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Topic: Using N-channel MOSFET to control LEDs? (Read 2 times) previous topic - next topic

zenwebb

I'd like to sink about 16 LEDs through an N-channel MOSFET, and I'm curious if I've set up the schematic correctly. I've only used transistors and Darlingtons before, so I'm not 100% sure if I've got this circuit just right. Do I need a resistor to ground? If so, what value?

The MOSFET I've spec'ed is: http://www.digikey.com/product-detail/en/DMN3404L-7/DMN3404LDICT-ND/2052805

Grad student, creative technologist, OSHW engineer
http://jason-webb.info

CrossRoads

Basically yes. If any one LED has a different voltage drop than its 3 neighbors, the current thru them will be unequal and may lead to failed parts.
The part you selected is not a Logic Level MOSFET, but it should work okay

42m? @ VGS = 4.5V
Check your planned current thru it to make sure it will not overheat.
Having a pulldwn to Gnd on the gate will keep it off during reset/sketch startup when the Arduino outputs are inputs and undefined (i.e. floating).
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

zenwebb

Cool, I am planning on using all the same LED, with a maximum current draw of 1.6A. On the Digikey product page it is listed as a "Logic Level Gate", what's up with that?

I am planning to use PWM to vary the brightness, which I expected to not be a problem. Am I right about that?

Would 100ohm suffice for the pulldowns?

Grad student, creative technologist, OSHW engineer
http://jason-webb.info

zenwebb

*bump* Would 100ohm suffice for pull-downs, or would I need to spring for a 1k or 10k resistor?
Grad student, creative technologist, OSHW engineer
http://jason-webb.info

fungus


*bump* Would 100ohm suffice for pull-downs, or would I need to spring for a 1k or 10k resistor?


100ohm is much too small - an Arduino pin wouldn't even be able to pull the gate high (5V 100 Ohms allows 50mA to pass, an Arduino pin is only rated to 40mA).

10k is much better.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

zenwebb

So, just to get my thoughts straight; 100ohms in series with the gate, with a 10k resistor going to ground for the pulldown?
Grad student, creative technologist, OSHW engineer
http://jason-webb.info

CrossRoads

No, 200  or 220 in series with the gate, 10K pulldown.
100 would let way too much current flow from the arduino pin.

Part has good specs for logic level use. Often you can find a part where the data sheets says right up front it is designed for logic level operations or similar words.
I did not see that when I looked, but the low Rds with Vgs = 4.5V is what you after.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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