Driving a big LED with a MOSFET

F

fungus

Guest
I'm trying to drive a big LED (3W) with a
MOSFET. Eventually I want to drive it
with an Arduino but so far I'm failing to
get the right numbers in my test circuit.

My test circuit is like this:

+5
|
[R1] (8 Ohms)
|
D
G -[R2]--- JUMPER
S
|
LED
|
GND

I can connect the jumper cable to
+5V or ground and it switches on/off
as expected.

My MOSFET is one of these:

http://www.datasheetcatalog.org/datasheets/120/171065_DS.pdf

R2 is 220 Ohms (it's probably not even
needed, I just put it in there while I'm
testing...)

The problem is that when I connect the
jumper to +5V and measure the voltage
drop across each component it's not
what I expect and the LED current is
dimmer then it should be as a result.

The voltage drop across R1 is about 1V,
the voltage drop across the MOSFET is
1.5V and the voltage drop across the
LED is 2.5V.

I expected the drop across the MOSFET
to be very close to 0V. Does anybody
have an idea why it isn't?
 
On Mon, 21 May 2012 06:36:45 -0700 (PDT), fungus <tooby@artlum.com>
wrote:

I'm trying to drive a big LED (3W) with a
MOSFET. Eventually I want to drive it
with an Arduino but so far I'm failing to
get the right numbers in my test circuit.

My test circuit is like this:
Click to expand...

This would work better:


+5
|
|
LED
|
[R1] (8 Ohms)
|
D
G -[R2]--- JUMPER
S
|
GND
Click to expand...
 
On May 21, 9:36 am, fungus <to...@artlum.com> wrote:
I'm trying to drive a big LED (3W) with a
MOSFET. Eventually I want to drive it
with an Arduino but so far I'm failing to
get the right numbers in my test circuit.

My test circuit is like this:

+5
 |
[R1] (8 Ohms)
 |
 D
   G -[R2]--- JUMPER
 S
 |
LED
 |
GND

I can connect the jumper cable to
+5V or ground and it switches on/off
as expected.

My MOSFET is one of these:

http://www.datasheetcatalog.org/datasheets/120/171065_DS.pdf

R2 is 220 Ohms (it's probably not even
needed, I just put it in there while I'm
testing...)

The problem is that when I connect the
jumper to +5V and measure the voltage
drop across each component it's not
what I expect and the LED current is
dimmer then it should be as a result.

The voltage drop across R1 is about 1V,
the voltage drop across the MOSFET is
1.5V and the voltage drop across the
LED is 2.5V.

I expected the drop across the MOSFET
to be very close to 0V. Does anybody
have an idea why it isn't?
Click to expand...
You only have about 2.5 volts of gate-source voltage, so the FET's not
getting turned on very hard. Can you move the LED above the FET so
the source is grounded? `

George H.
 
On Monday, May 21, 2012 3:48:18 PM UTC+2, George Herold wrote:
On May 21, 9:36 am, fungus <to...@artlum.com> wrote:
I'm trying to drive a big LED (3W) with a
MOSFET. Eventually I want to drive it
with an Arduino but so far I'm failing to
get the right numbers in my test circuit.

My test circuit is like this:

+5
 |
[R1] (8 Ohms)
 |
 D
   G -[R2]--- JUMPER
 S
 |
LED
 |
GND

I can connect the jumper cable to
+5V or ground and it switches on/off
as expected.

My MOSFET is one of these:

http://www.datasheetcatalog.org/datasheets/120/171065_DS.pdf

R2 is 220 Ohms (it's probably not even
needed, I just put it in there while I'm
testing...)

The problem is that when I connect the
jumper to +5V and measure the voltage
drop across each component it's not
what I expect and the LED current is
dimmer then it should be as a result.

The voltage drop across R1 is about 1V,
the voltage drop across the MOSFET is
1.5V and the voltage drop across the
LED is 2.5V.

I expected the drop across the MOSFET
to be very close to 0V. Does anybody
have an idea why it isn't?

You only have about 2.5 volts of gate-source voltage, so the FET's not
getting turned on very hard. Can you move the LED above the FET so
the source is grounded? `
Click to expand...
Yeah, that was it...thanks!
 
On Monday, May 21, 2012 4:27:41 PM UTC+2, John Larkin wrote:
This would work better:

+5
|
|
LED
|
[R1] (8 Ohms)
|
D
G -[R2]--- JUMPER
S
|
GND
Click to expand...
Yep. With the components in that order
I'm getting enough current to overheat
my resistor... :)

Thanks!
 
"fungus" <tooby@artlum.com> wrote in message
news:6bfdb91c-3a2a-4448-9d99-6eb588f08d08@googlegroups.com...
On Monday, May 21, 2012 3:48:18 PM UTC+2, George Herold wrote:
On May 21, 9:36 am, fungus <to...@artlum.com> wrote:
I'm trying to drive a big LED (3W) with a
MOSFET. Eventually I want to drive it
with an Arduino but so far I'm failing to
get the right numbers in my test circuit.

My test circuit is like this:

+5
|
[R1] (8 Ohms)
|
D
G -[R2]--- JUMPER
S
|
LED
|
GND

I can connect the jumper cable to
+5V or ground and it switches on/off
as expected.

My MOSFET is one of these:

http://www.datasheetcatalog.org/datasheets/120/171065_DS.pdf

R2 is 220 Ohms (it's probably not even
needed, I just put it in there while I'm
testing...)

The problem is that when I connect the
jumper to +5V and measure the voltage
drop across each component it's not
what I expect and the LED current is
dimmer then it should be as a result.

The voltage drop across R1 is about 1V,
the voltage drop across the MOSFET is
1.5V and the voltage drop across the
LED is 2.5V.

I expected the drop across the MOSFET
to be very close to 0V. Does anybody
have an idea why it isn't?

You only have about 2.5 volts of gate-source voltage, so the FET's not
getting turned on very hard. Can you move the LED above the FET so
the source is grounded? `
Click to expand...
Yeah, that was it...thanks!
vvvvvvvvvvvvvvvvvvvvvvvvv

The other correct answer would have been; use a P-channel MOSFET.
 
Ian Field wrote:
The other correct answer would have been; use a P-channel MOSFET.
Click to expand...

Sure. Throw out parts he already has, just to please you.


--
You can't have a sense of humor, if you have no sense.
 
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:g82dnc0aEtM-4SbSnZ2dnUVZ_vydnZ2d@earthlink.com...
Ian Field wrote:

The other correct answer would have been; use a P-channel MOSFET.


Sure. Throw out parts he already has, just to please you.
Click to expand...
I expected better from you!!!
 
Ian Field wrote:
"Michael A. Terrell" ?mike.terrell@earthlink.net? wrote in message
news:g82dnc0aEtM-4SbSnZ2dnUVZ_vydnZ2d@earthlink.com...
?
? Ian Field wrote:
??
?? The other correct answer would have been; use a P-channel MOSFET.
?
?
? Sure. Throw out parts he already has, just to please you.

I expected better from you!!!
Click to expand...

A whiner like you isn't worth the effort.


--
You can't have a sense of humor, if you have no sense.
 
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:NYOdnfxLd_LlnCHSnZ2dnUVZ_tudnZ2d@earthlink.com...
Ian Field wrote:

"Michael A. Terrell" ?mike.terrell@earthlink.net? wrote in message
news:g82dnc0aEtM-4SbSnZ2dnUVZ_vydnZ2d@earthlink.com...
?
? Ian Field wrote:
??
?? The other correct answer would have been; use a P-channel MOSFET.
?
?
? Sure. Throw out parts he already has, just to please you.

I expected better from you!!!


A whiner like you isn't worth the effort.
Click to expand...

So don't fucking do it then - dumbass!!!
 
Ian Field wrote:
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:NYOdnfxLd_LlnCHSnZ2dnUVZ_tudnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" ?mike.terrell@earthlink.net? wrote in message
news:g82dnc0aEtM-4SbSnZ2dnUVZ_vydnZ2d@earthlink.com...
?
? Ian Field wrote:
??
?? The other correct answer would have been; use a P-channel MOSFET.
?
?
? Sure. Throw out parts he already has, just to please you.

I expected better from you!!!


A whiner like you isn't worth the effort.

So don't fucking do it then - dumbass!!!
Click to expand...

yawnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn


--
You can't have a sense of humor, if you have no sense.
 
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:q8CdnSJOYZbwbSHSnZ2dnUVZ_qGdnZ2d@earthlink.com...
Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:NYOdnfxLd_LlnCHSnZ2dnUVZ_tudnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" ?mike.terrell@earthlink.net? wrote in message
news:g82dnc0aEtM-4SbSnZ2dnUVZ_vydnZ2d@earthlink.com...
?
? Ian Field wrote:
??
?? The other correct answer would have been; use a P-channel MOSFET.
?
?
? Sure. Throw out parts he already has, just to please you.

I expected better from you!!!


A whiner like you isn't worth the effort.

So don't fucking do it then - dumbass!!!


yawnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn
Click to expand...

Was that your nickname at school?
 
Ian Field wrote:
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:q8CdnSJOYZbwbSHSnZ2dnUVZ_qGdnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:NYOdnfxLd_LlnCHSnZ2dnUVZ_tudnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" ?mike.terrell@earthlink.net? wrote in message
news:g82dnc0aEtM-4SbSnZ2dnUVZ_vydnZ2d@earthlink.com...
?
? Ian Field wrote:
??
?? The other correct answer would have been; use a P-channel MOSFET.
?
?
? Sure. Throw out parts he already has, just to please you.

I expected better from you!!!


A whiner like you isn't worth the effort.

So don't fucking do it then - dumbass!!!


yawnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn

Was that your nickname at school?
Click to expand...

I've told yopu that I don't date guys, so stop all of your
touchy-feely crap!


--
You can't have a sense of humor, if you have no sense.
 
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:2pWdnXg07OxQkCDSnZ2dnUVZ_t2dnZ2d@earthlink.com...
Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:q8CdnSJOYZbwbSHSnZ2dnUVZ_qGdnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:NYOdnfxLd_LlnCHSnZ2dnUVZ_tudnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" ?mike.terrell@earthlink.net? wrote in message
news:g82dnc0aEtM-4SbSnZ2dnUVZ_vydnZ2d@earthlink.com...
?
? Ian Field wrote:
??
?? The other correct answer would have been; use a P-channel
MOSFET.
?
?
? Sure. Throw out parts he already has, just to please you.

I expected better from you!!!


A whiner like you isn't worth the effort.

So don't fucking do it then - dumbass!!!


yawnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn

Was that your nickname at school?


I've told yopu that I don't date guys,
Click to expand...
So why are you going on about it?!!!
 
"Ian Field" <gangprobing.alien@ntlworld.com> wrote in message
news:MG8vr.356239$JQ.342080@fx02.am4...
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:2pWdnXg07OxQkCDSnZ2dnUVZ_t2dnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:q8CdnSJOYZbwbSHSnZ2dnUVZ_qGdnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:NYOdnfxLd_LlnCHSnZ2dnUVZ_tudnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" ?mike.terrell@earthlink.net? wrote in message
news:g82dnc0aEtM-4SbSnZ2dnUVZ_vydnZ2d@earthlink.com...
?
? Ian Field wrote:
??
?? The other correct answer would have been; use a P-channel
MOSFET.
?
?
? Sure. Throw out parts he already has, just to please you.

I expected better from you!!!


A whiner like you isn't worth the effort.

So don't fucking do it then - dumbass!!!


yawnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn

Was that your nickname at school?


I've told yopu that I don't date guys,

So why are you going on about it?!!!
Click to expand...

Anyway - who's yopu? Has he been after your rusty bullet hole again!
 
On Wed, 23 May 2012 10:13:17 -0400, "Michael A. Terrell"
<mike.terrell@earthlink.net> wrote:

Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:NYOdnfxLd_LlnCHSnZ2dnUVZ_tudnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" ?mike.terrell@earthlink.net? wrote in message
news:g82dnc0aEtM-4SbSnZ2dnUVZ_vydnZ2d@earthlink.com...
?
? Ian Field wrote:
??
?? The other correct answer would have been; use a P-channel MOSFET.
?
?
? Sure. Throw out parts he already has, just to please you.

I expected better from you!!!


A whiner like you isn't worth the effort.

So don't fucking do it then - dumbass!!!


yawnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn
Click to expand...

Sounds like you're not getting enough sleep lately.


--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
 
"John Larkin" <jlarkin@highlandtechnology.com> wrote in message
news:vebqr7ldrf9s5cnsuvij8kais225a3hru1@4ax.com...
On Wed, 23 May 2012 10:13:17 -0400, "Michael A. Terrell"
mike.terrell@earthlink.net> wrote:


Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:NYOdnfxLd_LlnCHSnZ2dnUVZ_tudnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" ?mike.terrell@earthlink.net? wrote in message
news:g82dnc0aEtM-4SbSnZ2dnUVZ_vydnZ2d@earthlink.com...
?
? Ian Field wrote:
??
?? The other correct answer would have been; use a P-channel MOSFET.
?
?
? Sure. Throw out parts he already has, just to please you.

I expected better from you!!!


A whiner like you isn't worth the effort.

So don't fucking do it then - dumbass!!!


yawnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn


Sounds like you're not getting enough sleep lately.
Click to expand...

The voices in his head keep him awake!
 
John Larkin wrote:
Michael A. Terrell wrote:

yawnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn

Sounds like you're not getting enough sleep lately.
Click to expand...

It's very hard to get any sleep when you're in pain 24/7 I haven't
slept more that four hours at a time in over six months. A VA nurse
asked how bad the pain was. I told her I was looking for a chain saw to
cut my swollen & bloody legs off. She said, "That's just a two or maybe
three, on a scale of 1 to 10." One VA doctor told me that I was already
on a high dosage of pain medicine. 400 mg of Gabapentin three times a
day doesn't do anything for any pain except diabetic neuropathy. He
insisted that was a very high dose, and also refused to give me a few
days prescription for Ambien, to try to break the erratic sleep
pattern. He claimed that I would become addicted to it. My last
prescription for Ambien was over 12 years ago. :(


--
You can't have a sense of humor, if you have no sense.
 
Ian Field wrote:
The voices in his head keep him awake!
Click to expand...

You can only speak for yourself. The only sound in my head is
Tinitus, and I wish that I could give that to you to go along with all
your other problems.


--
You can't have a sense of humor, if you have no sense.
 
On Wed, 23 May 2012 17:58:15 +0100, "Ian Field"
<gangprobing.alien@ntlworld.com> wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:2pWdnXg07OxQkCDSnZ2dnUVZ_t2dnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:q8CdnSJOYZbwbSHSnZ2dnUVZ_qGdnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:NYOdnfxLd_LlnCHSnZ2dnUVZ_tudnZ2d@earthlink.com...

Ian Field wrote:

"Michael A. Terrell" ?mike.terrell@earthlink.net? wrote in message
news:g82dnc0aEtM-4SbSnZ2dnUVZ_vydnZ2d@earthlink.com...
?
? Ian Field wrote:
??
?? The other correct answer would have been; use a P-channel
MOSFET.
?
?
? Sure. Throw out parts he already has, just to please you.

I expected better from you!!!


A whiner like you isn't worth the effort.

So don't fucking do it then - dumbass!!!


yawnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn

Was that your nickname at school?


I've told yopu that I don't date guys,

So why are you going on about it?!!!
Click to expand...
---
Because you don't understand that "no" means "no"?

--
JF
 
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