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MOSFETs and PWM

J

John Larkin

Jan 1, 1970
0
Just wanted to be sure. The NTE part I mentioned turns on hard at 4V drive.
They aren't the cheapest things, but so far I like them ok for a jelly-bean
part. Do you have a favorite cheapie logic-level part in this current
range?

Not up in the amps. I usually whack the big fets with a gate driver,
10 volts maybe.

At lower currents, I use a lot of 2N7002's, pretty good a 5 volts.
They cost about 3 cents each and can switch 50 volts in a couple of
ns.

FDV301 is good at 3.3 volts drive, also fairly low current.

There are tons of logic-level fets around these days. Maybe somebody
else has suggestions.

John
 
J

jasen

Jan 1, 1970
0
On Sat, 24 Feb 2007 13:03:23 -0600, "Anthony Fremont"

I'm a complete amateur WRT MOSFETs, so I'm finding this discussion
interesting. I hope you'll bear with me if this question sounds dumb:


If you're driving the gate from 5V CMOS logis (eg; a micro), can you
get around this by putting in a series diode after the logic output, &
a large (1M+) pullup resistor from the MOSFET gate to your 12V (or
whatever) relay supply?

no. typical cmos outputs have a diode to vcc so you'll not get the output
above with a reasonable pullup resistor and a series diode vcc+1.2V

one easy way to get more voltage use a npn transistor and a pullup
resistor, this also inverts the signal so you'll need to adjust the
micro's program


.-------------------------------------------------------------.
| This is an ascii schematic, if the diagram appears garbled |
| try switching to a fixed-pitch font (courier works well) |
| pasting it into notepad works well on ms-windows. |
| or in google groups "view source" (found under options) |
`-------------------------------------------------------------'


+v
| |
| |
[1K] ||--+
| ||<-.
+------||--+
/ |
|/ |
uC +-[1K]--| |
|\| |
~\ |
\ |
| |
--+---------+--- 0V


Bye.
Jasen
 
J

John Fields

Jan 1, 1970
0
If I was actually going to try and switch 30A with it, I guess I'd use a
gate-driver of some type. I have some in my shack that are made by
Microchip (IIRC) that I used for tinkering with some IGBTs. They have very
fast switching times and will supply much larger currents (>3A) during the
transition. The NTE transistors that I mentioned seem to work fine being
driven by a PIC at 5V. I was only switching a couple of amps though without
a heatsink.

---
I've used these:

http://www.micrel.com/_PDF/mic4451.pdf

with excellent results on many occasions.

Once even to PWM a motor!

BTW, some work came in and I had to put the pulse counter code on
the back burner, and to top it all off my emulator seems to have
gone south. Aarghhh...
 
@nowhere.com> wrote:
I'm a complete amateur WRT MOSFETs, so I'm finding this discussion
interesting. I hope you'll bear with me if this question sounds dumb:
If you're driving the gate from 5V CMOS logis (eg; a micro), can you
get around this by putting in a series diode after the logic output, &
a large (1M+) pullup resistor from the MOSFET gate to your 12V (or
whatever) relay supply?

no. typical cmos outputs have a diode to vcc so you'll not get the output
above with a reasonable pullup resistor and a series diode vcc+1.2V

one easy way to get more voltage use a npn transistor and a pullup
resistor, this also inverts the signal so you'll need to adjust the
micro's program

.-------------------------------------------------------------.
| This is an ascii schematic, if the diagram appears garbled |
| try switching to a fixed-pitch font (courier works well) |
| pasting it into notepad works well on ms-windows. |
| or in google groups "view source" (found under options) |
`-------------------------------------------------------------'

+v
| |
| |
[1K] ||--+
| ||<-.
+------||--+
/ |
|/ |
uC +-[1K]--| |
|\| |
~\ |
\ |
| |
--+---------+--- 0V

Bye.
Jasen-

Try John Popelish solution back in 2002 covering this subject.
(It'd DejaVu all over again)

+12V---+-----+-----+
| | |
10k | LAMP
| |/c |
+---|npn |
| |\e |
| | |-d
+-|<--+----| Nfet
|/ c |-s
logic-|npn |
|\e |
com----+-----------+
 
A

Anthony Fremont

Jan 1, 1970
0
John said:
On Sat, 24 Feb 2007 16:10:44 -0600, "Anthony Fremont"


---
I've used these:

http://www.micrel.com/_PDF/mic4451.pdf

with excellent results on many occasions.

I beieve it, looks like there's enough drive there to turn on about 30 or 40
of my NTEs. ;-)
Once even to PWM a motor!

Was it for an elevator? ;-)
BTW, some work came in and I had to put the pulse counter code on
the back burner, and to top it all off my emulator seems to have
gone south. Aarghhh...

Yeah, yeah, yeah, sure John. (;-) <---- It's a joke! It's a joke!
I know how it goes.

I've been tinkering with my propellor clock ideas again. Most of the
electronic problems are easy enough to solve, it's the mechanical ones that
keep getting in the way.
 
J

John Fields

Jan 1, 1970
0
I beieve it, looks like there's enough drive there to turn on about 30 or 40
of my NTEs. ;-)


Was it for an elevator? ;-)
 
J

John Larkin

Jan 1, 1970
0
On Sat, 24 Feb 2007 13:03:23 -0600, "Anthony Fremont"

I'm a complete amateur WRT MOSFETs, so I'm finding this discussion
interesting. I hope you'll bear with me if this question sounds dumb:


If you're driving the gate from 5V CMOS logis (eg; a micro), can you
get around this by putting in a series diode after the logic output, &
a large (1M+) pullup resistor from the MOSFET gate to your 12V (or
whatever) relay supply?

no. typical cmos outputs have a diode to vcc so you'll not get the output
above with a reasonable pullup resistor and a series diode vcc+1.2V

one easy way to get more voltage use a npn transistor and a pullup
resistor, this also inverts the signal so you'll need to adjust the
micro's program


.-------------------------------------------------------------.
| This is an ascii schematic, if the diagram appears garbled |
| try switching to a fixed-pitch font (courier works well) |
| pasting it into notepad works well on ms-windows. |
| or in google groups "view source" (found under options) |
`-------------------------------------------------------------'


+v
| |
| |
[1K] ||--+
| ||<-.
+------||--+
/ |
|/ |
uC +-[1K]--| |
|\| |
~\ |
\ |
| |
--+---------+--- 0V


Bye.
Jasen

Why use a bipolar to drive a fet? Use a fet to drive a fet!


+v
| |
| |
[1K] ||--+
| ||<-.
+------||--+
| |
| | |
uC --------| |
| > |
| |
| |
| |
--+-----------+--- 0V



John
 
A

Anthony Fremont

Jan 1, 1970
0
I would feel dirty if I used slip rings. ;-) Ultimately, I'm hoping for an
inductive hook-up to transfer the power. I'll need something on the order
of 50-100mW max to power the PIC and LEDs, average should be much less
especially when simulating analog hands.

Roger on the walking thing, the noise of the air turbulence picks up quickly
as you raise the RPMs too. One last little thing, we wouldn't want to
impale any innocent bystanders. ;-) I've been using some high brightness
LEDs and even using 1K current limiting resistors, they are easily visible
with a 1mS pulse. I believe this will give me decent enough looking pixel
at ~1000 RPM.
 
J

John Fields

Jan 1, 1970
0
I would feel dirty if I used slip rings. ;-) Ultimately, I'm hoping for an
inductive hook-up to transfer the power. I'll need something on the order
of 50-100mW max to power the PIC and LEDs, average should be much less
especially when simulating analog hands.
 
A

amdx

Jan 1, 1970
0
Anthony Fremont said:
I would feel dirty if I used slip rings. ;-) Ultimately, I'm hoping for
an inductive hook-up to transfer the power. I'll need something on the
order of 50-100mW max to power the PIC and LEDs, average should be much
less especially when simulating analog hands.

Roger on the walking thing, the noise of the air turbulence picks up
quickly as you raise the RPMs too. One last little thing, we wouldn't
want to impale any innocent bystanders. ;-) I've been using some high
brightness LEDs and even using 1K current limiting resistors, they are
easily visible with a 1mS pulse. I believe this will give me decent
enough looking pixel at ~1000 RPM.
How about the rotating transformer from a vcr? I don't understand what your
doing, but I always thought there should be some use for the nice
transformer
with good bearings. Probably need to be run at a fairly high frequency.
Mike
 
L

Lionel

Jan 1, 1970
0
I would feel dirty if I used slip rings. ;-) Ultimately, I'm hoping for an
inductive hook-up to transfer the power.

The obvious, easy way would be to use a small DC motor, mounted in
reverse, so that the rotor is attahed to your plinth (or whatever) &
your 'blade' is attached to the stator. You'd spin the rotor via a
second, conventionally-mounted motor, & power the electronics by
rectifying & filtering the AC from the brushes of the spinning motor.
I'll need something on the order
of 50-100mW max to power the PIC and LEDs, average should be much less
especially when simulating analog hands.

This sounds like a lot of fun. I hope it works out for you!
 
R

Randy Day

Jan 1, 1970
0
maxfoo said:
John Larkin wrote:

[snip]

The FDC655 lists 33 mOhms Rds(on) at 4.5v, which
seems more than adequate. I think I'll go with it.


I find it hard to believe that this little super sot-6 package can handle
6.5Amps, Hell I've blown mosfet in soic-8 packages rated a 4amps. too bad
digikey doesn't sell it, I like to try to see where it explodes. ;^D
http://www.fairchildsemi.com/ds/FD/FDC655BN.pdf

That was my impression too. We'll see if it lives
up to its billing. ;)
 
L

Lionel

Jan 1, 1970
0
no. typical cmos outputs have a diode to vcc so you'll not get the output
above with a reasonable pullup resistor and a series diode vcc+1.2V

I think I haven't made myself clear enough. I'm talking about putting
a diode in series with the micro output, with the anode towards it, &
the cathode to the MOSFET gate. The pullup resistor would be between
the junction of the diode & gate, going to the relay Vcc. The only
potential problem I can envisage would be that the logic-low output
voltage of the micro would be raised from gnd by the amount of the
diode drop. In summary, what I'm suggesting would make the CMOS output
of the micro look like an OC TTL output, only raised by a diode drop.
 
A

Anthony Fremont

Jan 1, 1970
0
Thanks John. Sorry I didn't get back to you sooner, I had to watch the
NASCAR race. :)
 
J

Jamie

Jan 1, 1970
0
Lionel said:
I think I haven't made myself clear enough. I'm talking about putting
a diode in series with the micro output, with the anode towards it, &
the cathode to the MOSFET gate. The pullup resistor would be between
the junction of the diode & gate, going to the relay Vcc. The only
potential problem I can envisage would be that the logic-low output
voltage of the micro would be raised from gnd by the amount of the
diode drop. In summary, what I'm suggesting would make the CMOS output
of the micro look like an OC TTL output, only raised by a diode drop.
I can't see how the Gate will ever get a signal from the uC that way?
with the cathode toward the gate, this would mean that the uC will only
be acting as a source and not a sink . Since you have pulled up the Gate
input via a R, you need a sink to pull it down. Switching the diode
around with the cathode towards the uC will solve that how ever, then
you get higher voltage from the Vcc that i assume you may not want in
the uC io ?
What you need is a simple NPN transistr to be driven from the uC IO
used to pull down the pull up Resistor from what ever Vcc source level
you need to force the FET on.

Maybe i'm over looking something in your message ?
 
L

Lionel

Jan 1, 1970
0
I can't see how the Gate will ever get a signal from the uC that way?
with the cathode toward the gate, this would mean that the uC will only
be acting as a source and not a sink .

<slaps self> D'oh! Yes, you're 100% correct.
Dunno what I was thinking there. Thanks for correcting my idiotic
mistake in such a tactful manner. :)
 
A

Anthony Fremont

Jan 1, 1970
0
Lionel said:
The obvious, easy way would be to use a small DC motor, mounted in
reverse, so that the rotor is attahed to your plinth (or whatever) &
your 'blade' is attached to the stator. You'd spin the rotor via a
second, conventionally-mounted motor, & power the electronics by
rectifying & filtering the AC from the brushes of the spinning motor.

That sounds similar to the diagram that John F drew, but his uses a motor
with a hollow shaft if I'm interpreting his drawing correctly. If I
understand you, you are suggesting the drive motor be offset and use a belt
to spin the outside of the other motor (generator).
This sounds like a lot of fun. I hope it works out for you!

Thanks, it'll be fun, and I'll learn some new things, no matter what
happens. ;-)
 
A

Anthony Fremont

Jan 1, 1970
0
amdx said:
How about the rotating transformer from a vcr? I don't understand
what your doing, but I always thought there should be some use for
the nice transformer
with good bearings. Probably need to be run at a fairly high
frequency. Mike

I'm not sure what you mean. Are you talking about the head or is there
something else?
 
A

Anthony Fremont

Jan 1, 1970
0
Anthony said:
I'm not sure what you mean. Are you talking about the head or is
there something else?

Never mind, I went and looked it up. Way cool, I never took a VCR that far
apart before. I wonder how hard it would be to connect to the read/write
head wiring.
 
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