Back to Basics Video Tutorial: Voltage/Current in Capacitors and Inductors

[w2aew]

I put this video together by request, and thought I'd share it here.  It is a "Back to Basics" tutorial style video that describes the voltage and current relationships in capacitors and inductors.  It covers the terms "leading" and "lagging" which are often used to describe the voltage and current in these devices, and shows a simple test circuit to visually observe the phase relationships in these devices. 

[Anonmis]

Thanks for this quick little guide. Your explanations/demonstrations were a good addition in order to understand this topic better.

[George Lee]

as describe in this video,in capacitors,current leads voltage while in inductors,current lags voltage.
thanks for this movie.

[JuiceKing]

Thank you for this video!

What values of frequency, capacitance and inductance did you choose for your demonstration?

When I try to reproduce the results myself, I've found that the phase shift between the two oscilloscope channels varies depending on the frequency of the signal applied. Why would it be anything other than 90 degrees? Watching your video again, I can see that it's not exactly 90 degrees for your test, either. Why is that?

Thanks!

- Ken

[KTP]

Thank you for this video!

What values of frequency, capacitance and inductance did you choose for your demonstration?

When I try to reproduce the results myself, I've found that the phase shift between the two oscilloscope channels varies depending on the frequency of the signal applied. Why would it be anything other than 90 degrees? Watching your video again, I can see that it's not exactly 90 degrees for your test, either. Why is that?

Thanks!

- Ken

The problem is that breadboards have lots of stray capacitance and inductance which will have a larger effect on the circuit as you increase the frequency.  Also, there is inductance in any capacitor and capacitance in any inductor...unless you go to Radio Shack and buy the ideal varieties (do they still sell those?)

[w2aew]

Thank you for this video!

What values of frequency, capacitance and inductance did you choose for your demonstration?

When I try to reproduce the results myself, I've found that the phase shift between the two oscilloscope channels varies depending on the frequency of the signal applied. Why would it be anything other than 90 degrees? Watching your video again, I can see that it's not exactly 90 degrees for your test, either. Why is that?

Thanks!

- Ken

The problem is that breadboards have lots of stray capacitance and inductance which will have a larger effect on the circuit as you increase the frequency.  Also, there is inductance in any capacitor and capacitance in any inductor...unless you go to Radio Shack and buy the ideal varieties (do they still sell those?)

And - I used a 100ohm resistor to sense the current, so that add a small "in phase" component to the current, and the probes themselves add some capacitance, in addition to the capacitance and inductance of the breadboards.  I believe the values I used were about 220uH and 200pf, frequency around 700-800kHz.

[w2aew]

I added one more video to show how the these phase shifts combine under resonant frequency conditions of series and parallel LC circuits:

[IanB]

Nice clear video.

That scope shows the traces apparently as "fat" lines. Does it draw the data with pixels of varying intensity to produce a smooth "anti-aliasing" effect like you sometimes see with text on LCD monitors, or does it do something simpler than that?

[w2aew]

That scope shows the traces apparently as "fat" lines. Does it draw the data with pixels of varying intensity to produce a smooth "anti-aliasing" effect like you sometimes see with text on LCD monitors, or does it do something simpler than that?

The traces are "fat" are due to a couple factors.  100MHz scope without the BW limiting on means that there's some noise that fattens up the signals.  Plus, this is an older scope design - nearly 10 years old.  In this era, these basic entry-level digital scopes didn't have terrific screen resolution, so a few pixel wide trace looks fat.  Plus, the basic scopes of this era did not have the graphic processing to do the proportional intensity, so all "pixels" have the same intensity, unlike today's scopes that perform 'live' intensity grading to mimic the old phosphor displays of yesteryear.

[JuiceKing]

Thank you for this video!

What values of frequency, capacitance and inductance did you choose for your demonstration?

When I try to reproduce the results myself, I've found that the phase shift between the two oscilloscope channels varies depending on the frequency of the signal applied. Why would it be anything other than 90 degrees? Watching your video again, I can see that it's not exactly 90 degrees for your test, either. Why is that?

Thanks!

- Ken

The problem is that breadboards have lots of stray capacitance and inductance which will have a larger effect on the circuit as you increase the frequency.  Also, there is inductance in any capacitor and capacitance in any inductor...unless you go to Radio Shack and buy the ideal varieties (do they still sell those?)

And - I used a 100ohm resistor to sense the current, so that add a small "in phase" component to the current, and the probes themselves add some capacitance, in addition to the capacitance and inductance of the breadboards.  I believe the values I used were about 220uH and 200pf, frequency around 700-800kHz.

Right! My trigonometry is, ahem, a little rusty and I had imagined that the addition of these phase-shifted waves would make something other than sine waves, but of course they just make other phase-shifted waves of different amplitudes. That explains why when I tried it I saw amplitude vary so much with frequency; it wasn't resonance as much as simple wave addition. I made a spreadsheet (attached) to simulate this and persuaded myself that it's true. I'm sure it's very simple to show this symbolically, but I was at my computer so I gave it a try with this simulation.

[w2aew]

My trigonometry is, ahem, a little rusty and I had imagined that the addition of these phase-shifted waves would make something other than sine waves, but of course they just make other phase-shifted waves of different amplitudes. That explains why when I tried it I saw amplitude vary so much with frequency; it wasn't resonance as much as simple wave addition. I made a spreadsheet (attached) to simulate this and persuaded myself that it's true. I'm sure it's very simple to show this symbolically, but I was at my computer so I gave it a try with this simulation.

You got it!  Your mathematical model in Excel illustrates numerically what I was describing graphically on paper. 

[dcel]

Excellent videos! Some people should have been teachers, like yourself and Dave.

Chris

[kb3yyn]

Excellent video. Thanks for the information.  I was wondering what probe you were using for your signal generator.  I tried the circuit with an inductor and resistor and the sig gen output and the voltage across the resistor were in phase.  Does it matter what type of probe you use with the sig gen?  I have a wavetek 182 and let the ground float when I tried the circuit.

[w2aew]

Excellent video. Thanks for the information.  I was wondering what probe you were using for your signal generator.  I tried the circuit with an inductor and resistor and the sig gen output and the voltage across the resistor were in phase.  Does it matter what type of probe you use with the sig gen?  I have a wavetek 182 and let the ground float when I tried the circuit.

On the signal generator, I didn't use any probe - just the 50ohm coax with a pair of alligator clips at the end.  You wouldn't want to do this at RF frequencies, but it's fine at the relative low frequencies I was using (a few hundred KHz or less).  I did not float the ground on the generator or the scope probes (which is why I had to do the math trick in the 2nd video to "see" the voltage across the inductor).