Hi Welcome to Fundamentals Friday Today, we're going to take a look at a rather unusual and often little known aspect of ceramic capacitors. In fact, it's a really undesirable aspect and a real trap for young and old players alike if you're not aware of it. So let's have a look at some Basics first and then we'll very quickly. Then we'll jump over to the bench and I'll show you.

so I won't tell you what it is yet. I'll let you figure it out on your own. but uh, you know a capacitor, right? Simple RC Charge Circuit Like this. very common application for capacitors.

Uh, RC Time constant charges up, gets to a certain threshold and you can detect that and you can use them as timers. Very typical, Uh. application. Not that accurate because as we're aware, capacitors are not particularly high tolerance devices.

and if we take a look at, uh, some typical ceramic capacitors, we're only talking about ceramic capacitors here, then, uh, they basically divided into several classes. Class One capacitors are your NPO and your Co G type and a couple of more obscure ones that you've probably heard of. And as you might know, they are actually very stable capacitances with temperature. Okay, so once you've actually measured them, then they stay pretty stable.

They basically do not change with temperature, so they're typically your low values. You know your one nanofarad and under you can get in these M Cog types, but your larger capacitance values. You know your hundreds of nanofarads. your microfarads, all your modern ones.

You know you can get like hundreds of microfarads now in little surface mount ceramic packages. The technology is absolutely incredible anyway. I Won't go into a huge amount of detail, but basically you've heard of these codes before. Things like uh, X7r for example, what does that mean? Well, uh, there's a three-letter code like this.

It's part of the Um Eia Code system and X for example. Uh, the first letter represents present the low temperature so - 55 and the next digit here represents what the high temperature limit is. In the case of X7r, it would be Uh - 55 to+ 125 C and the r part is the tolerance of the capacitor this case, X7r plus - 15% Pretty basic stuff. or an X5r for example, is basically the same capacitor but with a slightly lower higher temperature range.

And you're familiar with these things. And as you might know, if you're an experienced designer, picking something like an X7r is a relatively stable uh, high value capacitor to use a relatively stable dialectric. So you you might use, say, an X7r or an X5r in an RC time constant like this. But aha, wait until we get to the bench.

I'll show you a trap. It's not just about the tolerance here. Now if we look at the Uh RC circuit which I'll use in the example on the bench in a minute, you're familiar with the charging Uh graph, charging response of a capacitor and time constants one time constant or t as it's known is the simple formula R Time C And you know a basic building block circuit. Basic building block formula you should be familiar with and one time constant is the point.

Here it is One time time constant is on the X-axis here, and the percent of charge is on the Y AIS here. And if we put a step voltage in like this, then our capacitor will charge up with a a Um exponential Uh formula. The main formula is actually down here, but you can simplify it. We won't go into details on that.

We'll just use our simple R * C formula today and we'll be measuring this one time constant period. One time constant is where the voltage charges up to roughly 63.2% of the final voltage. Why that? that particular number? Well, it has to do the fact that uh, when you plug 1 E to the power of minus one up here, that's the number that actually spits out of the formula and it's useful. And basically a capacitor is essentially fully charged.

You could say after roughly five time constants or five times RC a capacitor is fully charged. So if you know your resistance, you know your capacitance. You can figure out how long it takes to fully charge that cap or get to one time constant. Pretty simple, pretty basic, and of course, there's more to this as well.

There's a class three down here using yet more exotic Uh materials and yet more unstable materials. Again, for ceramic capacitors, so pretty much class one is the most stable Class 2. They're okay, but as we'll see, there's some few traps in there and well, class 3 is even worse. So let's not go there.

What we'll take a look at today is an X5r or X7r capacitor on the bench and we we we won't be taking frequency into account as well. It'll be nothing to do with that, it'll just be the capacitance this tolerance figure here. So if we have a look at, say, the X5r capacitor we're going to go look at on the bench. Now it should measure plus - 15% Does it Well, Yes and maybe no.

Come with me. Okay, the first example we're going to look at is a typical 08 5 Class 2 ceramic capacitor from Mada one of the Uh top manufacturers. And here's the full part number. Got this from Uh Fels and it's a multi-layer uh ceramic capacitor x5r basically the same as XR 7R.

It's just slightly lower temperature nominal 10 microfarads and 6.3 volt rated so one of these you know, low voltage rating ones. You typically only use them on 5 Vols or 3.3 Vol rails or something like that. Now it's a nominal 10 microfarads with with a tolerance defined by this letter. Here In this case it is K and I'll show you the data sheet in a second.

that is Plusus 10% Now that's not the X7r, the X7r value of Plusus or X5r value of Plus - 15% We saw before that's its temperature or change the temperature coefficient over that full temperature range. So, but this is the initial tolerance as you'll measure it and buy it from Mada straight out of the packet here. So there you go. You have to decode these rather complicated uh part numbers for these things and you'll notice that the 102 is the value there.

10 Microfarads and we've got K Number Eight. So we have to drop down to number eight and see what that is. And here you go. Here's the full table available from Mada and K.

There it is Plusus 10% So assuming that we measure it under the defined measurement uh conditions for this capacitor, then we should measure 10 Microfarads plusus 10% Okay, now what frequency are we supposed to measure this at? What do they the specs Define it as well. Here it is capacitance. If it's greater than Uh 10 Microfarads, there it is at 0.1 khz 100 Hertz There you go at a 1V uh test voltage Plusus 2 Vols RMS So well let's use our LCR meter. Here it is I've surface mount solded one of the capacitors onto this little adapter board so we can plug into our breadboard.

No, it's completely cooled down from the temperature change. It's at room temperature. We're like 23 see here in the lab. So the temperature is not going to change during this measurement and 100 Htz Here we go.

Look, we're practically bang on to our 10 Microfarads value. Okay, let's round that to 10 microfarads. Okay, this is spot on. Okay, so let's Mount that on our breadboard here.

We got our bang on 10 microfarad capacitor there. We've got a 1K resistor in series with that, so we' got that RC circuit I showed you on the board before so we should be able to see the Uh charge of the capacitor there. and I've got that hooked up to my function gen here and I've got it output in the Square wave. So we get a nice big step uh change from 0 to 5 Vols Remember this is a 6.3 rated cap so we're safely under that Uh 5 Herz repetition rate with a 20% uh 5 Herz frequency with a uh 20% duty cycle.

So that will allow us to see the charge and discharge cycle of the capacitor. And here we go. If we look our scope here, let single shot capture that. Bingo There is the charging of our capacitor.

Now there's our green step change on the input there and we're charging up, charging up until we get to 100% and we're basically then dropping it back down. And we don't care about the discharge period here. we're only going to be looking at the charging period so it's got enough time to charge up. So there it is.

there's our beautiful charging waveform. Now, from this, we should be able to turn on our cursors, measure one uh, time constant here, and from that one time constant equals R * C. We should be able to verify that that capacitance is correct. So if we go into our select our cursors here, let's go the Y right down the bottom like that Y 2 Let's Uh, take that what we want because we've got a 5V uh signal here.

We basically want that to be uh, 63.2% of 5 Vol So that is 3.16 Vol. So we set our cursor our Delta y to 3.16 Vols. We'll get close enough. Okay, just in the ball 3162 There we go.

So that's the Delta difference between there. so that height is 63. uh, 2% or one time constant. Now we go into our X here and we want our X right at the start period period there, and X2 cursor right where that intercepts the waveform.

That's pretty close to spot on there. and we're getting 10.6 milliseconds as the difference there. Aha, let's plug that into the formula. All right.

So what do we have here? We measured precisely. Basically, we rounded it to precisely 10 microfarad. Uh, t = r * C. We measured that time constant to be 10.6 milliseconds = 1K * C.

We're trying to calculate C rearrange the formula 10.6 milliseconds over 1K = 10.6 microfarads. Hey, that's not bad. That's pretty close Ry Uh, 6% out there still with inside that uh, 10% uh value that we you know the data sheet value. Yeah, it's a little bit off.

it is 6% to what the our really accurate agilant LCR meter told us. But hey, you know you could put that down to uh, the error in the cursors and you know things like that eyeballing this sort of stuff. So eh, we're going to call that near enough. Not a problem.

Okay, do it one more time for uh, 0 to 6 Vols and I've done the cursors here and we basically getting 10.1 milliseconds or translates directly into 10.1 microfarads. Everything's hunky dory. But what happens if we change this volage and we add a DC bias voltage in here? So our low level is is our waveform is going from 0 to 6 volts. As you can see, here's our ground reference level here: DC coupled uh, inputs and 1, 2, 3, 4, 5, 6.

So we're getting six divisions there. What happens if we actually set this thing? So our low level here is say five volts. So it's going between 5 and 6 Vols So the capacitor always has has a constant bias voltage of 5 volts on it. Let's have a look what happens.

So if we do our single shot capture there, There we go. There's our ground level down there still exactly the same as was was before: 1, 2, 3, 4, 5 volts. but now our for and now our waveform is now superimposed biased 5 Vols up. But hey, that should not affect our formula at all.

The time constant doesn't matter. Uh, about what the initial value is. it's all about that step change. It makes no difference.

Trust me, the formula ain't wrong. It's been around forever and it you can rely upon it. It's 100% correct. So let's go in and measure that, shall we? H So let's take that up a little bit more, shall we? And let's bring that right down to the here say bring that waveform down to here and then we can capture that again.

And bingo, we can go in there and measure that. I'll even bring out the horizontal a bit so we can get a reasonable amount of accuracy in there. Once again, we go in there and we use our curses. Now we've got to set.

Oh, let's set our y1 minimum our Baseline here right down the bottom. No problem. Y 2 Let's go up because it's a 1vt change we're looking for 63.2 Delta here. Difference: 63.2 There we go.

Excellent. So that's fine and we're going to X1 over here. X1's already set. it's that cursor over there so we won't mck around with that.

X2 Bring that over to the point where that waveform crosses there. What do we get? a Delta a time constant, time times difference there. Once again, we still getting to that 63.2% one time period. But look at this.

4.82 milliseconds. You whack that back into the formula. You get 4.82 microfarads. Our capacitance has halved.

So if we go back to our full formula here for the Uh charging curve of a capacitor, we'll actually just change it slightly. V0 here is actually V origin. It's the start of where the voltage starts at. Okay, so it doesn't necessarily have to be zero like we saw in this case.

it's starting from 5 volts and going up to 6 volts. The formula still holds. I Mean you know you read that in any textbook and it is not wrong. Okay, it is absolutely spot on.

And of course, uh, tour is RC And well, so what is varying? Okay, our start voltage is staying the same. We've measured it with our oscilloscope. There it is. we're going up to the 6 Vols and uh, we where we were measuring the time constant.

Uh, we were measuring the time period correctly. That 63.5% of the total change. Now our resistor in here are. well.

a resistor is probably the world's most basic component. These things don't change. Okay, so it's it's a fixed. know 1K So what is left? What must be changing? Because we've measured all the voltages, we've confirmed them with the oscilloscope.

Well, you guessed it. See, the capacitance is changing. It has changed so it's dropped from that Uh value we had before of Uh 10.6 microfarads. It's dropped down to 4.82 microfarads with that 1V range with that 5vt offset.

and it's got nothing to do with the 1V range instead of 5 volts. We can confirm that again. Let's go back. remove the DC bias and check that one Volt range.

It's not the range of the voltage, it is the DC bias which is causing this capacitance to drop. And here we go just to confirm. I've set it back to 0 to 1 volt. there.

and there it is. There's our 0 to 1 Vol waveform and what do we get? So it's exactly the same apptitude as before and we get look bang on 10 milliseconds. Wow, bet you didn't know that these Class 2 ceramic capacitors changed their capacitance based on the applied DC bias level. Unbelievable.

Who knew? They don't tell you that in the data sheets. The bastards. But not only that, the capacitor also changes with the applied DC Uh voltage level as well. not just a DC bias offset, so the AC level as well.

In fact, the capacitance can go up, not decrease as we've seen with an an applied Ac voltage depending on the level, depending on the Uh construction technology and the dialectric used in a particular type of capacitors. and it can even change fairly drastically between the same family with different size capacitors. Unbelievable. So the same family, the same type, the same XR7 rating or whatever It is, it can change.

Man, hate capacitors. So are you shocked? Well, you should be. Now let's go on to a different type here. This is a pretty horrible Y5 V uh ceramic cap minus 20% plus 80% initial uh tolerance 10 microfarad, 16 volts and uh, let's measure it and see what we get.

So I measured a value on the LCR meter of 8.65 microfarad. So let's see what we get on the scope. I won't bow you with the operational details. So here you go: 7.8 microfarads, 7.8 milliseconds, exactly the same resistor everything else, so it's reading quite significantly lower than that 8.6 we were getting before and well, that's not a mistake.

As we've seen, capacitors can vary with bias voltage and also other things like the just the basic applied voltage. Now let's check this same y5 V capacitor from 0 to 10 Vols instead of 0 to 1 vs 1. Once again, there's no DC bias here at all. What do we get? Well, look at it, it looks substantially different.

And there we go. Look at that 4.6 milliseconds goodness. Our capacitance once again has like halved. pretty horrid.

But look at this. the wave shape. the charging waveform has actually changed. It is not that sort of you know rapid rise and then the Decay Like that it sort of goes up like this and then decays.

Totally different to what we saw before. Ah so these Wi5 Vcaps are absolutely horrible. and there's some weird, you know, physics going on here. Um, based on the dialectric and the construction of these uh, y5v capacitors, they're nothing like those X7r that we got before or even like this same wif V But down at 1 volts, all we've done is now Chang it to 10 Vol There we go.

we're 2 Vols per Division And like the characteristic of the charging of this thing has changed. Oh man, you definitely don't want to use these for any sort of timing application. And what happens if we add a bias level? We'll add a 9vt DC bias level. Remember this is a 16vt cap.

This cap will only go up to 10 volts maximum output voltage on a square wave. So we're going from 9 to 10 so we're going back to that 1V uh difference that 1V change. But let's have a look at the waveform and this is what we get now. Yes, we're back to that uh characteristic shape that we saw at 1vt.

that proper, you know the curve you actually expect for the charging of a capacitor. But look what, we're at 1.5 milliseconds, 1.5 microfarads, it's dropped by. you know, not quite an order of magnitude. But jeez, it's getting there.

and this is for a 16v r ated uh, you know WiFi VC cap and that we measured the Val measured the capacitance of with our good LCR meter. it's hopeless. Now, the real problem with all this is that this is rarely mentioned in data sheets. There are exceptions to this.

Some manufacturers do actually remind you of it every now and then, but hey, sometimes it's almost next to impossible to find for your particular manufacturer. Now, this particular Mada capacitor we're actually using here. Uh, we use first of all, the 0805 here, the 6.3 Vols 10 Microfarads. There it is there.

and if you jump on over to the product uh page here. By the way, it's not in the data sheet. you have to actually go to the manufacturer's product page for that specific capacitor. And here, look at this.

Look at this graph right here. Maybe I could, uh, zoom into that? Perhaps not that great. But here it is the capacitance. Oh no.

Here it is the DC bias characteristics of the capacitor. Look zero is like this is the change in capacitance I.E the drop in capacitance with the DC bias level. And that's exactly what we saw here. And look at that graph.

It can drop as much as 60% by the time it gets to 6.3 Vol So it's 6.3 volt rating. It's dropped by a massive 60. The capacitance has dropped by 60% It's unbelievable. And look over on this side here here are the Ac voltage characteristics I was telling you about I Don't have time to um set up experiments to uh verify this one today I Want to keep it a bit short? as short as possible.

Probably be gone long enough already. Anyway, the capacitance change with Ac voltage RMS look quite significant up to like plus 15% minus 30% right down at low signal levels. Unbelievable! Who knew this stuff right? And um AVX are quite, um, decent. They actually remind you of it here.

it is capacitance change versus bias voltage. Look at the DraStic drop off in this particular one I Mean this is just awful. Look at this: Uh, this is for one of their general purpose multi-layer ceramic capacitors. So the it actually goes up a bit.

As I said, it can actually go up at small DC bias levels and then it it drastically drops down to you know once again, these are high voltage caps. but it can drop way way off 80, even 90% or more of your nominal rated capacitance just by adding DC bias or by uh, changing that Ac voltage. And anyway, you can get some interesting, uh, little obscure articles here which tell you all about the physics and stuff like that behind all, how how all this sort of stuff works and how the various crystalline structures work. And look, did you see know that the crystalline structure changes with temperature? Look at that.

This is for certain type of dialectric and construction Uh, which is found in typical Class 2 and Class 3 uh multi-layer ceramic capacitors. So depending on the temperature, it actually changes the crystalline structure and then the DC bias it goes into try and explain the physics of how that actually works and how the DC bias affects the Um actual capacitance. So anyway, I'll link in this stuff down below. Check it out and there's a lot more to it.

Some manufacturers don't even mention it, but wow, it's a can be a real trap. Capacitance ain't capacitance. And here's an example of Um AVX actually showing you the effects of voltage. In this case, it's the AC Uh voltage.

Capacitors change versus Ac voltage as well and you can see how the capacitance can actually increase uh drastically. You know, 50 odd per or so depending on the Ac voltage applied to it for these multi-layer ceramic capacitors. So yeah, really tricky business. Now a couple of Manufacturers namely Uh Mada and AVX Um.

these are the two of the better manufacturers in this field for analyzing and trying to correct. You know they're always improving their manufacturing processes and stuff like that For uh, to you know, try and eliminate Um this kind of effect although it's next to Impossible with the Class 2 capacitors. but at least they're aware of it and they do allow you tools to actually simulated it as well. The Um Mada website has a tool called Sim surfing and it allows you to actually uh, plot this stuff with bias values and all that sort of stuff.

and AVR as well they have uh, spy cap, uh software which will do a similar thing as well. So you can play around with uh, these simulation tools, but there's nothing better than actually whacking it on your bench and seeing it for real like we did today. And I won't bore you with the details I Leave it up to you to uh, experiment with this. but if you uh, drop that DC bias level like say, one volt at a time.

So if you went from from 9 to 10 and then 8 to 99 and 7 to 8 and 6 to 7 and so forth and dropped it down or increased it, then if you plotted that and measured your capacitance at each value, you would get that similar characteristic huge big characteristic drop in uh, capacitance versus your bias voltage. Exactly like the manufacturers tell you if you can find their data. So there you go. That's an interesting fact that not a lot of uh even experienced design Engineers know about because well, they just throw their capacitor in.

they assume it works. Yeah, they know about all sorts of other characteristics of the ceramic Uh caps and all your temperature coefficients and mainly all they care about is temperature. But what a lot of people don't think about is that capacitance can change with voltage as well. Not only DC bias, uh, voltage uh, that can have a drastic effect, but also the applied Ac voltage as well.

So it's not just like RC time constants like this. Of course, if you're using the same Uh circuit for you know you're doing filtering and things like that, it can really matter based on the signal level. It can be really quite critical. so you have to be very careful with how you use these modern ceramic capacitors.

They're fantastic with all this wonderful materials technology that goes into and give you incredibly High Capac in incredibly small uh volumes for SMD and stuff like that. But yeah, there's a few downsides. and it's not just temperature voltage as well. Trap for young and old players alike.

Certainly now this only applies to Class 2 and above ceramic capacitors. It does not apply to electrolytic capacitors, tanms, and uh, class one, Npos and things like that. So yeah, and there's of course I've done videos on other uh traps with these ceramic capacitors as well. p Electric uh effect of course.

Well, I'll link that in down below if you haven't seen that video. So there's lots of stuff to think about here. Man, so many traps. got to be careful.

Electronics Design ain't as easy as it seams on the surface. You dig deeper and deeper. and deeper. And well, if you're in a critical application, this can be a really big deal.

So there you go. Hope you enjoying that video. There's so much I can do on this subject. I Can measure you know? and hey, let's not even get into CH frequency, uh, changes and and all sorts of other stuff thrown into the mix.

Oh goodness. could do hours and hours of videos experimenting with this sort of stuff. but you should, uh, get to the bench and have a play around with it yourself. It can be rather FAS fting So there you go.

Um, hope you enjoyed that one. If you want to discuss it, jump on over to the Eev blog. Forum link is down below if you like fundamentals Friday Please give it a big thumbs up even though well, it's actually Saturday I'm shooting this on yeah I'm a bit late Anyway, catch you next time.