All You Need To Know About MOSFETS To Fix Stuff! How Mosfets Work Fail Test In & Out of Circuit

hi guys welcome to another learn electrolytes repair video in this video we're going to look at mosfets so this is all you need to know about mosfets to fix stuff before you watch this video i would suggest if you haven't already seen it that you watch or you need to know about transistors to fix stuff because we're going to start by comparing a mosfet with a bipolar transistor mosfet in fact is a type of transistor and mosfet stands for metal oxide semiconductor field effect transistor which is a bit of a mouthful so we'll call them mosfet mosfets are quite analogistic transistors in some ways so with transistors you have two types npn and pnp but with mosfets although we have the equivalent of npn and pnp we actually have four types so let me explain to you what we have first we can look at the differences between field effect transistors mosfets and bipolar transistors and then we can look at how to test them with our multimeter or component analyzer and lastly we can have a look at what goes wrong with them normally and how we can test in circuit so the first thing we need to look at is some schematic symbols for mosfets so you may well know bipolar transistor we have npn like so and we have pnp which can draw this way up the difference being the way the arrow points and the pens are the base collector emitter and i'll do this one the other way so base emitter collector and that will come clear why i drew this away in a minute the equivalent of an npn transistor is an n-channel or n-type mosfet the symbol for your n-channel mosfet is like this and we can put a circle around this and we have the three pins we have gate we have drain and we have source and these are basically analogous to the pins so the drain effectively is like the collector of a transistor bipolar source like the emitter bases like the gate there are a few variations on this symbol the gate connection can be the other way up usually is in fact so usually this would go such a way but it really isn't important so we can have this sort of symbol and we can also have a symbol like this one where the gate connection actually showed in the middle so we have again one thing you might notice and i always found a little bit strange as with an npn bipolar transistor the arrow is pointing outwards from the circle but with an n channel mosfet it's pointing inwards it's the opposite way around at first you might look at this and because it's pointing inwards think it's a p-type it's just something you need to get used to so these are n-channel mosfets whichever symbol you prefer to use with a p channel it's much the same apart from the direction of the arrow so i'll just draw the one but again we can have the various variations of this and now the arrow actually points the other way outwards which again to me is a little bit strange if you like drain source gauge so those are the symbols for your bipolar transistors and for your mosfets and these are p channel or p-type mosfets i'm sure i don't have to draw to the two versions because you can see what they would be like you will also find that mosfets and bipolar transistors can work basically the same so the same package the same encapsulation can be used for both and it's normally impossible to tell just by looking at the device if you don't know the part number whether it's a bipolar transistor or a mosfet i'll give you an example so these are a transistor of power transistor 2st 2398 and you'll see 100 volts that's 100 volt rated collector to base 6 volts base emitter maximum 2 amp and you can see the devices and this is what they look like now these are mosfets so this is an n channel mosfet 50 n06 um drain source 60 volts very similar to the rating we have here uh voltage gate source is 20 volts i'll come to this in a moment these are maximum ratings by the way and you will see this can handle 50 amps compared with two amps but physically they look the same so as i mentioned it can be very difficult just by looking at the device what it actually is we can look at them with a test meter and that even with a multimeter we'll be able to tell the difference from one to the other or we could use a component analyzer but we'll come to that a little bit later now let's have a look at the things that are different between mosfets and bipolar transistors rather than the things that are basically the same mosfets and bipolar transistors also have the equivalent uses so they can both be used either as a switch or as an amplifier the first major difference between your mosfet and your bipolar transistor a bipolar transistor is a current controlled device so the current flowing from the base to the emitter will control the larger current flowing from the collector to the emitter and the ratio between the control current the base current and the collector current is called the gain and for reasons i said i didn't know on the previous video for which i was chastised gain is actually called hfe and i'm going to tell you now why it's called hfe and not for instance g or g8 again and hfe actually stands for hybrid parameter forward current gain common emitter and even after we get up i'll defend myself for not knowing what it stood for the reason being because i don't need to know okay i'm not circuit design it the hfe is the gain as far as i'm concerned and the only time i really need to note is if i have some thought condition where i have a problem because the gain is too low and my component analyzer will tell me that but besides we're talking about mosfets today so let's look at the differences the bipolar transistor then is a current controlled device and it will have a gain which is expressed as hfe which effectively is the number of times the collector current is than the base so for example if the base current was 2 milliamps and the current here was a 140 milliamps that's a gain of 70. so that's what it is your mosfet however is a voltage control device it works in the same way if you like but instead of passing a current through the base emitter to control the current with this you just apply a voltage to the gate now on a bipolar transistor the input resistance of the base emitter is low the input resistance is low this is why we need to connect resistors like so on our transistor when we apply a base voltage otherwise because the resistance is aware it will just pass a massive current for the base emitter and burn the junction out the transistor will fail in the case of a mosfet you can actually connect the gate volt the gate directly to the drain voltage as long as you do not exceed the maximum parameter so in this case the maximum voltage between the gate and the source would have to be less than 20 volts so you can't take this above 20 volts if you imagine this is connecting to your ground 20 volts is the maximum that doesn't mean to say it takes 20 volts to turn the mosfet on most mosfets will switch on or at least start to conduct with a gate voltage of about 2 volts to 3 volts once you get to this voltage it starts to conduct but unlike the transistor where the base emitter voltage cannot go above around 0.7 volt otherwise we'll destroy the device in this case you can continue to increase the gate voltage it will reach a point where it's saturated maybe at 4 volts 3.5 volts it cannot turn on anymore but you will not cause a problem by increasing the gate voltage further until you reach the maximum rating and you can find that from the datasheet so your mosfet is voltage controlled and the input resistance is high is high the gate is effectively an open circuit with a transistor your base current is going to be in the range of microamps into milliamps with your mosfet the gauge current is going to be practically nothing in the pico amps range it's probably so low that without special equipment you can't even measure it so this if you're thinking about it may beg a question why do we have resistors on the gate of a mosfet if effectively it's like an open circuit so no current can flow from the gate to the source well the reasons for that i'll come to a little bit later but for now and i will come to it let's just carry on where we're going we'll put this question on one side and i will answer that for you for now let's continue with the differences between the bipolar transistor and the mosfet so another difference relatively speaking your bipolar transistors the switching speed onto off is slow it takes an amount of time for the base currents to be emitted to start flow it will stop flowing and that limits the switching speed with your mosfet the switch speed is fast this is because you only need to apply the voltage no current needs to flow although technically again i'll talk about resistors so we have that difference and the last main difference is that an mpn or a pn trans pmp transistor are unidirectional so the current flows one way between the two okay in the case of your pmp is emitted collector in the case of your mpn is customized the unidirectional a mosfet is bi-directional so when you turn a mosfet on by applying voltage to the gate current can flow in both directions that is the basic differences between bipolar transistors and mosfets with a p channel mosfet just like the pmp transistor the gate voltage has to be below the source voltage that needs to be negative with respect to the source to switch the mosfet on but this does not mean that you need a negative voltage supply to switch on a p-channel mosfet and this can confuse people so i'll just explain it to you a typical circuit so this is with an n-channel mosfet we have the gate a drain source you can connect this to ground and we'll connect this to a load well let's put a light bulb here with a light up and this to a positive supply to switch the mosfet on i'll just complete the symbol i won't be lazy today let me go to complete symbol okay so to switch the mosfet on we need to apply a positive voltage to here and this could be three volts four volts something in that region that will turn on so we can take for example from here a resistor to the positive supply and we can take a switch to ground and we can make this quite a high value resistor 10k or something so when we close a switch we ground the gate and the thing switches off yeah when we open the switch this resistor pulls the gate voltage high it goes above three volts in fact it will go up to the supply voltage and it switches on and that's how it works that's quite intuitive with the p channel mosfet we need to think of it like this so just like the pmp transistor tends to be used this way up with a positive voltage rail here this is much the same so we now have our p channel mosfet we'll put that we'll put the load now if we'll put it here let's put it to you we have our p-channel mosfet our symbol and here we can put our light bulb and we can go to the ground and this is a positive supply now if we connect the resistor here so this this is your source this is your drain this is your gate the other way around yeah this is your gate this is your drain this is your source with the resistor holding the gate voltage up to the source this is switched off to switch this on we simply have to take the gate voltage through our little switch to ground as long as the maximum voltage doesn't exceed the voltage gate source let's say we have a 12 volt supply we can do that otherwise we would use a resistor in here but again in this case we close the switch the gate voltage comes down and the light turns on but notice we only have a positive supply there's no negative voltage we're just making the gate voltage to be lower than the positive supply for example if we have a resistor in here and depending on the values of the resistor the voltage here could be 12 volts the voltage here could be eight volts put the resistor in and again we'll have our little switch in the circuit so when we switch the switch on now because of the ratio of the two resistors this goes down to eight volts it's now four volts and negative compared to the source it turns on but notice there's no negative voltages if we put our multimeter between here and here we'll see eight if we put our multimeter between here and here we'll see 12. yeah positive voltages but the gate is lower than the source once you get your head around that you'll find that pmp transistors and p-channel mosfets become quite simple to visualize how they're working now let's have a look at some mosfets with our multimeter so we're going to put the multimeter onto diode range and this is one of the n channel mosfets that we were looking at earlier we have on this mosfet now we'll just draw it out so we can see what we have we have gate drain and source and this is quite a common layout for mosfets we'll also have a p-type of mosfet so we have one here this happens to be a surface mount one so the tab is the same connection as the middle pin which is cut off basically so here we have our p channel mosfet little tab on it a short leg two longer legs and again we have the same layout gate drain source they don't have to be this way but this is a very common layout for mosfets let's check them with a multimeter so we'll take our meter and we will first measure between the drain and the source this is the n channel and type and this is the p-type save any confusion with what we're doing okay so we're the end type let's have a look between the drain and saucer with the positive to the drain which is how it would normally be used in a circuit so the source is open circuit okay drain sources open circuit if we go the all the way around and then negative to the drain you'll see it reads like a diode junction and i'll explain this in a moment now if i touch the gate with a negative lead compared to the source nothing happens okay the transistor is just switched off and i caught the wire and i did it too soon but you saw what happened so with the negative connecting to the gate it's turned off but we know that the mosfet needs a positive voltage on the gate to turn it on so let's go from the source to the gate touches yeah now watch what happens it's now turned on yeah it's like a short and it's bi-directional and it stays turned on until i touch the gate with a negative lead and then it goes back to be an open circuit and like a diode junction why is it doing that well first of all our tester is actually applying a voltage to the gate if i take another multimeter on volts range you will clearly see that as the case here is my fluke meter on voltage range and this is on diode mode so let's see so we can get the two reds together and the two blocks together and you can see that the multimeter and diode range is putting out 3.9 volts this tells you it's supplying enough voltage to turn on the gate of the mosfet which is why the mosfet is turning on that's why it turns on it should also tell you by the way if you're worried about the well-owned short circuit finder we looked at on the previous video and you're worried that like a high voltage of 3.9 would damage graphics cards and such like that work on 0.8 volts can i ask you why you never worry about using your multimeter on those cards but that's just to prove something else we were talking about it's nothing to do with this video so we can see why the transistor turns on because we apply a positive voltage to the gate compared to the source positive to the gate and it turns on the question is now why does it stay turned on why does it not only turn on when there's positive voltage on the gate because there isn't any now yeah i've taken the meter off there's no positive voltage on the gate why is it still switched on yeah until we put the negative on the gate well the reason for that is actually quite simple your mosfet basically the gate acts like a capacitor so when you connect your positive metally to here effectively this is a capacitor internally in the gate and the mosfet and when you put your meter lead from here to here here's our meter with the leads when we do that we charge the capacitor up so what happens now is the capacitor holds the charge when we take them we take the meter probe away there's nothing to discharge the capacity of it stays charged up therefore the mosfet stays switched on and that is very very important when it comes to fault finding and the problems we can have with mosfet so just remember that it's like a capacitor and that's the end type and the p-type is the same just back to front if you like so with the p-type this is the gate if i go between the drain and source and one direction i read like a diode junction which i'll come to the minutes and the other direction i read nothing you do circuit if i now put the positive lead on the gate compared to the source yeah i've touched it with a positive lead this still leads like a dire junction you still really die junction in one direction and open the other because it's a p-type let's put the negative lead on so now the gauge is below the voltage of the source now let's see what happens see it's switched on zero in both directions and it will stay switched on until we apply a positive voltage or we short the gate back to the source and we discharge the little internal capacitor yeah it's about to be in the diode junction again and open circuit the other way that is how you test mosfets with the multimeter now the question is why do we see a diode junction in one direction between the source and the drain well the reason for this is because this isn't the complete symbol for a mosfet the actual symbol for the mosfet and i'll after we draw is i can fit it all inside the circle it's like so this is the n-type and channel okay and we have from here to here a diode internally the drain source gate and likewise for the p-type it's exactly the same thing so we have again the mosfet are obviously pointing the opposite direction the gate but the diode is still in the same direction and this is where i made a mistake so i'll just correct this and then we can continue so the first image which was uh this one this is actually correct and this image is for an n channel mosfet so we'll just try it right on the screen that's your n channel okay the p channel actually the diode goes the other way around so we'll just uh correct that one that's here so we i get to the right place on the screen so okay so the diode actually in this one goes this way around first time i've played with this particular toy so we can just draw it in there and we can put a circle around these as well okay so now we still have the source and we have the drain pin and of course we have the gate here but the diode is the opposite way around and this is your p channel so you might wonder why did he put a diode inside a mosfet well the answer to that actually is that they don't this diode is basically there because of the way the mosfet is actually made so this is called a parasitic diode or a body diode and what you're actually seeing is the p n junction between the two waves of silicon pitap and n-type that make up the mosfet so now we know why there is a diode junction inside our mosfets let's continue now you might wonder why do they put a diode in the mosfet and you also might wonder if you know anything about buck regulators and circuits that are switching relays and inductor coils is it there to take away what's called the back emf well the answer to the first question is they don't put the diode in the mosfet the diode is there because of the way that the mosfet is constructed it's a by-product of the construction of the mosfet that's why the diode is there it's not deliberately put there it's just a by-product of what the mosfet actually is internally and the second question about is it there to stop emf damaging the mosfet when the inductor switches off the answer is no and i have to just interrupt this again that is not what the body diode is therefore even though google will tell you that's exactly what it does yeah well it doesn't do that and i'm now going to show you exactly why it is not doing that because if you look at the circuit for what you were thinking okay you're thinking of something like this round okay inductor coil yeah the diode if you think about it which is in here there's actually in the wrong place the diodes you were thinking about actually goes here okay if that's beyond some of you guys by the way i did a whole video on inductors all you need to know about inductors to fix stuff and if you watch that video you'll understand what i'm drawing here but for the rest of you who know about that can you see now that diode is not that one it's in the wrong place in the circuit let's just have a quick talk about why do we need a gate resistor on a mosfet or gate resistors okay why do we have the gate resistors well the reason is down to the fact that this acts as a capacitor so if you were driving a mosfet just as a switch effectively just switching it on and off the capacity doesn't the capacitance isn't really a problem until you get to high frequencies so a capacitor passes ac but blocks dc when you get to high frequency switching that capacitor becomes a load on your switching device so if you have some sort of driver sending pulses into here the capacitor will effectively be charging every time you get a pulse then putting a load on the signal and what that will tend to do is cause the signal not to rise straight up but to rise up slowly as the capacitor charges and end up with something like this which is probably not what you intended so the resistor is basically to limit the current that's charging the capacitor and these capacitors are by the way they may be 2 000 picofarads something like that sort of value um which is um about two nanofarads you know it's a couple of nanofarads that's the value this will depend from one mosfet to another and the data sheet would tell you so the idea of the resistor is to effectively limit the current this is passing by switching on and off you can probably realize the higher the value of the resistor the longer it takes to charge the capacitor so the slower the switching speed but it puts a lower load on whatever is driving it that is a matter really for circuit designers it's not for us we just want to fix this stuff but i thought i would just mention if you've always wondered why we do have gate resistors on mosfets does a mosfet have gain like a transistor has gain when it's used as an amplifier and the answer is it doesn't in the same way because there's no gate coming there's no current flowing so there is no current gain but what a mosfet does have as a parameter called r d s on and this is the resistance between the drain and the source when the mosfet is turned on i was just testing some with a multimeter you saw and i was reading effectively zero across the mosfet but in actual fact they don't have a resistance of zero when they turned on but it can be very very low it can be into the microohms or tens of micro ohms so to find a mosfet with an rds order of something like 20 micro ohms or milliamps or that is not unusual milliamps i'm saying microbes milliamps so rds is effectively the resistance when it has switched on and it's a very light resistance so your main important parameters of a mosfet is how much voltage can it withstand how much current can it pass how much wattage can it dissipate the rds on should be low and the gate capacitance that's the main parameters that affect the performance of a mosfet this can become quite important especially in amplifier circuits so when we're looking at amplifier circuits and also sometimes in switching circuits where we have many many mosfets in parallel as a pulse width modulator so they're all sharing the load then these factors all have to be balanced we can read some of these using the component analyzer so let's have a quick look at our mosfets with the peak analyzer let's see what it tells us this is our n channel mosfet so i'll just switch the analyzer on let's connect the cables to our mosfet we'll put the green on the gate why not green for green gate forget the two g's together but it doesn't matter the analyzer really doesn't mind what's gonna tell us enhancement mode and channel mosfet and we'll come to an enhancement in the movement which tells us which pin is which the gate threshold so this is the voltage in which it turns on the test currents and that's basically giving us those parameters it's not telling us the rds on it's not telling us the capacitance of the gate there are analyzers that will and that becomes more important when you're trying to match transistors and amplifiers i don't at the moment have a tester that will do that although maybe we could build something so with one of our p channel mosfets so again this is the gate i put the blue wire on it this is the source and then the drain is the actual uh metal tab or the middle pin which is sure because i don't think i think i can get yeah i'm on let's see what i told us enhancement about p channel mosfet again tell us which pin is which voltage thresholds slightly different the test curves the component analyzer was telling us something interesting that these are enhancements type mosfets and this leads into what i was saying that there are actually four types of mosfets and these ones the enhancement type are the most common but we also get a depletion type of depletion mode mosfets and again they come in p channel and n channel so with a depletion mode mosfet this is a symbol i'll just put the arrows in so again this is a p type gate source drain and we'll have the end type as well so we'll just do the other one gate drain source the difference between an enhancement and depletion type is effectively they are opposite so with the enhancement type you have to put a voltage on the gate to turn it on either positive or negative with respect to the source depending if it's p channel of its n-channel and that's what we've just been looking at with a depletion type it's the opposite way around so with no voltage on the gate the mosfet is switched on it reads effectively zero ohms and to switch it off you put a voltage on the gate either positive or negative enthalpy channel so that is the difference now the depletion type mosfets depletion modes are not very common so much so i don't actually have one here you will find them in some type of circuit so you need to know about them but i don't have one to show you and i must admit i haven't come across anything i've been working on i'm guessing some of you guys will be able to tell us where you will commonly find depletion mosfets otherwise honestly guys i wouldn't worry about until you find one and then panic [Laughter] no seriously when you find one now you'll know what it is as i'm still on this page i'll just mention something that should be obvious but maybe isn't because a mosfet has this internal diode built into it it means as if you apply a reverse voltage between the source and drain so in other words the source is more positive than the drain it will conduct and it will conduct regardless of any if there's any voltage on the gate or not the mosfet cannot switch the current off in that direction so really you need to think of a mosfet as acting like a diode in one direction when there's nothing on the gate and like a short circuit in both directions when there is a voltage on the gate this ability of a mosfet to be able to conduct in the opposite polarity if you like is sometimes used by circuit designers so it's something you need to keep in mind because in some cases you cannot understand the circuit operation unless you understand the role of this diode and the fact that it will conduct when it has a reverse polarity across the source and drain let's have a look now at some of the things that go wrong with mosfets much like transistors they tend to fail in two ways they either go short circuits or they go open circuit and like transistors again they can go short circuit from the drain to the source or from the gate to either or the whole thing can go short circuit so short circuit mosfets is a very common failure and quite often as well they will explode they will go open circuits so they can burn off explode sometimes they can look like they've exploded and still be short but these are the main failure modes of mosfets they're not so likely to fail in unusual ways like bipolar transistors do but you can get mosfets that fail where the rds on resistance between the drain that source on goes high this sort of problem you're not likely to find without taking the components out of circuit mostly speaking you will have these problems the open circuit ones can be quite hard to find unless they've physically blown you can see their bone the short circuit ones are quite easy to find in circuits and normally like just about everything else the parts that are doing the most work fail the most often so it's the larger power devices the output devices which tend to fail having said that the number of other problems with mosfets that you need to know about one of them is esd the electro sensitive devices electrostatic sensitivity basically mosfets especially vintage ones can be damaged just by touching the gate with your finger and normally they should be kept when they stored and a little bit of conductive foam like this not in a plastic bag anti-static bag yeah sure so mosfets can be prone to esd damage when you're working on mosfet circuits especially in amplifiers and such like it's a good idea to keep that in mind and try not to touch the gate when the device is out of the circuit once it's in circuit then the gauge resistor will effectively protect the gate from high voltage like i was switching get on with my multimeter if you touch the gate with your finger and you have a lot of static charge in you it's easy to put more than say 20 volts between the gate and the source that's why they get damaged but once the gate resistor is there in circuit it really protects the device which brings me to gate resistors and this is quite important you could see that the mosfet would turn on using my multimeter when i applied some voltage to the gate so i touched the gate with my multimeter i'm sure he goes about 3.8 volts it turns on and when i take the multimeter away it stays turned on because the effective internal capacitor stays charged up this can be a problem especially in vrms but also amplifier circuits and many places you'll find mosfets if you disconnect the driver so normally you'll have some sort of pull-up resistor and reverse register go to whatever device is driving the mosfet you have a circuit something like this and this device will be connected to ground so when there's no drive to the mosfet the internal resistance of this device is holding the gate low the problem occurs when this resistor goes open circuits and this often happens when the mosfet went short circuit so you've changed the mosfet and you haven't realized that this has gone open circuit what happens now you might think well the mosfet's got no drive yeah the resistor isn't there so when i power up it's got no drive well it might have a pulp resistor and that'll instantly cause the mosfet to turn on at which case because the circuit isn't there that mosfet comes on and stays on and whatever circuit it's supplying power runs down through here and the whole thing goes short again okay but even if you don't have this resistor so you've got a circuit we'll put something back in again so you've got a circuit like so no pore position just a gate resistor going to whatever was driving it the driver i see portsmouth modulator whatever and intuitively that goes to the ground same thing happens the mosfet was short so you've changed the mosfet you've found the short circuit mosfet i haven't realized this is open circuit in this case you could reasonably expect well there's no drive it can't turn on because there's no drive to the gate yeah but you saw yourself that any voltage applied to here stays here it doesn't go away because of the capacitance so it's quite likely even because you've got all the pcb tracks near to the one that you've got there's a little bit of interference a bit of capacitance between them because with two pcb tracks yeah close together what have you got you've got a conductor you've got a conductor and you've got an insulator between them a dielectric yeah and if this conductor is connected to your gate of your mosfet yeah and the resistor that's here is open circuit so there's nothing else connecting to the gate it's not unlikely that switch it on a few times and the surge of coming from here will induce some voltage into this or that lack of capacitor there'll be some voltage on the gate and this will now turn on yeah and your 12 volts coming into here assuming this example is a book regulator going off to uh your gpu or something like that yeah through it book regulator this will turn on the 12 volts comes down through here straight into your gpu or cpu or whatever doesn't want 12 volts it wants one volt or 3 volts yeah so it's very important to understand that not only can mosfets fail because the gate resistor fails but they can also fail in a way that is catastrophic to whatever is being connected the other side of the mosfet so the upshot of this is that you need to check for open circuit gate resistance especially when you had a short circuit mosfet one good way to do this is to do what i did you saw me with my meter effect for you going from the source to the gate and then measuring between the drainage source so i'm using my meter you've just seen me do it i'm using my multimeter to turn the mosfet on yeah one mosfet well let's do it again it's so cool one must have turned off touch the gate one mustache turned on it'll stay tuned on until we discharge the gate that is a very cool thing to do with the mosfet out here but you should not be able to do that with a mosfet in your circuit connected to the portsmouth modulator because unless this gate is open circuit when you apply some voltage to here yes if we used another meter maybe this is switched on but once you remove the voltage from your multimeter the resistor which is effectively connecting from here to ground will discharge the voltage and the mosfet will switch off so you should not be able to turn on mosfets in circuit like i was just doing out of circuits and this is so reliable it's something you should test so when you've changed your mosfet your shorted one try what i've said try to switch it on by applying voltage to the gate and remember p channel is all the way around yeah and make sure that your cord switches on that way if you can switch it on that way find the gate resistor and if that's okay have we got what's wrong with the driver because it may be that when this went short it didn't boil the resistor but it blew this and this is now open circuit inside so that's a very important point with mosfets and this problem is called floating gate and for the same reason never remove the driver or disconnect the driving circuits and switch the device on you're asking for trouble as i've just described so can we test mosfets in circuit with our multimeter well let's have a go we have some mosfets here this is actually a vrm for a motherboard so this is effectively the voltage supply for the cpu and these are mosfets now if you're not familiar with these you may wonder why do they have eight legs on them why is there eight pins and the reason is with these are basically four of them with a drain three of them are the source and the other one is the gate so the pins are actually all connected together if you look here you can see i just take the test meter yeah you'll see they connected together they're connected together the one that reads the resistance is the gate and then the other side are all connected together okay i'll just get a good connection you'll see okay sometimes these devices can actually be two mosfets in one and dual mosfets so that's something you need to check from the part number but i'm sure you can see here we have four pins all together three pins all together and then this is the gate and as i was just describing the mosfet should be switched off well there's a diode in one direction as we know i'm reading some resistance but bear man there's other components in the circuit and if i go on to the gate which is here and then back onto the mosfet i should not be able to turn this off we have the diode junction okay a little bit of resistance other way so this is what i'm saying you should not be able to turn them on like i was doing out of circuit if you can you've got a problem let's have a look at some other um possibly mosfets i'm sorry i just caught the meter okay so we have here a lot of either transistors or mosfets or either diodes in fact actually i know from the port number some of these actually double diodes these w2f and wv3 recognize a part number so let's have a look for some other ones these may be transistors or diodes or mosfets again and it's very hard to tell so we can have a look but normally speaking in circuit you cannot tell if you have a transistor this would be the basal gate on the red bin nothing there nothing there clear the opposite way nothing there nothing there between these two do we see a diode junction the other way yes so that one i think we can say is a mosfet because all we can see is the diode junction in one direction but it's not a very reliable way of testing to be quite honest and you will find i mean that's the diode junction there and zoom on here no but again these could be just diodes these could literally just be diodes so i'm sure you can see from what i'm showing you it's not really practical to test for mosfets and circuits unless they are short-circuit short-circuit mosfets especially in the voltage regulator modules when you have many many mosfets in parallel as a topic in itself and i made a video showing how to trace short circuits in this type of circuit which i'll link from this one so does this mean we can't test mosfets in circuit well actually no it doesn't really mean that because we know that the conduction of the mosfet is controlled by the gate voltage positive or negative with respect to the source if we know we have a mosfet and we measure the voltage on the source and the drain especially the voltage difference between the source and drain then we can quickly realize whether the mosfet should be switched on or off and if it's switched on then the drain and source would be basically the same voltage so i would suggest the best way to test mosfets in circuits is using your multimeter in volts mode with the device powered up and there we have it that is not everything there is to say about mosfets but i'm sure that's everything you need to know about mosfets to fix stuff i hope you enjoyed this learning electronics repair video and i'm looking forward to see you all soon on another one so there's one thing left me to say no and that is ciao for now guys

Channel: Learn Electronics Repair

Views: 196,926

Keywords: learn electronic repair, electronic repair, school, lessons, course, training, free, fault finding, diagnosis, component level repair, test transistor

Id: LNu00nMBaaQ

Length: 55min 24sec (3324 seconds)

Published: Thu May 19 2022