Decoupling Capacitors - Simply Put

capacitors have many uses throughout circuits but one of the most common and important use is what are called decoupling or bypass capacitors and sometimes filter capacitors recall that capacitors store energy by having two conductive layers and the insulating layer between that causes charge that's applied to kind of stick to it and so what ends up happening is as charge flows through the circuit and the capacitor is connected in parallel it tends to smooth out the voltage it resists changes in voltage different capacitors have different capacities capacitances they have different construction which means they have different response rates basically each capacitor of a different type and size responds to different voltage change rates and causes a different effect on those voltage change rates think about a graph of voltage over time you have one axis which is time and one axis which is voltage so up on this side is positive voltage and down on this side is negative so you might have your alternating current goes between positive and negative and positive and negative so this would be voltage changing over time this could be a signal now if you hook a capacitor up to this very little is going to happen because the capacitor is gonna charge back and forth and back and forth because it'll charge one end being positive but then when that end becomes negative it'll basically suck the charge back out of it and charge it the other way so the capacitor won't do much so while capacitors do have a purpose in alternating current we're going to leave that for a far future video right now we're going to discuss capacitors most common usage in direct current so here's our voltage over time graph again but this time it's only positive so we've got zero and positive and there is no negative so let's say you had a direct current over time gonna be kind of like that there's gonna be some waviness and of course there's waviness because of my hand but let's say your current goes like doo-doo-doo and this is where everybody in the neighborhood all turn their tvs on at once because all of a sudden the Andy Griffith marathon was starting so the power company had to spin up their little extra generator for a second to make sure that they could feed the neighborhood so you might view this as a signal it's not a wild no but it's a signal and this happens all the time there's all kinds of variations in little ups and downs and this is why I love battery so much because a battery isn't affected by anything outside of itself it just has a voltage and over time it just goes nice and smooth but batteries obviously have their limitations so wall power it is so what does a capacitor do to this well I'll just draw it below so you can see it just imagine the numbers are the same so a capacitor would do pretty much nothing here because the capacitor would be charged actually let's pretend this is the start of time so at the beginning if this is what the rest of the circuit season you have this capacitor at the start at the beginning you're gonna be like down here and then go mmm because the capacitor is charging and it's gonna be a lot quicker than that but you get the point so the capacitor is going to suck up the voltage right at the start and then it will be fine and the circuit will continue on but then this is gonna happen and the voltage is gonna go but now the capacitor is there charged and ready and so the power supplies voltage has dipped but the capacitor is all charged up so it's gonna go like wait no I got it and it's gonna go up and then it's gonna feed the circuit and the power supply Oh kick back in and the capacitor will charge back up and then there you go so it smoothes it out another example let's go back to our alternating current so we discussed rectifiers and that's one common use of these capacitors a full bridge rectifier takes your alternating current and turns it into direct current which looks like this so what is a capacitor to do well the capacitor is going to turn it into something that kind of looks like this and depending on the capacitors you use it'll you know be a lot better it's almost like a sawtooth wave I just have trouble drawing it but the point is you can see it basically smoothes it out if you add more capacitors bigger ones and smaller ones it'll end up looking more like this now what's this variance you know this variance can end up being small enough that it doesn't even matter that's the point it won't be perfect you can't get it perfect but if this variance has no effect on your circuit well then you're good aren't you so basically what happens is the capacitors suck up enough charge to charge up and then as every dip happens they supply some and every rise happens they charge back up so overall though slightly lower value but you can compensate because you're taking the wall voltage of 120 to 220 and bringing it down to five so you've got plenty of headroom so you can put through as much power as you need to make sure that it always supplies it so like you have your linear voltage regulator that puts it out you know you give it seven volts and then it always regulates it down to five or you have a switching power supply so if there's a lack it just switches faster we'll discuss all those things when we discuss those types of power supplies but this is what a capacitor does now some internals real quick and I'm not going to be doing any math there's no math here I'm going to show you a mathematical thing but we're not gonna do math so relax if you have taken any medium level course in math including high school that's where I learned it you may have heard of something called a Taylor series basically you can have imagine here you know your graph of y equals x you know you've got a plot well let's say y equals a whole bunch of crazy stuff that you can make head or tail of and it's really complex and it's really hard to calculate and it wraps up all the concepts but you can approximate it with something called the Taylor series 800 plus a 1 X 1 plus a 2 X 2 plus derivative now the superscript the number on the top is exponentiation X to the power 0 is 1 so this is just a oh and now this is a 1 times X to the 1 is X and x squared X cubed the subscript just labels the value so that's easy to remember if the number is higher that's exponentiation if the number is lower it's a label like you know V in or you know X first or whatever you want the subscript is a label and in this case it matches the exponent so it's easy to keep track that's why we did it but the point is this is what's called a polynomial and you create higher and higher and higher and higher orders and you can approximate any graph you want so for example you know if you've only got the x to the 0 let's say it stops there well that's a line and then you've got a parabola and then you've got one of these and then you've got you know one of these and it gets more complex and you can tailor the numbers no pun intended so that within a range that function approximates the function you want to greater and greater and greater efficiency well there's another kind of approximation called the Fourier series the Taylor series is just a polynomial the Fourier series is sinusoidal you know your sine and cosine these functions you know your sign is gonna look like this and your cosine is gonna look like this sine and cosine are exactly the same thing one's just offset a little bit the cosine starts at one the sine starts at zero they're exactly the same thing it's called a different phase we'll discuss that later but point is it's waves waves in the ocean electromagnetic waves you know your alternating current is a wave waves that's all it is up and down you can approximate functions with waves as well you may have heard of something called a fast Fourier transformation FFT this is for signal analysis frequency analysis and all this other stuff this is how filters work like if you have a high pass or low pass filter in your software what happens is it splits a complex wave using the fourier system and combines it back down after adjusting the individual frequencies so let's say you have this wave alright and then let's say you want to make this wave with other easier to work with waves so let's say you have like this one and it's so tall and then you've got this one and it's so tall you've got this one it's so tall and then you've got this one it's so tall basically you've got waves of different amplitude amplitude is how high it is and different phase phases how much it's shifted all right so you've got if you have a wave like this and then if you have a wave like this it's shifted over a little bit that's slightly out of phase so just like you have the Taylor series where you have you know X to the 3 with a number multiplied at X to the 4 X to the 5 and you just adjust essentially the amplitude of that part of the Taylor expansion that's the same thing with the Fourier expansion you just take a wave of a different frequency and you add them together frequencies and phases with amplitudes so every frequency has a face and an amplitude you add them together and you'll get a more complex wave and you just keep adding terms and adding terms and adding terms until it's as precise as you want so real-world example would be my voice my voice is made up of different frequencies if I go high it's higher frequencies low is lower frequencies but let's say I'm snapping while I'm talking like this those are combined you're hearing them as one complex waveform but your brain can obviously separate them you hear two different things this one is a lower pitched sound it's lower waves so if I wanted to filter that out let's say that I was to make sure that all of the frequencies of my voice are much higher than this so what I do is I use the Fourier math which obviously we're not going to discuss right now it's very complex but really cool though but I split it when you split it you get the frequencies amplitudes phases and you look at the frequencies of the low stuff and of the high stuff right you've basically taken this and split it apart into its constituent parts right instead of seeing an atom of oxygen you see a bunch of protons and neutrons and electrons and all this other crap just spread out on a table right in front of you and then you say okay well I want to get rid of the snapping so I just delete all of the lower frequency stuff I'm going to pick a threshold everything below this delete and then you smoosh it together again and it won't be perfect because there's some variation but for the most part you will have done what's called a high-pass filter it allows the high frequencies to pass through so that is one thing called a noise filter have you ever heard of a noise filter where like if you have you know hissing in your microphone and what you do is you go in a program like audacity and you select some of the noise and then you say delete that noise and you get some weird artifacts but it mostly deletes the noise and leaves the voice you know good enough so what it does is it takes that noise and it does a Fourier transformation to figure out what constitutes that noise what are the frequencies amplitudes phases of that noise and obviously it varies per second but it gets an approximation it gets a rough one so then you do a Fourier transformation on the whole sound delete the ones that were the noise and smush it together again so you know again it's not perfect but it works out this is actually how capacitors work do different capacitors if you imagine now it doesn't actually you know do some sort of computation here it's physical properties but it works the same if you imagine a voltage signal being split into its constituent parts by the Fourier series different capacitors will allow different frequencies of that signal to pass largely unmolested and the others will be smoothed out so you could have high ones low ones whatever capacitors can be used as filters like that a larger capacity capacitor will tend to filter out lower frequency noise whereas a low capacity capacitor will tend to filter out high frequency noise so we had our rectified AC signal sixty times a second you can see that's pretty low frequency so you get a big capacitor and it's gonna smooth out these big ol lumps and it's gonna leave you with still a noisy signal but much smoother so now you've got noise that's smaller bumps and if you split this up into your Fourier series you'll see there's a lot of high frequency components so then you get your smaller capacitors that don't have as much capacity but they have a faster response they start supplying voltage and charge faster are gonna smooth that out so that's why you use a big and a small you smooth the low frequency stuff and you smooth the high frequency stuff this is what capacitors are used for most of the time smoothing so you'll see something like a voltage regulator let's just pretend this is a voltage regulator then you have your wall power let's pretend this is already rectified and filtered and this is this is a nice you know higher input from a wall board or something and then you've got your actual circuit over here and you're gonna have some capacitors let's say you've got one there and you've got one there and you're just gonna connect them in parallel obviously a much simplified diagram but you'll see that the capacitors are in parallel they're not stopping anything from happening like the series regulator is but this one is a big one and this one is a small one so what happens here this one is in charge of see this is before the regulator this one is in charge of making sure that dips in the power supply from the wall when everybody turns on their washing machines at the same time our smooth dealt so this one sits here charged big capacity slow response but big capacity and then the power dips and this thing kicks in you know make sure you have your your diode in there so it doesn't backflow and then so it's gonna supply the regulator mostly well enough until the power kicks back in and this is going to smooth out those lumps this one is responsible for the high frequency noise the little what you call ripple and this is what's going directly into for example your integrated circuit that wants a nice clean voltage so this one is in charge of making sure that the unregulated power supply is there that this thing has everything it needs to work with this is the seven volt and to make sure this thing stays as close to seven volts as possible so that this thing has its head room where it's taking seven to five so it always has that Headroom to make sure it stays at five and then this one takes care of all the little variations that this thing can't stop because this thing this thing is the powerhouse it's making sure that it's rough and ready and and and make sure that the cement is still flowing out of the truck and doesn't worry about the little lumps this is the filter getting rid of the little lumps you know the one little guy with the low brush to make sure that the integrated circuit gets its nice clean supply because this one doesn't have to worry about supplying when there's a power outage this one is just making sure that when it goes a little high when it goes a little low smooths it out and you'll see this all over the place every time you see a capacitor in parallel that's probably what's happening so this is what a bypass or decoupling capacitor is for they call it decoupling because the idea is whatever the power supply else were in the circuit is doing going up going down whatever you know some other part of the circuit kicks in and the power supply goes trying to supply it this capacitor in this part of the circuit because you might have more than one voltage regulator but this capacitor in this part of the circuit is going to handle that just like I said if your household gets a dip because everybody else turned on their TVs it's the same thing if some other part of the circuit suddenly turns on like the system decided that it's getting too hot so it turns on the fan and the fan starts sucking power and you know the the main regulator transform or whatever has trouble keeping up immediately the other parts of the circuit such as the control circuit are going to have their big capacitor that's going to supply voltage exit extra voltage to catch up in the split second it takes for you know whatever other main power supply to kick back in and catch up and this is just the guy cleaning up after so that's why they call it decoupling capacitors to make sure that this part of the circuit whatever it's doing is mostly functionally separate from the rest so that whatever the rest of the circuit is doing this guy doesn't have to think about it and they call it a bypass capacitor for roughly the same reason I think decoupling is a little more clear but yeah but you know how I said these capacitors respond to different frequencies it's the same thing with sound how I was saying you can filter out high and low parts of the voice you do the same thing in sound circuits and you can use capacitors to do that so you find the right capacitor and for the right voltage range that is going to get rid of the parts that you don't want and you do it just like this and it's essentially the same thing you know the capacitor is supplying voltage when it's low it's taking voltage when it goes back up but the effect of it instead of thinking of the voltage as supplying a circuit think of the voltage as a signal to make a circuit do something and so you're smoothing that signal instead of the supply and the only real difference is how many amps are going through so it's exactly the same physical principle you're just using it in a different way you're doing something different with it you can smooth your rectified AC power to make sure that your integrated circuits are happy you can smooth certain frequencies out of a signal to make sure that let's say you have a regular speaker and then you have a subwoofer so you could split the signal run a high-pass filter on one that goes to the regular speaker so it's not trying to replicate all these low sounds it's just doing what it knows best and then you do the low-pass filter you know just whatever other capacitor and of course filter circuits get more complex but you could do it with just capacitors it would be good enough to functionally perform but then you have your low-pass on the other side that sends just the low signals to the subwoofer so you essentially split the signal by duplicating it and then cutting half of one and the other half of the other using capacitors now I can't demonstrate this right now because I don't have an oscilloscope so right now you're going to have to take my for it but as time goes on I will sneak in demonstrations of this as we make circuits because capacitors are everywhere decoupling capacitors everywhere and every time they show up you'll see it and eventually I'll have my oscilloscope and I can show you directly until then be seeing you

Channel: Simply Put

Views: 24,588

Rating: 4.9575758 out of 5

Keywords: simply put, simply, put, capacitor, capacitors, decoupling, bypass, filter, filtering, fast, fourier, transformation, frequency, electric, electronic, electrical, electricity, circuit, circuits, power, supply, ripple, rejection

Id: BpuCv4hfYZU

Length: 17min 59sec (1079 seconds)

Published: Fri Aug 17 2018