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Simple LED question

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poodlenuggets

New Member
Attached is a schematic of an LED circuit I thought would work, but when I hook it up, the LED only dims when the gate of the NFET is set to GND. I've taken it off the MCU port and shorted the gate to GND, but the LED is still slightly on and draws 300µA of current.

Also attached is the datasheet of the NFET. Am I not understanding how to use a transistor?
 

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  • LED.GIF
    LED.GIF
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  • NFET BSS138.pdf
    121.1 KB · Views: 119

k7elp60

Active Member
Your resistor R1 is to high a value for a 3V supply. For a red, yellow, or green LED with a Vf of approximately 2V you need a calculated value for R1 of 50Ω. But I would use a standard value of 68. The fet should turn on with a + voltage from the MCU. Are you sure you have the source connected to ground and the drain connected to the LED?
 

poodlenuggets

New Member
I've checked it over many times because it confounds me that this simple circuit is not working like I thought it would. What does work is the attached schematic - I measured on the multimeter and the current drops to 0µA when the gate is grounded, but it's certainly not the setup I remember from school.
 

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  • led2.GIF
    led2.GIF
    2.3 KB · Views: 120

k7elp60

Active Member
I've checked it over many times because it confounds me that this simple circuit is not working like I thought it would. What does work is the attached schematic - I measured on the multimeter and the current drops to 0µA when the gate is grounded, but it's certainly not the setup I remember from school.
Perhaps you are confusing a mosfet with a Jfet. The Jfet current will normally increase dramatically when the gate to source voltage is 0.
A n channel mosfet requires a + voltage between the gate and the source to turn on.
I think if you use the original circuit you posted with a change if the value of R1 to 68 ohms it should work fine if the mosfet is good.
 
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