I'm building a circuit that stays powered for some time after an additional power source is switched off. enter image description here

+5V is always enabled but I only want LOAD to be powered when VCC (12V) is enabled and for some time (around 5 mins) after it is disabled. To achieve this I'm using a capacitor C1 which keeps NMOS open. There are some questions:

  1. Is this circuit looks good enough or are there some issues here?
  2. R1 + R2 defines how fast the C1 discharges (and given the desired time, defines C1 capacitance). How big resistance can I choose for R1? I don't really care about how fast it works but my guess is that with too big value it may fail.

Appreciate any help.

  • 1
    \$\begingroup\$ potential issues: 1) as the voltage across C1 drops, the NMOS will not switch off abruptly, it will gradually switch off. Your load needs to be able to handle that. 2) 5 minutes is at the edge (maybe even over the edge) of what you can reliably do with such a simple RC timer. I would stay under 1 minute, if longer is needed use a microcontroller or oscillator+counter. \$\endgroup\$ Commented Nov 27, 2020 at 8:14
  • 2
    \$\begingroup\$ OPEN (as in "open circuit") means "disabled" in EE; CLOSED (as in a closed or conducting switch or device) means "enabled" in EE. It's not advisable to use fluid analogies when talking about EE problems. As noted and hinted, if your load resistance normally takes a lot of current, as the MOSFET gradually turns off (deactivates or opens), there is a distinct possibility that the MOSFET will overheat and die. \$\endgroup\$
    – Andy aka
    Commented Nov 27, 2020 at 10:05
  • \$\begingroup\$ In my simulated scenario, if the Load pulls 1A, the NMOS will reach peak wattage at 1.25W. Should be managable. \$\endgroup\$
    – JonathanG
    Commented Nov 27, 2020 at 11:26

1 Answer 1


Some assumptions were made by me during simulation, but the NMOS will gradually turn off after 375 seconds. It will take approximately 20 seconds to fully turn off under my simulated conditions. YMMV.



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