I couldn't find a definitive answer.

What's more suitable to drive a 5v relay from a 3.3v MCU, a MOSFET or a BJT Transistor? NPN or PNP?

I will be using a Schottky SS34 as flyback diode. Relay consumes 75mA. I can use NPN transistor MMBT3904LT1G or N-Channel MOSFET BSS138 for turning on the relay, emmiter/source to ground, would that be ok?


1 Answer 1


The BSS138 is not characterized for Rds(on) with 3.3V drive, and the MMBT3904 is not characterized for Vce(sat) at 75mA so neither one is great.

There are plenty of better parts such as MMBT4401 or AO3414 that are properly specified to work reliably. Either one will work fine, with the proper choice of base resistor etc.

  • \$\begingroup\$ I have some AO3400, can I use that? If not I'll get MMBT4401. Will it make any difference for this application either an N-FET or NPN BJT? Thanks. \$\endgroup\$
    – Luna
    Jun 18, 2018 at 5:57
  • \$\begingroup\$ The AO3400 is spec'd at 2.5V & has very low Rds(on) so it would be excellent. Put 1K series gate resistor and 100K pull down ideally. The BJT is cheaper in large quantity otherwise no advantage and it will require a lot of base current to get low drop. \$\endgroup\$ Jun 18, 2018 at 10:52

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