Meeasure capacitance wiht oscilloscope - Page 1
Forum section: electronics / beginners

author: Adrian_Arg.
author_rank: Frequent Contributor
timestamp: February 11, 2020, 05:11:44 pm
content: Hello friends, I have a problem, I have watched some videos of how to measure the capacity, I follow the steps of the videos, function generator, square wave frequency 1Khz, amplitude 5v and in other cases amplitude 1v, components a resistor of 1000 ohm, and The capacitor to measure. I perform the steps you show, I don't get the results, I check the values with other meters and these are correct. what am I doing wrong.in spanishHola amigos, tengo un problema, he mirado unos videos de como medir la capacidad, sigo los pasos de los videos, generador de funciones, onda cuadrada frecuencia 1Khz, amplitud 5v y en otros casos amplitud 1v, componentes un resistor de 1000 ohm, y el capacitor a medir. realizo los pasos que muestras, no consigo los resultados, compruebo los valores con otros medidores y estos estan correctos. que estoy haciendo mal.capacitor 222 K (2.0nf)capacitor 104

author: StillTrying
author_rank: Super Contributor
timestamp: February 11, 2020, 05:39:34 pm
content: I make the first one 22nF, and the second 470nF, if they're out are you sure the 1k is 1k.

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 11, 2020, 06:47:52 pm
content: Quote from: StillTrying on February 11, 2020, 05:39:34 pmI also calculated 22nF on the first one.

author: Adrian_Arg.
author_rank: Frequent Contributor
timestamp: February 11, 2020, 08:44:58 pm
content: Thank you for answering, this is the case with many, some good and others are not even close, the controls with two meters and are almost in measure. I'm missing somethingin spanishgracias por contestar, asi me pasa con muchos, algunos bien y otros ni se aproximan, los controlos con dos medidores y estan casi en la medida. de algo me estoy perdiendo

author: StillTrying
author_rank: Super Contributor
timestamp: February 11, 2020, 10:15:38 pm
content: Don't bother with the triangular waveform for now, just use the 0% to 63%I use the fine Y control to make the waveform exactly 6 Divs high, one of the Divs is then = 66% and 1/5 div below that Div is = 63%.Then Cap value = time(0% to 63%) / resistance should give a quite close cap. value result.It has to be the 1k being 10k, are you reading a 4 colour band resistor as a 3 band.No te molestes con la forma de onda triangular por ahora, solo usa el 0% al 63%Uso el fino control Y para hacer que la forma de onda tenga exactamente 6 Divs de alto, uno de los Divs es entonces = 66% y 1/5 div por debajo de ese Div es = 63%.Entonces el valor de Cap = tiempo (0% a 63%) / resistencia debería dar un límite bastante cercano. resultado del valorTiene que ser la 1k siendo 10k, Estás leyendo una resistencia de banda de 4 colores como una banda de 3?

author: David Hess
author_rank: Super Contributor
timestamp: February 12, 2020, 11:30:00 pm
content: My preferred method is to use a sine wave source and two channels of the oscilloscope to find the frequency where the phase lag is 45 degrees.

author: Adrian_Arg.
author_rank: Frequent Contributor
timestamp: February 13, 2020, 02:01:14 am
content: Quote from: David Hess on February 12, 2020, 11:30:00 pmand how do you do that?

author: Caliaxy
author_rank: Frequent Contributor
timestamp: February 13, 2020, 03:02:45 am
content: Your second screen capture shows a capacitor that doesn’t discharge completely between pulses and doesn’t charge 100% by the end of the pulse either. If you don’t know when the capacitor is 0% charged and when it’s 100% charged, how do you know when it reaches 63%? Try reducing the frequency until you see the capacitor charging 100% and discharging completely between pulses, then measure the time constant and calculate again.As far as the first screen capture is concerned, as previously suggested, check again (measure) the resistor value - it might be 10k.

author: David Hess
author_rank: Super Contributor
timestamp: February 13, 2020, 03:14:22 am
content: Quote from: Adrian_Arg. on February 13, 2020, 02:01:14 amSet the capacitor up in the same RC circuit but driven with a sine wave. Attach the probes before and after the resistor.Now you can measure the point where the output voltage is reduced to 0.707 of the input to find the -3dB bandwidth as the frequency is increased but it is more accurate to measure the point where the phase lags by 45 degrees and most DSOs can make this measurement automatically. Once you know that, you can calculate the capacitors value from the resistance and frequency.The time domain method using the exponential waveform works but measurements of time are generally easier and more accurate than measurements of voltage.

author: Adrian_Arg.
author_rank: Frequent Contributor
timestamp: February 13, 2020, 02:49:00 pm
content: ;)ok, I'll try to see if I find a video, thanks

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 13, 2020, 03:55:19 pm
content: Quote from: David Hess on February 13, 2020, 03:14:22 amFor kicks, I played around with both methods this morning. My Siglent does XY but I have not found a way for it to do the phase calculation for me. I have to adjust until the ratio of two vertical readings are 0.707. You adjust freq, measure, calculate...adjust some more. Tedious.The time-domain method is a little less tedious because you are just measuring.

author: Adrian_Arg.
author_rank: Frequent Contributor
timestamp: February 13, 2020, 05:02:34 pm
content: ok, I could not find any video or frequency domain, or the phase

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 13, 2020, 05:16:26 pm
content: Quote from: Adrian_Arg. on February 13, 2020, 05:02:34 pmMake a simple RC circuit with the input into the resistor and the capacitor to ground.With channel 1 of the scope, connect to the junction of the R and CInput a sine wave to the dangling end of the resistor.Input the same sine wave to channel 2 of the scope.Set the scope in XY modeAdjust the input frequency until you get a waveform as shown in the attached figure.For a 45° phase shift. sin(45) = 0.707Adjust the input further until C/D = 0.707Multiply that frequency by2π, multiply by the resistor value and then invert to get C--That's how I did it anyway...never used this method before today--

author: Mortymore
author_rank: Frequent Contributor
timestamp: February 13, 2020, 11:30:35 pm
content: Decided to play with the 45º phase shift option, using C=820nF and R=100 OhmScope CH2 only at R, and CH1 R+C (or measuring the generator output if you wish to see it that way)The generator set for sinusoidal 4Vpp and frequency was set until the scope shows the desired 45º shift between the 2 scope inputsAn in XY modeC= 1/(2*Pi*f*R) = 1 / (2*Pi*1900*100) = 837.66*10^-9 or837nF

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 14, 2020, 12:20:22 am
content: Quote from: Mortymore on February 13, 2020, 11:30:35 pmCool! What scope is that?

author: Mortymore
author_rank: Frequent Contributor
timestamp: February 14, 2020, 09:17:33 am
content: Quote from: Wimberleytech on February 14, 2020, 12:20:22 amIso-Tech IDS-2074E (hacked), same as GDS-2074E from GW_Instek.

author: Adrian_Arg.
author_rank: Frequent Contributor
timestamp: February 16, 2020, 04:26:53 pm
content: thanks for your explanation Mortymore, the method works very well, only in some capacitors 152j, 582, and others, all between 250v and 450v the method fails, I will see that I do wrong.in spanishgracias por tu esplicacion Mortymore, funciona muy bien el metodo, solo que en los unos capacitores 152j, 582, y otros, todos de entre 250v y 450v falla el metodo, ya vere que hago mal