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I am building a lamp using 5 1-Watt LEDs, an 18V DC power supply and an Arduino for dimming/controlling it. Im using a simple design (below) for the constant current driver (as was used here for example). The MOSFET I have is not logic level (it's a FQP50N06) but it opens enough at 5V to pass the 350mA that feed the series of 5 LEDs.

This circuit works fine when the Arduino is On; my problem is when the MC is turned Off, and the PWM pin is (I guess) floating, the LEDs are still powered (though slightly dimmer). I am looking for a way to modify this circuit so that M1 shuts down when the MC is Off. (Assuming that the power supply V1 is still On.)

Any idea? thanks!

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ You should not allow FET gates to float. Put a large resistor to ground from the gate of the FET. That way when it is not actively driven, it will pull the gate down to ground and turn completely off. \$\endgroup\$ – jbord39 Oct 7 '16 at 13:00
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You should add a resistor (say 100K) from the left side of R1 to ground. That way it will not divide down your already too-low gate voltage.

If you are actually removing the supply voltage from the Arduino this will not work if you are breaking the ground connection. If you break the 5V connection the internal protection network on the ATMega chip will supply a few volts. Only break the +5V side, if you must break something.

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There may be leakage bias thru R1 from your Arduino "switch" , so a shunt R across Vgs of FET (or Vf to ground ) should pull it down. Try 100k towards 22k. R1 can also be reduced if you need to go to 10k.

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