Find out value of pF capacitors

Other Hardware General Electronics
1

hi there

I'm looking for a way to calculate value of capacitors of pF(pico Farad) range. the thing is when I use a RLC meter, looks like the device is limited to high ranges of pF to nF and so I can not measure value of for example 1pF...
I used this technique

Circuit

using a microcontroller, I generate 8MHz pulse and pass it through first: a Lowpass filter and then: a peak detector
C1 is the value I'm trying to measure and it is around 1pF to 20pF(I do not knoe the right value, using this circuit I'm trying to measure it).
I've wired the circuit on a breadboard
then I use the equation Vo=Vs(1-exp(-t/tau))
Vs=3.3(STM32F103)
t=(here)62.510^-9
tau=R1C1 and R1=4100ohm

I use a multimeter at the end of circuit where you can see "equivalent voltage"(I use a multimeter because it seems that it has no effect on my circuit for changing values due to very large input resistance ect.)

there are some issues:
1-it seems like the breadboard itself has some pF capacitor adding to my measurements(even for example if I just insert a piece of wire in a hole of breadboard)
2-it seems like the frequency also effects the value of output voltage and so the calculated value of C1.

can anyone help me with suggestions?

2

Do you know how little 1 pF is ?
This is old but still useful: https://www.gammon.com.au/forum/?id=12075

I think that measuring 1pF is not possible with an Arduino board.

1 Like
3

You really need to read this:
https://wiki.epfl.ch/carplat/documents/rcl_agilent.pdf

4

Hi, @ALAA72

At that low a capacitance, even the test leads and tracks/wires in the tester can add to the value, not just in fractions of pF, but 10's of pF.

Have you Googled;

how to measure picofards

Tom... :smiley: :+1: :coffee: :australia:

2 Likes
5

IIRC soleless breadboard rows have 5 - 10 pF capacitance between them. Skipping a few rows may help but I don't think it is needed. Just measure with and without the cap in circuit. But all other things nearby must be the same. Your hand nearby may do a great difference (this is how capacitive touch sensors/screens work).

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6

Even just the 1N4148 adds 4pF.

1 Like
7

yeah I was thinking the same, but what do you think about this article
How to make a capacitance meter using Arduino

8

right, those are the things that bother my measurements. yeah I've googled
the article I've posted for @Koepel sounds good. I haven't tried it yet.what do you think about it?

9

for real? what else? and how do you say that?
this is really a good note! what should I do to eliminate these factors?

10

It's interesting. I have a DMM that measures capacitance, and also an LCR meter to make finer capacitance measurements, so I don't need anything like this.

Note:
" Range 0.1 μF to 3900 μF"

0.1 uF is 100,000pF

11

I know, I try to go away from the circuit. do you think I should wait for steady states as well?I mean the effect od temperature, etc...

12

What else besides one of the most comprehensive, accurate and readable overviews of impedance measurement that you will probably find on the internet? You complain that it's not enough? Strange... unless I misunderstood...

You seem very fixated on this goal, and seem to be ignoring clear indications of problems by reflecting them back at us to solve. This problem has been around for longer than any of us has been alive. You are not going to make any breakthoughs today. It's also why we don't have easy, bite sized solutions for you.

13

I checked the article you mentioned. I'm using the same method except that 8MHz is too high for oscilloscope to capture, besides every oscilloscope adds ~20pF to measurements which degrades the situation even more

14

No you aren't ... that is just an incredible statement. The common method uses vector analysis of sine waves.

15

yeah, but LCR meters are also limited to ~100pF or nF which can not help...
note that in the article I mentioned you can see the setup for 1pF-1nF

16

You seem to believe that the right words will get you to your destination. They won't. You need to talk to the hardware using its language.

The article is very naive if it claims to measure 1pF with that apparatus.

1 Like
17

right :sweat_smile:
still trying to get to the hardware language, I'll keep trying. and your help will guide me through for sure :pray:

18

We can't guide you through. I would say, guide you away. This can't really work. Not well.

19

AA72]
Why are you trying to do this?

  • 99% of the time capacitor values are not important and a higher value will usually work.

  • They are not worth salvaging.

  • If you're trying to reverse-engineer something there may be other ways of determining the correct (or minimum) value.

1 Like
20

And do not forget that any wire is an inductor as well...