MOSFETs and How to Use Them | AddOhms #11

MOSFETs and How to Use Them | AddOhms #11

Hi, I’m James,
the Bald Engineer.
Both of these are transistors.
One is a BJT and the
other is a MOSFET.
Can you tell the difference?
If not, that’s OK because
in this AddOhms video,
we’re going to take a look
at MOSFETs, how they work,
and how you can use
them in your circuits.
So let’s get going.
This video is part of a
two-part series on transistors.
The previous one covered Bipolar
Junction Transistors, BJTs.
While this video covers
Metal-oxide Semiconductor Field
Effect Transistors,
known as MOSFETs.
Generally, you’ll use
a BJT for small loads,
say less than one
amp of current,
while MOSFETs are
well-suited for applications
with much higher current.
For the video on BJTs,
For now, let’s move on MOSFETs.
Moss is a flowerless
plant that typically
grows 1 to 10 centimeters.
Some mosses grow up
to 50 centimeters
and can be commonly
found on trees.
Wait a minute, this
is the wrong script!!
Hold on…
The tree sounds right.
OK, Here’s the other stuff.
OK, let’s try this again.
MOSFETs belong to a family tree
of field effect transistors
or known as FETs.
There are JFETs,
JFETs actually work a little bit
like a BJT, which we’ve already
talked about.
For this video, we’re
focused on the MOSFET,
which has two types
of modes called
depletion and enhancement.
A depletion mode MOSFET works
like a normally closed switch.
Current can flow when
no voltage is applied.
Applying a negative
voltage actually
causes the current flow to stop.
An Enhancement Mode FET works
like a variable resistor.
They come in N channel
and P channel types.
Enhancement mode FETs are by
far the most common transistor
used today, so let’s
focus in on them.
Here’s the symbol for an
N channel enhancement mode
MOSFET, and here’s a
TO-220 style transistor.
The pins of a MOSFET are
identified as the gate,
the drain, and the source.
The field effect
part of their name
suggests they work
by voltage, compared
to BJT, which works by current.
When voltage is applied between
the gate and the source,
current is allowed to
flow between the drain
and the source.
Here’s the really cool
thing about MOSFETs–
they are variable
resistors controlled
by voltage, which
means depending
on the voltage applied
between the gate and source,
the resistance between the
drain and source will vary.
With a low voltage at
the gate, the resistance
from the drain to
source is very high.
It’s kind of like
an open switch.
As we increase the
voltage at the gate,
we pass a threshold
voltage, and then
the resistance from the
drain to the source drops,
and it drops very quickly.
The key difference
between a MOSFET and a BJT
is that the output current
isn’t a multiplier of the input
because MOSFETs are
all about VOLTAGE.
Since the resistance is
between the drain and source,
it is known as RDS-on
and can always
be found in the
MOSFET’s datasheet.
For example, this
is a FQP30N06L.
Let’s take a look at its
datasheet from Fairchild.
We can see that
RDS is given when
there are two different
voltages from gate to source.
At 10 volts, the on-resistance
will be about 27 milliohms,
and while at 5 volts,
the on-resistance
is only about 35 milliohms.
That’s pretty small
when you think about it.
We picked this
MOSFET on purpose.
It is known as a
logic level MOSFET,
because the voltage
from gate to source VGS
is lower than 5 volts.
In other words, the threshold
to turn the MOSFET on
is low enough to be used by
an Arduino or Raspberry Pi.
Not all MOSFETs are
logic level compatible,
so it is very important
to check to see
what the VGS threshold is
before using it in your circuit.
Since you will
probably use a MOSFET
in high current
applications, it is
important to check how
hot it is going to get.
Here’s how we calculate if
we need a heat sink or not.
The formula to
determine how much power
the MOSFET dissipates
is resistance
times current squared.
In this case, the
resistance is RDS on,
and the current is whatever
your load will draw.
Let’s use an example of a motor
that draws one amp of current.
This means we
multiply 35 milliohms
by one amp squared
to get 35 milliwatts.
OK, now we need a few more
things from the datasheet.
First, we need the junction-
to-ambient coefficient, which
is r-theta-ja, and in this case
is 62.5 degrees C per watt.
We also need the maximum
junction temperature,
which in this case is 175
degrees C. Using this formula,
we can calculate
the maximum power
the transistor can dissipate
without using a heat sink.
We take the maximum
junction temperature
minus the ambient
temperature, which
is going to be 25
degrees C, and divide
by the thermal resistance.
This gives a maximum
dissipation of 2.4 watts.
In our example, we are only
dissipating 35 milliwatts,
so we’re safe to operate
without a heat sink.
Now you might be wondering,
how can the number we calculate
be 2.4 watts when the datasheet
clearly said 79 watts?
And that’s a really great point.
The 79 watts is if
we had the ability
to cool the transistor
case to 25 degrees
C, which means you have to be
using some kind of heat sink.
But we’re going to cover more
on that in a later video.
Let’s review what
you need to know
to use a MOSFET as a switch.
Number one, find out which pin
is the gate, drain, and source.
Number two, look
at the datasheet
to determine the threshold
voltage, which is going
to be shown as VGS or VTH.
Find the drain to source
resistance or RDS-on.
Number four, look at R-theta-ja and the maximum junction
temperature to calculate
how hot the MOSFET will get.
all lowercase,
to download a simple PDF form
you can use to calculate these
MOSFETs are cool little devices,
but they’re also a little bit
So we’ll cover them in more
detail in future videos.
Make sure you follow
us or subscribe
to know when new video
tutorials are released.
If you visit, you can also
get show notes for this episode,
as well as other Addohms
If you have any questions
about MOSFETs or ideas
for future videos, send them
our way and keep watching.
Maybe we’ll cover them
in a future video.

100 Replies to “MOSFETs and How to Use Them | AddOhms #11”

  1. When looking at a datasheet, do NOT use the value of Vgs in the section "Absolute Max." This section is for conditions that will damage the MOSFET. More information here:

  2. Mistake: 79W is derived from R_JC witch is 1.9 C/W and you can calculate it for every case temp. not only 25C.
    by: (175-25)/1.9=~79C; or for every possible case temp T you can get: (175-T)/1.9; so for T=-10C you can dissipate more than 79W.

  3. Current ME student doing hardware research for a mechatronics project. Wish they taught us more about these applications use in school. Too much theory. Not enough hands on.

  4. I'm always confused when people talk about things "drawing" current. Things don't "draw" current; they "allow" current to pass thru. The amount of current they allow to pass thru for a given voltage is determined by their internal resistance.

  5. Anyone ever run into a ss10pl10 transistor? I need to find a replacement. Can find anything on it on the web. Company does not reply.

  6. Amazing video for both engineers and beginners who are learning to prototype. And the occasional people who kind of forgot how they operate :/

  7. if u wanna teach anybody anything u should at first explain what a transistor actually is before u tell us what for different types of transistors exist. your lecture makes absolutely no sense. unless its your attention that the viewers learn nothing.

  8. I found a lot of MOSFET datasheets at

  9. Used a power depletion fet to drain the bank on my CD welder(through power resistors) either when switched off or wanting to weld with a Lower voltage. It was expensive relative to other parts but not having a relay on an otherwise solid state design was oh so satisfying. Nice video only thing I would have added is if you want to high side switch one can avoid a lot of headake using a floating supply on the gate and an opto isolate to signal it. Esp practical for one offs where a few bucks isn't multiplied by hundreds or thousands

  10. Would you be able to explain why the drain is called drain and source is called source and do you have any content on using P channel vs N channel MOSFETs ?

  11. Thanks James! Got here from Rossmann Repair Group. Thanks so very much and keep up the fantastic commentary! B'H.

  12. My uncle is an Electrical Engineer. I have asked him many times about mosfets as I have been watching him working on circuits since my childhood. But every time his lectures just flew over my head. Then I searched youtube and got my answer.

  13. Okay… I've been watching videos on how to make 18650 battery packs and this video pops up as suggested. I thought I would watch it and see what I could learn. Waaayyy over my head. 😂

  14. Thanks for the video, very informative and easy to understand. Is it possible to gate the mosfet with an audio signal, as that is AC, would that work? I've been attempting it, but it will only drain when I touch the gate input, it's not responding to the audio at all. I tried with different levels, quite some voltage output. Perhaps mosfet only responds to DC, is that right? Thanks!

  15. Wow good video, just found your channel and I am about to watch a lot of your vids and give loads of likes! I appreciate the work you put in so much, therefor a HUGH THANK YOU to you!

  16. When to use P=R*I^2 , and when do you use P=U*I !?

    The datasheet of my MOSFET gives Rds=1.2 ohm to 5 ohm.
    Im pulling around 80 mA.
    The Vdd = 11-17 V

    If a use the formula described in this video I get P=0.032 W, if I use the other formula P=1.3 W

  17. 2:46 if there's no load between 12V and drain, can the MOSFET explode? If there's no load, all available current is gonna squeeze through that thing and make it pop, right?

  18. Strange, I do the calculation as stated, but my mosfet still got too hot and seems like it got damaged
    I was using it IRLB3034PbF to control the 12V 18A heater, when it started smoking and got really hot. I did poorly connect it though (by using alligator clips), was that the problem?

  19. Great video. I blew a mosfet and need to replace it. I figured while I'm doing that, I may as well upgrade and replace them all. 3077 would be nice, but I don't know if they are the same size. I imagine that they come in many different sizes? I'll be upgrading some other parts in the motor controller while I'm at it. Time to order a solder sucker and some heat sink paste to improve dissipation. 51 yrs old and this will my first time playing with electronics.

  20. A great video. Very well explained. But I would'nt recommand taking the maximum temperature for calculating thermal dissipation. In my experience, the cooler you keep your transistors, the more reliable they will be. Never believe datasheets. I don't care about what professionnals are doing in commercial products. In my opinion, commercial products are not reliable enough. I'm tired to replace under rated components and to fix under cooled designs.

  21. Mosfet design its everywere i noticed it at the dollar store floor design i wonder how that works

  22. @7:24 The symbol you shown is for an depletion mode MOSFET. The enhancement mode MOSFET has broken lines from source to drain.

  23. Liked you t-shirt. Haven't done any PCB layout in a 20+ years but even back then you couldn't trust the autorouter. I'm surprised that it's still the same.

  24. Are these voltage specific? I need a mosfet with a gate of 5V at .5amp to power LED lights plugged into a 5V power outlet

    Your visual diagrams were amazing

  25. Dammit! I thought H2O2 was heavy water, which relatively is safe to drink. 🙁 I forgot that's H2O also but with deuterium instead of plain Hydrogen. 🙁 looses 1 point from geek cred

  26. Can you explain the selection or gate resistor (value) subjection to turn on and turn off time with gate to source voltage?

  27. I like how easy your videos are to understand but they are so short that you don´t cover a topic fully. Even if you were to cover them sequentially which you don´t by the time the next video comes out the person/learners have already forgotten what was covered in the last video.

  28. Nice video sir. How to use MOSFETs as variable resistance to measure the IV characteristics of solar cells?

  29. what i dont understand is why are mosfets used in iphones and computers…surely they dont use a lot of current…why not just use a transistor.

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