Today we're going to use an oscilloscope to look at the three different effects
that capacitance can have on electrical systems
Hi and welcome to the 2-Minute Guru
My name is Daniel Bogdanoff. Earlier in this season we've been
looking pretty much at DC systems. We've looked at resistance, voltage, and current
Now it's time to look at AC systems and today we're going to look at capacitance
We're going to look at the three effects that capacitors have on your system
using an actual capacitor, but capacitance is actually present pretty
much every electrical system so understanding what those three effects
are will help you analyze and diagnose your circuits later on. Our circuit setup
today is super simple. We have the wave gen from my oscilloscope running through
a resistor and that's probed with channel 1. And then we're going to put a
capacitor in parallel with that resistor to see what happens.
The first effect that capacitance has on a system is it's going to resist a
change in voltage and store charge according to the formula Q= C*V.
So let's add the capacitor and compare before and after. I'm going to
save this waveform using a reference waveform so I hit the reference waveform
save, save to R1 and then I'm going to display it on screen. So when I turn off
channel 1, you can actually see that waveform there. Now let's turn channel
1 back on and add the capacitor
And you can see if I zoom in here, this
region my capacitor is charging up. It's resisting that step-up in voltage. Here
my capacitor is starting to discharge so all of that charge stored up when the
voltage was high is slowly trickling out of my capacitor after my source voltage
is already gone low. So if you see your system resisting voltage changes or
adding some charge back in after the voltage has turned off, you know that's a
capacitive effect. And this effect is roughly how switch mode power supplies
operate. When the power is supplied to the capacitor it begins to charge up and
then when it's turned off, the capacitor discharges that energy into the system
And that leads us to the second effective capacitance frequency
filtering. There's two different ways to look at this. First let's look at our
signal from before. Remember that a square wave is made up of an infinite
number of increasing frequency sine waves. When I remove the capacitor, we get
a nice crisp edge, but when I add the capacitor, we see that that edge gets a
little rounder, and that's because the capacitor is filtering out the higher
frequency components that make up that nice square wave. Another
way to look at this is with a pure sine wave. At lower frequencies, the capacitor
is going to absorb those frequencies. It's going to charge up the capacitor
and discharge the capacitor as the segment goes up and down, but if I crank
up the frequency, we're going to see a smaller amplitude of that signal on the
capacitor which means those frequencies are passing right through the capacitor
So it's filtering out the low frequencies and letting the high
frequencies pass through it. The ability of capacitance to filter out frequencies
is widely used in electrical engineering. A couple examples would be getting a
cleaner power rail, you can filter out some of the ripple on that or separating
frequencies out for audio applications. The third effect of capacitance is that
it can change the phase relationship between voltage and current. So I've
hooked up a current probe to channel two and I put the scope in averaging mode
like I'd like to do with this current probe, and you can see right now my
voltage and current are in phase- they're both rising and falling at the same
point in time. But let's see what happens if I add a capacitor. You can see that as
soon as I add the capacitor, the current shifted forwards in time so the current
is now leading the voltage or the voltage is lagging the current thanks to
that capacitor and we can use the oscilloscope to measure that so I'm
going to hit "measure" and I'm going to add a phase measurement. It's all the way
at the bottom. And we can see that I'm getting about a 45 degree phase
shift and that's because my capacitor is in parallel with my resistor. If I pull
my resistor out, I should get to see about a 90 degree phase shift from
my capacitance and we do see a roughly 90 degree phase shift between voltage
and current when I have just a capacitor. And the ability to control the phase
relationship between voltage and current is really useful especially in power
where you're concerned about instantaneous power, which is current
times voltage. You can actually use capacitance to adjust your phase to get
the maximum power out of your systems. So those are the three effects of
capacitance that you can put into your engineering toolbox-
storing charge, filtering frequency, and changing the phase between voltage and
current. That's all we have time for today.Thanks for watching! My name is
Daniel Bogdanoff. Make sure you subscribe to the Keysight Oscilloscopes YouTube
channel and check out our free eBook, download "Six Essential Tips for Getting
the Most Out of Your Oscilloscope." I'll see you next time.