How to measure a capacitor with an oscilloscope.
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- Опубликовано: 28 сен 2024
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I go through a practical exercise myself to measure the value of a capacitor with an oscilloscope. This is to prepare for a future tutorial and reviews of LCR meters.
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Great video.
Like many people have said, when you are measuring such a tiny tiny value capacitor, then there are a ton of other stray capacitances that confound the measurement - the probe, the wiring, the breadboard, they all have values in the pF range so you are actually trying to do something pretty difficult. These stray values don't affect the measurement of the bigger capacitors much because the stray values are so much smaller in comparison.
When measuring something small like this, you might want to connect a bunch of them in parallel (the capacitances will add) and then divide by the number of capacitors you used to get the answer for a single one of them.
Andre Sant'Anna Thanks very much for your input Andre.
Even the capacitance of your bum on the chair....
@@muppetpaster The capacitance of the bum on the chair is inversley perportional to how interesting the video is and like a capacitor it will short to ground as soon as the interest peeks and it will need to dump its load..
Great explanation and teaching skills. I'm a 67 year old newbie ham radio enthusiast from the UK. I lived in SA for around 27 years. Keep up the good work. Totsiens and 73s.
10 years on, you remain one of the best, if not the best teacher in the subject matter. Great presentation, insightful instructions and knowledge sharing. Thank you so much for your tutorials. I am sure many amateur diy enthusiasts like myself have benefitted immensely because of your generosity and great work.
Apart from your cool spirit, one thing I love about you is your humility. knowing and accepting that you can also learn from your students is a great attribute of a good teacher. Thank you so much.
Man, this was a great lesson.
I’m glad I saw the difficulty you had because it mirrors my experience.
Big help.
Well ... here we are in 2024 and it's still one of the best tutorials, simply because it is utterly honest. We all struggle at times and this was so typical of life amongst we technicians when striving to prove our stuff and ourselves. Today, I have a little gadget that ws purchased around 2010, and it will measure capacitance to within a fraction of a puff quite accurately (it says), in about 15 seconds. However, theoretically, just walking into a room changes things. Thank you for a splendid video ... greatly appreciated.
EXPLANATION OF THE pF ERROR
When measuring a very small capacitance value (in the pF range) your measurement is influenced by the parasitic capacitance of the oscilloscope (~10-30pF), of the oscilloscope probe (~10-20pF) and of the test stand (~xxpF).
You should measure the parasitic capacitance of the system first (without holding the wires with your hands), then connect your capacitor, measure the new value and subtract the parasitic value from the compound value.
I presume you had a 55.2pF parasitic capacitance in the measurement system which added to the 4.7pF you were trying to measure (59.9pF=4.7pF+55.2pF).
In the case of larger capacitors (nF range or larger) the effect of the parasitic capacitance is irrelevant: 220nF+55.2pF=220.0552nF so basically no difference.
Also, capacitance does NOT change with frequency although impedance DOES!
No electronic component is purely resistive, capacitive or inductive. This however is another discussion! :)
I think that is why he starts with explaining that he is saving up for a good and precise LCR-meter.....
And how would you measure the parasitic capacitance? Connect the function generator through a resistor directly to the scope?
"You should measure the parasitic capacitance of the system first (without holding the wires with your hands), then connect your capacitor, measure the new value and subtract the parasitic value from the compound value."
With what frequency should I measure ESR in circuit then?
@@thuglifescorpion What do you mean?
An impressive arsenal of tools on your workbench, and lots of know-how! I think a 5 minute version based on the [title] of the video would be helpful, but the long version does show you're qualified as a cyber-teacher. +Awesome
The acquire modes will be the next topic I cover in the oscilloscope tutorial series..
Thanks for the feedback.
Great job Martin! thank you so much for explaining this very important concept. I like the way you go, explain the concept first then go to real world example, better yet, step by step. It is nice too that you along the way make mistakes and comment on that later so we can learn from that too. Way to go! keep up with the good work!
Great explanation of how to do measurements on the oscilloscope plus doing a step by step on calculations.
Like it mate, Like you I make the same mistakes you do; unedited video is great. Thanks, I learned from it, at the same time as enjoying the the video's journey.
I enjoyed this and learnt from it. Im at a very basic level but like to stretch out into the hobby beyond building blindly from other peoples schematics. Im glad you kept all the content in and gave me the chance to see you process through the challenges. Thank you
Thank you for the slow explanation of the issue. I can understand everything. People often speak too fast and cannot explain things well.
Thank you very much, best regards.
An alternative measurement method is to use a sine wave and a two channel scope, Place the channel 2 across the cap, and the channel 1 across the resistor (R) and capacitor (C) in series. Adjust the frequency (F) until channel 2 is 0.707 of the input (channel 1 value), At this frequency the capacitive reactance is equal to the resistance of the resistor i.e. R=1/(2*pi*F*C). As we know R and F we can calculate C. A digital storage oscilloscope will display accurate peak voltages of the channels. The phase angle could also be measured as it is exactly 45degrees.
Thanks for your post Barry.
My pleasure, thanks for the feedback.
Instead of using 63.21% (t = rc), you can use 86.47% (t = 2rc), because slope of voltage on capacitor is smaller there, so it is easier to find out exact spot for it. After that you do the same, but devide result by two. c = t/(2r). Also it is use as wide time division as possible.
Other value to try is 75.00% (t = 1.3863 * rc), or 80.00% (t = 1.6095 * rc) which should be also easy to setup on a scope.
Another approach is to take data to computer, and fit exponential function (or linear after taking logarithm). This should be much more accurate and also give you estimation of error.
These are great suggestions. The exponential fit will give the best results, but obviously requires a bit more work.
Using the method in this video, you were relying on the capacitor being nearly saturated by the time it hit the end of the waveform. In particular, you took y2-y1 to be the completely charged voltage. In the process you effectively measured two voltages (bottom to top of the waveform, and bottom to 63%) and one time (beginning of waveform to 63%V).
There is an in-between option that would offer improvement without much more work. By adding one measurement, you can ignore the top voltage, and get better results. The big advantage is that you can use bigger resisters, since you no longer care about saturating the capacitor during a pulse, which makes for a slower rise time and better measurements. The procedure is as follows:
1) As before, pick a time that puts the voltage about halfway up the curve. This doesn't need to be precise. Forty percent or sixty percent up the curve is fine. Write down the time (measured from the beginning of the waveform) as t1.
2) Write down the corresponding voltage (measured from the bottom of the waveform) as V1.
3) Now choose t2 equal to twice t1. In other words, if t1 was 1.00 milliseconds, choose t2 to be 2.00 milliseconds. This does need to be precise in order to take advantage of an algebraic trick, and to avoid solving a nonlinear algebraic equation for the final answer. Just double t1, and measure V2 at t2.
4) The time constant, RC, is equal to (V1*t1)/(V2-V1). Note that t2 does not appear in the equation, because it is assumed to be double t1. The equation is derived from the assumption that the voltage history has the form V(t) = Vfull*(1-e^(t/RC)) and plugging in (t1,V1) to give a first equation a and (2*t1,V2) to give a second equation. Solving the pair of equations for RC gives the simple ratio above.
5) Fix my algebra and repeat step (4). I did the algebra by hand, which means I screwed it up.
One more note for the future: you did a nice job of eyeballing the measurement locations, except that you measured from the top of the noise band for the top voltage, and from the bottom of the noise band at the low voltage. In this procedure, the errors roughly cancelled each other out, but in most cases your results will be better if you measure from the middle of a noise band.
Thanks for the video! I'm coming back to electronics after many years, and it's great to see this kind of tinkering in a lab. Lots of fun.
Thanks a lot to you both for your comments, they helped me a lot! :-)
after watched ur video,i found my school lecture is a piece of shit. u have patient,and good attitude. really like the way u speak. keep it up
Dear MjL,
The probe capacitance must be observed when measuring capacitance < 100pF. Probe settings for 10:1 will give a higher impedance than 1:1 and thus lower capacitance.
I know this is an old video but I found it very educational. I really appreciate this video. Also I read what Andre said in his comment. That does make a lot of sense what he said. But still I did learn quite a lot of this video I'm going to experiment with that myself. thanks for the tutorial. :-)
Martin, Ignore the negative comments, 25,000 people learn something every video.
Nice video, but what about when using electolitic capacitors and the positive and negative?
Thanks for the input and feedback...appreciated!
Mr Lorton, I echo the compliments on this video. Would hasten to point out that those attempting to this on a scope without cursors are royally screwed!
But then again, the killer is determining the "e level" or the 62.3% point.
But great kudos for showing us this method and your "failures" as well.
Awsum video very understandable and educationall and im trying to learn basic electronics love the way you explain things keep up the good work
Thanks for the suggestion...I just bypassed the decade box and no change..I will see if I can test to see if the probe might be the issue nest.
I suggest The reason for the noise is because the test circuit has such a high total impedance and there is no shielding. I suggest that the value of the resistor and the impedance of the capacitor need to be approximately the same to get a readable slop. In this case the resistor is about one meg-ohm then the total circuit impedance will be more than 2 meg-ohms which was much higher than the clean waveform test circuit impedance. You will need shielding to get a noise free waveform. You would need to make sure to decade resistor box was shielded as well. Or you could implement a bandpass filter on the scope to remove the line power frequency noise and the higher frequency noise. A much cleaner waveform could be derived if you raised the testing frequency. This would allow a lower resistor value and because the capacitor value would be lower in impedance the total circuit impedance would be lower. This would load down the noise that is induced more and thus clean up the waveform. But you would have then to be worried that the capacitance would not be linear. That is to say the capacitance may not be the same at 1 kHz as it is at 100 kHz. Linearity of the capacitance is sometimes encountered with old or defective capacitors.
Reducing the lead on the resister will also help. Nice video.
Thank you very much for that video and a very clear explanation. Especially at this difficult time to provide any Laboratories to the students, that video may bring a big addition to any physics course. Only I would add the next: to provide the circuit diagrams to each experiment. You showed the connections at the knots, but the entier diagram would help to understand the entire connection. Thank you very much again for your dedication.
For those without a function generator on hand, there are a couple of other options available. Most, if not all, scopes have a built in probe compensation square wave output with a known voltage and frequency. Another option for those with a storage scope is to use the single shot mode of scope triggering, and a toggle switch, or pushbutton to apply the charging current from a known voltage source. You'd want to be sure that the capacitor is fully discharged before hand.
Thanks for the feedback.
Thanks very much for the feedback.
Thanks for the video. Makes me want to purchase an oscilloscope even more. :)
Good question! Let me add that to the list.
I am a beginner with the oscilloscope ... and I am a teacher (both high school and adults at night school). Your delivery and explanations are outstanding. I rarely see an instructor deliver the sort of simple detail that a beginner needs....but you, my friend, did. Thank you and congrats.
The problem is that water is used as analogy to electricity instead of gas, because water is practically incompressible contrary to gas. A capacitor should bee seen as a gas cylinder instead of a bucket. This way you may do a better analogy, like this:
V = Gas pressure;
Q = Gas quantity;
C = Capacity of Gas storage for a given pressure, and so Q/V.
Like a gas cylinder, greater pressure (V) means that greater quantity (Q) may be stored, however, if you increase V (gas pressure) too much, it blows up, so, besides C a capacitor has also defined the maximum pressure V it can handle before it blows up...
This is why you should think in electricity as Gas flow instead of Water flow.
Thanks for the post...I checked it several times to make sure and it is 1M Ohm.
Hi Mjlorton. I have a question for you, cause I didn't found any information on the internet.
My question is: do you know to use a oscilloscope to capture a very short signal (with trigger I suppose), this kind of signal can come from any experiment device, appearing during one microsecond, and capture it only when it appears on the probe.
If you know, Can you explain how do to it in any video or something like that?
This video saved my lifeeeeeee! Thank you for such an in depth video
Hi Alexander, I will be posting a video on this exact topic in a day or two.
I have a TEK p6205 probe, it lists "750 MHz active FET probe, Input impedance 1 MΩ // 2 pF" on its specs, I believe your probes come with much higher capacitance than 2 pF. as you can see, they are of the same order of magnitude as your capacitor. actually for such a tiny value you are measuring (4.7 pF), a lot of other factors also need to be considered. so the large "tolerance" of your second measurement is expected.
For input/output on your calculator, using engineering mode makes it easier to convert back and forth between mega, micro,pico etc
I would say you can consider the point where its fully charged and then make an approximation of ' 5T=RC ' and work your way in finding C, this works fairly good given then 1 to 10% tolerances of normal resistors :)
Good Video!!
finally I have seen someone explaining the things how must be done!!!!
The resistance value on bottom of calculation written wrong.
Value was said to be 1 meg in series of res. box of + 1K ohm = 1,000,000 + 1,000,=
1,001,000 should give you 5.99 pf
Great video! I like the way you start off asking the questions of why use a resistor, and wonder about the entire reasoning behind this test and its results. Those are the questions that I run through my mind as well, and I need to know why things are as they are, not just mimic a result from what I watched.
I'm an enthusiast myself though I'm not nearly as advanced as you are, (and probably most of the people who watch this video), but I made a point to save your link to all your videos. I have yet to watch this entire video, (I have to go on an errand), but I'm looking forward to it.
BTW Have you ever been to Mt Kilimanjaro? I haven't but I'm 62 and I want to go there within the next 5 or 6 years, (if the planet lasts that long ;).
Thanks again,
Rich
Very nice job sir.
Great job, very well explained.
The basic concept is sound, but a few comments though:
- you swapped 63.2 and 62.3 percent at some point.
- you have to take the output impedance of the signal generator into account
- the error margin of the measurement cursors is way to big at the given timebase.
- 4.7pF will be swamped by the capacitance of the cables and passive probes. That's why this method with the given equipment doesn't work on small value capacitors.
Thanks very much for the input Thijs, noted.
Consider if you will, the capacitance of the probe, as well as the loading effect of test equipment used, which will of course, effect your results.
I have that same label-maker. It works well. Though I had to take apart a cassette in order to re-roll the black transfer tape.
Using this method for measuring very small values of C has a major potential problem: inductance in the resistor. Even metal & carbon film resistors have some inductance. The resistive film is actually in the form of a coil.
Excellent video, Univ Prof should learn how to teach these things from you.
Thanks for explaining - how to measure capacitor using scope.Are there scopes with built in function generators too? What would benefit people more is if you could present a simple block diagram on paper and show how u connect the wires for doing the measurements.
If you set your cursors at very top and bottom of trace, then your p-p value actually depends on your intensity setting! Turn your intensity up and down to see what I mean. Better to set them at center of trace. I hope this description makes sense. I refer to how you set the cursor at the 20:37 mark. (MJ, I cant believe that in 5 months you have not responded to this comment. And now a full year later, still no response. If you do not agree with my analysis, then please explain how I am wrong. I promise to cheerfully listen to your position.)
Thanks for the video. This really helps me learn how to use the Siglent SDS1052DL. Could you expand on how to use the average button in the acquire mode?
I find large discrepancies between an LCR meter and a DMM on electrolytic capacitors and will use this method to check and see which is closer - of course frequency comes into play. Have you noticed this? Measure on DMM and then LCR and compare results. Again only on electrolytic types for the most part.
Very nice tutorial. Mad scope skills!
Very Well Done Video..Thanks Martin!!!!!!!!!!!!!!
Persistence, very interesting video. Thanks
Thanks for your post.
Martin,
I just purchased a Rigol DS1102E Scope. I tried to replicate your method used in this video, but can not seem to place "X" and "Y" cursors on the screen at the same time. It appears that either "Y" or "X" can be selected, but not both. Is this correct or am I possibly missing something? My scope does have a Track function, which does allow both an "X" and "Y" cursor to perform tracking functions. Any wisdom you could provide would be greatly appreciated! Thank you in advance! :)
Sir I am a1st year year engineering student. I will be very grateful to you if you send a video of following experiment. DETERMINATION OF STEADY STATE AND TRANSIENT RESPONSE OF RC CIRCUIT. PLEASE SEND VIDEO.PLEASE EXPLAIN THE CIRUIT OF THIS EXPERIMENT.
I also posted a mini teardown of another (more decades, higher precision) decade resistance box on Martin's forum. Look under the "Product Reviews" section. As far as I know this one is made by the same manufacturer in Shanghai, China.
two questions,,,,,1.- you can check te capacitance only whit internal resistence the function generator (50 ohms) or is necesari add the other resistor? 2.- test the any capacitance whit same frecuence? or the frecuence change whit diferent capacitance for test?
You explained that very well ... Subscribed
Great Video - where does the exact value of 0.632 come from ?
I know old video. On the paper you wrote 62.3 but during the test you used 63.2 was that a mistake or did I miss something. Thanks for the education.
Hey, Martin I am a big fan of ur work, I have learned so many things from you. Anyways I just wanted to ask can we do the same thing with supercapacitors for capacitance or theres a different way for it. Would really appreciate if you can make one video on How to measure capacitance/ charging / discharging of supercapacitor .... Thanks
and other question, your function generator, output impedance is 50 ohms ?, and your oscilloscope is to 1 Mohm ?? this difference is correct? and not Affect end to measure ??.. thanks for your videos.
So when the square wave is down to 0, the capacitor is discharging itself through the resistor and INSIDE the function generator ? Is that ok ? What would be happen with higher voltages/currents ?
I suppose you could put a diode in series, and take its voltage drop into account.
Just asking because I'm pretty new to these things.
Thomas Cain have you figured out how to get the average on your siglent scope?
very educative. Thank you Regards
Нет проблем с определением емкости конденсаторов без большой утечки. Есть проблема измерения емкости с водяным диэлектриком, у которого собственная утечка значительна, и определение емкости становится трудным заданием. Если учесть, что сама утечка водяного конденсатора еще и нелинейная.
Thanks for the post William...appreciated. Cheers, Martin.
Some of us need the full length video to understand the subject. Were not all quick to pick up every topic.
Hi there, I tried to test a capacitor with an oscilloscope as the way you did but I failed to do so I don't know what was going wrong even though I applied 1kh and 1vpp on the cap but unfortunately no curve shown on the oscilloscope, would you tell me please why is that thanks
No problem with video length. The alternative is to remove footage that some of us findd
You don't babble. very nice vid thanks
The problem starts with measuring time accurate in the first place. Your scope might provide something like +-30ppm to +-100ppm accuracy from it's Quarz, cause it might not contain an OXCO of some hundrets of Dollar. Next problem is that your resistor might not be one of the higly precise (0.005% Tolerance - 2ppm per C°) type for some hundrets of Dollar per piece as well. Not to mention parasitic & stray capacitance and resistance in your setup caused by probes and else. Any capacitor charging while measured by inaccurate time and also inaccurate input resistance will not allow to create any result of reasonable precision. But at least the principle is very correct and well explained. Well done! Perhaps you try at least some precision resistors of 0,1% tolerance to it's value for the charging. By the way oscilloscopes are very bad devices for measuring DC in general!
Your second measuring is very bad. You better get the mean of the ripple instead of peak amplitudes to get at least somewhat close to around of a value of 4.7pF. From the Timing values the ripple does not seem to come from neon lights. Those are about 44kHz mostly. And that did not seem to match your ripple. Perhaps that was some resonance between your probes and the cap? But just guessing...
Great video.
@mjlorton, on the background, I see that you have a AIM-TTI generator, why did you choose it over for example Rigol series?
Do you plan to use a 100khz (or higher) frequency to show the reactance curve of a cap?
How accurate are RCL testers for such tiny capacitances? (such as the 4.7 pF )
Based on the overwhelming positive response says that the human brain is very well capable of concentrating 45 minutes (which is not very long to begin with). And when it comes to "casual" part, doesn't it become more so when it actually is a bit "airy"?
I have a 1000uf capacitor using 1000ohm resister and can’t get any where near a square wave it’s very spiky wave dense in the middle, maybe I need to lower the resistance or raise it or change the settings in the frequency generator.
thank you so much, electronics is amazing!!
Excellent video, Thanks
Thanks, you got me thinking now.
Nice video! really enjoyed it.
Exxcellent, keep up the good work.
...physical resistance adds to the impedance of the generator 50 ohms? or I can remove the physical resistance and do it with the output impedance of the function generator 50 ohms?... and your function generator is set to square wave with offset positive DC to reference to ground? 0v to 1v for electrolytic capacitors?
All in all a good tutorial on capacitors.
One comment.
Quit using all the zeros and switch to Engineering notation.
Engineering notation is a form of Scientific notation based on metric units,
pico, nano, kilo, milli.
If your calculator can not handle Engineering notation, get a new one.
BTW,
Capacitance = Time / Resistance (as you said)
= 50 u seconds / 1 k Ohms
or 50 nano Farads.
The difference between the package label may be the normal variance in
batches of capacitors or also in your measurements of time.
what is the racetrack on the side of the cap,?
Thanks Martin:)
I got lost, how did you get 1000 ohms for resistance?
So wonderful...that was alot of fun :) ...thanks very much
Great video thanks
If you overlap vs with vc, you will see that pass the 63% mark, the current is slowing in transit.
Sir- Can you give me advice on buying an Oscilloscopes? I can't justify a large amount of money for this until I become more efficeint circuit building, design and testing. Looking for entry level to electronics that would be a good purchase to further my knowledge of the finer points of Oscilloscope. Thanks in advance! Bill