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### Topic: LED driver circuit (Read 3439 times)previous topic - next topic

##### Dec 27, 2009, 04:57 pm

I am trying to build a lamp using a tilt user interface, I have the sketch and the interface figured out, but since I don't want to run 110 AC through my Arduino I decided to use an LED. Is this circuit right in terms of controlling my 3 watt LED?

#### Grumpy_Mike

#1
##### Dec 27, 2009, 07:53 pm
Two points.
1) You need a 100R resistor in the gate to limit switch on current.
2) You can't control a 3W LED like this because you need something to limit the current. A resistor or just a low voltage won't hack it. See this discussion:- http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1234273497

#### retrolefty

#2
##### Dec 27, 2009, 08:29 pm
In addition to GM's points, you need to make the type of MOSFET device known. Most won't turn on completely with just the +5vdc output of a digital output pin when high. A logic level MOSFET is required for direct connection to a output pin.

Lefty

#### westfw

#3
##### Dec 27, 2009, 09:50 pm
And a 3.3V supply is low for most modern high-power LEDs (it might prevent you from blowing things up without the current limiting resistor, though.)

I would think that you could get away without a gate resistor for most moderate-power logic-level MOSFETs...

#### Grumpy_Mike

#4
##### Dec 28, 2009, 11:30 am
Quote
I would think that you could get away without a gate resistor for most moderate-power logic-level MOSFETs

I am sure you would get away with it but you would exceed the absolute maximum current output for the output pin for a very short length of time. Who know what long term damage that might do, maybe nothing but maybe it will reduce the lifetime of the arduino.

#### westfw

#5
##### Dec 30, 2009, 03:58 am
Quote
you would exceed the absolute maximum current output for the output pin for a very short length of time.
Do you know the practical gate capacitance limit below which you no longer need to worry at all?  After all, inputs to additional CMOS logic gates are also capacitive, and no one expects you to need current limiting resistors to connect "reasonable" fanouts of additional logic...

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