Correct me if I am wrong
There are 3 modes for mosfet: 1, cut-off; 2, active (triode); 3, saturation; When VgsVt, it's mode 2; when Vgs>vt & Vds>=Vgs-vt, it's mode 3;
For N-mosfet, the drain is positive side, and source is the negative side, and the load should be connected to the drain side.
Here are my doubts
1, P0903BDG n-mos has 3 Vt values: min, typical, max (1, 1.6, 3V), which one should I use? Since this Vt is so critical here.
2, I setup a simple testing circuit with 4.07V Vdd -- red led -- 100 ohm -- drain, source -- ground and have a 50k pot connected to the gate.
According to Vgs>vt & Vds>=Vgs-vt and so when it's in saturation mode, Vds should be equal to 0v in my circuit. so Vds>=Vgs-vt == 0=Vgs-(1-3), and the result for Vt should be equal to (1-3) when it's in saturation.
Here are My measurements: when Vgs<1.2v, it's in cut off mode; When Vgs=1.3v, the led start to light up slightly; Vgs=1.3-2v = active mode; when Vgs=>2.1V, it's in saturation mode. When it's in sat mode, Vds=0, 0=vgs-vt, 0v=2.1v-2.1v. so Vt=2.1v for this mosfet? It can't be because when Vgs=1.3V, the led is already On. SO I am very confused here. What exactly is Vt? what is the exact problem here?
3, I want to operate this simple LED circuit, so when the pot is at 0 position, the led is off; when I turn the pot slightly, the led is slight on; and when I turn the pot to the other end, the LED is totally on with the mosfet in saturation mode. 1.2-2.1v is the range with my measurement, but how do I do it mathematically?
4, This should be a 50W mosfet. Yesterday, I tested this mos with a 12v vdd and a 12v power led. 12v-led-drain/ source-ground/ gate-10kohm-12v; and the current is just 550ma. It's not bright enough if I were connect the led directly to 12v power source. what is the problem here?