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Topic: My Mosfets not turning on... (Read 2470 times) previous topic - next topic

Jeremyvnc

Hey all,
  I hate to be the one to post another MOSFET problem and I've searched  through the other posts to see if anyone else has had my problem to no avail.  So, I've got a MOSFET (Digikey: 497-2477-1-ND).  I'm using it to control an array of IR emitters, each at 100mA.  My issue is, when I turn on the output of the arduino, the Mosfet does nothing.

I have it connected to the arduino through a 1K resistor on the Gate and the Gate pin also has a 100K resistor to gnd.  The Source side is to gnd and the array is connected on the drain side.

The Array is comprised of 48 IR emitters with this configuration: 5V->led->led->18ohm resistor ->Drain of MOSFET

This Mosfet is probably overkill but it's all I had in my bin at the time.

Any suggestions would be greatly appreciated!

Thanks,
Jeremy

retrolefty

Your problem is that you didn't select a [glow]LOGIC LEVEL [/glow]n-channel MOSFET transistor. Your mosfet requires at least +10vdc to fully saturate the mosfet on. Don't confuse the gate threshold spec (gate voltage where the mosfet just starts to conduct) with the gate voltage value needed to be fully on. As the Arduino digital output pins can only reach +5vdc, a logic level mosfet is required. Most mosfet manufactures put an L in the part number to show it's a logic level device, but checking the data sheet is the best method.

Lefty

Jeremyvnc

Ah, that would make sense.  So looking around, I see this one:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=2SK2503TLCT-ND&itemSeq=84819135&uq=634063550797420201

That looks promising.  What do you say lefty? Btw, anything else seem out of wack to you?  My experience with these buggers isn't high.

retrolefty

#3
Apr 09, 2010, 04:59 am Last Edit: Apr 09, 2010, 05:02 am by retrolefty Reason: 1
That should work fine. Sounds like you will be using an external +5vdc power source for the total current demand of all 48 LED strings. When using external power sources, you must wire a common connection between the external power's ground side and the Arduino ground pin. Otherwise the MOSFET will not operate, as the source/gate voltage potential will not be felt by the mosfet.

Logic level N-channel MOSFET transistors are my favorite method of interfacing things that needed higher DC voltage or higher DC current then an Arduino can supply directly.

Lefty

Jeremyvnc

Retro, thanks for the advise!  The part about the common gnd is covered.  I'd say it's too simple of a thing to forget but I remember my college days... Ouch.

-Jeremy

Jeremyvnc

Hey Lefty,
  I'm having some issues with the LED array not lighting.  I have taken out all other LEDs minus 1 set (2 LEDs) and instead of the mosfet that is on order, I've tried using a Darlington Transistor and a 2N3904 and both of them just make the 2 LEDs barely light.  When I use my multimeter on it, the arduino output when using the darlington drops to around .8-1 volt.  On the 2N3904, it drops to 1.2-2 volts.  On the collector side, the voltage should drop to 0 when the transistor is on and it should be around 3.7 when it's off.  It currently has  about 2 volts when on, 3.7 when off.  I took out the 100K resistor and I've tried taking out the 1K resistor for the Darlington.

Any Ideas?  Here are the schematics of the array and the mosfet circuit:


Thanks for any help.  I'm just trying to get something going while I wait for shipment.

-Jeremy

retrolefty

#6
Apr 10, 2010, 01:32 am Last Edit: Apr 10, 2010, 01:34 am by retrolefty Reason: 1
What is the forward voltage drop of your IR LEDs. +5vdc may not to enough voltage to bias both LEDs on, plus the curreent limiting resistor and finally including the transistor emitter/collector voltage drop. Darlingtons do suffer from a higher collector/emitter voltage drop as I recall. You can try jumpering out one of the two LEDs in your series string and see if the other gets brighter.

The MOSFET should have a better time with it as they have a very low source/drain voltage drops.

Lefty

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