Guide: Properly picking and using MOSFETs!

[@Sonnevs]

This video was very useful to me. As an auto technician, I am seeing more MOSFET's failing in engine ecu"s. Replacing these in house has become easier to do with helpful vids like this. Thanks!

[@LynkedVideos]

@1:04 - If you can't share a common ground, you just need to use an optical coupler in conjunction with the MOSFET. An opto-coupler provides isolation via an IR diode and phototransistor in a common package. If you can wire an LED properly, then you can wire an opto-coupler. In any case, a pull-down resistor is advised on an N-channel enhanced MOSFET.

[@neildarlow]

This is probably unnecessary but I will mention that the MOS (in MOSFET) requires that you handle the device with a degree of care. Despite their significant current carrying capability they are static-sensitive devices.

[@PaulWayper]

Another way to make sense of 'Drain" and "Source" is that they're for electrons. Electrons, which are negative, flow into the source and flow out the drain. So the actual electron flow is the opposite direction to that which we normally think "current" flows.

[@jeric_synergy8581]

@Billi Vandori , for actual current, yes. For "convention" current, it's probably right, but it sure is confusing.

[@caducoelho2221]

Electron point of view: Source: provides electrons to the drain. Drain: receives the electrons. Source -> e -> Drain. Conventional current: Source

[@Firefly-nz2xq]

Such a great video once again. I had a really bad MOSFET on the Heated Bed output on my Rampsboard. So after watching your video I took the opportunity got an IRF1404 from the local electronics supply and replaced the old one. Now its running perfect and at ambient temperatures. Thanks Thomas for sharing knowledge.

[@rich1051414]

Thanks, I have a few IRF1404's I picked up awhile ago because of it's specs. Couldn't think of anything wrong with it for this application. I usually use them for PWM applications.

[@edinfific2576]

@@rich1051414 IRFB7446 is even better than IRF1404 because it has lower ON resistance, lower input capacitance, and much faster switching, including much improved intrinsic "body diode" switching time.

[@MinhTran-wn1ri]

@5:00 Wait so Rds(on) increases as driving gate voltage, Vgs, decreases? And higher Rds(on) means higher power loss (P=I^2R) @7:13 Pins of a MOSFET. Controller voltage applied at gate. Electrons flow from Source to Drain (Current/positive charges flow from Drain to Source). @7:36 MOSFET OFF: Vgate - Vsource < Vthreshold @7:36 MOSFET ON: Vgate - Vsource > Vthreshold @1:21 Pick MOSFET based on load operating voltage and load current draw @1:57 Vdss and Id specs assume you can provide active heat dissipation. It doesn't mean the MOSFET can passively dissipate heat up to those those values to prevent failure. @2:36 Rds(on) is a MOSFET's internal resistance when switched on. Use P=I^2*R and what you know about the load current to compute thermal power generated (which needs to be dissipated). @3:03 Rds isn't an exact value multiply it by 1.2 (+20% derating). @3:52 Thermal resistance Junction-to-Ambient spec tells you how much the device heats up per watt of power dissipated (take value, multiply by power, and add ambient temperature 25C) @5:00 Rds(on) is the Static Drain-to-Source On-Resistance. As the driving voltage, Vgs, decreases Rds(on) increases which means higher power loss. @8:03 Sample circuit with MOSFET driven by Arduino. 1:100 voltage divider biased.

[@jasonkeiter2700]

WARNING. My top label plate was on backwards such that I un-knowingly connected 120ac to the DC side of relay. Using small screwdriver pry off label plate and make sure led is lined up with round window. Bought SSR-60 DA off Ebay.

[@danielkempf5688]

Thank you, thank you, thank you...I was trying to control a DC load with an AC SSR. Wasn't working. Now I will try the MOSFET route. Again, thank you.

[@matthewtilney9596]

Hey Thomas, grate video, the fist one that actually neatly explained everything I needed for my project without getting into too much detail or leaving out critical things (like heat dissipation which I'd have otherwise not known about).

[@bumv2]

The last FET in the list has a TO220 equivalent named IRLB8743 and on stock for about 1 euros (in Budapest).

[@gametabulas]

These hand drawing sketches helped me alot. I am building a arduino triggered multistage coil gun and this helped me alot.

[@bgable7707]

NICE, I just started looking for this type of explanation. THANK YOU, very clear, and nice diagrams on the wiring of the parts!

[@Strawberrymaker]

A small fix for the resistance: I think the Radds or ramps-fd uses actually two mosfets: A smaller one to translate the 3.3V Logic from the Due to 5V and THEN that mosfets switches a beefier mosfet. quite a neat hack to make the price still cheaper (but consume more space)

[@Crytek1337]

6:30 even if you only have 5 volts from a controller, you can use a voltage divider so its at least as the minimum voltage for what the mosfet needs to work.

[@AdrianCiubotariu]

the last schematic is what I really needed to see explained - I was curious about the optional 100R. Thanks for that

[@dejayrezme8617]

+Adrian Ciubotariu I'm a newbie but was also curious how this works. What I gathered (and take it as a grain of salt) is this: The 100 ohm resistor is to make sure that there isn't too much current flowing out of the output pin of the arduino. The gate basically works like a capacitor that is charged to switch the mosfet. So some current will flow into the game. The pins of the arduino are only rated for something like max 40mA so the 100 ohm limits that. Afaik the 10k is so the gate charge can dissipate again when turned off and the mosfet switches off correctly.

[@ProtonOne11]

+Adrian Ciubotariu Gate resistors are very nice not only to prevent your driving source to get destroyed (a gate of a mosfet behaves basically a capacitor, so in the first instant your driver switches on, it looks like a short circuit to the drain!) it can, by limiting that initial burst of current, help to reduce electromagnetic interferences too. (in a simplyfied view: less harsh current impulse means less emissions and with that fewer possible problems). If you would go really deeper into all the interference topics, you would actually start to look at the electromagnetc impulses travelling along paths in your circuits inducing electrical currents in the conducters, but thats another topic for another day... Anyway, there is of course a trade of with increasing that resistor at the gate, especially once you have your heater PWM controlled, so it switches on and off all the time thousands of times per second. The gate resistor limiting the current flowing into the gate is now causing that capacitor to charge slower, so your FET stays into the transition between ON and OFF state longer. Now, thats where a FET really gets into trouble, because at that transition it has to dissipate a LOT of energy (the value of the drain-source resistor is changing from gigaohms down to milliohms, or the other way around.) A rule of thumb i use is to go with roughly 33 Ohms for gate resistors, but as usual, a rule of thumb is just that, and can be just as wrong as it can be right, depending on what you are doing and who is involved. (gate impdances and capacitance, conductor impedances and losses, driver output impedances...)

[@AdrianCiubotariu]

thanks to all for the reply, I received way more than I initially asked, but that's the + to the community, not just people printing, with general knowledge, but people with good understanding of all elements involved. reading all of this I realised I studied it, back in the day, but not enough, it seems. how it just went away, no idea... there were a lot of "duh, dummy!" moments :) Thanks to Dejay Rezme ExtraBase and, of course, *****

[@bryanst.martin7134]

Lets see, current is the flow of electrons (- charged particles) towards the positively charged potential. Source is the source of electrons, drain from the valve is the path towards the + potential. Yet they still teach the inverse. It's a lot easier to learn if you keep with the basic physics. Other than that you have an excellent program going on here. I wish I had run across this when I was building my system during the same time as this post. I was unfamiliar with MOSFETs at the time. And partner was doing the programming and had the darnedest time understanding the use of controlling the ground path as opposed to the potential side. We finally got the simulator up and working, all 3 tons of it. This info could have saved a few hundred bucks. And many hours.

[@earthdevelopmentedsce7350]

In order to switch on a mosfet the gate must have a voltage difference with the source NOT THE DRAIN as the video says. The mosfet datasheets do indicate the resistance depending on the VGS (voltage between gate and source ). When the mosfet is N type the source will be at a lower voltage than the drain and so the gate voltage must be positive compared to the source.

[@SuperCozMick]

Hi Tom,Great video as always. I particularly like the IRLB8743 for driving high current loads like heater beds but you need to be wary of the lower VDSS of 30V and the fact that the gate is not logic level. I run this HEXFET as a direct replacement for the heater MOSFET in my cheap RAMPS board with great success. Way cooler as the RDS(on) at 10V is 0.0032ohms. Not sure what it is exactly at 5V but I think its around 0.0035ohms which is way better than 0.022ohms on the IRLZ44N.

[@faheemhabib550]

SuperCozMick hi sir this heated bead mosfet irlb8743.did u make one...i tryd buymt it keeps heating and shorting out the mosfet pls help with a diagram thank u

[@nihaludeenkaisome2796]

Superb explanation sir, what is the duty cycle of this mosfet

[@fatbambi]

I use the Microchip TC4420 (DIP8), driver chip for driving my MOSFETS, so you can feed the signal from the arduino/pi into pin 2 with a 10k pull down (it works with 3.3 or 5v logic level) then connect the gate of the MOSFET to pin 6/7 (with a 100ohm resistor), connect supply to pins 1/8 gnd to pins 4/5. then you can fully turn the MOSFET on with your full supply voltage. (12/24V). (lowers the Rds on resistance).

[@fatbambi]

sorry my bad the TC4420's won't work with 24V.

[@azlanikram9393]

Great video specially for beginners like us who don't even know at least some basic part nos of these while all other youtubers assume we somehow know it.

[@AmadonFaul]

Thank you so much for this! I thought there would be way more to using a MOSFET than this. I actually cooked the heated bed MOSFET on a RAMPS 1.4 a while ago and had to get a new board. I now use a mechanical relay (in a handy dandy box I designed, with headers, some LEDs to show when the bed is powered or not, and a fan header). I may redesign it using a MOSFET now that I understand them better. Of course I'll have to worry about heat more then. I have a question. So how quickly can a MOSFET switch? Can I stop using bang bang mode with my ramps and use real PID settings instead if I use a MOSFET? Smoother control of the power, basically a PWM output instead of just on or off. Thanks for all you do!

[@scottwillis5434]

Switching speed depends on the gate drive. Down around 10ns for a power device using 'reasonably strong' gate drive. That means a specialized MOSFET gate drive circuit or chip, and you need to account for losses during switching (MOSFET isn't fully on, so you get high VDS and high RDS when the MOSFET is transitioning from on to off and vice-versa), etc. You will want an oscilloscope and probably an adjustable bench power supply while developing such a circuit, to help figure out what's going on.

[@ProtonOne11]

Just some piece of information i miss in your video about MOSFETs. There are N-channel and P-channel types. VERY different types using completely different driving techniques, usually for switching applications deciding if you are doing a high-side or low-side control of your load (cutting the 12V going to your heater, or cutting the GND coming back from the heater, in simple words). For those more familiar with the good old bipolar transistors, it's similar to the NPN compared to the PNP transistors. N-channels are easyer to control and usually offer a better performance (generally lower on resistance). So, just don't even think about using a P-channel FET or any other fancy application specific FET (JFET,...) not actually made for the task at hand. And you should probably consider adding a blocking diode in parallel to your heater. Depending on how inductive your heater is, it can 'backfire' when you disconnect the supply while the FET is still turned on, and the diode helps to kill that induced voltage in a safe way.

[@Rich-Flies]

Ok - You've convinced me that I should be using a MOSFET rather than an SSR. I've already melted a 40A DC-DC SSR trying to power my 200W heater, and you couldn't have come out with this video at a better time. It looks like this MOSFET will work well. So how do I know how much cooling I'll need? I've already melted an SSR, so I'm rather hesitant to take this thing with the much smaller package and expect it to flow the current, even though it is rated for far more. I do have a couple of heat sinks that I bought for the SSR, though, so I'm wondering if that will be sufficient, and how I should wire up the fan to cool it.

[@gustawfita1315]

Gate doesn't need higher voltage tham drain. Even connecting gate with drain is enough to switch it on.

[@OtherDalfite]

This is false. On an n-channel mosfet, gate must usually be around 10 volts higher than source to switch on. On low-side configurations it's fine to just drive it at vcc. In high side, you need a way to step the voltage above whatever will be coming out of source. Bootstrapping IC's or a bootstrap capacitor can be used for this

[@scottwillis5434]

Gate to Source voltage is what turns the MOSFET on or off. It starts to turn on at the "threshold voltage". If you hook Drain to the Gate, that's a specialized circuit used in some circumstances (too long to write here). Whatever the Drain to Source voltage is doesn't affect whether the MOSFET is on or off; it interacts with how 'on' the MOSFET is to determine drain current.

[@inhdungho9509]

Thank you very much, I used an Arduino to drive a MOSFET but it got hot so quick and broken. Thank to this video, now I understand why it was broken

[@DanBader]

Hey Thomas, great video as usual but I have a correction and a recommendation. At the 7min mark you start talking about wiring it up. You mention "how hard could it be, it's only got three pins". I think you missed a chance at sharing an important safety message here. Yes, while it only has three pins it's important to note that most of the time the heat sink is electrically connected to one of those pins. Letting it touch other things in the circuit is a real fire starter concern. Also, a few minutes later you said that to turn it on the voltage at the gate needs to be significantly higher than the drain, pretty sure you meant to say higher than the source for this N-channel mosfet.

[@stevecxj6]

Love the hand-drawn sketches! I'd like it if they were up a bit longer . . .

[@nexusprime4704]

Thank you for the video. I am attempting to find a very simple, small and reliable way to power a portable LED array (40 parallel 3.1v 40mA each / 1,600mA total) from one or two 3.7v LIPO batteries. I was considering a LDO Regulator but the MOSFET seemed like a simpler solution. It also gives me an option of adding a PWM via a 555 timer if I need a dimmer. From my understanding, even a variable resister can be used to vary the gate voltage and adjust the current if I do not use a PWM. I am very new to electronics and probably do not have the skills to make a switching power supply, from what I have read. My question is how much heat would a MOSFET generate to take a LIPO Battery (3.7V) and allow a low side MOSFET to act as a drain to ground for the 1,600mA array. If there is a lot of heat can MOSFETS be paralleled in order to spread out the heat? If they can be paralleled, should a balancing resistor be used? Any suggestions or corrections to anything above would be greatly appreciated.

[@bosston8888]

Hi +Thomas Sanladerer Nice short guide! However on sketch at 8:03 I'd change one thing.. When powering MOSFET gate 2 resistors work as voltage divider. In this case voltage drop is really small, but in my opinion you should connect optional resistor between gate and 10k pull-down resistor.

[@zolatanaffa87]

Pawel WozniaK...relative of Steven :-))

[@волковдмитрий-ц2ш]

Depending on switching frecuency, if above 1 Khz, then a resistor is required on a mosfet gate, else the microcontroller output will die. Seen on spice simulation (about 100mA spikes on gate, due to miller capaticance between drain and gate). An this is worse if you drive an inductive load

[@DIY_ALAM]

hi i got faulty ER1606F mosfet is there any alternative of this pls help?

[@michelbillen9233]

Thank you for the nice and easy vid! It was a great help for me!

[@scripttic]

Great video, everithing on spot, love how you chose to show datasheets and source of informations.

[@Dc_tech386]

Your right I bought some mosfet on Amazon thinking the irf z44n was a good logic level mosfet as I connected the 6 volt pwm sine wave to it and power up my inverter everything just get really that the midge internal system failed and the I thought the data sheet gos for all z44n mosfet but I was wrong

[@SianaGearz]

Hey Thomas, how about the inductive back-EMF of the heated bed? There's a simulation on Reprap wiki (not sure if it should be trusted) that with measured actual inductance of Prusa PCB and wire, the spike on the ST MOSFET employed in RAMPS is if memory serves well above its rated voltage! And the types that you suggest like IRF3708 are rated even lower.

[@peterfitzpatrick7032]

Does the internal diode not handle this Siana ? 😕

[@brianmcintosh504]

Great video, please do more like this explaining how to select and implement electronics components

[@km5405]

important bit - at 5V many many power mosfets will only hit a fraction of their min on resistance. - you would probably want logic FETs for the arduino....or a npn and a pullup to a 12v line.

[@martinez1701a]

Would I be able to drive a mosfet through something like a 5v voltage regulator?

[@faramier321]

If the wiring only requires up to 2 additional resistors then why do the prebuilt 3d printer mosfets have so many SMD components on them? Not sure what they all are, but it looks like multiple resistors and a couple other components I'm unfamiliar with? Is it to somehow make up for the cheaper mosfets they use?

[@JasperJanssen]

Given that most electronics packages already have a mosfet-switched 12V output that just isn't *quite* up to snuff, how about just adding a second mosfet behind there? That way you're driving the power mosfet with 12V and you can be pretty sure it goes into saturation.

[@Wolves200]

Hey Tom; thanks for your videos-- I have a MKS base V1.2 and I´m upgrading my printer for a larger print bed and dual extruder, now dual extruders are supported by the board, but not the heat bed, how did you wired your second PCB heat bed on your printer; I recall you saying that you have double heat beds and you are able to shut one down if you are not using that print area. Thanks

[@Karatektus]

super video. hätte ich das mal vor drei tagen gefunden als ich mir nen mosfet rausgesucht und bestellt habe :D

[@KennethScharf]

Well you could make use of some of the cheaper mosfets by driving them with a bipolar or smaller logic level fet used as a buffer running off of the 12v supply. I've got enough 2n3904's and 2n3906's in the junk box to pull that off.

[@mmawad100]

HI , can i use a 10v vgs mosfit to be driven from the ramps stock mosfit ?

[@jacobstone6306]

I normally don't comment on videos; however, this explanation is excellent!

[@Jrobt3223]

Hi there. I don’t know if I’m biting more then I can chew. I bought a ebike kit. I put one together for my son and it was so cool that I put one together for my self. The one I put together for my self runs a 1500watt hub motor. On the first ride we took I was out 7 miles and it quite . I shut it off and drove about a half a mile and it blue a fuse. I put another fuse in it and it blue that one to. I seen a guy checking the board and it was pretty much the same. I have two mosfets blown. The numbers on the mosfets are 110n7f6, gf549mb, chn813. What would be the best I can replace these with?

[@SchwettyBawls]

He says that Gate goes to the RAMPS output pin, I assume the heatbed + from my RAMPS. Where does the heatbed - go? Is the GND near the control board referenced in this video the heatbed - ? Or is it the GND from the ramps plug itself (where does the heatbed - go in this case)?

[@ianide2480]

There are other considerations when deciding on when to use a heatsink as well. If PWM is involved...... There's the actual slope of the square wave, and of course you cannot see this without a semi-decent oscilloscope. Luckily you don't really need to worry about this in a heated bed scenario, I wouldn't use PWM for a heated bed anyway. A simple on and off will suffice to keep it up to temperature, but I have seen the option to use PWM in some firmware. But I digest. =P (I know it's digress) Arduino PWM can warm up a MOSFET because of its switching speed, (MOSFET spends to much time in a linear range, not fully on or off, which causes extra heat) but this is handled quite well by even a small heatsink. The extra heat can be cured by a MOSFET driver circuit or a properly set up totem pole driver, but a small heatsink is probably the easiest. For my money IRL3803 is the way to go for 30v or less, with 7.5 milli Ohm at 4.5v gate Voltage. Up to 100v I generally use IRL540NPBF which has slightly more resistance but it's not horrible, coming in at 53 milli Ohm at 5v gate voltage. Both are less than $1.50 at digikey.com The IRL3803 is hard to beat for it's super low on resistance though and I have used both of these in 12v applications and the heatsink never gets warm, but this does not stop me from using one. Call it overkill or whatever but heatsinks are cheap and can be pulled out of old electronics for free (my preferred method)

[@RexxSchneider]

I really doubt that the IRL540NPBF has a maximum Rds of 53mΩ at 5V Vgs. The Vishay datasheet that I have (Document Number: 91021, S11-0510-Rev. B, 21-Mar-11) shows a maximum Rds of 77mΩ at 10V Vgs, and it doesn't seem possible to have a smaller Rds with a lower drive voltage. What is the datasheet you are drawing your information from?

[@edinfific2576]

At 7:37 you are talking about voltage between gate and DRAIN, when you meant to say between gate and SOURCE.

[@chinmeya8811]

sir please could u post videos of properly selecting mosfet,gate driver and gate resistor

[@scottwillis5434]

Great video! For completeness, when he writes "12V" or "24V" he means VDC.

[@syedhussain3463]

Hi I need RU6888R any replacement.. Because I can't find in India any equivalent

[@syahrulzahwan]

Wjat type of mosfet should i use for a really fast switch

[@pdegroote]

If you want silmilar specs as IRLR8743 you may look at IRFB7530PbF. It is available in TO-220

[@byxlettera1452]

Why do you suggest only 100 ohm (optional) for the Gate? Is it not too low? What do you think to use 270 ohm to protect arduino control board's pin instead. Maybe I am wrong and I have missed something. (Scheme at 8:31) Very good video, well done. Thanks.

[@tookitogo]

by XLETTERA Well, basically, the gate is a capacitor that needs to be charged to turn the MOSFET on, and discharged to turn it off. So the value of the resistor is a balance between protecting the MCU's output pin on the one hand, and on charging/discharging the gate quickly, so that the MOSFET spends less time partially on, when resistance is far higher. This is why it can make sense to use a dedicated gate driver IC, whose whole job it is to take the small input signal, and then drive the MOSFET gate hard, to make it turn on and off very quickly. Another approach is to use two transistors ("push-pull" or "totem pole", this is called) to drive the gate. This allows peak currents of several amps (!) to/from the gate, which the MCU itself could never do, though this current is only moved for literally a few nanoseconds.

[@byxlettera1452]

Antonio Tejada Thank you for your reply. So 270ohm is not completely wrong. My point of view was protecting the Mcu first. I will use a bang bang mode for the heated bed. So I think speed is less important. Anyway I got what you mean. Thomas is always very clear and precise. Thank you again Tejada

[@byxlettera1452]

MikeOnTheBox At the end I decided to use 270 ohm and I is working like a charm. Better to be safe then sorry. Thank you

[@user-rs8zg8ey2b]

7:35, I think you meant, the gate needs to be higher than the source, NOT drain?

[@francisdery2212]

Sir,thanks for your tutorial.But i am confuse when you said the gate voltage need to be higher than the drain voltage.What i know i think a MOSFET needs a small amount of voltage to turn on the MOSFET while the drain is connected to the supply voltage

[@Anonymouspock]

Francis Kuuderb It's an error. The diagram that's up when he says it is correct.

[@ecobob101]

What would the difference be if the heater was moved between the Source and GND? Wound the Mosfet get hotter maybe?

[@MomoCoder]

Is it always bad to use mosfet in infinite mode / without the gate pull down res?

[@Mu1e0]

Is it OK to use the 12V heated bed output from the board to switch the mosfet? Also does the heated bed require a separate power supply or can it share one with the board?

[@Engineerd3d]

I love the hand drawn sketches. However a picture or a camera zoom into a completed circuit would help quite a few people. There are also some 160A mosfets out there(as per your calculations these should be able to handle around 100A), maybe you should cover those for people who need help wiring up a high amp heated bed.

[@anullhandle]

Bruno M. , Old post but in general ignore the front page marketing bs and head for the graphs and other detail info on the data sheet. You'd need some beefy drive circuit to deal with the gate capacitance and proper board layout to pull off a short 200amp pulse.

[@supernielsen1223]

Im a big fan of your channel you have offered help when i needed it, you are there for the viewers thats very nice. Just an idea out of the blue, you seriously need to review some of those delta printers out there! An example could be the mass portal! :)

[@mmawad100]

can i use 10v vgs mosfit to be driven from the ramps stock mosfit by the 12v from the power supply ??? & by the way thanks for the professional work as always (y).

[@martinda7446]

Re 'infinite cooling' at 47A, well that is of course a silly thing to say. This device is safe at 47A if you keep the junction temp below 175 degrees, and if used as a switch with the on resistance being 0.02 ohm you will need a heatsink to dissipate 50W max.

[@sankijuhcphawa6583]

What does it mean when people ask when they see a MOSFET, is this a 12V or 5V MOSFET? Reference to which part of the Data sheet.?

[@styrishrodrigues]

What about IRFP460? Is that a power mosfet?

[@jitendrakumarsaini947]

Which mosfat to replace MOSPAT d2494 can be used

[@Yotanido]

So, what about good old electromechanical relays, then? Wouldn't those make much more sense for those kinds of applications? Especially when controlling something high-powered with a µC or something.

[@mechanicallydev4536]

You can't PID with mechanic relays, because they're very slow to switch on/off.

[@SianaGearz]

As to PID comment. 1. Yes, you can force a looong duty cycle, of like 10s at least, and PID is just going to improve its performance. 2. Current firmware doesn't to PWM control on the heated bed anyway. 3. Bang-bang control is considered sufficient for heated bed. I think the magic words are cost and wear. A MOSFET is cheap, and it should be possible to choose it such that it has a very long service life, longer than the device as a whole. Yes, i know, they do degrade and fail, so maybe there's a little bit too much of cheapening out going on. These contacts on the relay always suffer wear though, and eventually it will melt or catch fire. The typical MOSFET failure mode is easy enough to catch with a fuse, because it fails short while it explodes, so you can at least prevent actual fire, but how do you catch increased resistance and smoke coming out of the relay? What if its resistance raises to same as bed's and suddenly you have a 60W heater in a tiny little enclosure?

[@stojcsicspeter1198]

You could use the mosfet for heating the bed.

[@angelramosexperience8698]

hi tom i need some help if I have a couple of different stepper motors laying in my room and I do not know the specifications is there a way to identify them? They are all, 17 type motors and I really want to find out the differences but I don't see any indicator on them that Will told me any information have you made a video on how to identify different stepper motors?

[@rholt2]

i think i'm a geek in that i totally enjoyed watching the entire video. sheesh. great vid, as are most, if not all of the dozens oyour Vids i've watched. Russ from Coral Springs, Fl, usa

[@iforce2d]

Can confirm, Russ is a geek :)

[@filipgd123]

Thanks, for your video. I need to connect 20x30 heated bed to 25A so now I know what to do.

[@rokibsumon2160]

I have a 650v ups and its use IRLB4132 mosfet. But I can not find this mosfet in local maket please tell me a alternative mosfet of IRLB4132.

[@ccv3237]

I loved your instructing video, very useful. Thank you very much. I would only recommend you not to rush too much in each of the various important details you mention all across the video, and indeed, your drawings are all very useful, especially i liked the last one with the battery. It helped me much better how to hook things properly. Thanks for posting this. I subscribed & liked it.

[@reyhaneheskandari837]

so how we drive MOSFETs which are not logic level gate drive? and thank you for your video : )

[@dalefirmin5118]

See my reply to James Johnson.

[@thatcrazywolf]

Let's say you have some mossfets that take a higher Vgs. I've seen some people use an npn transistor connected to a 3.3v microcontroller to provide say 12v to the gate of a mossfet to lower it's RDS on. I just haven't seen a good explanation of how to do that

[@dalefirmin5118]

Just use a low power MOSFET as a driver, like the BSS138, which has a Vgs(th) of 1.3v typical. You will need a resistor to limit the inrush current and then it will draw no current from the MCU to stay on, unlike the gate current of a BJT. The lower the Vgs(th), the more susceptible it will be to noise so if you use the BSS138, keep it close to the MCU.

[@sunildalvi82]

Can some one elaborate the part where it's mentioned that not mounting the 10K resistor would make mosfet act as a resistor and make it burn.Thanks in advance.

[@Nuttavoot]

If no 10K resistor the MOSFET may not switch off that means it will always on and act like a resistor.

[@raykent3211]

Sunil Dalvi sorry for being late to this party... as others have said, we need to drain charge from the gate to ground to get clean switching from on to off and we want to do it fast. The arduino output pin will do that quicker than a 10k resistor, so it's redundant. The pin is active high, sourcing current, and active low, sinking current. No worries. Except if weirdly, you've switched the port from output high to input.... or the connection is broken. In these cases the fet could fry without the resistor to discharge the gate, so put it in just in case. Also if you go via an optocoupler (highly recommended!) they generally won't both source and sink, so the resistor is essential.

[@brantwinter]

Can you please explain the optional 100 Ohm resistor on the gate ?

[@iforce2d]

Just to limit the current that can flow out of the arduino - those pins are rated to 40mA output max.

[@abandonedcranium6592]

2:53 I always thought "constant current" would be anything greater than 1 second... I'm a novice btw. Since electronic systems can operate so fast, I assumed anything over 1 second was a long time!

[@PaulWayper]

Upvote for "fire on mosfet" diagram :-)

[@haz939]

It's difficult to find a high current mosfet that has a reasonable RDS(on) resistance at 3.3v.

[@neildarlow]

+haz939 Very much so! The level of rDS(on) is heavily dependent on the magnitude of vGS. For 3.3V systems some form of level translator is required. A 5V powered 74HCT125 is a good starting point.

[@ProtonOne11]

+haz939 There are several options to get around that. A simple level shifter that first converts the 3.3V logic signal into a 12V signal to then drive the gate is definately a very good and usually pretty simple solution. There are specific gate driver circuits (integrated) from a lot of the major silicon vendors, you just have to check what kind of logic thresholds they have at the input side. These are more mostly more specific to drive high frequency applications, so thats why they state how much current they can sink in or srouce out of a power mosfet gate. (Reduce losses by switching the FET as fast as possibe) It starts to get a bit tricky again once you work with 24V on your power side, since you have to take care not ever to go above the maximum G-S voltage rating of the power FET (usually between 20V to 25V), so you need some intermediate gate drive voltages between your 3.3V logic and 24V power supply, apply some tricks to limit gate voltage after the level shifting, or again look for other components that are solving that problem.

[@Strawberrymaker]

+haz939 or something easier (which is used in things like the radds) is to use a second mosfet which can operate at 3.3V which will control the main mosfet. With that middleman-Mosfet, the Rdson value isnt as relevant, because a normal mosfet wont drain much current from the drain pin. But then you have to use a pullup resistor for the middleman-mosfet and also a pullup for the main mosfet. the middleman one is going to be pulled high, which means the second one is going to be connected to ground, so what is connected to it is also grounded. here is an example schematic (uses a smd mosfet as a middleman) puu.sh/mPJdV/24d317a88d.png

[@jamest.5001]

could you recommend a MOSFET for switching a 15 amp inductive load. at up to 30volts. and I'm so new at this. and self taught by ppl like you. I don't really understand how to select gate resistors. I don't totally understand how to select resistors for anything. the basically zero current of a gate of a FET. really throws me off. also could a led be added to the gate to ground? without any bad effects? I will be switching on the fet with a zener of about 27 volts. to activate the gate when the voltage rises to about 27 volts. it is a regulator for a alternator. using two FETs . one turned on with a switch. then another to ground the gate when the voltage rises to about 27v. when in operation it works like a pwm. pulsing power to the field. keeping the voltage below the zener voltage. the field windings can pull about 12 to 15 amps. depending on the resistance in the carbon brushes. they are about 1/4" by 5/16" and up to nearly 1" long. I font know the resistance of them in circuit or otherwise. but most of the time they stop operating when they get to about 1/4-3/8" long. they don't have enough tension to keep in contact. maybe I should try checking. I guess it would have to be calculated . there is no way of testing while assembled . thanks if you can help it would be awesome and appreciated . thanks again....

[@deadshot534]

Hello sir, if we are only allowing 2.0A collector current but Vgs is 5.0V? What will be the Rds now? I can't see the graph clearly sorry

[@jitendraparit9374]

Sir I am working on... battery management system..in this I am using..14 cells in series..and 10 in parallel..now for balancing can u suggest .good MOSFETs..

[@sangtao1245]

Could U tell me which one could be alternative for APM3095p (SMD), please!

[@PauloRicardoBlank]

Nice guide, thanks for sharing your knowledge !

[@sparky7071]

If your using a MOSFET then I would add a fuse to the circuit as it is not going to like an accidental short on the output!

[@ethanschofield8082]

My brain got fried during this video, but I think it will come in handy pretty soon thx a lot!!

[@travisturner6761]

Can this be used on a Prusa Mk2 to heat the bed faster?

[@radiorob007]

am confused after this video, I think other viewers also! I can put a FET behind a FET? so the FET on ramps1.4 sends another(better) fet? or I have to solder the fet from ramps1.4 and change to a better/stronger one? or use ramps1.4 no more and go directly to an Arduino? Friendly greetings from the Netherlands Rob.

[@MarkusOsmers]

+R. Rob. Yes you can put a MOSFET behind your ramps board. You are using the output as your signal to switch your MOSFET which is connected to a propper power source. This way you can have any power switched not limited to the max Output of your Ramps-Board, but limited to your MOSFET and your connected power supply. This is often usefull for a heatbed which often draws more power than the typical boards provide.

[@radiorob007]

+Markus Osmers (mo22) thank you Marus! Regards from nederlands R.Rob.

[@peterfromm]

I know this is an old discussion, but I'm struggling with the same question. From what I read in other places, it is not straightforward to connect a MOSFET to the existing heated bed output (which is driven by a MOSFET). Something about the signal being inverted: www.reprap-diy.com/connect_sevenswitch_to_ramps Can any one confirm or deny this?

[@radiorob007]

Yes I think it would be smart in order to draw a clear schematic of it! perhaps even with multiple possibilities maybe? Regards. Rob.

[@Djhg2000]

You should also mention you're talking about N-channel MOSFETs. There are P-channel MOSFETs which go on the high side of the load.

[@evvaldis]

I wanted to say same thing. Because P-Channel is also widely used, but in this application you need N-Channel.

[@ConnorWeller]

Can a 12v bed not be wired into the mains with a ssr? I bought the one from your last video and was planning to hook it up.

[@ProtonOne11]

+Connor Weller Seriously? Mains is 230V AC or 110V AC, depending where you live. Hook that directly on your 12V bed and you make a direct short circuit, have an AWESOME little firework with blown/burnt heated bead, blow out the fuse of your house (or at least the section you hook it up to) and maybe even start a little fire and damage you, your printer, your house or any nearby 3rd party. DONT EVEN THINK ABOUT IT! A 12V heated bed is designed for 12V DC (or less), any higher voltage can and will get you in trouble. If you don't even realize that, please don't even think about doing any wiring that involves mains voltages (LETHAL!) and restrict yourself to plug in only finished products with already attached connectors in main sockets.

[@ConnorWeller]

+ExtraBase thank you so so much. For some reason I thought it could Handle the increased voltage. I guess I'll use the ssr for another project. Again thank you so much

[@MartinVincentBloedorn]

Superb guide! Drawings were also very good!

[@mrgb306]

would you happen to know which mosfets i would need for an american bass 500.1 amp?

[@trendkilla254]

Is there any reason I couldn't just get a 12v relay from a car and use another computer power supply to power the heated bed?

[@Javafiendcuppajoe]

You REALLY do not want to use a car Relay. Mechanical devices fail, and it would really suck if it LATCHED closed, then you bed would not turn off.

[@Coffee2clutch]

Super nerdy, I give you a thumbs up because anyone possessing this knowledge could likely kill me with their brains alone.

[@daren99]

hi what can this 60s380ce mosfet transitor can be replaced with

[@DavidGrayOK]

Thank for a great tutorial. The "heated bed" I have in mind is in my wife's greenhouse - LOL!

[@Cambria399]

But it I use PWM with a 100 ohm gate resistor, won't that slow down the mosfet switching and cause loss and waste heat?

[@noobulon4334]

(Answering for posterity) It will, but the gate resistor is to save your arduino's output from the current spikes of charging the gate capacitance. If you need to use pwm at high speed consider using a mosfet driver to keep the switching fast

[@rich1051414]

What about the IRF1404? It has a 0.0035 ohm RDS(on).