Mosfets

I've got an NPN mosfet (tip120). The collector is connected to Vcc, the emitter to the anode of an led and its cathode to ground. What i dont understand is why when i connect base to 5V it and collector to 3.3v it lights up more than when i connect base to 3.3V and collector to 5V.

Also, do you guys know where to search for certain mosfets, cause i want to build a simple h bride, but i dont know which to use.

Look up the difference between a MOSFET and a BJT

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The TIP120 is an NPN darlington pair - ie a bipolar device, not a MOSFET.

MOSFETs come as p-channel or n-channel.

You have the wrong circuit for your TIP120, you must low-side switch a load with an NPN, not high-side switch - you have an emitter follower circuit, in which the emitter voltage follows the base voltage (less 2 diode drops).

So emitter to ground, collector to LED cathode, LED anode to +ve supply (with suitable current limiting resistor in series).

And the base connects via a 1k resistor to the controlling pin. Because darlington pairs have a switching gain of about 1000 a 1k resistor is usually fine for any load.

A darlington pair is an arrangement of 2 BJTs (bipolar junction transistor, or just "transistor").

MOSFETs are completely different in function to bipolar devices, then respond to an electric field produced by an insulated gate electrode - no current flows in the gate at steady state, the device is voltage controlled. You cannot interchange the two types of device.

The LED needs a current limiting resistor. Without it, you can fry the LED or the transistor, or both.

You can also fry a transistor with 3V or 5V across the base-emitter junction. Like mark says, that needs a current limiting resistor too.

[u]MOSFET driver circuit[/u]

[u]Transistor driver circuit[/u]

(In both cases, you can leave-out the diode if you're not driving an inductive load.)

Thanks guys. Sorry about the tip120, i saw someone's post about mosfets and they talked about this transistor.

-How to calculate the base-emitter junction? I'm still confused what resistor I need?

-In a p-channel mosfet, in what direction the the current flow? From drain-source or source-drain? And how do you turn it on, with a positive signal or ground?

Thanks!

icedgoal: Thanks guys. Sorry about the tip120, i saw someone's post about mosfets and they talked about this transistor.

-How to calculate the base-emitter junction? I'm still confused what resistor I need?

Calculate the base-emitter junction? Huh? Its a transistor, it has a base emitter junction sitting there, why would you calculate it?

-In a p-channel mosfet, in what direction the the current flow? From drain-source or source-drain? And how do you turn it on, with a positive signal or ground?

everything is the exact opposite of n-channel