Differentiator Circuit Absolute Beginner - Page 1
Forum section: electronics / beginners

author: SewingYard
author_rank: Contributor
timestamp: March 13, 2016, 02:52:10 am
content: Hi to all on EEV Blog, I'm fairly new to electronics but ive played and tinkered for many years gone, I know how to use a multimeter my math is good and my physics is good and I've finally decided to take the massive leap into electronics big time and hopefully take up a career in it in 5 years time after taking a degree course.I've bought The Art of Electronics and the Student Manual and I'm starting to work through it but here lies my problem, I'm new to oscilloscopes and signal / waveform generators and no matter how many videos I watch I'm really just not getting it.The book wants me to play around with making an RC differentiator circuit and analyse it on a scope with a square wave so I can see the charge and discharge curves and analyse the cycle.So, I went online and got myself a Rigol MSO1104Z-S Oscilloscope with built in Waveform/Signal Generator, I lovely piece of kit and I must admit someone skilled will probably be drooling over it but to me it doesnt do much yet apart from show if something has a voltage or not, ive also looked at the Signal Generator by looping the output to the input with a BNC cable and switching it on so im familiar with zooming, panning etc... but not much more yet.I have a Constant Variable Power Supply and lots of bit and bobs with a breadboard for testing.So, I figured from the book and couple of other YouTube videos playing with capacitors () I get a 1k resistor and a 2.2uf capacitor connect the whole lot together as follows:+5v ..... resistor ...(scope signal).(signal generator signal). capacitor ..(scope gnd).(signal generator gnd)... -Now, I expected to see the oscilloscope as a nice square wave with a curve at the leading edge and to see this repeated over and over again.However I dont see this, I just see the line go up to +5v then stay there still i switch off the power then it drops to 0vI'm really not sure what I'm doing wrong.Do I need a voltage to drive the whole thing or will the signal generator charge the cap for me?Why would the cap constantly charge and discharge as it says in my book?Why can i see a square wave from the generator when i take out the capacitor but it disappears when i include the capacitor?The book wants me to attach a 7.5k load to test later down the line, how on earth would I attach a 7.5k load to a circuit?Am I out of my depth?Should I be starting elsewhere with a different circuit to be familiarised with the oscilloscope better?Sorry about all the questions, I'm a newbie so I assume its allowed?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: March 13, 2016, 03:28:14 am
content: Don't use the 5V power supply. Look at the schematic presented at about 12:00 in the video you linked. The signal generator is supplying a +5V - 0V pulse train. When the pulse is at 5V, the capacitor charges through the resistor. When the pulse goes to 0 volts, the capacitor discharges, and the cycle repeats at the frequency chosen for the pulse train.

author: SewingYard
author_rank: Contributor
timestamp: March 13, 2016, 04:38:19 am
content: Hi,Thanks for the response, I really appreciate you taking the time, I have removed the power supply from the circuit. Does this mean that the signal generator is actually putting 5v through the BNC cable? I tried to measure this with a multimeter and it was not showing any voltage output which is what made me wonder.Now for another silly question.I've got my 2.2uf capacitor that I'm trying to test, the wave was all over the place with using a Frequency of 1khz like in the video, I have changed to 1hz and I'm now getting a square wave then ive added a 33k resistor and I'm getting a wonderful curve that I can use for the calculations.What I dont understand is why I would need such a drastically different Frequency?Many thanks.

author: SewingYard
author_rank: Contributor
timestamp: March 13, 2016, 04:59:37 am
content: So, I done the calculations and hope the following looks right?T = 70msR = 33kGives: 2.121212121212121e-6Makes it: 0.000,002,121 fOr Estimated 2,12ufThe capacitor is 2.2uf I'm assuming that's ok with error allowance?Thanks again.

author: IanB
author_rank: Super Contributor
timestamp: March 13, 2016, 05:22:41 am
content: There are many questions here.Firstly, the signal generator is indeed producing a voltage signal. The amplitude of the signal will be adjustable at the signal generator.The signal generator will also have a (limited) maximum power output. If you load it down too much it won't be able to generate a clean signal.By its nature the signal generator is producing an AC signal, and a capacitor looks like a short circuit to AC. So if you connect a capacitor directly across the output of the signal generator it will short circuit the output and stop any reasonable voltage appearing.When you do the experiment of the kind you are performing you need to put a resistor in series with the capacitor in order to limit the current and present a reasonable load to the signal generator. (The resistor and capacitor form an RC network, which is also a first order filter, and is something you will encounter a lot.)There is no reason why you should not be able to do the experiment at 1 kHz, as long as you have a suitably chosen resistor and capacitor. In electronic terms 1 kHz is a very low frequency. At higher frequencies into the MHz and beyond there are special considerations about signal paths and impedances and wave propagation that will mess up simple experiments like this. But 1 kHz (and up to 10's of kHz) should be fine.It seems like you got a successful result when measuring a capacitor, so good job on that

author: danadak
author_rank: Super Contributor
timestamp: March 13, 2016, 11:24:36 am
content: This might help -http://www.electronics-tutorials.ws/rc/rc_1.htmlThe RC time constant (Tau) will basically control what setting of frequencyyou should use for the scope generator.As a guide t = RC, 5 time constants = ~ circuit charged / discharged to 99%of the final voltage. So if we have a square wave and want to see most of thecharge discharge cycle we would want it 5 Tau high and 5 Tau low. So frequencyof generator would want to be ~ f = 1 / (10 * Tau ) = 1 / ( 10RC ).Pick and R that will not load down generator too much, say 1 K, then calc C neededfor the 1 Khz you want.Regards, Dana.

author: SewingYard
author_rank: Contributor
timestamp: March 14, 2016, 09:11:56 am
content: Hi,Thanks for the replies, and thanks danadak for the link.I don't seem to be able to get this to work still in the 1khz frequency, only when I drop it right down to 1hz do I seem to get any kind of a decent curve, I've tried several resistors small to large.It must be something simple I'm not understanding properly, even after spending till 5am on it, lolAny suggestions?Thanks

author: danadak
author_rank: Super Contributor
timestamp: March 14, 2016, 01:39:54 pm
content: We have a 1 Khz square wave. We want to see RC charged to 5 time constants, when square wave is high,and discharged 5 time constants when it is low.So 1 Khz = 1 mS = .001 secs. T = R x C, but we want 10 TC to occur in 1 mS. So .001 sec / 10 = .0001 secs.Use the calibrator output as the square wave, it typically is 1 Khz. So choose an R = 100Kso .0001 s = 100000 x C or C = 10-9 F = .001 uFSo here is what I get on scope, attached.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: March 14, 2016, 01:46:20 pm
content: Whereas the squarewave >> R >> C >> Ground is an integrator of sorts,here its configed as a differentiatorsquarewave >> C >> R >> GroundRegards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: March 14, 2016, 02:37:26 pm
content: Reducing cap by a factor of 10, differentiator case, produces much narrower spike, more "ideal"differentiator, but we loose amplitude because C and R form a V divider, and the C reactance, forsmaller C, much higher. So less signal gets coupled to R.Regards, Dana.

author: SewingYard
author_rank: Contributor
timestamp: March 14, 2016, 08:32:07 pm
content: Dana,Thanks again, you should have a PayPal account for donations!I went out and diversified my capacitor collection (along with £120 of other "stuff" lol)I think I've cracked it now, its looking beautiful in the 1khz range and I've also switched my probe and scope to x10 which i noticed you have on the examples and its looking much better.Thanks again, loving this forum and the videos too, they're immensely informative.I'm sure ill be asking many more inane questions over the coming months and years until I've graduated from the Beginners forum!