I am working on a design where the input (48 VDC) has to be supplied to I/O when a particular condition is met and the rest of the time it should be grounded.

A relay is ideal for this, but the PCB doesn't have space to accommodate that as the size of the overall board is 20x20 mm which has to fit one RGB LED, an LDO, a 5-pin connector, a TSSOP20 Micro and caps and resistor. Since the current requirement is under 50 mA, a relay isn't suitable for this design.

The switching voltage will be typically 48 V and 60 V (max). The ground is common between 48 V and the microcontroller which will be controlling the switching circuit.

I am thinking of using either an NPN & PNP switching circuit or an N-channel MOSFET circuit to achieve this. Here are the two circuits that I am planning to use. I am not sure if it's the best way and whether there is anything that needs to be taken care of. Which part should be used for such an application?

NPN-PNP circuit:


MOSFET circuit:

enter image description here

  • \$\begingroup\$ If you have a space constraint, please describe the constraint to avoid having people provide solutions that won't fit. \$\endgroup\$
    – vir
    Commented Sep 19, 2022 at 17:38
  • \$\begingroup\$ I have updated my question with details. \$\endgroup\$
    – MICRO
    Commented Sep 19, 2022 at 17:49
  • \$\begingroup\$ How much power does the switched device need? \$\endgroup\$
    – jonathanjo
    Commented Sep 19, 2022 at 17:57
  • \$\begingroup\$ Not more than 50mA. \$\endgroup\$
    – MICRO
    Commented Sep 19, 2022 at 18:05
  • 1
    \$\begingroup\$ The NPN/PNP circuit is useful here, could also be implemented using N- and P-MOSFET. Forget the other circuit, it won't work. \$\endgroup\$
    – Jens
    Commented Sep 19, 2022 at 18:24

1 Answer 1


In order for your second circuit to work the gate of the MOSFET would have to be 48 + Vth Volts, perhaps as much as 52 V.

The first circuit will work fine.

The circuit that I present is a favorite. The BJT is easy to drive from the modern low voltage circuits. The P-channel MOSFET switch is used for its low saturation voltage delivered by low \$R_{DSon}\$. 50 mA is easy to handle. The circuit will dissipate very low power. Choose \$R_{2}\$and \$R_{3}\$ to provide the \$V_{GS}\$ specified to get the low \$R_{DSon}\$. 10v for \$V_{th}=4V\$ is typical, be sure to set for the FET that you choose. Surface mount parts will be a good choice for pace optimizing.


simulate this circuit – Schematic created using CircuitLab

Edit: After a second look The protection for the base is not required.

  • \$\begingroup\$ How does D1 provide protection to the base of BJT? What's the purpose? Can I connect it to +5 V because micro is also running on +5 V? Which diode to use, 1N4007? \$\endgroup\$
    – MICRO
    Commented Sep 19, 2022 at 21:38
  • \$\begingroup\$ After a second look , the protection is not required. \$\endgroup\$
    – RussellH
    Commented Sep 19, 2022 at 23:01
  • \$\begingroup\$ Okay, that makes sense. Which part to use for the P-channel FET and BJT? The +48V is a typical voltage and the maximum can go up to 60V. \$\endgroup\$
    – MICRO
    Commented Sep 20, 2022 at 12:45
  • \$\begingroup\$ Breakdown voltages >80V. I don’t have a preference. \$\endgroup\$
    – RussellH
    Commented Sep 20, 2022 at 16:58

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