All You Need To Know About TRANSISTORS To Fix Stuff! How Transistors Work Test In & Out of Circuit

hi guys welcome to another learning electronics repair video in this one we're going to look at transistors in particular bipolar transistors so all those who are going to comment that i didn't talk about mosfets and junction fats and all sorts of other things that's because i want to talk about bipolar transistors today so here we have it all you need to know about bipolar transistors to fix stuff so what's a transistor well it's a semiconductor device um they come in all shapes and sizes in a few variants but basically they all have three pins on them apart from the ones i have four so they really all have three pins on the three legs and um the symbol for a transistor is this and this is an npn transistor there's another type of transistor if you like which is the mirror image or opposite of this one it's the pmp transistor so a pnp transistor has a very similar symbol but the arrow points the opposite way okay these two if you like a complementary to each other they work in opposite ways so all the voltage and uh polarities and the current flows on this are mirrored on this one and p and p and p the three legs are called bass or b commonly uh abbreviated to collector or c and emitter or e so on this one we'll put as the number r b c e that's how you'll see them you also when you find transistors on circuit boards and the actual circuit board has their component markings printed on them commonly these days a transistor will be marked as q so you'll have q followed by number q1 q10 q345 so this is just the reference number for the component not the value of the component of the type on older circuit boards you might find them quite often not just tr-1 tr7 and so on but certainly in more recent stuff and probably for the last 30 years to become honest they tend to be marked this way so that's one way to identify a transistor i'll show you some physically at the moment but unfortunately there are lots of devices that look like transistors but are not necessarily our transistors so it can be a bit difficult sometimes look at a pcb to know what the device is without testing it let's have a look at a few transistors to see what they look like and then let's test a few and we'll test them out of circuit initially so we're going to do this using a semiconductor analyzer because they are cheap to buy these days and they're very handy and very useful but we'll test them with a multimeter as well so we're going to have a quick look at how we identify a pnp transistor how we identify an npn transistor and how we identify which pin is which and how we identify whether it's actually a good component or not when you test the transistor with a multimeter it basically looks like two diodes and i'll just show you this now so an npn transistor to your multimeter looks like two diodes connected like this like so so if we put the positive lead of our emitter on the base we will actually see effectively two diodes and with a pnp transistor it's the same but basically just the opposite way around so with a pnp transistor it looks like two diodes like this okay and this is your collector this is your emitter and this is your base now just because a transistor looks like two diodes doesn't mean that you can make a transistor out of two diodes you cannot actually make a transistor that way but this is electrically how it looks to your meter now we know this let's have a look at at least a couple of transistors and see what we can see here is the test meter and here's a transistor this is a tip41c i can't remember if it's pnp or npn so let's see if we can identify which type it is and which pin is which um so we'll go from one of the pins and i'll put the positive weed on it and we'll go to either the other two so here we see we get a reading 0.643 that is the junction the semiconductor junction the voltage dropped across it and because we see a junction we know one of these two leads must be the base so let's see which one it is is it the middle one well if we move the red to the other end we should see the same or similar and we don't so this is not the base this one must be the base oh it's a faulty transistor so we go to the other end and again we need point six four five six four three six four five you notice these slightly different readings between the other two legs we read nothing okay the open circuit and if we go the opposite way around so the black is on the base we know this is the base we get nothing so the only time we ever see a reading is with the positive to the base okay because the positive is on the base this is npn i think npn the base is always a different one so p okay that's an npn transistor the other thing we can work out is that this reading is a little bit lower than this one the one with the higher reading is the base emitter junction so just make a note of this rather than just trying to remember it all in one go this is our transistor and we know because we worked it out base collectory meta now with the positive on the base what we should see is the diode junction going to the two or the legs so with the positive the red lead on the base to the collector we read in about 0.6 here and to the emitter we read it about 0.6 0.6 0.6 okay to there and to there everything else means open circuit that's the only time you will see a reading that's how you know where the base is it's the one that goes through the two and because it's the red lead it is n p n okay because it's the red lead on the base the second thing i just pointed out between the base and the two orders that i get are reading the one to the emitter is one of the highest six four three six four five so we know this is the emitter now we can test this in a moment without a component analyzer just to be certain let's test another power transistor this is a big one so let's stick it in let's actually draw this again and let's mark the readings as we go along so you can see how to actually do this okay so there's your three legs there's the transistor because this one had the base on this leg this might well be the same layout so that's probably what you should try first is it the same way out okay so let's have a look and let's see if it's npn so we'll put the red on here which we think might be the base yes we get a reading so again we know one of these two is the base to this end yes we get a reading again so we know it's like that one and this is the slightly higher reading okay everything else any other polarity opposite way round between any of the pins will need nothing that's a good transistor and we know once again base collector emit it and this was the slightly highest reading so that's the emitter i've got one more similar shape to this that sounds like it's just a small one which is a similar sort of thing so again let's draw each and then let's see if we can work out which one is which we go so we got my three legs okay let's go we'll try it again the same way let's see the same way positive to the base yes okay but today there's nothing 2.4 okay ah now this is reading differently from here i get a reading to here okay you see it and here i get a reading to hear this has a rather unusual reading 808 595 595 it looks like with the black in the middle this is reading five five eight and this is reading five nine three which kind of suggests that that is the base in the middle and it's a pnp that means nothing that means nothing so in that respect you see it nothing to either end black in the middle five nine three five five nine so in that respect this loops like base collector emitter with the negative to the base makes you conduct so this looks like it's a p n p transistor okay that's how it looks to me but this one if we go between the other two legs base collector we're getting some readings it which suggests either this is a faulty transistor or there's something unusual about it so put that in the component analyzer we'd have to put the question mark against it it looks like a pnp transistor and the base in the middle but it's a little bit odd let's look at two more and then we'll get our components analyzing we can double check so here is a small signal transistor i don't know which pin is which i can draw it so we have a flat bit at the bottom i'm doing from underneath and we have three legs coming out okay that's what we have and if you can see it's flat in the middle i'll zoom down a little bit you can see it properly there we go so once again we need to find the base so we're looking for one pin that conducts the other two so let's start with this end one we don't know where it is let's start with the end let's go to the middle that doesn't conduct with the positive on the ends does it go to there yes go on to it again dropped it check there meters working okay oh well i got a bleep and then it didn't look the white blips okay so positive for this end doesn't seem to do anything let's try with the positive to the other end so let's turn this over let's go with the positive to this end and again we're looking for something that conducts no no so positive to either end doesn't seem to do anything let's go with the positive in the middle let's try it again yes that conducts okay and that conducts as well so it looks like the base is in the middle and it's readily so it's positive so it's n p n we think the base is in the middle and whichever letter is in the middle n p n is an npn if it's a black lead it's a negative always the base in the middle p and p so we have six seven eight six seven seven so it was looking like i have to turn it around to match the orientation of this it's looking like this is the base and the one that was slightly higher is the emitter which i think is just dropped it again whereas they've missed the one the slight higher really six seven six six seven eight so put it around the way it might be yeah this is the emitter and this is the collector and it's with the positive here the positive lead so it's n p n okay that's the transistor if that's the case then from either end shouldn't mean anything in either direction because it's collected three meters it doesn't mean anything okay and with a negative to the middle to the base it shouldn't read anything either way make this short try yeah nothing nothing okay so we know that one is npn last one moved a little bit another one works the same we'll try and draw the thing the same way up so this way up so again it has a flat edge so it's round these has three legs coming out okay let's see if it's the same so we know the base was in the middle and the positive lead went to the middle so positive leads to the middle subway velocity meter there you go positively to the middle what happens nothing nothing so either the base isn't in the middle or it's the opposite polarities of pnp let's put the black leading limit what we got now yes we've got a junction there so one of these two pins is the base okay it's reading 696 is it the middle one the base is it pnp yes 697 so this one is the same way out base emitter collector but it only conducts with the black lead in the middle and always in black negative red positive yeah so this is p n p so there you go that's how you measure them with a multimeter very commonly these type of transistors will have the base on the left-hand side very commonly obviously not always because this seems like it's in the middle these type of transistors to be quite honest it could be anywhere there's no really set way out i think every possible combination has been made by some mercury order but let's now look at the uh component analyzer and let's see if we were correct in identifying all of these transistors they all appear to be good transistors we know if they're npn or pnp and we think we know the way how to create uh collector base emitter let's see if we were right and let's see what this thing is yeah now let's look at these transistors with our component analyzer i have two here so this is the very cheap one i got from aliexpress it was about six or seven euros and this is a peak atlas dca dc85 i've had this about six years old seven or eight years even and uh this is my tool of choice i picked this one up probably cause i'm just so used to using it and i find it very easy to use but this is like 50 60 euros and this is about six it was maybe eight or ten times the price and i don't actually think it really gives you any advantage well let's use them both and then we can have a week so we'll do them in the order we check them so this was the first one and we thought this was an npn when the bass was positive yeah i didn't write it on we thought this was np and and we thought this was npn and npm pmp that's that's what we think we've got okay so let's have a look at the first one and we'll see so we just put the leads on in any order it doesn't matter which order we put the weeds on here just put them on and then we can press the analyze button let's see what it thinks of it so test npn silicon transistor it gives us a bit of information so the green is the base and the red is the emitter and you can see it matches the layout that we worked out with the multimeter based on the emitter has a slightly higher voltage sometimes occasionally we'll find the transistor that that doesn't work and the base collector basically meter the same and there was only 0.01 difference on this but there was enough to tell is it okay so that's the actual transistor this will also give us the gain i'll talk about it in a little bit hfe and it just tells you the test current is 2.5 milliamps and it's telling us the voltage base emitter is 0.68 that's the voltage which the transistor turns on again we'll talk about that in a moment test current of 4.73 okay leakage current leakage is basically if the transistor is passing coming when it shouldn't so it's a good thing to use an analyzer now let's use the other one as well and it'll probably give different readings but i'm expecting it'll tell us the same thing so it's testing it again it's an npn transistor it's giving us a gain and that doesn't give us any more no that's what it gives us but that's good enough base collector emitter again so that confirms what we just read with the other one so that's good and the multimeter givers are correct let's get this big one these big transistors tend to have lower gain by the way i will say um than the smaller ones let's have a look but obviously you can pass higher currents that's the thing and sometimes with sand higher voltages as well it's normally they're coming to the wattage that specifies the size okay let's run test another npn silicon transistor is the blue one the emitter yeah blue one emitted again it told us uh it told us it's an npn note the gain is lower that's common for a power transistor like this that's the base emitted voltage just as i said we'll talk about and that's it so we know it's another npn transistor we can do this with the little component tester this is where it's a bit inconvenient because you can't get the thing in there yeah you'd have to in fact kind of like tuck some wires on so i'm actually not going to bother but you could do it yeah you could do it it's just not so convenient to do it maybe if we bend these two pins then we might just be able to do it there's there we go let's see we can actually hold this in place enough to get a reading now we shall see the dial let me try again yeah it probably isn't making the contact that's that's the top and bottom of it yeah i don't think there's anything wrong with us not getting a good contact against a transistor but we know it's a good one so look at this rather odd looking device that we found let's see what this thinks of it so again we think the base is in the middle on this one so we'll put the blue on the base this time it doesn't matter which one we think the base is in the middle we think the emitter is on the right hand side but we're getting some strange readings between collectors meta what's it say it's a darlington transistor okay that's why we have some strange readings and i'll explain what one of these is in a moment if i have a very high gain normally we got the uh woo is the correct that we we we completely read this wrong on a uh multimeter okay the red is actually the base so the multimeter would not read this diode protection during collateral emitter resistance shot between base emitter gain not accurate due to basically resistance to 16. so that is not actually a bipolar transistor hence the strange readings we got but it's under the remit of this video so we'll talk about it um this one we think the emitter is on the this ends we'll put the blue wheel on the emitter and by the way i'll just come back to that old one just check it at the moment on the other components analyze to see what it thinks okay so we've got the green on the base we've got the blue on the emitter that we think is the emitter and we think it's an mpn transistor battery i've got some batteries for this i don't don't think i've ever changed the battery this i bought it npn transistor that was correct uh it goes this way up on our diagram of the flat side so red is the emitter which is correct gain 245. um i can't remember why hfe stands for gain so he'll tell me well i can go and google it turn on voltage basically meter 0.72 okay that's that one so we identified that correctly on our multimeter let's have a look on the uh little analyzer now let's have a look at this darlington transistor as well so here we go transistor in what you think you see this can't recognize it it doesn't recognize it it just says it's an mpn transistor yeah with a forward voltage as that is 1.1 volts this test does not recognize this we can check this part at the moment and let's see if it actually is a darwinian transistor yeah i have a part number let's go to this one so met her should be in pin one but he's in pin two okay so your message should be pin one pin to be sorry let me look at the right way around check yes he misses this answer he met him pin three my mistake again not making a good contract you see why i prefer the other one yeah you're missing pin three bass pin two in the middle so we got that right npm last one you want to do it in this one first as we always so again with a flat end this way round emitter should go into pin three see yeah you met one pin three yeah oh yes forward voltage that thing is let's go one more time with this we know we know this will find this so uh i'll put it the same way as i had it so the blue is on the emitter green on the base red on the collector okay yes pmp which is what we thought it was and green as a base okay yeah blue as the emitter so once again the multimeter identified all the pins for us so really the only uh strange thing that came from that really is this one which is a darlington transistor which basically is two two uh transition one package by the way i'm just gonna check that because this component analyzer doesn't seem able to determine what it is here is the data sheet for that strange device it was this one mje 700 that's the actual device the transistor it's in that type of case and if you have a look at this it basically shows you what's inside this so effectively it has two transistors internally connected whereby the emitter of one goes to the base of the other one and the idea of this as it basically increases the gain so the total gain the current gain is the gain of this transistor multiplied by the gate of this transistor you will see it has an internal diode and it has two internal resistors from base to emitter and this is what the peak analyzer was telling me and said he couldn't give me an accurate gain figure because of these resistors and that he's had an internal diode whereas the el cheapo component testers just could not recognize this um so there is another good reason why i actually prefer to use the peak analyzer so there we have a few examples and you can see that the um multimeter could identify all of them apart from this it can give us completely wrong pen out and he had some strange readings the uh i'll cheapo could identify all of them apart from this so in waterways it was no better than the multimeter was yeah this one could identify all five let's just have a quick look at the few transistors so transistors come in all sorts of shapes and sizes these are some older ones you like little tin cans little top hats basically uh on all the equipment you will find those there's more of them here sometimes like this but a much larger tin can basically um you'll also find i mean these are these are more the plastic ones but they're bigger um so really these transistors will come in all sorts of uh shapes and sizes but with all the plastic ones again i like that sort of shape yeah and uh we have i'll just show you these are old power transistors package called t03 or t03 probably obsolete for any current devices but you'll find these especially in old amplifiers it is another one similar to these if you find transistors with a metal tab here here on the back almost always you'll find that metal tab is actually connected to the collector which is the middle pin we can just check this we'll put the meter on here and we can just go to uh continuity range and we can have a like so this metal tab is connected to the middle pin okay on all of those is this one got one as well this darlington is it connects to any of them it goes to the middle pane again yeah it goes to the middle top this means normally when you find one of these mounted on a heatsink which is what they're designed to do that either the heatsink will be at the same voltage as the collective of the transistor the middle tab or there will be a micro washer or a plastic rubbery type gray washer between here and the heatsink and if this type not only will there be a washer and heat sink compound usually there'll also be a plastic insert in here so the bolt going through doesn't contact the metal it's very important when you're repairing stuff to watch out for that and put the insulators back in when you replace the transistor otherwise you'll short it to the heatsink and you'll just blow something up again most likely i'll just show you one more you're unlikely to find these these are vintage components really but this is a photo transistor so effectively by shining light on it you can change the gain of the transistor or make it effectively pass more currents or less currents by the intensity of light so that's a photo transistor i'll just mention now seeing this as everything you need to know about transistors effect stuff these transistors we've been discussing are silicon transistors and they have as you can see with the test meter when we measure it these voltage drops across the the semiconductor junctions of about 0.6 for a volt roughly speaking if you're working on vintage equipment you may also find uh germanium transistors these were used certainly in the 60s 70s and these transistors have a voltage drop of about 0.2 volts across the semiconductor junctions base 3 meter based collector you'll only come across them in old equipment i would say i can't i'm trying to think are they specialist uses for germanium transistors these days i don't know but i haven't studied that so maybe somebody can correct me but just be aware you may come across these on very old equipment and to be quite honest they tend to have some strange failure modes which you don't see in modern transistors it's probably a topic to itself so i'll i'll leave it at that with that one but if you are going to be working on old very old amplifiers and transistor radios and these sort of things then you do need to know about them let's have a look at how transistors work and not from the physics of how they work what's an analogy about the work so an npn transistor can be considered using the flow of water so imagine we have a water pipe a wide water pipe a mains water pipe and we have coming into it a smaller water pipe and this water pipe is connected to a pump that's gonna apply pressure to the water pump in there in here we can imagine now that there's a valve so the valve effectively is pivoted and sits here and as you apply pressure you open the valve allowing water to flow through here but until you apply a certain amount of pressure no water will flow it takes a certain amount of pressure to overcome the inertia the mass of the valve before it will open this valve is connected to another valve inside here effectively that can move in and out quite how it does without the water leaking we won't go into but it's attached like this so as you open this valve it pulls this valve open and this is the larger pipe so basically with a small amount of water flowing through here by varying the pressure you can control a larger flow of water flowing through here and that's basically how your transistor works this would be your base the control terminal the valve this is your collector and this is your emitter this is for an npn type transistor okay so that's basically how it works there's a few things that you should just consider here which are important first of all no water will flow through here until you reach at least a certain pressure here to start moving the valve okay that's the first thing the second one the current the water flow flowing through the collective emitter is greater than from the base of the emitter okay and the current that's flowing through the emitter is actually the sum of that and that because this water the small amount is also coming through here so flowing through here you have both the flow of water coming from the base and the flow of water coming from the collector okay that is basically how an npn transistor works i'll show this now electrically so all we've got to really do now is consider it pressure to be voltage and water flow to be current electrical and then we can consider how a transistor works so we'll do the npn first this is probably the most common way of using the transistor so we have the collector connected to a positive supply via a resistor or this could be effectively a load a light a relay a small motor it's always okay and this is your base so basically we have to use resistors this resistor or the resistance of the load sets the maximum current that can flow through the transistor if you exceed the maximum current for that type of transistor you'll burn the transistor out it can only handle a certain amount of current and the same with the base you could only have a certain amount of base current as well so normally you have a set up something like this and this has a small positive supply okay this is the big positive supply this is your small with the semiconductor junks and that's from the base 2 meter we've just measured with our test meter and it measures about 0.6 volts so basically you have to apply 0.6 of a volt here before anything can happen at all so while the voltage here and let's just do a little graph out for graphs not very technical graphs this is voltage and this is current amps okay and this is on the base this is the base yeah so the base voltage has to go from zero let's make it somewhere zero point three point six point nine yeah this is your base voltage going up okay until the base voltage reaches about six nothing will happen basically no current will flow at the point which is six the current goes up very rapidly so by the time it's going from point six to 0.7 it's probably gone from not conducting at all to its maximum conductance it's now switched on and it could not switch on any more this area here before the transistor can switch on before you get to 0.6 volts this is called cut off okay there's only one there's only one t in it cut off yeah the transistor is cut off so it can't switch on once you get to this point here it cannot switch on anymore it's saturated or more common too and it's hard on some of you get a lot out on it it's harder it's not hard on yeah so that's basically how your transistor behaves i'm hoping you can see from this a transistor is basically a current amplifier it amplifies the current so for a small control current flowing in through the base you can control a large current from the collective emitter and that's basically how an npn transistor works transistors are generally used in two ways although the circuits they are using can be very varied the transistor itself is actually really in two ways it's either acting as a switch so you either have no volts on the base no current flowing or you have enough voltage 0.6.7 for the transistor to be fully conducted so it's either off or it's on and it's nothing in between that's one way it can be used as a switch the other way a transistor can be used as an amplifier so with an amplifier circuit basically you have a usually two resistors to a base this is very much like that before let's put a load let's put a speaker okay a loud speaker is your positive supply okay this is ground and this is ground and with this you basically set up i mean there's a pack with 9 volts 12 volts 100 volts whatever but with these resistors you set the transistor the voltage at the base to be halfway through this you know this this bit between 0.6 and 0.7 where it goes from being fully off the point fully on you can set it for example to be about halfway so the transistor's half on and half off okay if you now maintain that dc voltage and you apply an ac signal through a capacitor so you're not affecting the voltage capacitors past ac not dc so you can now put a small signal on here and it'll just vary the voltage between about 0.6.7 yeah between this point in this point and effectively for your small signal going in you get a big signal coming out roughly between whatever voltage supply is and ground okay so a signal here maybe 0.1 of a volt peak-to-peak in this circuit can give you a signal the same signal pick-to-peak with 9-volt amplitude the other thing you need to just realize about transistors and this is whether you do a switch or whether with an amplifier i'll draw a circuit with some voltages and you'll see very simple base emitter resistor collector okay that's your transistor let's put plus 12 volts on it okay and this is your base same as we do before we'll use this as a switch and we'll put a voltage on here when there's no voltage on the base so this is zero the transistor is switched off it's not there okay so what voltage will be on the collector if the transistor isn't there well basically no current is flowing so the voltage here will be the same voltage as the other end of the resistor because no current flows through the resistor there's no voltage drop across the resistor so when this is zero the voltage here is 12 volts okay now look what happens when we put some voltage on here let's put some voltage on here we'll put enough voltage into this here so the base of this goes to 0.7 volts okay this is now passing current and this is now passing a larger current okay the current being set by the value of this resistor what voltage does the collector know well basically now this end resistor has 12 volts on and this end of the resistor via the transistor is connected basically to ground there'll be a small voltage drop across this when it's switched fully on this doesn't go to the voltage there zero you might have 0.1.2 a volt so the voltage here will be about points one of a volts point two a volt okay so look what's happening when the voltage here is zero the voltage here is twelve when the voltage goes up to 0.6.7 the voltage here comes down to zero when you switch it back to zero this goes back up to 12. so it's switching you're switching it but you're inverting the polarity can you see that when the transistor is turned on this has a positive voltage on it and when it's turned on from the collector this is held down to ground and the same applies with this amplifier i'm sure you can see that now so as this signal goes more positive the transistor turns more on an actual fact what you see is that the waveform this is getting more positive on the collector here this is getting more negative okay it's upside down it's inverted as this goes through zero to more negative this goes to more positive and so on so a transistor is a switch as an amplifier and this also inverts the saber i made uh a couple of videos and it was two or three i went them from here so i made the series uh transistors for beginners where i went much more into the general use of transistors but for this tutorial remember we just want to fix stuff i think that's really all you need to know about npn transistors now let's have a quick talk about pmp transistors and something that confuses a lot of people and to be going this confused me when i was being and if it confused me i'm sure you would confuse a lot of others so let me tell you about the pmp transistor here's our npn transistor we'll use it as a switch same as we were just doing would you apply straight down boy okay and then what was your pnp transistor so here we have plus positive okay so we know we've just looked at it to switch this transistor on you need to apply on here about 0.6 of a volt positive with respect to the emitter okay the base has to be a higher voltage than the emitter by about 0.6 to switch it on with a pnp transistor it's actually the other way around so with a pnp transistor we'll draw it like this and we can in fact put away here although it doesn't have to be it could be in this leg and we'll have the same thing okay now with the pnp transistor instead of the emit the base having to be more positive than the emitter to turn the transistor on with a pmp the voltage on the base has to be more negative than the emitter so you have to apply the voltage here about minus 0.6 to turn this on and when you turn this on the voltage on the clutch will basically go to the same voltage the emitter it's almost like a short okay so you might think from this a pnp transistor needs a negative voltage supply to work okay what's the ground that is not the case a pmp transistor needs to be based to be more negative than the emitter it doesn't have to be a negative voltage with respect to ground so for example if the supply here again positive 12 volts okay to switch this transistor on the base has to be as i say 0.6 negative of the emitter so the voltage on the base has to be about 11.4 volts positive with respect to ground okay so that's something you just need to get your head around when i was ever getting it took me some time i thought pmp transistors need negative batteries they don't yeah they don't it's all relative to the emitter i'll talk about i'll talk a little bit more about this when we look at how to test transistors in the circuit now let's have a look at what goes wrong with transistors we want to fix stuff yeah so we've had enough of them complicated theory what goes wrong with them this is what we want to know and how do we find the defaults okay so the main ways the transistor fails is to go short circuit okay they go short and they can go short between any or all of the terminals so you can go short base to emitter they can go short based to collector and they can go short collected to emitter and any combination of all three collector base emitter and that's all the combinations so they can go short between any of the terminals or all three of the terminals the other way they fail is the opposite they go open circuit and again they can go open circuit between any of the terminals or all of the terminals so that's the main ways transistors fail they can also fail in as much as they go low gain i mentioned this so the transistor is low gain is imagine like this is opening and shutting it's working yeah but there's some slack in this cable uh no not some slack better say this cable is like rubbery it expands it'll stretch you okay as well as moving this it can stretch so at the right pressure it starts to turn on but because the factors can stretch it's like elastic this doesn't move as much as it should do and even when this is fully open this is not open enough so effectively the current flowing through here is less than it should be when this is fully conducting it's low gain okay and while i've still got the diagram we can just look at another failure which i'll tell you about which is leaky so with the one that's leaky imagine the rubber seal on this thing is gone so even when it's shut a small amount of current still flows through here yeah a bit like your toilet system yeah the valve shut but it's still trickling water into it so those are two other common ways that transistors can fail so we have low gain and we have wiki transistors can actually fail in quite a few esoteric ways they can go noisy uh they can become temperature sensitive so the game is varying too much as temperature changes they can do some pretty weird things to be quite honest and the weird things they can do i think you'll find them more commonly and analog circuits such as amplifiers radio frequency circuits and these sort of things i'll be quite honest with it that's something of a specialist subject on its own so although this is all you need to know about transistors to fix stuff i will say i i need to do another video specifically for those type of circuits that discusses the problems where we have noisy transistors and problems with gain and amplifiers in particular transistors where the gain isn't matching so you have a number of transistors all in parallel effectively handling the load and the gain between the transistors doesn't match two other things i'll mention um transistors can fail especially in amplified circuits again where this um i chose another page yeah this one if there's a problem with these resistors these are called bias resistors so the idea of the bias is it just holds the transistor in its quieten state partially conducting or just on the point where it will start conducting and the problems with these bias resistors can affect the way the transistor works if it's not biased properly so that's something you need to just bear in mind and the other one is that you often find transistors connected like this this type of circuit um so again we have a resistor ground and we'll have a resistor and then this is connected to another transistor like so and in various formats yeah this sort of thing and maybe this is connected to something else maybe this is connected to a pmp transistor coming in this way from the 12 volts yeah so you can get this type of circuit and with this type of circuit we can get a failure whereby if one of these goes short circuits it will damage the next one and that will damage the next one so you get this cascade effect where one transistor failing will cause all the others to fail in the circuit that's common again in audio equipment can we test transistors in circuit yes we can and there are two ways in which we can do it this will not always allow you to find a faulty transistor but we'll certainly find the short circuit one and you may well find an open circuit one as well so in those type of faults then yes we can test them in circuits and to do this we need to use the diode mode on our multimeter so we're in diode mode now let's have a look there's a few old circuit boards here this is offered old atx and there are two transistors down here now if we try to test these in circuits i'll just find where they are i can see three pins here we should be able to see the semiconductor junctions but we may not always be able to so let's have a look well there's a semiconductor junction there okay and there's one there so this is looking like it's the base and these look like the collectors normally in the middle on this type of transistor so we'll try the voltage method five four zero five six that's the emitter the higher voltage ones if we go to the way nothing that's fine there's nothing we see a reading here another diode junction now that is almost certainly due to another component around here that's causing that reading and again we see them between the collector emitter but not that way so you're tending to see more diode junctions than you would expect on the transistor let's have a look at the little nothing there see that's reason that way like 0.610 if it's the same as that this should be the base we would imagine yeah he misses are we seeing that nothing that way so you can see we we we can test them but we've seen more diode junctions than you would expect well if you have a short it will be obviously there's a short circuit there so shorts you can find when you get a situation like this there are two ways really you can test the transistor you can either take it out of the circuit board if you suspect it or if you just want to test them you can take the transistors out and just test them out circuits that will give you a very good indication you'll also be able to see the gain as well um so you'll be able to for example in some circuits amplifiers you may have to match the gain so you'll get like four or eight transistors driving the output and they have to be matched again so you can also do that with the transistor testing um let's have a look at one or two more so let's trying to just find some uh surface mount transistors so smg transistors look like this let me zoom down you can see them a little bit better these appear to be smd transistors this little package it's called sot 23 which is small outline transistor sot 2 3 is the size 2 millimeters by 3 millimeters and you find a number of other larger packages especially for higher powered components so these could be smd transistors but the problem is that there's lots of devices made in this package they could be mosfets which we'll do with another complete tutorial on they could just be diodes either a single or a double diode in the package they could be a bipolar transistor the best indication that they're going to be transistors of some sort is that the port number is q not the part number but the identification number q662 q661 yeah so let's have a look at these and see if we can see any semiconductor junctions normally with these the pin that's on its own is the collector and usually this is the base and this is the emitter but i'll say usually i mean these looks like these two pins are actually connected to each other which is not uncommon to find so let's have a look let's see if we can see any junction so we'll go from here and we'll go to here well that means open so this is the base it's not an mpn transistor here that just gives a medium it's not a diode julting it's not a semiconductor junction so this one give we can't say any semiconductor junctions there what about between this spinning this one no no now let's try it with the negative on this end i haven't tried this yes we'll put the negative to here no no so we get a little bit of a reading even there but we're not seeing any junctions so you can see straight away that we have a situation where they either a these are not bipolar transistors or b the default the only way to find out what they are is they have a code on here this is not a part number it's an smd code the easiest way to find if you are going to find them is obviously use google and if you search for smd code and then the size of the package sot-23 yeah if you search for that you will probably find what they are it might be only the first couple numbers in this smd code actually the part number two possibly a date code but by that method so smd code sot-23 or sot space 23 yeah and that will probably find you the port number now once you get the part number from the code you can then look up the data sheet and figure out what that actually is the other possibility is just to take it off the board they're quite easy to unsolder you may have to tag some little wires on this and use a components analyzer and see what it is um so that's uh one there's another one here let's see is this a transistor we don't know again this is on a thick track so it's probably the emitter this is probably the base but you could be a mosfet again so what's up with what we've got nothing there nothing there for the other way around yeah a reading but it's not let's have a conductor junction okay so again we can't read anything from that and we'd have to look up the code again smd codes yeah sot23 and now we'll see if we can find out what that is so you can't test transistors in circuit i've shown you two examples but it'll probably only reliably find short circuit transistors for you we can see that testing transistors and circuits can be problematic you can normally spot a short circuit transistor or short circuit junction occasionally in circuit even that will let you down so you can sometimes get circuits similar to i've shown you where you have uh resistors on the base i'll draw an npn transistor again this type of circuit and this resistor can be a low value it can be sometimes a very low value and then this type of circuit if you measure across the base emitter you'll probably see a short or near to short and in fact it's the base resistor that you see the only way to find that out is to take the transistor out so if you take the transistor out and it's actually okay we'll see if some low value resistor and occasionally you'll find a similar circuit but there's a coil here an inductor coil maybe even a transformer this sort of thing and again this coil can read practically short to your multimeter so sometimes that will let you down are there any other ways of testing transistors and circuits well yes actually there is and this is actually probably more reliable so this assuming there's no shorts look for shorts first is to actually use the voltmeter to test the transistors and there's a few simple rules if you fold these rules that will get you a long way so one of them is that between the base and the emitter you can't have more than 0.6 of a volt okay if you've got circuits these resistors are getting very dodgy there we go something like this okay and we'll put a load in here for the speaker okay so if you measure more than 0.6 of a volt say 0.7 of evolve between the base and the emitter you can say with a very good degree of certainty that the basic meta jump says open circuit now when i say measure more than 0.6 i don't mean that if you find a transistor with more than 0.6 volts on the base it's faulty because you can have a circuit commonly like this one put some resistors in we put our transistor in and we have a resistor going into ground and we have either diode in fact we can actually go like this okay this sort of circuits or again we can put some sort of everything here in this sort of circuit if you measure from ground so you put your black lead on here and you put your red lead on it of your meter you'll take your multimeter and you measure it okay you might find like four volts or something here it doesn't mean that this junction is open circuit it just means that the emitter isn't ground so the most reliable way to test the transistor is this first test and let's use a different chord pen is there a short between any of the terminals on the transistor base emitter collector if there isn't a short measure from the emitter not from ground from the emitter to the base and put your probes here on your test meter okay so you're measuring directly base two emitter do you see more than 0.6 volts if yes so if it's greater than 0.6 let's say 0.7 i mean quite a bit greater one volt or more yeah if it's more than that you've got base emitter there's open circuits you've got a faulty transistor okay this is npn we'll do pmp in a minute well third one if the base emitter is near to zero certainly less than 0.6 yeah if there's less between directing the base emitter with the emitter probes positive to base then the transistor is off okay so if we measure from here emitter to collector directly across the transistor okay put your multimeter here now if the transistor is off less than 0.6 on the base then we should read a voltage across here and the voltage probably is a voltage between ground and the power rail because no current is flowing so it's effectively the same as measuring here and here so the transistor is off then you'd expect between collector and emitter to be a voltage it's off yeah number four base emitter is about 0.6 volts okay the transistors on 0.6 positive now if we measure between the emitter and collected this transistor should be turned on so the voltage across here should be almost zero maybe point one or point two okay so the transistor is on and collector emitted equals about 0.1 volt 0.2 volt something like that so that's how to test a transistor by measuring voltages you can measure from ground guys you can put your black lead on here you can measure the voltage on the emitter and then you can measure the voltage from ground to emitter ground to base subtract this voltage from this one so you if this is reading three and this is reading 2.4 you know it switched on but it's easier to do it this way okay again you can measure from ground to emitter ground to collector and depending on the difference you can work out if the transistor is conducting or not but i would suggest at least if you're a beginner do it that way and you won't go far wrong okay pmp transistors the same as it applies but with pnp and number two yeah so base two emitter this is again with your red lead on the base yeah always put your red lead on the base okay base three meter now if it's more than minus point six yeah minus one minus two minus three base emitter it's open the same as two there it's just minus yeah the same with this one i think we need to write it out again but number three f base do we met it yeah i will match that again if it's about naught or it's certainly less than minus 0.6 the transistor is off and between the collector emitted expect to find the voltage because it's pnp you next find the collector is actually going to be lower than the emitter rather than you and then the last one to say if base emitter is about minus 0.6 then the transistor is all and between the question method should be very low voltage again the collector should be slightly lower than the numerator because it's pnp so if you look at those basic rules you can test most transistors in circuit so there we have it all you need to know about transistors to fix stuff mostly amplifier circuits particularly audio amplifiers or f amplifiers are kind of an art within themselves there's a number of topics specifically to that type of circuit that are really outside the scope of this video but don't you worry i will do at some point um a video or you need to know about amplifiers to fix stuff so for now i think i'll give you what to go at you can see if you make a note of this the best way to test the transistor in circuit is with the device powered up and by measuring voltages other than the fact you can find shorts okay guys hope you enjoyed that short video and i'll see you all soon on another electronics repair video ciao for now guys

Channel: Learn Electronics Repair

Views: 152,450

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

Id: GDDwTbM9DLw

Length: 67min 45sec (4065 seconds)

Published: Sun May 01 2022