![]() ![]() When driving a motor, you’re unlikely to notice much difference between an Arduino driving the gate at 5 V and a Raspberry Pi driving it at 3.3 V. The difference here is, admittedly, slight. ![]() For example, at a V GS of 1.5 V, R DS(ON) is 840 mΩ, while at 4.5 V, it is just 235 mΩ. Referring again to the SSM3K56FS, the reader will notice that the on-resistance value, R DS(ON), is dependent on V GS. There is a further point to consider when reading the datasheet. With an Arduino supplying 5 V to the MOSFET gate the switch turns on, V2 emulates the 5 V output from an Arduino I/O pin, while R2 is used as the load instead of a motor (we’ll ignore the difference between a resistive and inductive load). To demonstrate this, the circuit has been simulated in LTspice. A resistor (1 MΩ) between the gate and the ground ensures that the MOSFET remains off should the control signal from the Arduino become detached. Thus, we can use a 5 V Arduino output pin connected to the gate of an SSM3K56FS, connect the source to the ground, and attach a motor between a 15V supply and the MOSFET’s drain. Such MOSFETs can be used as low-side switches, meaning they are placed between the load and the circuit ground in a simple low-voltage DC application. For example, the Toshiba SSM3K56FS, a small high-speed switching device, gives V th as between 0.4 V and 1.0 V when the drain-source voltage (V DS) is 3.0 V and for a drain current (I D) of 1 mA. Pull out any n-channel MOSFET datasheet, and you’ll quickly find this value. The voltage at which this occurs is the threshold voltage, V th. N-channel MOSFETs require a higher voltage on their gate than the voltage on the source to turn on. ![]() Low-Side n-Channel MOSFETs for Switching MOSFETs, specifically enhancement-mode MOSFETs, come in two types: n-channel and p-channel. A MOSFET enables boards, such as an Arduino, to control large loads, ![]()
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