EEVblog #908 - Zener Diodes

hi welcome to fundamentals Friday today we're going to take a look at one of the most fundamental components in electronics the Zener diode let's take a look at it now you no doubt familiar with the regular silicon diode and we've got our Schottky diode as well same thing for the purposes of today's talk but we're going to look at the differences between a regular diet and a Zener diet and we're going to take a look at how the Zener diode is useful in a bunch of different applications now your regular diode like this you no doubt familiar that it conducts current in one direction only it only allows current to flow through hence why it's got that sort of arrow shape pointing in the direction of the conventional current flow and if you try and reverse bias it it doesn't allow current to flow back up but the Zener diode is a bit different it works exactly like a regular diode and allows current to flow through like that and it has the same voltage drop you know not point 6 volts to a volt or there abouts but it has a special property where it also allows current to flow in the other direction you might say what uses that the whole point of a diode is that it blocks current going in one direction well there's something very interesting about the Zener that makes it useful let's take a look now you should be familiar with the characteristic curve of a regular diode like this Schottky diode is basically exactly the same except it has a smaller voltage drop and as you can see on this characteristic curve here also called an IV curve because it's I versus V current versus voltage you can see that until you get to about nought point 6 volts no current will basically flow through this so if you've got less than 0.6 volts on the diode of roughly it won't conduct anything won't allow current to flow but once it gets above that what's called the knee the diode knee here then it start can start to conduct current once you get above about null point 6 volts temperature and device dependent and current dependent it will allow current to flow that's it and if your reverse bias and put a negative voltage on ie you go on this side of the curve here no car flows it just stays flat like that easy now a Zener diode works exactly the same as a regular diode when you forward bias it when you have current going down like this and you get exactly the same characteristic response at the knee starts at about nine point six volts and it starts to conduct current but in the negative direction when you try and put a negative voltage on here ie you put the positive here and the negative here and you try and make current flow in the reverse direction a regular Dyer just won't conduct anything but we'll talk about that later but a Zener diode when you reverse bias it and try and make current flow in the reverse direction it will do nothing for a while it'll do nothing but then it'll actually start conducting like a diode in the opposite direction weird and the voltage that it typically does that is called the zener voltage VZ or it might be called the Zener knee voltage or something like that so now we can start to annotate our characteristic graph here with some industry terminology up here we've got the forward characteristic that is the characteristic of the Zener diode in the forward current direction like that with the forward voltage and naturally we've got the reverse characteristic down here which shows what happens when you try and push current in the other direction and you have reverse voltage on it with the Zener breakdown voltage now actually hinted before that regular diodes may have something a bit unusual with them - they actually act like Zener diodes kind of n so - ah key diodes as well will loop him in lump him in the same thing you know how I said when you reverse the voltage on them they don't allow current to flow that's what you typically think of a diode and that's typically how it kind of works in practice but ultimately you get to a breakdown voltage we'll call that V B here and at that voltage it's actually going to do a very similar thing it's going to break down or what's called Avalanche in the other direction very similar to a Zener diode so you might go Dave what's the difference between a Zener diode and a regular well regular diodes are not designed to work down in this reverse characteristic region their uncontrolled their horrible lecherous with temperature and all sorts of they're not designed to work down here Zener diodes have been specifically manufactured specifically doped to actually have a quite a reasonable controlled characteristic in the reverse breakdown region regular diodes and Schottky diodes don't so that's why he uses inner diodes and as opposed to regular diodes in the applications we'll see but just be aware that regular diodes can do it as well and that will be the maximum reverse voltage of the diode typically before it breaks down and the other thing with diode breakdown voltages is it's usually very high for example a one in nine one four diode 4148 it might break down it you know 70 or 90 volts or something like that you know really not a very usable voltage in practical circuit so it's effectively it stops current flowing in the other directing direction for most practical voltages you know a one-in-four double-oh-seven breaks down to the thousand volts for example you know so really they're not of practical value in that negative region but Zener diodes have been specifically manufactured and doped to actually work at very usable and very useful breakdown voltages anywhere from 2 volts up to 30 volts or something like that or even higher and you can use that low-end controlled Zener breakdown voltage for useful circuit applications beauty now just to completely mess with your mind sorry but no explanation on Zener diodes would be complete without actually at least mentioning this there are two different types of Zener breakdown the red one which we saw here is called Avalanche breakdown and that's what happens at high voltages in regular diodes but there's also another type of Zener breakdown actually called Zener breakdown and it's actually this effect the Zener effect the Zener breakdown where Zener diodes get their name from the person who founded it our physicist Clarence Zener way back before you were born now I won't go into the details of doping the pn regions and all the physics involved in actually manufacturing diodes it's outside the scope of this thing really I'm sure you can google it if you want to find out more but just be aware that the Zener effect the Zener breakdown is actually quantum tunneling so quantum physics is involved in this sort of thing hence why it was found by a physicist anyway the Zener breakdown occurs at roughly about 5 volts or below anything above roughly 5 volts is going to be a different effect avalanche breakdown so they're two physically different phenomenon happening inside the Zener diode or both or a combination of both could be happening if it's near to that sort of 5 volt region so you know like a 12 volt Zener diode will definitely be working as an avalanche breakdown and a 2.5 volt Zener diode would definitely be working in the Zener using the Zener effect it's called but you don't need to know that and most people just go the Zener effect even though there actually might be talking about an actual avalanche effect instead of a Zener effect if you're talking about Zener diodes so Zener breakdown voltage doesn't matter which voltage it occurs that it's the Zener effect and just a further clarification on the terminology here the reverse breakdown voltage I put VB before cuz I was talking about breakdown but in the datasheet you'll find that as VR for regular diet so just stick with VR now let's take a look at a typical implementation of a Zener diode here and we're using the reverse characteristic because if we use the forward characteristic Mars well just use a diode it's just a regular diode so that's why in a circuit like this oh I should put that that's positive and that's negative there so you use it in the reverse bias configuration so we're looking at the Zener breakdown voltage and now hopefully you can see why the Zener symbol is as it is look how it kinks up there and kinks down there it looks exactly like the characteristic bingo that's actually where the symbol comes from now some typical labels you'll find on a Zener diode datasheet are VZ which is of course there's then average the knee voltage that it nominally happens at and also there's the current going through the Zener which is called a Zed or it could be I test or something like that but typically or I Zed T for example and then there's this other weird one Z Z what is that Zed Zed of course is impedance so it's AC resistance so it's effectively the resistance of the diode under AC test conditions they usually specify it at a particular frequency there's actually an internal resistance in the Zener which you need to take into account when you implement it in your practical circuit and then of course you typically have a series resistor in series with your Zener as well so this resistor is effectively inside the Zener diode and this diode impedance or diode resistance is also known as the dynamic resistance because it's dynamic it does actually change is not a fixed value and you guess that it changes with current and also as with practically every component temperature as well and not just your ambient temperature the actual junction temperature of the Zener itself because zeners are used as power devices typically so they heat up they dissipate power hmm trap for young players you know how I said diodes don't actually conduct anything in the reverse direction well of course that's not true no component is ideal and zeners and diodes regular ones have the same thing they have a reverse leakage so I drew this as zero because well it effectively is you can't really see it on the graph typically because you're talking milliamps down here and the leakage is typically in the order of micro amps tens and micro amps things like that for pretty much both types until but as you can see the knee here is not a sharp bam knee it does start to taper like that hence a real knee is not just a right angle is it no shape like it me hmm that's the same thing so let's take a look at some typical applications for Zener diodes and there are two main applications the first one we'll take a look at is regulation ie voltage regulation because you can see that then why I've seen on the characteristic curve that the Zener produces a stable voltage once it hits that knee and that can be used for regulation so take a look at the classic configuration where we've got an input voltage here we've got a Zener dropper resistor here R Z and then we've got the Zener diode itself with its internal dynamic resistance remember that it's important and that produces a voltage drop across it called V Zed let's have a look for the particular case of a one in 4733 which might be a typical you know medium power voltage regulation Zener diode so let's take a simple example where we've got no load here so we're just generating a reference voltage here from an input voltage VN of 12 volts and with the load open so I've disconnected the load air so we're only getting we've got two components the Zener diode and the dropper resistor here so let's go to the datasheet to look at the test current ie the nominal current for the given particular type of Zener that we've got to produce the nominal voltage and in this case i ZT there's actually two version values i z t we're looking at i said T 1 here and ZZ T 1 they got multiple ones just to show you the difference in the dynamic resistance anyway i z t is 50 milliamps and z ZT at that 50 milliamps is going to be 5 ohms but we can actually ignore that as you'll see in a minute because it's a cup you know it's an order of magnitude at least less than our dropper resistor is going to turn out to be so we can just take it out of the equation to keep things simple so what value dropper is this 2 RZ do we need it's easy it's going to be the input voltage minus the voltage we want the Zener voltage five point one and that will give us the voltage drop across the resistor here and then we're just following Ohm's law resistance equals voltage divided by current so it's the voltage drop across our Z divided by L Zener current our 50 milliamps our test car because all the current flows through the Zener there is no load so it all flows through and you punch that in and that gives us a value of 138 ohms so if you add 12 volts in and you want a five point one volts from you Xena you'd use 130 roughly resistor you can now see why the internal resistance the dynamic resistance of the Zener five ohms doesn't really matter it's more than an order of magnitude out from 138 ohms especially when you got no load doesn't matter easy and that's all fine and dandy and assuming your temperature didn't change that Zener diode would happily regulate your voltage at five point one volts but the definition of regulation is keeping a fixed voltage regulating the output when your input here varies in voltage so let's go through again and see what happens so if we actually go into our data sheet here and have a look at the value for Z Zed I actually read the wrong value off the table it supposed to be seven ohms for this particular one but we'll just stick with five ohms you'll notice that at I said to that second test current which was a one milliamp instead of 50 milliamps the dynamic resistance is like five hundred ohms or something it's absolutely huge but the good thing about Z said you can read it from the table it's it is going to change with relative to the current are somewhat fairly linearly but you know you can take that figure as a fairly stable one for most practical design calculation so if we increase our voltage and increase our current through our diode let's just stick with the same five own value if we decrease our input voltage a bit then we can stick still stick with the five ohm value even though the dynamic resistance is going to go up a little bit you've got to work from something unless you had a full parametric graph from the datasheet which often you don't get so let's take the example where our input voltage changes from 12 volts we had before up to 15 volts we've still got the same resistor you can't change the resistor we're after it's in your circuit so input voltage changes how much variation on our Zener voltage do we get ie how good is the regulation of this thing well let's see we need to get the differential so with these figures fifteen volts hundred thirty-eight ohms and our impedance which now matters and comes into play because we're got a variation in our input or our load current in this case our low current hasn't changed but our input has certainly changed so our dynamic resistance comes into play here so we calculate our current again because it's going to change because their input voltages chain inch so at once again Ohm's law the voltage drop across our ZD here is the voltage on either side of it 15 volts minus our five point one volts we had before let's just take it as five point one before because we're just getting a rough differential here and divided by one hundred thirty-eight ohms Ohm's law 71.7 milliamps we had 50 milliamps before now it's gone up to 71 point seven so we've increased our voltage it's naturally what you'd expect but here's where we get the difference in the current from what we had before ie the Delta that's what that little triangle is don't be scared by the Delta it just means difference or change in so the change is the value we have now seventy one point seven milliamps the value we had before 50 milliamps we've increased our current we've changed our current we've got a delta our current change of twenty one point seven milliamps our currents going up what does that do on the output when you have the current flowing through here and it's higher than what it was before before we had five point one volts here V Z for our nominal dynamic resistance but now we've increased our current there's going to be extra voltage drop across this internal resistance so Delta VZ ie the change in our Zener voltage the regulation is equal to our Delta in our current our change in current times our resistance Ohm's law nothing fancy here equals 21 point seven million times five ohms a change of 0.1 one volts or there abouts so our regulator output voltage has gone from five point one volts to now five point two one volts 2% or thereabout so it's an okay sort of you know two percent voltage regulator yeah kind of does the job not terrific but okay so you can see how even with no load on the output Zener diodes aren't that great and the other thing you might have noticed we've got no load no load current at all but we're pissing away 50 milliamps or 70 milliamps just to regulate l 5.1 folds that's ridiculous you can use a 7805 regulator and regulate that 5 volts exactly the same and it takes bugger-all quiescent current this thing takes 50 or 70 milliamps quiescent current there hopeless as like a regular voltage regulator carrying no load so low currents they're very inefficient and the other thing we have to be careful of this is a 1 watt nominal power dissipation Zener are we within the power dissipation limits of this particular Zener diode well the power in the Zener diode is the voltage of the Zener diode times the current five-point-one times 50 millions quarter of a watt no worries and you might be thinking Dave that quarter watt is going to that's a fair bit of power in just a little package like that it's going to increase the junction temperature yes it will and that'll change the dynamic resistance and everything and it yea gets more complicated I could go into a lot more detail but I don't think we have time now unfortunately it gets a little bit more complicated because well the real world is a little bit more complicated let's add our load back and let's say our load has 50 milliamps we're still got our 5.1 volt Zener voltage that's what we're shooting for that's our regulation voltage your input is 12 volts again let's have a look we've got to figure out a new value of R Z because 138 ohms we had last time my ball is almost certainly not going to work for this particular current because we've got now got two currents one flowing down the load but we also have to maintain that test current that a bias current through the Zener diode remember there 15 milliamps we got from the datasheet so we still need 50 milliamps down here but we also have to account for our load down here and let's assume our load current is 50 milliamps then we've got 50 milliamps down here 50 milliamps down here Kirchhoff's current law we've got 100 milliamps flowing through our resistor here so we work out the resistor value the same way it's a differential volt it's the voltage across the resistor the difference which is 12 volts VM minus 5 but instead of just the diode test current we've now got the diode test current plus the load current so it's a total of 100 milliamps yeah work it out 69 ohms Beauty Goodyear but what happens if our load changes what if your power in a microcontroller that draws 50 milliamps during operation then it goes to sleep what happens well let's check it when IL drops to zero all of the current must flow through the Zener diode we've got our 69 ohm resistor here because it's in circuit we can't change it so our voltage across the resistor is 12 - let's assume it's still five point one volts doesn't change a huge amount the regulations ain't reasonable / l69 a.m. resist up much lower than hundred thirty-eight we had before with negative hundred milliamps all of that hundred milliamps is now flowing through the Zener Oh better check the power dissipation to make sure we still within limits of our Zener diode P Zed five point one times hundred milliamps it's now dissipating half a watt whoo it's still within our one what capability of ours in a dire but it may not have been we may have found if we use the hard wads enter in there at my Cook if we used a quarter watt Zener in there which might have just been enough before when we're dissipate in a quarter watt in here if our microcontroller went to sleep magic smoke would escape from our Zener hmm you can see how it starts to get complicated what if our load is changing all the time in our input voltage is changing all the time you have to redo all the calculations and check and get a compromise value for your Zener dropper resistor and it's not pretty there but they still so as for a voltage regulator for power in a circuit it's okay if you don't care about the efficiency of it and things like that but they as I said they have more use in sort of more niche applications within bigger circuits and things like that you know reference voltages and stuff like that but yeah that's how regulation works and you're still going to do even if you're using in low-power examples you're still going to do the same sort of calculations but this is why often especially in reference circuits and things like that you'll find that the Zener is actually powered from a constant current source so it's driving a constant current through and it's everything's much simpler but anyway that's basic Zener diode regulation can get quite complicated then if the temperature changes in the die temperature rises and I'll let you redo calculations as an exercise for when the resistance vary changes in the input value changes and the temperature rises go and find the temperature coefficient on the diode in the datasheet and have a play now the other huge application which I can spend an entire video on and I probably will in the future is clipping and more importantly clamping protection circuits so not only used in sort of like audio applications if you want to clip an audio waveform for example there might be some audiophile reasons why you might want to do that but one of the big ones is protection let's say you've got your IC here being a microcontroller whatever it is you can actually use zeners for protection they're actually pretty decent devices for protection and you have the series current limiting resistor of course and the zener can clamp the voltage let's say you're powering a circuit from five volts here you might choose say a 5.1 volt Zener like this and it can if you've got a huge spike and your input here make up 250 volts or something you choose a suitable resistor you calculate the power dissipation and things like that and you will it'll clamp it at five point one volts so you don't blow up your chip beauty and the good thing about this is that also in the other direction if this input goes negative what happens it acts like a normal diet it conducts and clamps it to nought point six volts below the rail here and we'll be able to demonstrate this now I won't go through the full math fer choosing the correct input value as I said separate video but take the one in 47:33 diode we have before the five point one volts it's got a nominal 1 watt capability but if you have a look at the datasheet it's also got a surge current as well for this particular one it's about 900 milliamps and that's like at five point one volt clamping that's like four-and-a-half watts but if you read the little asterisks down the bottom of the datasheet it tells you that only applies for 10 milliseconds and there's actually a typical you can get typical D rating curves and things like that here's an example of how you can dear eight the power and from that you can just calculate how much power up or pulse power you can actually get in a particular component but yeah that's some data sheets don't that at all and there's one neat little configuration which is two Zener diodes series here and what this does let's say I've got an audio waveform coming in like this it's going positive and negative it's an AC waveform then the Zener diode here is operating in the reverse characteristic this one's operating the forward characteristic so it's operating just like a diode here so when the waveform goes positive it's going to clamp it at the particular voltage of that Zener plus the naught point six volts drop across this other Zener which is acting in the positive region acting like a diode and it'll clamp your waveform at the whatever value you choose for these dinners and then when the waveform goes negative the reverse happens this one here is operating in the reverse characteristic this one here is operating as a diode it does the same thing it clamps your negative wave form at the Zener voltage plus not 0.6 volts and we can demo these sort of things and there's lots of other clamping applications and clipping circuits and all sorts of weird and wonderful configurations you can do with zeners this is one of their huge applications and check it out if we use our rode and Schwartz HMO scope here it's got a built in component tester and bingo we can get the characteristic curve that which voltage do you think this is oh let's have a look here we've got current on the vertical scale so it's an IV characteristic curve it slowly goes look up to 10 milliamps down to minus 10 milliamps here and voltage on the x-axis here hence the V so bingo we've got our nought point six volts there of our characteristic that's the forward characteristic and our reverse characteristic bingo it's about five points something ish it's actually a five point one volt Zener beauty and if we swap it around in the other direction what's going to happen you guessed it the characteristic is in the other direction nor point six volts here five point one volts there beauty now although this doesn't let me expand the scale here you can actually still see the knee in there is not a really sharp and that line there is not action leave vertical it actually slopes slightly in that direction due to you guessed it the dynamic resistance and if we just have a rudimentary example here with a 5.1 volt our Zener I just got out of the jump in Choza 1k droppers this to ant whatever I don't know the datasheet for it just put in a you know in nominal resistance value might be a bit higher but whatever anyway we can see that when we switch it on it's going to regulate at roughly five point one volts no worries and that's like seven the half volts input here so as you can see if we go down below the threshold it's just going to go down as well it's not going to regulate but anything above five point one there we go we're up at ten but you'll notice that it is actually going up and that's due to of course the dynamic resistance we just learned about what I've done here is drop the resistor by an order of magnitude down to 100 ohms or so and if we actually go right up to 15 volts here let's have a look we're now getting five point four volts but look it's actually increasing increasing increasing because the junction temperature of our Zener is going up so there you go whoopsie it's not regulating too well is it hmm and there's a bigger differential it went from five point to two volts up to five point four seven volts as opposed to with the 1k it went from if there was only a hundred millivolt change basically now there's like 200 plus millivolts differential big difference and a very simple clamping example here I've got a five point one volt Zener or 1k dropper and the yellow waveforms the input it's just a seven volt er square wave which just goes down though it goes between one and seven and the blue waveforms the output across the Zener diode and you can see both one volt per division the Zener diode clamps at output voltage at 1 2 3 4 5 volts I've got both channels set to the same ground positional reference there and the Clancy output nicely and it's going to do it very very sharply there's going to be no issues there whatsoever in terms of a response time and stuff like that but you can see if we go in there ah look at that that is our input and our output that is due to our dynamic resistance it's not going to fall it precisely it is not an ideal Zener diode so yeah but it's not going to overshoot it's never going to overshoot beautiful for clamping I'll just show you the difference between the 1k and the hundred ohm resistor here I've got a 1k in there at the moment and I'll keep the same time base and you can see it has a particular characteristic response let's pack back in the hundred ohm there we go it is significantly sharper now I'm going to show you the AC clipping there's the zero volt point for both waveforms blues the output again I'm feeding in 15 volts peak-to-peak on the yellow waveform there and you can see the blue one is definitely clipping at that bingo and what's a clamping at 2 volts per division and two four six it's so these are both 5.1 volt centers it's because of that additional diode drop operating the forward characteristic region that adds in there not quite not point 6 volts in this case it's going up to 1 volt and because of the current and everything else the characteristic but there you go it's out of the five point one two the diode drop and positive and negative clamping lead and just to show you that Zener diodes don't work at arbitrarily low currents I've got my Keithley current source here at five point one volt Zener I've got this is the decimal point this is the milliamp mozo at one zero that's ten milliamps okay and we get in L five point one volts it's you know hunky-dory I can go like warai towards getting a bit you know it's getting a bit how you're doing when we go up in current towards one hundred milliamps anyway let's drop it down a range okay so we now are one milliamp it's still working just fine let's go down to the my crap train so we're a hundred micro amps now look 100 mike is nine point four five volts is starting to drop what happens if go down to 10 micro amps not looking too good is it one micro amp nope so you can't go down to arbitrarily low current Zener diodes don't work at low currents like this even with a 10 volt compliance voltage look right 10 volts is plenty of compliant this is the maximum the compliance voltage means the maximum this current source will output even if I go up to a hundred volts compliant source okay it's a hundred volt power supply but it's a constant current of one micro amp it just can't do it it's not enough current for the Zener dilator anyway I hope you enjoyed that look at Zener diodes it's been much longer than I thought if I can maybe do this in 15 minutes now it's been at least double that sorry anyway if you want to cover Zener diode sort of that and we didn't even go completely in depth there yeah these sort of things take time fundamentals take time to learn unfortunately anyway if you liked the video please give it a big thumbs up discussed below all that sort of stuff hope you enjoyed it catch you next time today we're taking a look at a real basic building block circuit called the peak detector now what a peak detector is if you've got an analog input signal that you want to know what value it peaks at as the name suggests if you've got your it could be a voltage like that you want the positive peak voltage on that or negative it's much easier to do it with two simple components turns out all you need to do for a Peter Tech

Channel: EEVblog

Views: 374,404

Keywords: eevblog, video, fundamental friday, zener, zener diode, doping, pn junction, semiconductor, circuit, oscilloscope, test, tutorial, how it works, clipping, clamping, protection, reverse bias, current, voltage, transient, curve, characteristic

Id: O0ifJ4oVdG4

Length: 32min 34sec (1954 seconds)

Published: Fri Aug 05 2016