
values that have any practical meaning beyond E12 ohms or so. Most
practical insulation Rs may be around E12-E14 tops, unless you go to
s
Coincidental to all this micro-ohms and nanovolt talk, I've been doing
some severe large scale garage cleaning to thin stuff out. I found that
audio amplifier that I mentioned earlier, that is good for some LIA
reference driver applications. I also found my low-level measurement
notebooks, incl
Hmm. Alternating the direction of the current repeatedly and processing
the results - sure seems like that is fundamentally an AC measurement
too, despite using DC measurement equipment.
Ed
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I just noticed this discussion recently, so I'm late to the party, but
that never stops me from adding my one-cent's worth.
David, regardless of the aluminum and other material issues, I think
your initial idea of using a lock-in analyzer is definitely the way to
go. I'm very fond of LIAs, alt
I would recommend against trying to use cadmium - it's very toxic, which
is why Cd-based solders are rare nowadays. They are probably still made,
but for lab or industrial use with proper handling. If you try to alloy
it with Sn yourself without proper handling, you could get poisoned. You
can'
Sorry about the previous double-posting - had email problems.
There is another option that occurred to me last night, to get an
equivalent inductor from an accurate reference capacitor by using an
active circuit gyrator. The problem of course is that the circuit will
add errors too, diminishin
Haha, so it is legit - just a poorly described knock-off of the H-P
unit. I had never heard of this unit, but it looks like good info to
have, to replicate some equivalent reference inductors. Thanks for
finding this document.
Ed
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Haha, so it is legit - just a poorly decribed knock-off of the H-P unit.
I had never heard of this unit, but it looks like good info to have, to
replicate some equivalent reference inductors. Thanks for finding this
document.
Ed
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The method described in the document would not make an actual inductor -
it apparently fools the 4-port machine by providing the equivalent
signals that the inductance would have. You can avoid getting precision
inductors, and just go with this method (which I think is pretty clever
if it actua
Yes, the input DC bias current can be compensated for, as long as it
doesn't change too quickly, or isn't too big to make the input amplifier
circuits run out of dynamic range. In a digital meter environment, all
sorts of auto-zeroing and multi-sloping things and digital signal
processing can b
With modern digital readout meters, this can be very misleading in terms
of actual useable capability. In this case, using the specified highest
sensitivity 100 pA range, and six digits of digital resolution, gives
E-10 A/E6 or E-16 A, which is 100 aA for the last digit. But, looking at
the noi
You should include the effects of input bias current - the maximum
should be specified, and likely in the pA range at room temperature.
Just put a very high resistance from input to common, and read the
voltage to calculate the current at zero input. Likewise, you can
connect the resistor to va
Forgot about the chopper assembly - washing would probably not be good for it.
Ed
Yes, you are being a little too OCD about this. Instrument washing
issues come up often, and there are plenty of opinions available -
here are some of mine:
In the 419s that I have, the battery leakage crud ha
Yes, you are being a little too OCD about this. Instrument washing
issues come up often, and there are plenty of opinions available -
here are some of mine:
In the 419s that I have, the battery leakage crud has not gone beyond
the circuit boards or maybe the edge connectors. Cleaning the board
I agree these caps are the most failure-prone parts in the 3456A, and
also the 3455A. Sometimes they also cause damage to other parts like
the associated three-terminal regulators. Don't be surprised if
there's still more to fix after cap replacement - but usually easy to find.
I had one where
If you're talking about washing the board only, I'd say go for it.
Just run the hottest tap water on it, and a little liquid dishwashing
detergent, and scrub it thoroughly with an old toothbrush. Then lots
of rinsing and drying - compressed air can knock out a lot water from
parts that may trap
I agree with John on this one - put in another meter socket
downstream if you really want to experiment, and be sure to have a
main disconnect besides the utility's meter.
If you are changing the service entrance, you will likely have issues
with the local permits for it, and with the utility
Only specialized meters can provide virtually infinite input R at
voltages above the 10 to 20 V or so native range of conventional
amplifiers, so you have to use some kind of attenuator to cover the
higher ranges anyway. 10 megs and 1 meg (and sometimes 11) are the
traditional values used, with
Only specialized meters can provide virtually infinite input R at
voltages above the 10 to 20 V or so native range of conventional
amplifiers, so you have to use some kind of attenuator to cover the
higher ranges anyway. 10 megs and 1 meg (and sometimes 11) are the
traditional values used, with
I think you'll get better stability by using a resistive thermistor
as the sensor. As far as I know, all commercial oven circuits for
OCXOs and the like use RTDs, and not IC sensors, for best performance.
Ed
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If I understand what you're trying to do, I think you may be able to
run the experiment backwards to more easily figure it out. I'd
recommend focusing on junction to case characteristics first - the
case to ambient will have a lot of variables.
With the case attached to an "infinite" heat sink
Sometimes the old stuff is still the best. At room temperature, a
mechanical chopper combined with a high-ratio step up transformer and
high impedance follower amplifier is unbeatable for low noise, low
impedance signal amplification. The solid state devices that replaced
mechanical choppers in
This is now the third try at sending a message to the group. Here's
what I wrote before:
I sent this on the 18th, but it didn't show up, so here goes another
try. I don't know if something's wrong with my email. Please excuse
if the redundant original shows up too.
Original message:
Yes, it
I believe you'll find that those are not special low-emf relays -
they are also known as "printact" relays, used in some HP and
Tektronix equipment of the era for signal switching. Since they latch
in either state, no coil power is needed to keep it, so the
temperature can stabilize at ambient,
I agree with JL - it's either picking up an available frequency, or
making its own. Oscillation can happen with transistors, especially any
common-base amplifiers or emitter followers that don't have enough
degeneration - but that is usually in the VHF range. For MHz-type
oscillations, look for
Andreas,
I think your expectations are not realistic - even if you could make
such a reference, you could not transport its voltage to the ADC without
thermoelectric effects causing error that would swamp the performance.
To keep everything below the 1 ppm/deg C range you would have to put the
Don't bother with TC zeners, especially for battery operated equipment.
There are lots of nice IC references available from Analog Devices,
Maxim, Linear Technology, TI, an so on, that will run rings around TCZs
- you probably haven't seen the right one yet. The TCZs will only
provide near zero
Yes, very impressive - Solartron should hire you as a consultant to
design or improve their current products.
Ed
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That seems like a nice piece of equipment if it works or is easily
repairable. Why do you assume the displays are broken? If they used
gas discharge displays, they would likely be Nixie or Panaplex type,
which are pretty durable, and are still common enough to not be a
problem with procurement.
I would first look at the electromechanical aspects. Are there any
relays or connectors in the signal path? Next would be any device
sockets - they can be exercised by rocking the parts a little. Next
would be to inspect carefully for bad solder joints, corrosion, and
signs of physical damage.
I just junked out a very beat up old Fluke 803 differential
voltmeter, and found deep within, an old-school Cd/Hg standard cell.
It was well protected in an aluminum box, and wrapped in foam and
foil. It looks brand-new, and still measures around 1.018... V. I'd
like to keep this one as another
If you do modify the circuit, you will probably want to decrease the
series R. Even though the external loads are likely to be in the
megohm range, the feedback load is significant, and likely the
biggest part, except for fault conditions. For lowest noise and
suppression of effects due to bias
I'm not sure how much elaboration is needed, but here's some:
If you take all of the feedback from the output terminal, that's
better for DC accuracy by eliminating the voltage drop of the series
resistor, while still providing some overload protection to the
opamp. But, it also decreases phas
The simplest way to drop the output impedance without adding much
circuitry is to just change the series R to 100 ohms or so - that
would still give pretty good isolation from capacitive loading.
If the R is dropped to zero, the DC performance will be best, but
you'll have to worry about the a
Current regulator diodes are typically JFETs with gate-source tied so
that their Idss is more or less constant over a wide voltage range.
Anything in the 1 mA to 50 mA range can be replicated by selecting
common JFETs for the right Idss, up to 40 V or so, or by selecting
one with slightly highe
I recently acquired for cheap an old HP 740A DC standard/voltmeter of
1960s vintage. It seems to be pretty clean inside and complete except
for missing a small bias cell. I'm trying to find a manual - found
the "B" version one so far at hparchive.
Before putting in much effort to fire it up, d
Yes, the effect can be estimated quite easily. Also, keep in mind
that the 0.13 nV is the RMS noise, so the peak to peak excursions can
be around six times that, or almost 1 nV p-p. If the FS is 1 mA, then
it's about 1 ppm - one count on a six digit DVM, or ten times more
with each additional d
That which is more fundamental to the problem is the unavoidable (at
room temperature) noise from the resistors. Even a "perfect" resistor
with zero tempco has noise, so if you use resistors to measure
current with a high-precision voltmeter, eventually you reach a
resolution where the noise be
Does that matter on the AC buffer in this case? Maybe that's the
normal 1/f noise of the circuit down near DC.
If it is problematic, then you may want to try reconditioning the
low level signal relay contacts. You can rig up a circuit with a 9V
battery and a 1 k or so ohm resistor, temporarily
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