Author Topic: LC Tester Questions... (Read 4344 times)

ali6x944 · « **on:** March 16, 2016, 08:36:59 am »

Hi everybody,
i was experimenting with Alan Wolke's circuit to measure the inductance or the capacitance of unknown inductors & caps, in this video:
"#90: Measure Capacitors and Inductors with an Oscilloscope and some basic parts"-https://www.youtube.com/watch?v=74fz9iwZ_sM
but i didn't have 74AC14 Schmitt Trigger Inverter he used to build the oscillator, so i modified the circuit a little bit to work for me.
i use a basic LM339 comparetor to do this:
the circuit works fine but the second part is not working properly like it use to in the bread broad so I'm wondering what i did wrong?
to be more exact -the raise time is higher by 200ns and the fall time is longer by 195.65 ns too-

ali6x944 · « **Reply #1 on:** March 16, 2016, 08:45:15 am »

like if there is a cap to ground which cause this square wave not to have fast edges...

Alex Trofimov · « **Reply #2 on:** March 16, 2016, 09:52:30 am »

I tried to open your schematics with Altium Designer and it showed me just a blank document. Could you please include it as a picture (screenshot would work)?

ali6x944 · « **Reply #3 on:** March 16, 2016, 03:22:26 pm »

sorry for that...
but here it is:

danadak · « **Reply #4 on:** March 16, 2016, 10:05:34 pm »

This might be of interest -

http://icom.hsr.ch/fileadmin/user_upload/icom.hsr.ch/publikationen/RLC_Meter_EN.pdf

http://www.sillanumsoft.org/ZRLC.htm

Regards, Dana.

ali6x944 · « **Reply #5 on:** March 17, 2016, 07:12:00 am »

they are quit interesting to read but i can't do it because i don't have the right op amp for the job, but anyway I tried it out and got nothing, it didn't work, at lest the hardware seems to work nicely from the second schematic in first RLC artical u posted, the soft ware from the website didn't work, i ran it from windows 8.

danadak · « **Reply #6 on:** March 17, 2016, 10:56:43 am »

Maybe the software has to be run in compatibility mode, I will
try it under Win 7.

Regards, Dana.

danadak · « **Reply #7 on:** March 17, 2016, 11:07:52 am »

I installed and ran it as Win 7 64 bit, ran fine. Maybe of you run it
in compatibility mode (right click exe file, change compatibility settings)
it might work.

I did not have anything connected to sound card input however. Eg. I
could not run Z meter and measure anything.....as I do not have the
interface HW built.

Regards, Dana.

Alex Trofimov · « **Reply #8 on:** March 17, 2016, 11:29:28 am »

According to datasheets, 74AC14 can switch it's output in 3ns, while LM339 needs 300ns. That seems like the case, if I understood correctly.
I don't think, it's a breadboard stray capacitance interference, I've made some ballpark calculations and its not enough to cause this.

ali6x944 · « **Reply #9 on:** March 17, 2016, 05:18:54 pm »

as u said exactly, but how it used to work in the breadboard that's the question...
anyway see some photos of the output of the inverting schmitt trigger and i got this:

ali6x944 · « **Reply #10 on:** March 18, 2016, 10:29:28 am »

i checked the connection on the prefboard and redo some connections and i think i have fixed the inductance measure but still I'm not sure if the capacitance, and the capacitive effect to ground appear after the 1 meg resistor with no cap attached.
photos in order:
1.rise time after the 1 meg resistor -no cap attached-
2.wave form after the 1 meg resistor -no cap attached-
3.wave form before and after the 1 meg resistor (blue:before, yellow:after) -1nf cap attached-
4.wave form before and after the 1 meg resistor (blue:before, yellow:after) -0.1uf cap attached-
5.wave form before and after the 10pf cap (blue:before, yellow:after) -0.2mH inducter attached-
5.close up on wave form before and after the 10pf cap (blue:before, yellow:after) -0.2mH inducter attached-

ali6x944 · « **Reply #11 on:** March 18, 2016, 10:32:18 am »

photos:

ali6x944 · « **Reply #12 on:** March 18, 2016, 10:36:33 am »

last two:

Alex Trofimov · « **Reply #13 on:** March 18, 2016, 10:44:02 am »

The waveform after 1M resistor looks reasonable for me. The contacts can have some capacitance, say 10pF. t = R * C = 1MOhm * 10pF = 10uS. Bingo! Just like on the screen) Actually the scope itself have som input capacitance.

Quote from: ali6x944 on March 16, 2016, 08:36:59 am

the circuit works fine but the second part is not working properly like it use to in the bread broad so I'm wondering what i did wrong?
to be more exact -the raise time is higher by 200ns and the fall time is longer by 195.65 ns too-

Could you please specify on which exact point in the circuit you observe this?

ali6x944 · « **Reply #14 on:** March 18, 2016, 10:57:20 am »

after the 1 meg resistor

ali6x944 · « **Reply #15 on:** March 18, 2016, 04:07:12 pm »

I'm not so sure dose that it's as low as that, but still a good assumption to follow

I will do ferther testing and I will post the results...

ali6x944 · « **Reply #16 on:** March 20, 2016, 09:30:41 am »

these are wave forms befor and after the 1 meg resistor (blue:befor, yellow:after):
note: some meaurements are a screen measure so it is way of.

ali6x944 · « **Reply #17 on:** March 20, 2016, 09:52:00 am »

i also tryed redoing it but on a breadboard and i got this result:

Alex Trofimov · « **Reply #18 on:** March 21, 2016, 10:58:50 am »

Looks like the rising time is longer due to additional capacitance on a breadboard. Looks as it should be to me.
Maybe if you need shorter rise/fall time, you should use a lower value resistor? Say 1k instead of 1M?

ali6x944 · « **Reply #19 on:** March 21, 2016, 02:30:12 pm »

Sorry I forgot to mention that I have used a capacitor in this trial and its value was 0.1nf.
I'm really sorry

Alex Trofimov · « **Reply #20 on:** March 22, 2016, 10:56:38 am »

No worries, but I'm really perplexed now. No more ideas...
All I can say, is there's no black magic, there's capacitance, inductance, Ohm's law and all that sort of stuff everywhere. There is an explanation, somewhere..)


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Author Topic: LC Tester Questions... (Read 4344 times)

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