I have two MOSFET circuits; one is working and the other fails and I'm not sure why. I just want to turn on/off the LED, nothing special, no fast switching.

Below is the working circuit (series gate resistor):


simulate this circuit – Schematic created using CircuitLab

Below is the non-working circuit. By non-working I mean when I apply a '0' to the gate, there is 2V sitting on the gate and the LED dimly comes on. Why is this happening?


simulate this circuit

I understand that a MOSFET has high gate impedance and a gate resistor is not a necessity but I wouldn't expect the MOSFET to completely not work right. Am I missing something?

  • \$\begingroup\$ Is this the same circuit with and without the 10K gate resistor or two different MOSFETs? If the latter is the case, does it work with the gate resistor added? Also, what MCU are you using? \$\endgroup\$ – Fizz Nov 12 '15 at 17:51
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    \$\begingroup\$ I took a look at the output of the power supply on my 100Mhz scope and I see a ringing of 25Mhz on start up. It only peaks around 6.64V though. The datasheet says the gate resistance at 1Mhz is 4.4 Ohms and the part can only dissipate 1.1W. That may be the issue. Does this seem right? \$\endgroup\$ – Craigfoo Nov 12 '15 at 19:09
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    \$\begingroup\$ I had a 19" jumper from the power supply to the button then another 19" jumper from the button to the gate of the MOSFET. \$\endgroup\$ – Craigfoo Nov 12 '15 at 19:21
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    \$\begingroup\$ You are getting parasitic oscillations which can be destructive .Gate resistance is the accepted first line of defence against parasitic oscillations .This is why your version with the gate resistor survives. \$\endgroup\$ – Autistic Nov 13 '15 at 5:56
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    \$\begingroup\$ You're right on the LED resistors. I meant to put a 220 Ohm, just didn't do it when I drew the circuit. \$\endgroup\$ – Craigfoo Nov 13 '15 at 17:18

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