All You Need To Know About MOSFETS To Fix Stuff! How Mosfets Work Fail Test In & Out of Circuit
LER #243 All you need to know about MOSFETS to fix stuff This is the 9th video in this series looking at common components and circuits, from the perspective of a repair hobbyist / technician.
In each video I hope to teach you all you need to know about the most common way in which components and circuits fail, and how to test and diagnose these faults.
00:00:00 Chapter 1 - Introduction
00:06:47 Chapter 2 - MOSFETs vs Bipolar Transistors
00:13:59 Chapter 3 - Understanding P Channel MOSFETs
00:18:28 Chapter 4 - Testing MOSFETs
00:25:17 Chapter 5 - The Body Diode
00:30:23 Chapter 6 - Why We Need Gate Resistors
00:32:40 Chapter 7 - Gain vs Rdson
00:36:25 Chapter 8 - Enhancement & Depletion
00:38:57 Chapter 9 - Switching Properties
00:40:13 Chapter 10 - What Goes Wrong
00:41:56 Chapter 11 - ESD
00:43:23 Chapter 12 - Floating Gates
00:50:04 Chapter 13 - Testing MOSFETs In Circuit
00:54:55 Chapter 14 - Epilogue
Mentioned in the video - The Amazing $1 Short Circuit Finder
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Richard
Before retiring, I was an electronics troubleshooter for about 30yrs. I have worked with technicians and engineers from all around the globe. This man has taught me a lot in the past hour. Thank you Sir, but where were you 20yrs ago, when I really needed you, lol?
mind if i send you a mail sir? I’m graduating in a year from college
@@dhavies-ayo been a year! Hope you graduated and have undertaken a wonderful career! 😊
Well, been in electronics 43 years and still learned a little from this video.
Hi Richard, although I would call myself advanced when it comes to electronics, I love your "All You Need To Know About" series. And I still learn a lot through them. Sometimes I learn something new, but most of the time I remember something I had forgotten. Thank you for taking the time to make these series. Again, I love them!
Very interesting. I only ever had the most rudimentary of electronics knowledge as a teenager, with BJTs in switching applications being the limit of my skills. AC and amplifier circuits were completely beyond me, and I barely even knew what FET stood for, let alone how to use them. Now, mumblety decades later, I'm trying to learn some of these things again. Components and test equipment have become ludicrously cheap and accessible, something that was not the case in the early 1980s for me. I'm trying to go slow-and-steady, starting to dabble with JFETs before breaking into my stash of MOSFETs, but it's good to build up some knowledge for Future Me. One thing I hadn't realised until it was explained and demonstrated here in this video, was the concept of capacitance and a MOSFET remaining switched-on when current is removed. I genuinely didn't know this. I find myself wondering whether that retained charge dissipates over time (when out of circuit) or whether the transistor effectively remains "on" for a number of minutes, hours, days or weeks. Another part of this is a loose concept bouncing around between both my braincells, about this potentially being used as a form of (perhaps short-term) non volatile one-bit memory. I can see the entrance to a particularly deep and twisting rabbit warren nearby...
earlier today I sat down to watch a movie 10 minutes into the movie I got an email letting me know you'd uploaded a video so I turned off the movie and began to watch your video instead which was very interesting and I learnt something. thank you. later in the day I began to watch the movie it was very boring and I learnt nothing and it costs millions of pounds to make.🙂
hahaha I actually laughed out loud at this comment 😂
This channel is a little gold mine of information and clearly explained , I have searched high and low to get a clear understanding of such, so thank you so much for making this video as i have learned so much with in the hour and have not wasted years with all the complicated Bla Bla Bla in text books that fries the brain just like an over voltage Cap.
So well explained , no professor could explain it rather than equations on the board.
Hello sir, you are now my fulltime teacher! Nice to meet you!
One of your best videos in my opinin Richard - thankyou for posting.
Great video! I wish I had a teacher like this. All teachers started their courses with complex mathematical equations. They failed at teaching how to think intuitively of how a component is used in a circuit. It was all about solving equations and calculus 🙄
Thank you soooo much, I have been working in the field for 20 years, repaired a lot of equipment by using sight, and experience. Had I known this information I would have fixed a lot more equipment. Thank you for sharing in such simple terms.
Another top one Richard! I am amazed how much we can learn from you.
Pretty thorough and yet practical. Job well done sir 🙂
A really nice summary, thank you for being so to the point and clear.
Thanks to this gentleman, I enjoy electronics even more. Thank you for sharing your knowledge.
What a great teaching! Best ever found on you tube!
Hi Richard, I know I am a bit late to the party but I just joined so please forgive the tardiness. I feel the need to add in one point for those that might be new to mosfets, the gate resistor you have drawn in your mockup relay diagram is going to +Ve, that means that you are using the resistor to power the mosfet "on" there are other circuits in which the resistor will actually go to ground to turn off the mosfet, this depends on A: the circuit design and B: the semiconductor you are using to switch the mosfet with. In your circuit with the relay you are using whatever is driving the mosfets gate to "sink" the voltage from the gate to ground on other circuits with the resistor to ground the device driving the mosfet will be the "source" of the voltage for the gate to turn the mosfet "on" and the resistor will discharge the mosfet gate to switch the mosfet off.... The other point one might add, is that N Channel mosfets "typically" have lower resistances between drain and source compared to P channel mosfet and therefore also higher current capabilities sourcing P channel mosfets with low resistances and high current capabilities can become quite expensive compared to N channel mosfets of the same characteristics. Mostly it depends on the circuit use and design. I just thought that it might be prudent to add this point in.
Love your work, well explained detail to detail. As a troubleshoot technician I found a voltage difference on a transistor and could not comprehend what was going on internally until I watched this video. 💯👊
Wow, my head is swimming...but I know much more about the four types of mosfets than I ever did before. Inching towards making sense. Thank you so much.
Another gem of a class. Thanks a lot.
Wow...! What a beautuful presentation .....
Great video Richard you answered a lot of questions for me for sure,keep the good videos coming.
Thank you, I'm finding your teaching method easy to grasp and follow.
Again, a seamlessly delivered compendium of knowledge. If only they had taught us this way in the technical colleges of the late socialism in eastern europe... Cheers, Richard!
Amazing rich, Keep that knowledge flowing 🧠
An excellent and practical overview. Thank you.
I've seen soo many vidioes on mosfets - Yours is the best so far GREAT JOB keep it up .
Depletion mosfets are used where you want to make use of variable RDSon. They are used in LED drivers, linear power supplies,constant current sources, as a resistive load in logic circuits, or in the startup circuits of switched power supplies. It's just very easy to make a constant current source with a depletion mosfet, you just need only 1 resistor.
Awesome tutorial, loved it! Thank you!
These a brilliant tutorials. I'm trying to teach myself how to repair and even possibly design and build circuits, I can't tell you how much you've helped me to understand how components work. Your explanations just seem to click with me. I'm not too bright and have to watch two or three times to take it all in but thanks so much for posting these videos 👍
Great presentation Richard, thank you
Very clear and to the point lecture. Thanks a lot.
I was waiting for this ! ❤️
Thanks, Greatings from Portugal 🇵🇹🌟
Great video. Pure gold. Thanks again
Thanks for Sharing you knowledge Mr.... Great Thought!
your lessons are awesome, Thanks.
great vid learned alot from it in a easy to follow well explained vid thanks very much for sharing
fantastic channel!!! thank you! binge watching the videos now
Very good and this is an awesome explanation.
Loved this! It was very helpful! Thank You and please keep making such videos.
Thank you for the video. Very nice explained.
Great explanation, especially the in circuit cases and open gate hazard. I'm guessing you could test in circuit using a temporary weak gate drive so as not to harm the in circuit gate driver.
Your training videos are usually good, but I think that you excelled in this one. It was excellent - simultaneously informative and entertaining ( wow ...do I miss your type of *dry* humour having grown up in N.Wales and now living in North Carolina!!). THANK YOU!!!
great , thanks for sharing 👍
Excellent, thank you for this. Your videos are a big help, thank you thank you thank you.
Very nice vid, learnt a lot, Cheers from New Zealand
Thank you, this was quality info.
I would like to add that a resistor from gate to source is often added as a discharge path for the gate capacitance, its good practice with N channel so the 'switching on' with the multimeter wont work in a normal design that has a gate resistor to ground (source)...cheers.
Thanks for the info 🙂
Also as the gate is such a very high impedance (not resistance there is a cap in there) if left undefined it could drift and partially turn on, going in to the linear region with a reasonable current flow will over heat the fet in an switching application in no time at all. As your relying on the very low RDS(on) to limit or remove the requirement for a heatsink. Squeak!
@@TheEmbeddedHobbyist Squeak!
@@LearnElectronicsRepair There are actually a few fun project on hackaday for "mosfet based memory" ( I seem to recall, one managed to store something for a day, and someone built one and managed 15 minutes) JLC PCB TO THE RESCUE! ;) Eh first freebie circuit board might not be a bad plan to use a hackaday project, They usually contain at least ONE interesting talking point. Plus you could get some time flexing on EasyEDA, might be fun trying to figure out what equivalents from LSCC you can source. see if you get something 100% pick and placed? A challenge!
@@TheEmbeddedHobbyist Good point - I always think there's a resistor in there - much better to think it's a capacitor. I'll remember that from now on.
Wish there had been this channel years ago!
Nice explanation thank you
Thanks for all these superior quality videos! Could we get an episode about motherboards?
Wow, awesome video. Thank you.
Great video, Thanks!
Youare the best lecturer on all issues electronics in my humble opinion and you're a brit that's a plus, go Liverpool. Do you have any videos on tracing a problem where you don't have power to the VCC. I am dealing with a power supply for an Anton Bauer quad charger. And I cannot seem to trace the root of the problem. No Power to both switching chips, they share the VCC and it gets 16V. I traced it to a W1P transistor that gets 17V at the collector but it doesn't release it to the emitter which would go to the VCC as probably 16V.
very interesting.I learned a lot about MOSFETS. I know junction transistors and vacuum tubes.
Good video (as always)
Excellent!
This was effing awesome
Wonderful content all round, I've had a gander around your vids, but not found my problem...a meanwhile 750w server psu with just a relay pulsing on1 off3 seconds, a live wire touched the casing internally. Any ideas young sir? was thinking ptc/diode/feedback loop..
❇ Very informative 👏
Cheers Rich
Great video interesting thank you
Your videos are very informative and I'm grateful to you. However why you would leave errors in (like at 16:30) instead of re-filming and editing them out I don't understand.
Can you please do an all you need to know article on IGBT's? Really love the way your explanation made me understand easily. Thanks.
This not a pron channel.
Well done, sir. I was kind of fuzzy on how to bias MOSFETs. I want to 6:21 use one in a sample/hold circuit. 😮😮😮😮
Nice one.
4:26 - I just figured out a way to remember the difference in schematic symbols for N-channel and P-channel MOSFETs: for the N-channel MOSFET, the arrow point iN.
When testing, I just can't turn the gate on with the multimeter. But if I use my bench variable supply set at 9 V, it works fine. Another thing, probably mentioned somewhere else, but if you work with an antistatic mat it won't do to test the Mosfet over that. These mats are conductive and they will discharge the gate capacitor as soon as the terminals touch the surface.
fantastic!
At 4:35 you have just drawn the P-Channel version MOSFET. Since in all of the drawings the power has the plus on top, the source and drain seem reversed although the symbol in itself is correct. This is rather counterinuitive (for me at the least). Thanks for the video, very informative.
Finally!
I found your videos really educational for someone like me. I wonder if you check questions here. I got a question hoping to get your answer. I recently mistakenly connected an old PC power supply’s 12v output to a 26v battery, I hear two loud explosions and the supply was burned. I found the 16v rated 1000uf capacitors were blown open. But after replacing the two capacitors, the power supply is still broke. What else do you think could be damaged?
Thank you 🙏
Fantastically
Great video if you can also make a lesson about ipm driver
Very good video Richard. Can you make video for Mosfets selection criteria?
I can have a go but usually I use alltransistor.com as it will give you the datasheet and then also find substitute devices.
Of you have mosfet gate resistance value calculator formula please tell me
the fact that the pen is at 1% makes me both anxious and surprised is quite the feeling
Mosfet N- channel possible failed when it warm? Because when its cold I have negative signal and when it warm is nothing.
I just watched an ad that literally was explaining mosfets and thought i was watching the video for about 15 minutes hahaha
Hi I'm very new to the channel And absolutely thoroughly enjoying The videos So thank you for all the time you spend on them I was wondering if It's possible for you to do a video on 100v line amplifiers I have Recently purchased one to use as a outside garden audio system But it is very limited on output wattage I would really appreciate and love to see a video on these and how to Increase the output wattage Thank you
100V Line amplifiers are not intended to be high wattage, they are meant to distribute background or ambient sound over longer distances than 4 ohm or 8 ohm speaker systems, often using multiple speakers
@Learn Electronics Repair Thank you for your reply sorry I may have misled you my goal was to increase the wattage/ Output so I could have more speakers On the chain with different zones Rather than paying expensive prices for higher wattage/Output 100v amplifier I.e. a cheaper option But I would enjoy a vide on this subject To understand how they work either way Thanks 😁 👍
hi over there .I appreciate your video from Casablanca ,where i led you to open a technic school why not make a change ,
Hi Richard. Did you mean input resistance high on bjt? I'm lost on that point. To keep base current low would the resistors need to be high? Thankyou
I meant the input resistance (or impedance) of the BJT base-emitter junction is low, that is why a resistor is required to limit the base current. With a Mosfet gate-source junction it is very high resistance, practically infinite so a resistor is not needed, at least for the purpose of limiting gate current.
Thankyou for clarifying. That makes perfect sense to me know. All the best Richard
I have a hexfet part number IRF2204S that seemed to have been smoked from an overloaded small motor on tennisball machine .. rendered the entire PCB and system unresponsive to input
Another reason why resistors on the gate are a good idea is in the case of the gate going short circuit. Whenever I test MOSFETs or IGBTs on the bench with a load like, say, a 21W 12V auto lamp and a lead acid battery or a PSU. If the gate has gone short circuit, the package of the device will explode. If you put a resistor in there, it won't.
What's up with laptop mosfets when a laptop dies generally i go after the mosfets, i always mock them on laptops, but why they fail so much? but especifically on laptops i mean a desktop has a pc that has like 3 or 4 mosfet on the power supply to push a lot of power, i know that is not fair since them work at 3 digit voltage hence lower current but a desktop motherboard has mosfets for the cpu that are beaten up to ridiculous amounts of current and a desktop motherboard (at least in my experience) rarely dies of that. so what's is going on with laptop mosfets? they doesn't go overkill on laptops as on desktops?
Anyone have any tips for finding someone good locally who works on stuff like this? I have a several repair projects (I'm just a hobbyist) that I've traced the trouble to tiny little components on very crowded boards thst I don't have the tooling or skil to fix. They are old family items that I don't want to hand off to just anyone.
You brushed over the fact the FETs acts in the same way as an old style valve, it has voltage gain and can be operated in its linear region. They are used in amplifier just as much as they are used as switches. Also a point of note that can bite your butt is that logic level fets were designed for 5V logic and may not work on today’s 3.3V logic levels. You have to check the VGS(th) value in the data sheet to make sure it will switch on at 3.3V. Note that the low RDS(on) may not be met with a low VGS even if it’s above VGS(th) threshold. Also they may never be able to carry the current the data sheet claims. In practice you can’t make a heatsink that could carry the heat away fast enough. nice introduction.
I certainly know that MOSFETs act like thermionic valves ,but as most viewers would not understand the analogy yeah I skipped over it I could also have mentioned thermal runaway and the fact this is another area where mosfets are opposite to bipolar transistors . What you say about 3.3V logic levels makes perfect sense, though I never really thought about it as I am a circuit repairer rather than a circuit designer. Nice to see you liked the video. This is a big topic and the video is intended specifically for those who's primary intention is to fix stuff so I kinda concentrated on those aspects. There are plenty of videos on YT that will go more into the theory and circuit design aspects of MOSFETS
Heya not sure if I understand all of it gone watch this a agin on a later time stip but still learned a few things ( because of my autism I need to see it a few time to understand)
what would happen to the current if you were to feed a primary coil with the output of a secondary
as i tracked his pen 26:27 i was sure he will make a mistake 😁 ps an absolute masterpiece of teaching and he is very lucid with alot of experience , btw that diode is the result of shorting the source to the body of the transistor in order to avoid the turn on of the parasitic internal bjt (yes there is a bjt transistor in vertically and laterally diffused mosfets if i remember well) that is why altough a bidirectional device you are forced to connect it only in a single way since that diode Will Conduct
I don't know how I remember this since I was 5 or 6 but your logo is a recreation of the Rareware logo from Donkey Kong 64.
Hello rick. Perhaps you can make a video about pwm drive logic and the mosfet power sequence
Yeah I'll do AYNTKA VRMs at some point 😉
32:33 / Supposedly, the Gate/Dielectric/Channel creating a capacitor effect, can slow the switching speed due to charge time. So, they add resistors to the Gate. Doesn't make sense to me as in that case, it would seem that more current would charge the "capacitor" faster than using resistors and restricting current more ... slowing the charge speed.
I have an ebay listing for a powejack pswgt-1200. I have replaced the igbts a few times at high cost and given up. If you are interested in a challenge place a bit, it's for peanuts, and hope you can get to the bottom of the problem, I would be really interested. I'm done on it. Great channel, great info, keep up the great work. Cheers.
3:57 The way to remember that: The arrow always points to the N- type material.
Ahh OK, thanks.
My ups using 4 mosfet. Just buy 4 units and replace all irrespective if there are still good ones. Its cheap and all brand new that will longer.
NPN denotes the doping material (Base in Bi-Polar transistor) is P type, analagous to a P doped Channel FET, the opposite being true for an PNP and N channel FET.
3 videos. No notifications. What century is this??? Pretty sure it must be possible to manage to create service that can let me know when a video is posted, Google ;) Some random thoughts: MosFets and CMOS do not actually contain "Metal" anymore .... It's now just a name. (but you knew this) throwing it out there. The "Gate Drain and Source" Symbol is very similar to the older valve symbols. (Weren't those also controlled by same thing? a JFET symbol also looks the same? ) hmmm :) β == hfe, but hfe is the "technically right name for the docs". So meh, for all practical purposes use β. The Peak DCA75 seems to give a few more readings for the devices you mentioned. I have one of them, they are pretty decent. The windows software lets you curve trace, perhaps it does for yours too? Really appreciated that . was fun. thanks
Some random answers: Actually I didn't know mosfets don't contain metal anymore, but then I often don't worry how components are made, just how to find faulty ones Yeah I think a DCA75 is something worth having - especially for amplifier and inverter repair where you have a lot of power devices basically in parallel. I don't have one but would definitely like one. Is there anything else that has the same features (and cheaper?)
@@LearnElectronicsRepair In other news ;) My JLCPCB order is currently shipping. o7 Pick and placed 240 ridiculously small buttons for me onto 30 test board. Yay.
@@LearnElectronicsRepair I did reply buy seems like youtube remove the longer answer oh well.
@@twobob YT occasionally seem to have a problem with comments on this channel, personally I never delete any of them (including the ones that tell me I'm wrong) apart from the handful of porn links I always get in the first 30 mins of posting a new video, which in fairness I have nothing against (porn) it's just that this isn't a relevant place to post them.
@@LearnElectronicsRepair It wasn't you for sure. I mentioned an ebay item since you asked about alternates. That seemed to be the straw that broke the camels back ;) In a nutshell I don't have personal experience using other devices they do appear to be out there. Quality unknown, there was something interesting on UK ebay ( I wont post the link, search for "PC based transistor matcher/curve tracer mosfet and bipolar") hope it's not a fools errand. I personally was hoping you might design and build build an ESR tester. They are always handy.