LC Tester Questions... - Page 1
Forum section: electronics / beginners

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 16, 2016, 08:36:59 am
content: 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_sMbut 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-

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 16, 2016, 08:45:15 am
content: like if there is a cap to ground which cause this square wave not to have fast edges...

author: Alex Trofimov
author_rank: Contributor
timestamp: March 16, 2016, 09:52:30 am
content: 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)?

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 16, 2016, 03:22:26 pm
content: sorry for that...but here it is:

author: danadak
author_rank: Super Contributor
timestamp: March 16, 2016, 10:05:34 pm
content: This might be of interest -http://icom.hsr.ch/fileadmin/user_upload/icom.hsr.ch/publikationen/RLC_Meter_EN.pdfhttp://www.sillanumsoft.org/ZRLC.htmRegards, Dana.

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 17, 2016, 07:12:00 am
content: 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.

author: danadak
author_rank: Super Contributor
timestamp: March 17, 2016, 10:56:43 am
content: Maybe the software has to be run in compatibility mode, I willtry it under Win 7.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: March 17, 2016, 11:07:52 am
content: I installed and ran it as Win 7 64 bit, ran fine. Maybe of you run itin compatibility mode (right click exe file, change compatibility settings)it might work.I did not have anything connected to sound card input however. Eg. Icould not run Z meter and measure anything.....as I do not have theinterface HW built.Regards, Dana.

author: Alex Trofimov
author_rank: Contributor
timestamp: March 17, 2016, 11:29:28 am
content: 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.

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 17, 2016, 05:18:54 pm
content: 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:

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 18, 2016, 10:29:28 am
content: 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-

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 18, 2016, 10:32:18 am
content: photos:

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 18, 2016, 10:36:33 am
content: last two:

author: Alex Trofimov
author_rank: Contributor
timestamp: March 18, 2016, 10:44:02 am
content: 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 amCould you please specify on which exact point in the circuit you observe this?

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 18, 2016, 10:57:20 am
content: after the 1 meg resistor

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 18, 2016, 04:07:12 pm
content: I'm not so sure dose that it's as low as that, but still a good assumption to followI will do ferther testing and I will post the results...

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 20, 2016, 09:30:41 am
content: 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.

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 20, 2016, 09:52:00 am
content: i also tryed redoing it but on a breadboard and i got this result:

author: Alex Trofimov
author_rank: Contributor
timestamp: March 21, 2016, 10:58:50 am
content: 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?

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 21, 2016, 02:30:12 pm
content: Sorry I forgot to mention that I have used a capacitor in this trial and its value was 0.1nf.I'm really sorry

author: Alex Trofimov
author_rank: Contributor
timestamp: March 22, 2016, 10:56:38 am
content: 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..)