I have used commonly-available BJTs such as the 2N2222 and 2N3904 as switches by operating them in "saturation mode" from my MCU. I believe, however, that for these sorts of applications, a MOSFET is a more appropriate device. I have a few questions, however.
1) Does a MOSFET have a "saturation mode" like the BJT does? Is this "saturation" achieved by simply providing a high enough voltage on the base that the MOSFET is completely "on"?
2) Is it safe to drive the MOSFET directly from the MCU? I understand that the gate of the MOSFET behaves like a capacitor, and therefore draws some current while "charging", and then none thereafter. Is this charging current high enough to damage the MCU pin? By placing a resistor in series with the gate, I can protect the pin, but this will slow down the switch, possibly resulting in high heat dissipation by the MOSFET?
3) What is a common "hobbyist" MOSFET suitable for various low-power situation? I.E., what's the MOSFET equivalent to a 2N2222 or 2N3904?
Best Answer
Many power MOSFETs require a high gate voltage for high-current loads, to ensure that they are fully turned on. There are some with logic-level inputs, though. The data sheets can be misleading, they often give the gate voltage for 250 mA current on the front page, and you find that they need 12V for 5A, say.
It's a good idea to put a resistor to ground on the gate if a MOSFET is driven by an MCU output. MCU pins are usually inputs on reset, and this could cause the gate to float momentarily, perhaps turning the device on, until the program starts running. You won't damage the MCU output by connecting it directly to a MOSFET gate.
The BS170 and 2N7000 are roughly equivalent to the BJTs you mentioned. The Zetex ZVN4206ASTZ has a maximum drain current of 600 mA. I don't think that you will find a small MOSFET that can be driven from 3.3V, though.