MOSFET Why use a Gate and a Pull-Down Resistor?

[@abdullahshafi4432]

I was searching the whole internet for the explaination and here it comes. Thanks for the very detailed explanation. Have a great day

[@DustinWatts]

You are very welcome and I am glad for you that your search is over ;)

[@daniel3765]

Finally I understand how it works after many videos. Very thanks!

[@DustinWatts]

You are welcome!

[@Ech01378]

Simple, direct and perfect. Thanks for the didactic way of sharing knowledge!

[@DustinWatts]

You are welcome Emanuel!

[@coucouj2781]

Finally!!! I was looking for this a long time ago. The difference between gate-source resistor and gate-IC resistor. Thank you so much!

[@DustinWatts]

You are welcome! :)

[@MaxintRD]

Nice! A very clear explanation of how this works. I didn't know yet that a MOSFET can draw such a high current via its gate, so thank you for the warning and for showing how to protect the MCU pins. +1

[@DustinWatts]

Thanks Marcel!

[@FarmingwithZana101]

Best explanation I’ve heard so far

[@gedtoon6451]

The high current to charge the gate capacitance is only for a fraction of a second, but is best practice to use a gate resistor to limit it.

[@kobie1585]

Good video and a clear explanation Boss👍👍👍.... I just liked your video and subscribed to your channel.... Greetings from Ghana 🇬🇭, West Africa.

[@borsi99]

Thanks for this important Information! All the best to you, Boris. 🇨🇭

[@DustinWatts]

You are welcome Boris! Glad it was of some help!

[@KraussEMUS1]

Great clear description! Thank you!

[@KraussEMUS1]

I do have one question. I am using a stack of mosfets to throttle a miniature high voltage transformer and voltage multiplier. Often times the mosfets stick on for a while, the battery is connected with a small receiver. I am running the mosfets from a small mosfet driver that is connected between the receiver throttle output and the fets. I have added a small pull-down resistor of 10k value, and it seemed to help but, the fet still sticks on often times for a while. I'm going to try 1k now. Does that sound like a wise plan? Thanks ahead of time if you have any further ideas!

[@KraussEMUS1]

One more detail. When I test the circuit on the bench the mosfet doesn't stick on when the voltage multiplier leads are set to arc a little or way beyond arcing distance, the mosfet still does not stick on. When I connect the arrangement to a large ion producing surface, then the fet begins to stay on sometimes. Likely it is due to the proximity of the charged surface, or the load is higher than expected?

[@DustinWatts]

Both could be an issue. I would try to resolve both. A smaller pull down will drain the gate faster. But be sure not to exceed the power limit of the resistor. The rise and fall time of the MOSFET also increases as temperature increases. The temperature increases as the current from drain to source increases so that can explain it too.

[@alchemy1]

Perfect no nonsense explanation. Thank you.

[@DustinWatts]

You are welcome! Hope it helped somehow!

[@BrandonGlenn8867]

Thanks. I couldn't figure out why my fingers were messing up my mosfets

[@DustinWatts]

Now you know! :D

[@gilbertcuoco]

Nice! Very informative. Thanks!

[@DustinWatts]

You are welcome @wizzywizz !

[@actorsfan3004]

Dustine sir, thank you sooooo much for this brilliant work and explanation. It cleared my all doubts about mosfet switching. You did it practically and theoratically. Love from pakistan.

[@DustinWatts]

You are welcome! Greets from The Netherlands!

[@upendrachaudhari4635]

This was so good explanation...thank you

[@DustinWatts]

You are welcome Upendra!

[@upendrachaudhari4635]

Dustin Watts Thank you 😊

[@leonardoperalta2932]

Thank you Sir. I really appreciate it :)

[@DustinWatts]

You are welcome Leonardo!

[@democracy2005]

thanks man for making this video

[@DustinWatts]

You are welcome!

[@rajkartun5142]

Amazing video

[@DustinWatts]

Thanks for your compliment! Awesome!

[@rabeyahossain434]

I learned a lot, take a genuine love ❤️

[@CNoteZzz]

Now I know how they use MOSFETs in smps

[@michaeltina3866]

Thanks for this video... please could you do the same video for TRANSISTOR BJT

[@DustinWatts]

Good idea! I should take the time to do that :)

[@michaeltina3866]

Very nice video...it helps mee too much Thanks so much

[@vahagnmelikyan2906]

Didn't understand the last part of 100ohm resistor. Is mosfet back feeding high amps to controller?

[@DustinWatts]

No, it is to "protect" the microcontroller from trying to supply to much current to charge the gate. Current overdraw on a GPIO can cause weird behavior like reboots, or can even damage the microcontroller.

[@vahagnmelikyan2906]

@@DustinWatts thanks for info

[@DustinWatts]

@@vahagnmelikyan2906 You are welcome!

[@vahagnmelikyan2906]

@@DustinWatts but if adding a resistor wouldn't it cause a voltage drop? Or that's why using lower ohms ratings.

[@DustinWatts]

Well yes, but that is what we need. If there was no drop across the resistor there wouldn't flow any current to the gate of the mosfet. And as the gate charges, the voltage on the other side of the resistor rises and the drop gets lower and so does the current that flows. Basically we are charging a capacitor through a resistor. If you google "RC charging circuit" you can learn about how that exactly works. So "voltage drop" doesn't really apply the way as it would for example as a current limiting resistor with an LED.

[@hemanth_loky]

Great bro!! Love from Chennai :)

[@DustinWatts]

Thanks from The Netherlands! :)

[@Komputry]

great video!

[@DustinWatts]

Thanks!

[@jellejadoul8263]

Wow thank you, now I understand it!

[@DustinWatts]

Glad it helped!

[@hukkabukka]

Sir, can u please explain ZVS DRIVER circuit ?

[@hukkabukka]

Superb explanation.

[@eleazarbarruel1213]

Does the two resistor forms a voltage divider?

[@DustinWatts]

It depends on the location of the base resistor. One does create a voltage divider (as the circuit in this video does) and another configuration doesn't. I would that argue that if using 100Ohm and 10KOhm, the voltage at the output of the voltage divider is still high enough to not really make a difference. For example 100/10K will have 99% (in the case of this CR2032 it is 2.97V) of it's input voltage on the output of the divider.

[@otonomosblog5972]

if i use 12v battery, can i still use 10k ohm for pull down mosfet irfz44n or other resistor value

[@DustinWatts]

10K is fine. The reason for the pull-down resistor is that if there is no signal on the gate, the gate is not floating.

[@otonomosblog5972]

@@DustinWatts ok thankyou v much

[@cre8tivesriram342]

@@DustinWatts If the gate floats what happens?

[@DustinWatts]

@@cre8tivesriram342 Depends on the charge there is on the gate. Floating means not connected to anything. So if the gate floats, the MOSFET will stay in the same state it will be it was just before it became floating. Am I doing your homework for you? ;)

[@joeyhenry7312]

So how do you decide the resistor values and what value might be to low or to high?

[@DustinWatts]

The Pull Down resistor isn't really critical. A 10K pull down resistor is a popular value. The Gate resistor limits the current draw from the ATmega I/O. The value should be chosen so that it doesn't exceed the maximum current draw of 1 pin (ATmega328P = 40mA per pin max). Using Ohm's law you can calculate that @5V a resistor of 125Ohm limits the current to 40mA (5V/125 = 0.04A). So my advice, don't go lower then 125Ohm. The total maximum current the ATmega328P can source/sink is 200mA so keep that in mind if you are using the other I/O's of the ATmega. Also consider that the higher the Gate resistor, the slower the gate of the MOSFET charges and the longer it takes for it to fully switch on. Generally speaking a slow turn-on time is not good for your MOSFET as it can heat up too much.

[@dashiellbark-huss6806]

@@DustinWatts Is the 5V in 5V/125 = 0.04A the voltage of the battery? or the pin voltage on the ATmega? I'm using a teensy microcontroller and the pins go up to 3.3V but I'm using the mosfet so the teesny controls an LED strip with a 9V battery. So would the calculation be 3.3V/82.5 = 0.04A or 12V/300 = 0.04A

[@DustinWatts]

@@dashiellbark-huss6806 The calculation you are looking for is: TeensyPinVoltage/TeensyMaxCurrentPerPin = ResistorValue. I quickly looked up the maximum source current of the pins on a Teensy, which is 10mA per pin. In this case the minimum resistor value would be: 3.3V/0.01A = 330Ohm.

[@dashiellbark-huss6806]

@@DustinWatts thanks!

[@maicod]

I got a little scared when you connected the gate of the mosfet directly to the 5v rail but I see the gate can bear multiple amps.

[@DustinWatts]

Which 5V Maico? :) You mean the coin cell?

[@maicod]

oops no 5V around I know :) I was meaning the 3 V of the coin cell ofcourse

[@pabanoid]

Is this a same resistor that connect gate to emitter in IGBT circuit?

[@DustinWatts]

Depends on what circuit you mean. In this circuit is server to drain the gate so the MOSFET turns of when a voltage is removed from the gate.

[@pabanoid]

​@@DustinWatts Just did some research look like Rge and Rgs are the same, help discharge the gate charge, ensure the turn off.

[@cre8tivesriram342]

After using the Pull-Down Resistor why the LED doesn't turn ON even after touching the gate and positive of battery? Can you please give me an answer for this?

[@DustinWatts]

If you are using a different MOSFET, the voltage at the gate may need to be higher. Also this is for a NPN transistor. PNP require something different.

[@cre8tivesriram342]

If a low signal is applied to the gate it is dropped at the pulldown resisitor. So the MOSFET is at Cutoff. The MOSFET goes to saturation only when a High Signal is applied to the gate. What exactly is the low and high signal you are referring to? What are these low and high signals? Finally 1 last question please give me a clear answer. Why does the MOSFET doesn't turn ON when you use your fingers after using the pulldown Resistor? I have searched everywhere in the internet but I can't find any answers so please give me an answer.

[@DustinWatts]

@@cre8tivesriram342 High and Low are voltages. It depends on the MOSFET what is considered High and Low. But you could say 0V is low. 5V is high for example. Your second question: This is one of the other reasons for a pull down resistor. Only a small charge is needed to turn the MOSFET on. But you don't want to accidentally turn the MOSFET on. The pull-down resistor directs charge to GND so the GATE does not charge with small charges.

[@cre8tivesriram342]

@@DustinWatts So when you touch the Positive side of battery and gate with your fingers, the small charge required for the gate is supplied through your hand. This caused the MOSFET to turn ON. But when the pulldown resistor is used these small charges flowing through your hand is dropped at the pulldown Resistor. So the MOSFET didn't turn ON. Is that correct? But why does our hand supply only small charges why not the full signal?

[@DustinWatts]

@@cre8tivesriram342 Correct. There is a difference between charge and "signal" by which I think you mean voltage. In this case it is because we are dealing with a very low voltage. The human body resistance makes for a very small current. If the voltage was higher you would be able to measure voltage at the other end of your body. But now we are dealing with potentially deadly voltages, so please do not try that!

[@whynt]

hi how to calculate gate resistor value....

[@DustinWatts]

If you know the maximum current you want at the gate: R=V/I. The maximum current depends in the case of a microcontroller on the maximum current the chip can source. For an Arduino that is 40mA absolute max. It is recommended to not exceed 20mA. At 5V that would mean 5V/0.02A = 250 Ohm.

[@whynt]

@@DustinWatts Thank you sir ...understand the ohm's law

[@alf3071]

I have added this pull down resistor to my mosfet as you show but it still won't turn off, even if I connect the gate directly to ground it won't turn off, it turns on more!!, I have the CEP603al Mosfet and it worked before but now I can't turn it off :((( what's the deal? is it broken? if I touch it sometimes it turns off sometimes on this shit is making me lose my mind wait, if I connect the pull down resistor to the ground near the transistor it turns on but if I connect it to the ground on the usb of the breadboard power source then it turns off, wtf is the explanation for this? shouldn't these grounds be the same? it doesn't work on the ground pins either, just on the usb ground

[@DustinWatts]

To give a good answer I have to little information about your circuit. A simple check you can do is to replace the MOSFET and see if you get the same behaviour. Another thing is to build the simple circuit from the video. Also make sure you did not mix up the Drain and Source.

[@alf3071]

@@DustinWatts I just soldered a 10k resistor between the usb ground and the gate and it still doesn't work, it only works if I also touch it , this is weird af I think I will disassemble everything and try again tomorrow, or I will just replace with a transistor module because this mosfet might be broken, it shouldn't behave like this, there's no way it's good, it's probably shorted drain-source

[@alf3071]

@@DustinWatts all I did today was tranfer everything to a prototyping pcb board, it worked when it was wired janky, but I copied the exact same wiring and it won't turn off now

[@felipeherrera9396]

Gracias amigo que difícil era entender eso para los que no sabemos leer circuitos

[@DustinWatts]

De nada!

[@bilalasim3427]

I am a beginner. So, can any one plz tell what will happen is we use a low value resistor as a pull down resistor instead of a high value resistor ?

[@DustinWatts]

The simple answer is not much if you don't go to high. If you go to high the resistor can't ensure the line always being "low". If you go to low in resistance, it is more difficult to pull the line "high". You can see it magnet if the magnet is very strong you need a strong magnet to pull it away. But whether it is 10K or 15K or 4.7K makes barely a difference. If you are really nitpicking you also think about current that you are "wasting". The subject seems simple, but it would need a whole video to explain in depth. So thanks for the idea... :D And over the years some values got used so much that everybody uses them... 10K for example. But again, explaining it would take a whole video. Maybe even a chapter in a book :)

[@bilalasim3427]

@@DustinWatts Thanks for explaining, sir. By the way I just want to ask why you aren't making new videos.The videos which are currently on your channel are really good and educational.

[@santohalim1355]

why does mosfet stay on even after connecting the resistor to the ground?

[@DustinWatts]

At what point do you mean? In the video.

[@santohalim1355]

@@DustinWatts not in video but when I tried the same thing(real thing)

[@fooballers7883]

hmm....nice one. Txs

[@DustinWatts]

You are welcome foo :)