I'm having an issue currently with using an N Channel MOSFET. I would like to control a 12 volt LED using an Arduino Uno. The Uno puts out 5 volts. The issue I am having is that the 12 volt LED stays on but dims if there is no signal being sent from the Arduino. If there is no signal the 12 volt LED should turn off complete. The Uno is programmed with a simple blink program to test the LED. I connected the Gate to a pin on the Uno with a resister in between. The drain is connected to the 12 volt LED from the 12 volts being supplied from the bread board. Source is connected to a ground. There is another pin connected to the blue LED with the same code. So I knew when the 12 volt LED was supposed to blink.

I am using FQP30N06L MOSFET from spark fun. 12v LED. 12 volt wall adapter.

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  • \$\begingroup\$ Try connecting a resistor from the gate to ground (say ~10k). After you first switch on, the gate may stay charged up. So if you are not sending a signal to turn the mosfet off, you need a resistor to discharge the gate to ground. \$\endgroup\$
    – Big6
    Aug 26, 2016 at 4:34
  • \$\begingroup\$ @SixtoCabrera I added the resistor but it didn't make a difference. The 12 volt led stays on \$\endgroup\$ Aug 26, 2016 at 4:41
  • 2
    \$\begingroup\$ Do you know which is the drain and which is the source? Drain is the center leg... Just want to be sure. \$\endgroup\$
    – jonk
    Aug 26, 2016 at 4:46
  • \$\begingroup\$ Reversing D and S may mean the body diode is conducting. \$\endgroup\$
    – jonk
    Aug 26, 2016 at 4:48
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    \$\begingroup\$ @pipe: I haven't inspected the breadboard well enough to see that. So I'll take it from you that it is wired okay. Hmm. Does anyone make an oddball pinout for the TO220 and that device? (The PN2222A from OnSemi has a different pinout as does the P2N2222A also from OnSemi, just as one such example.) \$\endgroup\$
    – jonk
    Aug 26, 2016 at 4:54

1 Answer 1


but dims if there is no signal

It's hard to guess without a real schematic, but I'm making an assumption here: no signal means that it is left floating.

In that case, you need a pull down resistor from the gate to ground, to discharge the internal gate capacitance. Something like this:


simulate this circuit – Schematic created using CircuitLab

The value is not critical, it can be anything between 10 kΩ and 1 MΩ. You can also place it before R1 if you want.

  • \$\begingroup\$ I added the resistor but the 12 led stays constantly lit with no dimming. \$\endgroup\$ Aug 26, 2016 at 4:42
  • \$\begingroup\$ @MichaelNiebauer Maybe your MOSFET is damaged, or you have not connected things as described. Please update your question with a schematic diagram showing how you have connected things, maybe something else is missing. \$\endgroup\$
    – pipe
    Aug 26, 2016 at 4:45

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