MOSFETs have three terminals: the gate (G), the source (S), and the drain (D) (as shown in the figure).

The operating principle of MOSFETs is similar to that of BJT discussed in the previous issue, but there are differences.

In a BJT transistor, the current generally flows from the base to the emitter, determining how much current can flow from the collector to the emitter.

However, for MOSFETs, how much current flows from the drain to the other source depends on the voltage between the gate and the source.

Opening the MOSFET

When the voltage between the gate and the source is higher than the transistor's threshold voltage, the MOSFET opens. For example, if the gate-source threshold voltage is 2.1V.

In fact, the threshold voltage of MOSFET is the voltage at which it closes. Therefore, to properly open the MOSFET, you need a slightly higher voltage.

How high the voltage should be depends on how much current you want to pass. If it's just a few volts above the threshold for low-current things like opening an LED, it's usually enough.

However, even if you use a high enough voltage to allow 1A of current to pass, it doesn't mean you'll get 1A; it just means you want 1A to pass, and the actual characteristics of the circuit connection determine the actual current.

As shown in the figure, the value of R1 is not important, but it will work fine at around 10kΩ. Its purpose is to turn off the MOSFET. R2 is used to set the LED brightness, and for most LEDs, 1kΩ works well.

Closing the MOSFET

When a voltage is applied between the gate and the source, the voltage will remain until discharge, similar to a capacitor.

When there's resistor R1, the gate-source capacitance has a discharge loop, and the transistor will close again. If there's no R1, the transistor won't close.

Regarding Gate Current

If you want to control the MOSFET, for example, with a microcontroller, you need to pay attention to one thing: the gate current when the transistor is turned on.

As mentioned earlier, the gate-source of the MOSFET is like a capacitor; once charged, no more current flows through. So, after the MOSFET is turned on, there is no current at the gate.

However, when the MOSFET just turns on, there will be a current, just like when you charge a capacitor. In a very short period, there might be a lot of current flowing.

When choosing MOSFETs, pay attention to two points:

1. The gate-source threshold voltage. You need a higher voltage to open the transistor.

2. The continuous drain current, which is the maximum current flowing through the transistor.

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