Recent Posts

1

Metrology / Re: Measuring femto-level differntial capactiance changes

« Last post by Dr. Frank on Today at 08:01:07 am »

I'm as well an experimental physicist, so I suggest to find other applications for measurement of small displacements, than Gravitational Waves.

Anyhow, you'd need extremely stable capacitors, so I guess the design of such a capacitor cell would be the most crucial problem in your experiment.
It would as well be necessary to design a capacitance bridge configuration.. very delicate.. somehow reinventing the wheel.


In the 1990ties, at Aachen University (RWTH), a colleague of mine made measurements on 'Magnetostriction and Thermo-Expansion on High Temperature Superconductors', for his PhD thesis, using an absolute (direct) capacitive method to detect very small length changes. I don't remember the resolution any more, might have been on the order of nm.

We have used a GENRAD 1620 Bridge first, which allowed to measure 10^-5 pF or 10aF changes.

Later, we bought an automated and more stable electronic bridge, an Andeen-Hagerling AH2500, which allowed 0.5aF resolution, with comfortable GPIB readout. This brand is still available, I guess model 2700 is the most recent one. I propose to use one of these, used. Maybe it's possible to measure capacitor differences as well.

Frank.

http://www.andeen-hagerling.com/ah2500a.htm

2

Metrology / Re: Keysight B2980A series electrometer/picoammeter review/teardown

« Last post by Kleinstein on Today at 07:58:19 am »

With the leakage specs there is often a large difference between the actual leakage and the specs. The specs are kind of limits on how far the parts are checked. Even the typical specs are no really typical, but more a limit for batch checks and including plenty of margin to avoid recalls / rejects.

3

Metrology / Re: Measuring femto-level differntial capactiance changes

« Last post by charliehorse55 on Today at 07:54:03 am »

The first step would be to estimate a capacitance change when your capacitor's based system will be hit by a typical wave the LIGO/VIRGO/etc detect today.

It's somewhere around 10^-20 to 10^-24. LIGO measures somewhere around 10^-22

4

Metrology / Re: Millimeter range water level sensor

« Last post by jbb on Today at 07:52:02 am »

Is the tube permanently fixed, infrequently swapped or (for example) a disposable sample tube?

What came to my mind is capacitive sensing with sense electrodes on the outside of the tube. What’s the material and wall thickness expected to be?

Oh yes… if sample tube is transparent then maybe an optical method is possible…

5

Repair / Re: Need advice on replacing JFET for mic

« Last post by Greybeard on Today at 07:49:32 am »

Why don't you just replace your mic by a new electret mic for 1 buck?
https://www.mouser.de/c/?q=electret%20microphone

If you're interested in electret mic basics:
https://en.wikipedia.org/wiki/Electret_microphone
https://www.hobby-hour.com/electronics/computer_microphone.php

6

Metrology / Re: Measuring femto-level differntial capactiance changes

« Last post by charliehorse55 on Today at 07:47:23 am »

For best sensitivity discrete transistors (JFETs or maybe MOSFETs) have an advantage. The LT1128 and other BJT based parts are not really suitable because of there current noise, that is an issue, unless one operates at rather high frequencies.

I thought current noise won't be a problem because the source impedance is so low? A 3000F supercapacitor probably has a source impedance in the tens of milliohms, so the <1nA current noise becomes some pV of voltage noise, which would be orders of magnitude less than the 10-100nV voltage noise?

7

Mechanical & Automation Engineering / Re: gold plated brass or bronze rods ? (or silver)

« Last post by Overspeed on Today at 07:46:27 am »

compared the gold filled wire to nugold wire (red brass alloy).

To the eye it looks similar, but the actual gold wire is a better reflector. The price is much lower. The conductivity is fine for nugold.

However, I feel that the gold filled wire inserts better into the breadboard

Hello

Nugold is not corrosion proof at all and sensible to human sweat and finger contact as Nugold is a copper alloy , shiny is just 

Nugold even if that cheaper than 99.99 Gold metal , is costly see attachement .... 30 USD per inch for 16 Gauge wire ...... compare to 3 USD average cost for surplus aerospace quality copper / silver coated wire .....

In this thread you write that you are '' stingy '' as you reject to use the right abrassive or HSS tools and want to use diamond tools for every thing because ...... but you are ready to pay 1200 USD ( 40 inch ) of nugold rather than 3 or 5 USD for regular available copper wire silver coated AND for breadboard use ....

I will purchase more pop corn

Regards
OS

8

Beginners / Re: Required safety margin for Vds of mosfets (do I need to overprovision?)

« Last post by Siwastaja on Today at 07:42:56 am »

Slow switching by adding series gate resistor is obvious when fast switching speed does not matter. If you want to go extremely slow, it might be a good idea to add extra C in parallel instead of going to tens or hundreds of kOhm.

9

Repair / Multimeter FluKe 233 Remote Display Off

« Last post by papi on Today at 07:37:43 am »

Hi all,

I have 2 Fluke 233 in one the remote display does not turn on both when it is on the base and when it should communicate in RF ( 2.4Ghz) detached from the base. The two testers are dated and are from two different PCB versions but having one working I tried to make a comparison between the TEST points to simplify the fault analysis.

I suppose I have identified that the problem lies on the remote display since by exchanging between the two bases the working display shows ID error, so the base attempts communication (It turn on the red HV LED) but finds a different code in the MP430 micro.

I am sending test point verification scheme with comparison with the working model which has two additional Test Points, the TP217 and the TP216, the only differences are with the TP215 which on the non-working display is 0V instead of 3.3V and the IR diode DS202 which is in conduction instead of interdiction due to the conduction of the BJT PNP Q203 which is driven by pin 75 of the MP430 micro at 0V level instead of being at 3.3Vdc.

I am sending the picture of the remote Display PCB.

At the moment the display doesn't work and since I don't have any diagrams I don't know where to take action.

Do you have any suggestions.

Thank you very much for your help.

Andrea Papi

10

Beginners / Re: Required safety margin for Vds of mosfets (do I need to overprovision?)

« Last post by shapirus on Today at 07:30:39 am »

Run the circuit at highest input voltage, measure Vds with oscillosscope.

I actually did. No overshoots whatsoever, in my particular case. Even though it was wired on a solderless breadboard. That's because current isn't high and the transitions are relatively slow. That's why a tighter margin is probably ok in this case.

But, good info on general considerations.