Understanding MOSFETs: MOSFET as a Switch (UPDATED Supplemental Video)

Name: Understanding MOSFETs: MOSFET as a Switch (UPDATED Supplemental Video) | Open Video
Uploaded: 2026-01-19T16:30:29+00:00
Duration: 26 min 36 s

0:00
okay this is a video meant to be a
0:02
supplemental video for our blog post on
0:05
the topic of mosfets located on the
0:08
mbots website titled understanding
0:11
mosfets key Concepts and practical
0:15
examples I'll leave a link in the
0:17
description below to that reading
0:19
material if you're interested in this
0:21
particular
0:23
topic I've actually already created this
0:26
video once and posted it but thanks to a
0:29
very formative comment made by Rex
0:32
Schneider I was informed of a mistake
0:35
I'd made on the original circuit that I
0:38
created for this blog post I ended up
0:41
having to correct that mistake which
0:43
changed some of the component values for
0:46
this particular circuit here as well as
0:49
the uh
0:50
calculations which is why I'm creating
0:53
the second video here but all should be
0:56
in order now and all has been promptly
0:58
corrected on the site at least I hope
1:01
that it is I tend to make quite a lot of
1:05
mistakes especially with stuff like this
1:07
so we can only cross our fingers here so
1:10
it's it's always a learning experience
1:12
which I always appreciate that by the
1:15
way that mistake video is still up I
1:18
didn't take it down so if you're
1:20
interested in seeing it uh I don't know
1:22
why you would be interested in seeing it
1:25
I mean there's still some valuable
1:26
information in there I guess but it's
1:29
pretty much the same video that this
1:30
one's going to be but with the obvious
1:33
Corrections here so uh you can choose to
1:36
do so if you'd like to I'll I'll leave
1:38
the link to that video here uh too down
1:42
in the description
1:43
below anyway what we have here in front
1:46
of us now is a breadboard with our our
1:48
mosfet circuit and it's already uh all
1:51
connected up here this is the exact same
1:54
circuit setup that you can see here
2:01
and that circuit like I said is the
2:03
exact same circuit on the example switch
2:05
application for this particular mosfet
2:09
that we did on that post again I'll
2:11
leave the link down in the description
2:13
below which is where you can also find
2:16
the schematic for this setup as well so
2:19
if I zoom you in here a little bit and
2:21
I'll reposition
2:24
this maybe if I move this sheet of paper
2:27
out of the way real quick we'll be
2:28
needing that later just not right
2:33
now let me zoom in a little bit now that
2:37
we're zoomed in I can kind of briefly go
2:40
over the setup here so I tried to set
2:43
this up on the breadboard uh almost
2:46
exactly like it was uh in the schematic
2:49
that is on the website that's provided
2:52
on the website I know you can't see that
2:55
uh in the camera right now
2:58
but here I have
3:00
uh my battery source it's a 9vt battery
3:04
and the two terminals coming off that
3:07
battery the red terminal is the positive
3:09
Supply and the black uh lead or terminal
3:13
is the the negative Supply and if we
3:16
follow that red wire the positive Supply
3:19
it goes to the positive Supply rail or
3:23
power rail of the breadboard here and
3:26
then I've also continued that Supply
3:29
with using this orange jumper wire here
3:33
and continued it over to the other
3:36
positive power rail on this on this
3:41
breadboard and again going back to that
3:45
positive uh lead of the battery it goes
3:48
to the positive power rail to this
3:50
yellow jumper wire that goes to a lead
3:53
on this push button switch a normally
3:56
open push button switch on this end and
3:59
then the other turn terminal on the
4:00
other side of that push button switch is
4:03
in line with this yellow jumper wire
4:06
which is in line to the lead of this
4:09
resistor here this is resistor R1 in the
4:15
schematic again you can find that
4:17
schematic uh on the website I'll leave
4:20
the link to that down in the description
4:22
below this is the onek ohm resistor
4:25
resistor
4:26
R1 and the other lead of that resistor
4:29
is in line with the Gate of this uh 2in
4:35
7000 mosfet and the gate lead is the
4:38
center lead I know you can't see it from
4:40
The View that you're at but the lead to
4:43
this 1K Ohm resistor goes to the center
4:46
lead of this mosfet and that is the
4:49
gate also in line with the
4:53
gate is the lead of this resistor here
4:58
and this resistor represents resistor R2
5:01
from that example circuit in the
5:05
schematic this is the 2km resistor that
5:08
we find the value for in that example
5:12
switch
5:13
application and then the other lead of
5:16
this 2K Ohm resistor goes to the
5:19
negative Supply or negative rail of the
5:22
breadboard here and I forgot to mention
5:24
that the negative rail uh on this end of
5:29
the radbard is also connected with this
5:32
black jumper wire and it goes to the
5:34
other uh negative power rail on this
5:38
side of the breadboard so looking back
5:42
at the mosfet again if we look at its
5:46
drain pin which is uh from your view
5:51
above the circuit here it's this right
5:53
side pin and it's in line with this
5:57
resistor here these two two resistors
6:00
are in series and they're representing
6:03
my 200 Ohm resistor which is resistor R3
6:07
in the schematic these are two 100 ohm
6:10
resistors that when set in series add up
6:13
to 200 uh ohm or 200
6:17
ohms so that drain lead of the mosfet is
6:22
in line with this lead of this 100 ohm
6:26
resistor which is in line with this 100
6:29
ohm res resistor making two making up
6:31
200
6:32
ohms together and the end of this
6:36
resistor is in a point on the negative
6:41
Supply rail of the
6:44
breadboard looking back at the mosfet on
6:48
its source pin which is this left pin
6:52
from your view above here I know you
6:55
can't see the pin but it's on the left
6:58
side uh as the mosfet is currently
7:01
oriented and that Source pin is in line
7:05
with the anode or excuse me the cathode
7:10
of the LED and then the anode lead of
7:13
the LED goes to the positive power rail
7:18
on this side of the
7:22
breadboard and here in a little bit I'll
7:24
I'll give you a more close-up view so
7:26
you can see uh how everything's hooked
7:29
up and the placement of everything as it
7:33
is uh right now I know that it's a
7:36
little difficult to see how things line
7:39
up from a top view but again I I'll I'll
7:43
give you a different View later on in
7:45
this video so again this this was for
7:48
the switch application example in the
7:50
blog post and if I were to press the
7:54
button here we could see that the green
7:56
LED lights up
8:00
and this is all controlled thanks to
8:02
this in Channel enhancement mode
8:06
mosfet and again an enhancement mode
8:09
means that it's normally off so as you
8:12
can see here if I'm not pressing the
8:14
button the LED is currently off and
8:17
nothing's happening if I press the
8:20
button the LED comes
8:24
on and all that is explained in in that
8:27
blog post that I mentioned and again
8:29
I'll leave all that in the description
8:32
below but you can follow along in that
8:35
reading material and set up your own
8:37
circuit just like this one for the
8:39
switch application using the 2n7000
8:42
moset in that example we're given some
8:46
values of the circuit and we're asked to
8:48
find some unknown values uh in that in
8:52
this case we're asked to find the
8:56
resistor value of R2
8:59
which is this resistor right here this
9:02
is the 2K Ohm resistor and that's the
9:04
value that we found uh in that example
9:07
problem and we're also asked to find the
9:12
uh drain to Source voltage if you
9:14
haven't read that blog post all this may
9:17
not make a lot of sense right now so I
9:19
encourage you to go check out that link
9:21
and follow along with this example
9:23
circuit here everything would be much
9:26
more clearer if you did I just wanted to
9:28
show you this completed circuit the
9:31
working circuit here uh real quick and
9:34
kind of go over the circuit and share it
9:36
with you but before I let you go I just
9:39
wanted to go over something so I'm going
9:41
to move this out of the way and I'm
9:45
going to zoom you back
9:47
out and I want to bring back this
9:51
schematic here maybe I'll Zoom you in
9:53
just a little bit
9:56
more so in the example schematic or the
10:00
example problem on the blog
10:02
post were given values like the 9vt
10:05
power
10:06
supply uh we're given that we're going
10:10
to be using a drain current value of 20
10:15
milliamps we're given that the voltage
10:18
drop across the LED in the circuit is 3
10:21
volts and we're given values like uh the
10:25
resistor value for R3 of 200 ohms the
10:29
resist resistor value of R1 which is 1K
10:32
ohm and like I said we're asked to find
10:36
uh the value of
10:37
R2 and the value of the drain to Source
10:40
voltage now these values are already in
10:43
place here this is just to help me out
10:46
uh in discussing this example problem
10:48
but based on the values given what we
10:51
were able to do is uh to figure out the
10:55
voltage uh drop across resistor R3 since
10:59
we know the value of the 200 Ohm
11:02
resistor and we know the value of the
11:04
drain current of 20
11:07
milliamps looking at the circuit we
11:09
would know that uh the drain current
11:12
going into the drain of the
11:14
mosfet would be the same uh value uh of
11:18
the current coming out of the source of
11:20
the moset hence the source value is
11:23
equal to the drain value which is equal
11:25
to the 20 milliamps so in knowing
11:30
that the source current is 20
11:32
milliamps and the value of resistor R3
11:36
is 200 ohms then we can use ohms law to
11:40
find the value of the voltage drop
11:43
across R3 so if we were to multiply the
11:47
20 milliamps times the 200 ohms we would
11:50
get a value of 4 volts or that would be
11:53
the voltage drop across resistor R3 and
11:57
in that example problem on the Block log
11:59
post we were given that the gate Source
12:01
voltage is 2 volts so on knowing that
12:04
the gate Source voltage is 2 volts and
12:07
then us figuring out that the voltage
12:10
drop across R3 is 4 volts the sum of
12:14
these two voltages is 6 volts because
12:18
this we could take the sum of the two
12:20
voltages because they're in series here
12:23
so that means that the voltage potential
12:26
across these two points between the gate
12:28
and the negative Supply or ground of the
12:32
circuit you can see that all of the
12:35
negatives go to this node of the circuit
12:40
that we can say that these two points
12:43
between the gate and the S and the
12:45
negative Supply and these two points
12:49
across resistor R2 the voltage drops or
12:52
the voltage potentials should be equal
12:56
so that's why or how we got the value 6
12:59
volts across R2 because 2 Vols plus 4
13:02
volts is 6 volts that's the voltage
13:05
potential voltage potential between
13:07
these two
13:09
points which should be the same voltage
13:12
potential across these two points
13:14
because they're in parallel and voltage
13:17
Potentials in parallel with each other
13:20
are equal to each other so 6 volts
13:22
across these two points 6 volts across
13:24
these two points so now in knowing that
13:26
the voltage drop across R2 is six PS we
13:30
can't quite figure out what the value of
13:33
R2 is because we don't know the value of
13:37
R2 that's what we're trying to figure
13:39
out and also we don't know what the
13:42
value of I is through R2 but since we do
13:46
know the voltage across R2 and we know
13:48
the
13:49
voltage value of the source for
13:52
V1 then the voltage drops across
13:56
R1 and across R2 since this circuit here
14:01
is considered a series circuit this Loop
14:03
here the sum of these two voltage drops
14:06
should
14:07
equal to the source voltage of V1 now
14:11
even though it's the voltage drop across
14:14
R1 is given here in this print off we
14:17
didn't originally know what this value
14:19
was so in order to find out what the the
14:23
voltage drop was across R1 the sum of
14:27
whatever this voltage drop is plus this
14:30
plus this voltage drop should equal to 9
14:32
volts so if I have six volts here this
14:35
must be 3 volts across R1 because 6
14:38
volts + 3 volts is 9 volts which is
14:40
equal to the voltage value of the source
14:45
voltage of B1 so now that we knew what
14:50
the voltage drop across R1 is then we
14:53
can figure out what the value of the
14:55
current is coming from our source volt
14:59
voltage going through uh resistor R1
15:03
again we'd use ohms law for that since
15:06
we're given the value of R1 to be 1K ohm
15:10
we divide the 3 volts across R1 by 1 K
15:15
ohm and that would give us a current
15:18
value I of 3 milliamps and like I said
15:22
this Loop of the circuit is in series
15:26
because that current value is not going
15:29
going through the gate because uh a gate
15:33
of a mosfet acts as a capacitor so
15:36
there's uh we can just say ideally that
15:40
there's no current going through the
15:42
gate therefore the current is going
15:44
through this section of the circuit here
15:47
and this is a series
15:49
circuit so if this is a series circuit
15:52
whatever current is going through
15:54
resistor R1 is also the same current
15:57
going through resistor R2 and and that
15:59
is just the current I which we just
16:02
found to be 3 milliamps so now that we
16:04
know the value of I and we know the
16:07
value of the voltage drop across R2 we
16:10
can again use ohms law to find what the
16:12
value of R2 is and that's just the 6
16:16
volts divided by the 3 milliamps which
16:20
gives
16:20
us a resistor value of 2,000 ohms or 2
16:25
kiloohms although I didn't I didn't put
16:28
that back value in here but that's the
16:30
value of the resistor R2 that we were
16:33
able to figure out in that example
16:35
problem and then again we were asked to
16:37
find what the drain to Source voltage uh
16:41
was across the mosfet so all we would
16:45
need to do there is again use Kiros
16:47
voltage law since we know what the value
16:50
of the source voltage is here it's 9
16:54
volts we were given that the voltage
16:56
drop across the diode or the LED was 3
17:00
volts and we found that the voltage drop
17:02
across resistor R3 was 4 volts this
17:06
portion of the circuit is a series
17:08
circuit so the voltage drops across
17:12
these components of the circuit should
17:14
equal to the source voltage of the V2 so
17:19
3 + 4 is
17:22
7 so that means that the value or the
17:25
voltage drop across uh the drain and
17:28
source of the mosfet should be 2 volts
17:31
because 4 + 3 is 7 + 2 is 9 Vols which
17:36
is equal to the 9 volts uh the source
17:39
voltage for v2 so I just wanted to kind
17:42
of go through that real quick uh this
17:44
being from the exact same problem from
17:48
the blog post this is based on ideal
17:51
conditions and I thought it'd be neat to
17:54
go over the real world conditions
17:58
of our real
18:01
world uh circuit that we have here so
18:05
what I did is I took my
18:09
multimeter and I took the probes and
18:13
what I first did before hooking the
18:14
battery up to the breadboard is I took
18:18
my probes and I
18:21
measured the voltage across the battery
18:24
terminals and that gave me the source
18:27
voltage of 8.1 5 volts I'm obviously
18:30
using one battery here although it shows
18:32
two batteries in the circuit I'm using
18:35
one for the my circuit here on the
18:38
breadboard so in that case V1 and V2 is
18:43
8.15 volts which is the reading I got uh
18:47
from my multimeter reading and then what
18:50
I what I proceeded to do is before I put
18:53
the resistors into the breadboard is I
18:56
took my probes and and placed my
19:00
multimeter into uh its resistance uh
19:05
reading mode and I took uh resistor
19:09
reading values across each one of the
19:11
resistors out of the circuit and
19:15
uh for my 1K Ohm resistor resistor R1 I
19:20
ended up getting a reading of 979 ohms I
19:24
took a reading and that was this
19:26
resistor here the 1K ohm resistor I took
19:29
a reading of resistor R2 and got a
19:33
reading of
19:35
1,955 ohms even though it's labeled as a
19:38
2K Ohm resistor and then again I'm using
19:42
two 100 ohm resistors to make up for the
19:45
200 Ohm resistor value of
19:47
R3 and I took measurements across each
19:50
one of these resistors one being 88.3
19:54
ohms another reading for the other
19:57
resistor being 87. 9 ohms and the sum of
20:01
those two uh values gave me a value of
20:06
1762 ohms and not 200 ohms so those are
20:11
the real real world values of uh
20:14
resistor R1 R2 and
20:17
R3 and then with everything hooked up in
20:20
my circuit and the button pressed I took
20:25
voltage readings
20:26
across the L L and for the LED I'm using
20:31
I got a voltage drop of 2.05 volts I
20:35
took a voltage reading across the drain
20:37
and source of the mosfet and the voltage
20:40
reading I got from the multimeter was
20:43
2.51 volts and then I took a voltage
20:46
reading across resistor R3 which is
20:49
across these two uh 100 ohm resistors
20:53
here and I got a voltage reading of 3.45
20:57
volts so again again Kirk off's voltage
21:00
law says that the sum of the voltage
21:03
drops in this case across the diode
21:07
across or across the LED and across the
21:11
drain and source of the mosfet and
21:13
across resistor R3 those voltage drops
21:16
the sum of them should equal to my uh
21:21
Source
21:22
voltage which is 8.15 volts and if I
21:27
were to add to 2.05 Vol + 2.51 Vol +
21:32
3.45 Vol I get a value of
21:36
8.01 or 8.01 volts which is pretty darn
21:41
close to the 8.15 volts so we may be
21:45
able to determine that uh there's maybe
21:48
some voltage losses going on here in the
21:50
circuit uh we might have to consider the
21:53
internal resistance of the battery and
21:55
things like that but it's pretty much 8
21:57
volts equals 8 volts in this case which
22:00
turns out uh pretty good for this real
22:04
world scenario
22:07
and uh going back to taking voltage
22:10
readings I ended up taking my probe and
22:13
and making some uh voltage readings
22:16
across resistor R2 and I got a voltage
22:20
uh of 5.4 volts and then I took my
22:23
probes and I took a voltage reading
22:25
across resistor R1 and I got a voltage
22:28
reading rating of 2.69 volts and again
22:31
using Kiros voltage law the sum of the
22:34
voltage drops across these two resistors
22:36
should equal to the source voltage of
22:39
8.15 volts and if I were to add 2.69
22:43
volts plus 5.4 volts I end up getting a
22:48
voltage value of 8.09 volts which is
22:53
again pretty darn close to this 8.15
22:56
volts and again we're we might be uh
22:59
needing to consider some voltage losses
23:02
within this part of the circuit here uh
23:04
again the internal resistance of the uh
23:08
Battery Source so there there's that but
23:13
as for I just wanted to show this for
23:17
the real world uh values and their
23:20
readings using using a multimeter I just
23:24
thought it'd be neat to kind of show
23:27
that at least and not just go over your
23:30
typical
23:32
theoretical ideal circuit that was given
23:36
in the in the blog post but what I'd
23:39
like to do in future material or content
23:43
is put something out on the website and
23:46
and probably make a a video later on
23:49
that will be a companion to that content
23:52
on maybe some questions and answers on
23:57
circuits like this for mosfets uh
24:00
questions that I had when I first
24:02
started uh tinkering around with mosfets
24:07
you know maybe some questions like why
24:10
do uh some mosfet circuit diagrams show
24:13
two voltage sources and not one like the
24:16
one I'm using here do I need to use two
24:20
batteries in this case do I do I need to
24:22
use two 9volt batteries can I use one
24:25
9volt battery I'm obviously using one
24:28
here for for this one but when you're
24:30
starting out this may be questions that
24:32
you're asking yourself when you're first
24:34
learning stuff it's certainly questions
24:36
that I had like I said is it okay to use
24:40
one nvt battery for this particular
24:43
circuit or do I have to use two two 9vt
24:46
batteries is it okay am I going to get
24:49
different voltage readings would would
24:51
using a single 9vt battery effect
24:54
voltage readings throughout the circuit
24:56
instead of using two batteries or or
24:58
would those voltage readings across
25:00
components read the same either way so I
25:03
think it'd be
25:05
beneficial uh for some folks maybe maybe
25:08
not I certainly think it would have been
25:11
beneficial to to me for myself and and
25:14
my own learning experience when I first
25:16
started out so I'm thinking of doing
25:18
something like that later I'm just kind
25:19
of brainstorming right now but the main
25:22
focus is for this particular circuit and
25:25
I just wanted to kind of go through the
25:27
real world stuff not only the stuff that
25:30
was given on the blog post but I hope
25:33
this was beneficial to you uh so I hope
25:36
you go check out that blog post
25:39
understanding mosfets key Concepts and
25:41
practical examples and I'll leave all
25:44
that information that you need to know
25:47
uh for that down in the description
25:48
below and what I'll do now is I'll leave
25:52
you with another view of
25:55
pressing the button and watching the LED
25:58
come
26:00
on and like I promised I I'll get you a
26:03
closeup of this
26:06
too okay we're at a more closeup view
26:09
here of the circuit and I try to tilt it
26:14
as best as I could so you can kind of
26:16
get a a decent view of the connections
26:19
there but before I leave you I'll sit
26:23
here and press the button here again so
26:25
we can see the LED come on and watch the
26:28
circuit
26:31
working isn't that neat

Understanding MOSFETs: MOSFET as a Switch (UPDATED Supplemental Video)

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