How to measure a capacitor with an oscilloscope.
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- Опубліковано 20 чер 2013
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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..
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.
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.
Thank you for taking the time to explain!!! Keep up the AWESOME work!!! I learned quite a bit and I like how you take the time to explain things.
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.
Thank you Martin. I enjoy your videos very much and find them very enlightening. Above all you are extremely kind and sensitive to your viewers and assume them to be with it, this is one of the reasons you shine. As I view your presentations I sense a generous collaborative spirit as you plunge into your latest ideas with your great toys.
Man, this was a great lesson.
I’m glad I saw the difficulty you had because it mirrors my experience.
Big help.
Great video! The "babbling and stumbling" in some parts of the video is actually great for beginners like me.
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!
Thank you for your kind, intelligent, and inspiring videos.
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.
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.
Very methodical procedure and therefor a very good tutorial. I appreciated and enjoyed watching!
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?
i'm a computer engineer and i found this videos very useful for me ,thanks for illustration .
Thanks for the input and feedback...appreciated!
Martin, as a suggestion, I found another easy way to evaluate the capacitance with a scope. Using the same circuit as you have, set the frequency in such a way that the voltage across the capacitor will be half of the voltage across the resistor. Once there, apply the Sqrt(3)/(2*PI*F*R) formula. Quick and easy, less gymnastic with the cursors. Have fun!
I really like the way you have the handheld testing tools organized on the top shelf above your workbench. Inspiring!
I love this guy - thanks so much Martin for generously donating your time to educating others with your excellent tutorial videos... I've learnt a lot and it's set me well on the way :) Cheers
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 Great Video. Keep up the good work. I enjoyed the journey you made and the comments below of some...
My pleasure, thanks for the feedback.
Not sure why a 9 year old video popped up in my feed, but I enjoyed it. Two comments:
(1) Since you're measuring the rise time of a square wave, the frequency really isn't going to be a factor unless your rise time exceeds your cycle time.
(2) The noise is present on both the 47nf and the 4.7pf traces, but it's more filtered with the larger capacitor. You can still see the "fuzz" on the 47nf trace. A higher input voltage would probably help clear that up.
(3) you'll get a more accurate reading if you put your "Y" cursors in the middle of the "fuzz".
OK three comments LOL
ETA: (four comments) (4) the 4.7pF is probably inaccurate because you're down in the range of the capacitance of the scope leads and or the function generator, both of which will alter the rise time.
You are correct...the cheaper scopes can't do both at the same time...I will highlight this in a future video. Cheers, Martin.
Thank you so much for describing every detail, unlike most of electronic youtubers who tend to give important things for granted. To learn (at least from a mono-neuronic point of view as in my case) it's mandatory to understand to understand it's important to follow every step, so that i could even repeat it.
I know that will get to make and submit a very long video, but i myself really prefer this kind of method (in fact i don't even dare to watch short videos).
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.
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! :-)
Thanks, appreciate the feedback.
Martin, Ignore the negative comments, 25,000 people learn something every video.
this is brilliant, and again, thank you for investing your time
Great Job Mr Lorton,as always.Love the way you do your videos.Like I said you are one of the best tutors I have ever been privileged to see.And loved the part where you try to get the calculations to work for the 4.7pf..Really admire your honesty. Totally agree with the gentleman who talks about the oscilloscope probes adding their own capacitance...I had that happening to me at my work, once, This was with a RC circuit and the probe capacitance's were upsetting my calculations too.
Thanks for tutorials, It is Really Helpful, Good Luck :)
Thanks very much for the feedback.
This video saved my lifeeeeeee! Thank you for such an in depth video
Thanks for the feedback.
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
No problem with video length. The alternative is to risk removing footage that some of us find very helpful.
Awsum video very understandable and educationall and im trying to learn basic electronics love the way you explain things keep up the good work
Great job, very well explained.
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. :-)
thanks for your effort.
Brilliant video! Thank you SOOO much.
Great explanation of how to use an oscilloscope and measure a capacitor!
Great explanation of how to do measurements on the oscilloscope plus doing a step by step on calculations.
Very Well Done Video..Thanks Martin!!!!!!!!!!!!!!
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
I'm a heavy scope user you just can't beat a scope. When you can see a graph of what's going on is wonderful. I also use it in automotive testing. Often I tell people the issue what's wrong and since no codes they think I'm wrong. Months pass and they call back and say hey that's what it was. Due to heath reasons I'm getting back into board repair and it's better money and I don't have to be out in sun. I have a automotive scope pico 4425a I'm so happy I can still use this it was very expensive. Thanks for your video and information.
Martin, thx. All videos was for me realy usefull. Keep going...
Roman Kyselý My pleasure Roman, thanks for the post.
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.
Persistence, very interesting video. Thanks
Thanks for the post....good points...noted.
Very nice tutorial. Mad scope skills!
Thanks...noted and annotation added to your video. Cheers, Martin.
Nice and useful demonstration, it's great you take the time to do this. A couple of notes: in actual practice, you really need to zoom in more on the front of the waveform to get a more precise measurement, Also, it is often more practical (and faster to do) to increase/decrease the frequency of the wave generator to match the resistor / capacitance pair, intead of changing the resistor value (which could be a specially selected resistor).
Great video! Thanks!
Great video.
Thanks for the video. Makes me want to purchase an oscilloscope even more. :)
Thanks for your post.
Exxcellent, keep up the good work.
Very nice. Few observations: 1) That little capacitor surrounded by all those wires with alligator clips looks like a hostage out of a grade B movie. 2) when you divide by 1xxxxx no need to use a calculator. Just add x zeros to the right of the decimal point to your answer. 3) Even better use scientific notation so dividing nano (10^-9) by 1000 you would have 10^-9/10^3 = 10^-12. Simple and foolproof. Finally a cap charges in an exponential manner. the step voltage of a cap as a function of time is V(t) = Vs*(1 - exp(-t/RC)) where R and C are your cap and resistance values and Vs is your final voltage which is equal to your source voltage (in this case 1 V). You can reverse this equation and solve for C at ANY time t that you care to use. Taking V = 0.632 just simplifies the calculation. In general we have C = -t/(R*ln(1 - V(t)/Vs)) where Vs is your source voltage (in this case 1 V). Taking V to be 0.632*Vs simplifies the above to C = t/R because V(t)/Vs then equals 0.632 and ln(1 - 0.632) = -1. Voila!
You explained that very well ... Subscribed
Nice video! really enjoyed it.
Very nice job sir.
Excellent video, Thanks
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.
finally I have seen someone explaining the things how must be done!!!!
thank you so much, electronics is amazing!!
Awesome tutorial! I am really jealous of your work space.
very educative. Thank you Regards
Great video thanks
Thanks, you got me thinking now.
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 Martin:)
Good question! Let me add that to the list.
Good demonstration of capacitance calculation!
Thank you very much!!!
Good teacher! Very rare. Keep it up
Excellent ❤️
When you are testing that 4.7 PicoFarad cap, keep in mid that your scope probe has much more capasitance in it (typically 15-20pf).. also the scope probe has about 10M impedance that is close to your 1M Source side resistance that tries to charge to cap.
Also all those leads from sig-gen and resistor box add some capasitance parallel to the cap measured slowing down the charge curve.
Great video info
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.
So wonderful...that was alot of fun :) ...thanks very much
This was a great exercise. Even with a very basic scope I got reasonable results ( with practice ). If I had to identify a bag of small "mystery" caps I would put a bunch in parallel to reduce the error.
Thanks for the feedback Chris.
Great video thanks
always good, thank you ...
thanks for usefull recomandations
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.
grate video Sir , learning
Excellent
Thanks for the post...I checked it several times to make sure and it is 1M Ohm.
Very nice. Thanks
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
GOOD JOB THANKS
Baie dankie vir die antwoord my vriend.
That was good,,, thanks again.
nice one! thank u
Thanks for a great video, and is was not to long.
good
Thanks Martin
Reducing the lead on the resister will also help. Nice video.