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?
Driving a big LED with a MOSFET
Started by ●May 21, 2012
Reply by ●May 21, 20122012-05-21
On May 21, 9:36=A0am, 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 > =A0| > [R1] (8 Ohms) > =A0| > =A0D > =A0 =A0G -[R2]--- JUMPER > =A0S > =A0| > LED > =A0| > 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? ` George H.
Reply by ●May 21, 20122012-05-21
On Monday, May 21, 2012 3:48:18 PM UTC+2, George Herold wrote:> On May 21, 9:36=A0am, 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 > > =A0| > > [R1] (8 Ohms) > > =A0| > > =A0D > > =A0 =A0G -[R2]--- JUMPER > > =A0S > > =A0| > > LED > > =A0| > > 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? >=20 > 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? ` >=20Yeah, that was it...thanks!
Reply by ●May 21, 20122012-05-21
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: >This would work better:>+5 > | > | >LED > | >[R1] (8 Ohms) > | > D > G -[R2]--- JUMPER > S > | >GND
Reply by ●May 21, 20122012-05-21
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 > > | > >GNDYep. With the components in that order I'm getting enough current to overheat my resistor... :-) Thanks!
Reply by ●May 21, 20122012-05-21
"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? ` >Yeah, that was it...thanks! vvvvvvvvvvvvvvvvvvvvvvvvv The other correct answer would have been; use a P-channel MOSFET.
Reply by ●May 22, 20122012-05-22
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. -- You can't have a sense of humor, if you have no sense.
Reply by ●May 22, 20122012-05-22
"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!!!
Reply by ●May 22, 20122012-05-22
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. -- You can't have a sense of humor, if you have no sense.
Reply by ●May 22, 20122012-05-22
"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!!!
