Which pull-up resistor for this Mosfet ?

Hi, I have a N-channel FET type FQP30N06 which i am trying to use to drive some leds, which pull-up resistor should i use ? I use an ESP8266 which uses 3.3v and the voltage the led works at is 12v. I also use PWM to write to the Leds.

My problem is that the led stays on (using 10k R) even if i have not increased the output which leads me to believe that i don't have the right resistor.

/donnib

Try FQP30N06*L*

Note the 'L' at the end of it.

If you are going to apply a certain voltage to turn on an N-channel mosfet..... look for mosfets that have a "Vgs" threshold value that's substantially lower than the applied voltage.

Eg.... applying 3.3 V...... then look for components with Vgs (threshold) of say 2.5V .... or even 2.7V.

With the FQP30N06 (which doesn't have the "L" on the end), the Vgs threshold value of some sources say 4V or around there. Is 4V substantially lower than 3.3V? Answer.... nope.

My problem is that the led stays on (using 10k R) even if i have not increased the output which leads me to believe that i don’t have the right resistor.

You don’t have the right FET, you do not pull up the gate of an FET with a resistor anyway.

donnib: Hi, I have a N-channel FET type FQP30N06 which i am trying to use to drive some leds, which pull-up resistor should i use ? I use an ESP8266 which uses 3.3v and the voltage the led works at is 12v. I also use PWM to write to the Leds.

My problem is that the led stays on (using 10k R) even if i have not increased the output which leads me to believe that i don't have the right resistor.

/donnib

You should not be using a pull up resistor. You may have a series current-limiting resistor to the gate, and a gate-pull-down resistor.

For 3.3V use you'll require a MOSFET rated for 3.3V gate drive, most are not.

MosFets are voltage controlled devices. The gate draws essentially no current. The voltage you put on the gate creates a field in the device (hence the name).

Specific to your application:

Look in the datasheet for a specification called Vgs (threshold). In the Fairchild datasheet it is on page 2. It is the top line of "On Characteristics".

It is specified as somewhere between 2 and 4 volts. This means the FET will START to turn on (or turn on slightly) somewhere in that range.

Now look at Figure 2. It is telling you to get a few amps of FET current the voltage at the gate must be above 5V or so.

The long and short of it is that (as the others have said) you must find a MosFet with a Vgs below the 3.1 - 3.2 volts you will be putting on the gate.

Gate Resistor: It is a good idea to put a couple of kilo ohms in series with the gate so if the MosFet fails it will not take your processor with it.

Good luck.

You need to back up a bunch and explain why you want to pull up an n-channel enhancement mosfet.

Also, running a fet with an underpowered gate so that it is not fully on is a sure way to kill it, as that is the region it gets toasted.

The pullup resistor would make this n-channel mosfet operate as “normally on”, right? I guess… if that’s the intention, then ok.

Doubt it's even a problem with the mosfet, it's probably a problem in understanding how to use the ESP8266 (which we have no idea which board they are using). Wrong pin assignment, pin conflict, whatever.