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Driving a big LED with a MOSFET

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:

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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?

Reply to
fungus
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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 to
George Herold

Yeah, that was it...thanks!

Reply to
fungus

This would work better:

Reply to
John Larkin

Yep. With the components in that order I'm getting enough current to overheat my resistor... :-)

Thanks!

Reply to
fungus

Yeah, that was it...thanks! vvvvvvvvvvvvvvvvvvvvvvvvv

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

Reply to
Ian Field

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

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You can't have a sense of humor, if you have no sense.
Reply to
Michael A. Terrell

I expected better from you!!!

Reply to
Ian Field

A whiner like you isn't worth the effort.

--
You can't have a sense of humor, if you have no sense.
Reply to
Michael A. Terrell

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.
Reply to
Michael A. Terrell

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.

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You can't have a sense of humor, if you have no sense.
Reply to
Michael A. Terrell

So now you're going to start whingeing about how nasty nature has been to poor little you.

Pathetic!

Reply to
Ian Field

yawnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn

The only thing pathetic is you and your delusions of adequacy.

--
You can't have a sense of humor, if you have no sense.
Reply to
Michael A. Terrell

It looks like it's time for a baseball bat across his knees, since the rolled up newspaper to his snout didn't work.

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You can't have a sense of humor, if you have no sense.
Reply to
Michael A. Terrell

What a weirdo!!!

Reply to
Ian Field 

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Why?

You expected him to reply with something you would have liked, like a
baseball bat up your ass?
Reply to
John Fields

Nope - for the past few days he's been demanding that I give his arse a good pounding, then threatened to do my kneecaps when I refused!

You two had a tiff and he's missing the choccolate porridge?

Reply to
Ian Field

That option for the "Filter" you have, should be used. I have MT on it. I see him only in replies of others, and is still, that is to much.

Jamie

Reply to
Jamie

You really do have an ass fetish, like DimBulb. You two *must* be related.

Reply to
krw

Whenever JF runs out of arguments he always resorts to anal penetration insults - it seems pretty close to his first instincts.

Reply to
Ian Field

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