PLEASE NOTE: The schematic is slightly wrong. It is a MOSFET in my circuit, not the BC850 BJT. The actual part is: BVSS138LT1G

Link to MOSFET part

Problem Summary: In the circuit below, I'm turning on the relay by driving the gate of the MOSFET with a microcontroller GPIO pin - I believe it's 3.3V/12mA driving signal. However, when I do this my VDD_5V line becomes about 2.7V.

This seems like a fairly common relay driving circuit, so I'm mystified as to what is going on here!

My initial thought is add a resistor somewhere. I tried adding a resistor between the source of the MOS and GND but I can't get enough voltage for the relay to close (it needs 3.5V at least to open). I might try adding a resistor between pin 2 of the relay and the drain of the MOS, but this is on a PCB and it is tedious to cut traces so I want to be more certain of the approach to take.

The relay coil current from the specs is 74.9 mA (however when I turn it on with a power supply I get less than that... closer to 40mA).

5V Supply Part

Relay Part

Schematic Schematic

Layout Layout

  • 1
    \$\begingroup\$ Where does your 5V supply come from? Could you show the relevant part of the schematic? \$\endgroup\$ Jan 26, 2022 at 17:06
  • \$\begingroup\$ @JonathanS. Yes certainly! Thanks for the quick reply. Here is a link to the part -> 5V Supply Datasheet \$\endgroup\$
    – Bandito
    Jan 26, 2022 at 17:09
  • \$\begingroup\$ What happens to the input voltage of the buck converter when the 5V rail collapses? Also, does the FET get hot? \$\endgroup\$ Jan 26, 2022 at 17:15
  • 1
    \$\begingroup\$ It'll be fine if you supply 5V externally. Check that CR4 is installed correctly, by the way. \$\endgroup\$ Jan 26, 2022 at 17:31
  • 1
    \$\begingroup\$ Your CR4 footprint looks suspicious ... Seems to me like it's backwards. \$\endgroup\$
    – brhans
    Jan 26, 2022 at 17:40

1 Answer 1


For anyone curious about this - the problem was my buck converter. Unclear exactly what the problem was, but I'm assuming it was a faulty unit. I ordered a couple new ones to add and test.

  • \$\begingroup\$ Please edit your answer to explain what was wrong with the buck converter. \$\endgroup\$
    – Null
    Jan 31, 2022 at 18:42

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