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I'm trying to design a circuit to switch a voltage source between 12V and 24V for a load that draws 400mA. Initially I thought I could use a SPDT analog switch but the ones that can tolerate >24V have too high a Rds(on) (>20ohm) and therefore would drop too much voltage.

Instead I believe I need to implement this using transistors so I was thinking of a setup similar to this except using an NMOS instead of the BJT and where the on/off switch is controlled from an MCU (image sourced from https://forum.allaboutcircuits.com/threads/switching-between-two-voltage-sources.75408/):

enter image description here

Is there a neater solution to do this where one "enable" signal can disconnect the 12V and connect the 24V? The default configuration should be a 12V output. Any help would be greatly appreciated and schematic diagrams would also be useful.

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  • \$\begingroup\$ How exactly do you want to switch the sources? Mechanically? Or with some signal? If with a signal, what's the voltage of that signal? \$\endgroup\$
    – Ilya
    Mar 19, 2022 at 23:30
  • \$\begingroup\$ Is there any reason why you wouldn’t just use a diode in the path from the 12V (5V?) supply? That way the low voltage supply will be used when the higher voltage supply is switched off. \$\endgroup\$
    – Frog
    Mar 19, 2022 at 23:51
  • \$\begingroup\$ @Ilya it would be switched with a 3.3V signal from the MCU \$\endgroup\$
    – sophia_ash
    Mar 20, 2022 at 0:04
  • \$\begingroup\$ @Frog that definitely is a feasible solution, I suppose the only thing is I was trying to avoid the voltage drop of the diode. \$\endgroup\$
    – sophia_ash
    Mar 20, 2022 at 0:05
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    \$\begingroup\$ You could add a second bipolar to invert the signal again, so one rail is switched on while the other is switched off. Whatever your solution though, be careful to avoid momentarily shorting the supplies together during switchover. If you can spare an extra IO line from your micro you could add a short delay between switching one rail off and the other on (best done under interrupt mask to avoid the delay being extended by an interrupt). \$\endgroup\$
    – Frog
    Mar 20, 2022 at 0:13

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